is space travel time travel

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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Is time travel possible? An astrophysicist explains

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the  laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s  theory of special relativity  suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the  speed of light  – 671 million miles per hour! – will experience time slower than a person on Earth.

Related: The speed of light, explained

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is  6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves  wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical : Scientists have yet to spot one. It also looks like it would be  incredibly challenging  to send humans through a wormhole space tunnel.

Time travel paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by  throwing a dinner party  where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he  pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the  James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

This article first appeared on the Conversation. You can read the original here .

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Is time travel even possible? An astrophysicist explains the science behind the science fiction

Published: Nov 13, 2023

By: Sarah Hansen, M.S. '15

Written by Adi Foord , assistant professor of physics , UMBC

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago. https://www.youtube.com/embed/QeRtcJi3V38?wmode=transparent&start=0 Telescopes are a kind of time machine – they let you peer into the past.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

This article is republished from The Conversation under a Creative Commons license. Read the original article and see more than 250 UMBC articles available in The Conversation.

Tags: CNMS , Physics , The Conversation

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Is time travel possible? Why one scientist says we 'cannot ignore the possibility.'

A common theme in science-fiction media , time travel is captivating. It’s defined by the late philosopher David Lewis in his essay “The Paradoxes of Time Travel” as “[involving] a discrepancy between time and space time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival … is the duration of the journey.”

Time travel is usually understood by most as going back to a bygone era or jumping forward to a point far in the future . But how much of the idea is based in reality? Is it possible to travel through time? 

Is time travel possible?

According to NASA, time travel is possible , just not in the way you might expect. Albert Einstein’s theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light , which is 186,000 miles per second. Time travel happens through what’s called “time dilation.”

Time dilation , according to Live Science, is how one’s perception of time is different to another's, depending on their motion or where they are. Hence, time being relative. 

Learn more: Best travel insurance

Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories. 

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve space-time .

“In the moment that you carve the space-time, you can play with that curvature to make the time come in a circle and make a time machine,” Alonso-Serrano told USA TODAY. 

She explained how, theoretically, time travel is possible. The mathematics behind creating curvature of space-time are solid, but trying to re-create the strict physical conditions needed to prove these theories can be challenging. 

“The tricky point of that is if you can find a physical, realistic, way to do it,” she said. 

Alonso-Serrano said wormholes and warp drives are tools that are used to create this curvature. The matter needed to achieve curving space-time via a wormhole is exotic matter , which hasn’t been done successfully. Researchers don’t even know if this type of matter exists, she said.

“It's something that we work on because it's theoretically possible, and because it's a very nice way to test our theory, to look for possible paradoxes,” Alonso-Serrano added.

“I could not say that nothing is possible, but I cannot ignore the possibility,” she said. 

She also mentioned the anecdote of  Stephen Hawking’s Champagne party for time travelers . Hawking had a GPS-specific location for the party. He didn’t send out invites until the party had already happened, so only people who could travel to the past would be able to attend. No one showed up, and Hawking referred to this event as "experimental evidence" that time travel wasn't possible.

What did Albert Einstein invent?: Discoveries that changed the world

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USA TODAY is exploring the questions you and others ask every day. From "How to watch the Marvel movies in order" to "Why is Pluto not a planet?" to "What to do if your dog eats weed?" – we're striving to find answers to the most common questions you ask every day. Head to our Just Curious section to see what else we can answer for you. 

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Time Travel and Modern Physics

Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn’t this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics.

1. Paradoxes Lost?

2. topology and constraints, 3. the general possibility of time travel in general relativity, 4. two toy models, 5. slightly more realistic models of time travel, 6. the possibility of time travel redux, 7. even if there are constraints, so what, 8. computational models, 9. quantum mechanics to the rescue, 10. conclusions, other internet resources, related entries.

• Supplement: Remarks and Limitations on the Toy Models

Modern physics strips away many aspects of the manifest image of time. Time as it appears in the equations of classical mechanics has no need for a distinguished present moment, for example. Relativity theory leads to even sharper contrasts. It replaces absolute simultaneity, according to which it is possible to unambiguously determine the time order of distant events, with relative simultaneity: extending an “instant of time” throughout space is not unique, but depends on the state of motion of an observer. More dramatically, in general relativity the mathematical properties of time (or better, of spacetime)—its topology and geometry—depend upon how matter is arranged rather than being fixed once and for all. So physics can be, and indeed has to be, formulated without treating time as a universal, fixed background structure. Since general relativity represents gravity through spacetime geometry, the allowed geometries must be as varied as the ways in which matter can be arranged. Alongside geometrical models used to describe the solar system, black holes, and much else, the scope of variation extends to include some exotic structures unlike anything astrophysicists have observed. In particular, there are spacetime geometries with curves that loop back on themselves: closed timelike curves (CTCs), which describe the possible trajectory of an observer who returns exactly back to their earlier state—without any funny business, such as going faster than the speed of light. These geometries satisfy the relevant physical laws, the equations of general relativity, and in that sense time travel is physically possible.

Yet circular time generates paradoxes, familiar from science fiction stories featuring time travel: [ 1 ]

• Consistency: Kurt plans to murder his own grandfather Adolph, by traveling along a CTC to an appropriate moment in the past. He is an able marksman, and waits until he has a clear shot at grandpa. Normally he would not miss. Yet if he succeeds, there is no way that he will then exist to plan and carry out the mission. Kurt pulls the trigger: what can happen?
• Underdetermination: Suppose that Kurt first travels back in order to give his earlier self a copy of How to Build a Time Machine. This is the same book that allows him to build a time machine, which he then carries with him on his journey to the past. Who wrote the book?
• Easy Knowledge: A fan of classical music enhances their computer with a circuit that exploits a CTC. This machine efficiently solves problems at a higher level of computational complexity than conventional computers, leading (among other things) to finding the smallest circuits that can generate Bach’s oeuvre—and to compose new pieces in the same style. Such easy knowledge is at odds with our understanding of our epistemic predicament. (This third paradox has not drawn as much attention.)

The first two paradoxes were once routinely taken to show that solutions with CTCs should be rejected—with charges varying from violating logic, to being “physically unreasonable”, to undermining the notion of free will. Closer analysis of the paradoxes has largely reversed this consensus. Physicists have discovered many solutions with CTCs and have explored their properties in pursuing foundational questions, such as whether physics is compatible with the idea of objective temporal passage (starting with Gödel 1949). Philosophers have also used time travel scenarios to probe questions about, among other things, causation, modality, free will, and identity (see, e.g., Earman 1972 and Lewis’s seminal 1976 paper).

We begin below with Consistency , turning to the other paradoxes in later sections. A standard, stone-walling response is to insist that the past cannot be changed, as a matter of logic, even by a time traveler (e.g., Gödel 1949, Clarke 1977, Horwich 1987). Adolph cannot both die and survive, as a matter of logic, so any scheme to alter the past must fail. In many of the best time travel fictions, the actions of a time traveler are constrained in novel and unexpected ways. Attempts to change the past fail, and they fail, often tragically, in just such a way that they set the stage for the time traveler’s self-defeating journey. The first question is whether there is an analog of the consistent story when it comes to physics in the presence of CTCs. As we will see, there is a remarkable general argument establishing the existence of consistent solutions. Yet a second question persists: why can’t time-traveling Kurt kill his own grandfather? Doesn’t the necessity of failures to change the past put unusual and unexpected constraints on time travelers, or objects that move along CTCs? The same argument shows that there are in fact no constraints imposed by the existence of CTCs, in some cases. After discussing this line of argument, we will turn to the palatability and further implications of such constraints if they are required, and then turn to the implications of quantum mechanics.

Wheeler and Feynman (1949) were the first to claim that the fact that nature is continuous could be used to argue that causal influences from later events to earlier events, as are made possible by time travel, will not lead to paradox without the need for any constraints. Maudlin (1990) showed how to make their argument precise and more general, and argued that nonetheless it was not completely general.

Imagine the following set-up. We start off having a camera with a black and white film ready to take a picture of whatever comes out of the time machine. An object, in fact a developed film, comes out of the time machine. We photograph it, and develop the film. The developed film is subsequently put in the time machine, and set to come out of the time machine at the time the picture is taken. This surely will create a paradox: the developed film will have the opposite distribution of black, white, and shades of gray, from the object that comes out of the time machine. For developed black and white films (i.e., negatives) have the opposite shades of gray from the objects they are pictures of. But since the object that comes out of the time machine is the developed film itself it we surely have a paradox.

However, it does not take much thought to realize that there is no paradox here. What will happen is that a uniformly gray picture will emerge, which produces a developed film that has exactly the same uniform shade of gray. No matter what the sensitivity of the film is, as long as the dependence of the brightness of the developed film depends in a continuous manner on the brightness of the object being photographed, there will be a shade of gray that, when photographed, will produce exactly the same shade of gray on the developed film. This is the essence of Wheeler and Feynman’s idea. Let us first be a bit more precise and then a bit more general.

For simplicity let us suppose that the film is always a uniform shade of gray (i.e., at any time the shade of gray does not vary by location on the film). The possible shades of gray of the film can then be represented by the (real) numbers from 0, representing pure black, to 1, representing pure white.

Let us now distinguish various stages in the chronological order of the life of the film. In stage \(S_1\) the film is young; it has just been placed in the camera and is ready to be exposed. It is then exposed to the object that comes out of the time machine. (That object in fact is a later stage of the film itself). By the time we come to stage \(S_2\) of the life of the film, it has been developed and is about to enter the time machine. Stage \(S_3\) occurs just after it exits the time machine and just before it is photographed. Stage \(S_4\) occurs after it has been photographed and before it starts fading away. Let us assume that the film starts out in stage \(S_1\) in some uniform shade of gray, and that the only significant change in the shade of gray of the film occurs between stages \(S_1\) and \(S_2\). During that period it acquires a shade of gray that depends on the shade of gray of the object that was photographed. In other words, the shade of gray that the film acquires at stage \(S_2\) depends on the shade of gray it has at stage \(S_3\). The influence of the shade of gray of the film at stage \(S_3\), on the shade of gray of the film at stage \(S_2\), can be represented as a mapping, or function, from the real numbers between 0 and 1 (inclusive), to the real numbers between 0 and 1 (inclusive). Let us suppose that the process of photography is such that if one imagines varying the shade of gray of an object in a smooth, continuous manner then the shade of gray of the developed picture of that object will also vary in a smooth, continuous manner. This implies that the function in question will be a continuous function. Now any continuous function from the real numbers between 0 and 1 (inclusive) to the real numbers between 0 and 1 (inclusive) must map at least one number to itself. One can quickly convince oneself of this by graphing such functions. For one will quickly see that any continuous function \(f\) from \([0,1]\) to \([0,1]\) must intersect the line \(x=y\) somewhere, and thus there must be at least one point \(x\) such that \(f(x)=x\). Such points are called fixed points of the function. Now let us think about what such a fixed point represents. It represents a shade of gray such that, when photographed, it will produce a developed film with exactly that same shade of gray. The existence of such a fixed point implies a solution to the apparent paradox.

Let us now be more general and allow color photography. One can represent each possible color of an object (of uniform color) by the proportions of blue, green and red that make up that color. (This is why television screens can produce all possible colors.) Thus one can represent all possible colors of an object by three points on three orthogonal lines \(x, y\) and \(z\), that is to say, by a point in a three-dimensional cube. This cube is also known as the “Cartesian product” of the three line segments. Now, one can also show that any continuous map from such a cube to itself must have at least one fixed point. So color photography can not be used to create time travel paradoxes either!

Even more generally, consider some system \(P\) which, as in the above example, has the following life. It starts in some state \(S_1\), it interacts with an object that comes out of a time machine (which happens to be its older self), it travels back in time, it interacts with some object (which happens to be its younger self), and finally it grows old and dies. Let us assume that the set of possible states of \(P\) can be represented by a Cartesian product of \(n\) closed intervals of the reals, i.e., let us assume that the topology of the state-space of \(P\) is isomorphic to a finite Cartesian product of closed intervals of the reals. Let us further assume that the development of \(P\) in time, and the dependence of that development on the state of objects that it interacts with, is continuous. Then, by a well-known fixed point theorem in topology (see, e.g., Hocking & Young 1961: 273), no matter what the nature of the interaction is, and no matter what the initial state of the object is, there will be at least one state \(S_3\) of the older system (as it emerges from the time travel machine) that will influence the initial state \(S_1\) of the younger system (when it encounters the older system) so that, as the younger system becomes older, it develops exactly into state \(S_3\). Thus without imposing any constraints on the initial state \(S_1\) of the system \(P\), we have shown that there will always be perfectly ordinary, non-paradoxical, solutions, in which everything that happens, happens according to the usual laws of development. Of course, there is looped causation, hence presumably also looped explanation, but what do you expect if there is looped time?

Unfortunately, for the fan of time travel, a little reflection suggests that there are systems for which the needed fixed point theorem does not hold. Imagine, for instance, that we have a dial that can only rotate in a plane. We are going to put the dial in the time machine. Indeed we have decided that if we see the later stage of the dial come out of the time machine set at angle \(x\), then we will set the dial to \(x+90\), and throw it into the time machine. Now it seems we have a paradox, since the mapping that consists of a rotation of all points in a circular state-space by 90 degrees does not have a fixed point. And why wouldn’t some state-spaces have the topology of a circle?

However, we have so far not used another continuity assumption which is also a reasonable assumption. So far we have only made the following demand: the state the dial is in at stage \(S_2\) must be a continuous function of the state of the dial at stage \(S_3\). But, the state of the dial at stage \(S_2\) is arrived at by taking the state of the dial at stage \(S_1\), and rotating it over some angle. It is not merely the case that the effect of the interaction, namely the state of the dial at stage \(S_2\), should be a continuous function of the cause, namely the state of the dial at stage \(S_3\). It is additionally the case that path taken to get there, the way the dial is rotated between stages \(S_1\) and \(S_2\) must be a continuous function of the state at stage \(S_3\). And, rather surprisingly, it turns out that this can not be done. Let us illustrate what the problem is before going to a more general demonstration that there must be a fixed point solution in the dial case.

Forget time travel for the moment. Suppose that you and I each have a watch with a single dial neither of which is running. My watch is set at 12. You are going to announce what your watch is set at. My task is going to be to adjust my watch to yours no matter what announcement you make. And my actions should have a continuous (single valued) dependence on the time that you announce. Surprisingly, this is not possible! For instance, suppose that if you announce “12”, then I achieve that setting on my watch by doing nothing. Now imagine slowly and continuously increasing the announced times, starting at 12. By continuity, I must achieve each of those settings by rotating my dial to the right. If at some point I switch and achieve the announced goal by a rotation of my dial to the left, I will have introduced a discontinuity in my actions, a discontinuity in the actions that I take as a function of the announced angle. So I will be forced, by continuity, to achieve every announcement by rotating the dial to the right. But, this rotation to the right will have to be abruptly discontinued as the announcements grow larger and I eventually approach 12 again, since I achieved 12 by not rotating the dial at all. So, there will be a discontinuity at 12 at the latest. In general, continuity of my actions as a function of announced times can not be maintained throughout if I am to be able to replicate all possible settings. Another way to see the problem is that one can similarly reason that, as one starts with 12, and imagines continuously making the announced times earlier, one will be forced, by continuity, to achieve the announced times by rotating the dial to the left. But the conclusions drawn from the assumption of continuous increases and the assumption of continuous decreases are inconsistent. So we have an inconsistency following from the assumption of continuity and the assumption that I always manage to set my watch to your watch. So, a dial developing according to a continuous dynamics from a given initial state, can not be set up so as to react to a second dial, with which it interacts, in such a way that it is guaranteed to always end up set at the same angle as the second dial. Similarly, it can not be set up so that it is guaranteed to always end up set at 90 degrees to the setting of the second dial. All of this has nothing to do with time travel. However, the impossibility of such set ups is what prevents us from enacting the rotation by 90 degrees that would create paradox in the time travel setting.

Let us now give the positive result that with such dials there will always be fixed point solutions, as long as the dynamics is continuous. Let us call the state of the dial before it interacts with its older self the initial state of the dial. And let us call the state of the dial after it emerges from the time machine the final state of the dial. There is also an intermediate state of the dial, after it interacts with its older self and before it is put into the time machine. We can represent the initial or intermediate states of the dial, before it goes into the time machine, as an angle \(x\) in the horizontal plane and the final state of the dial, after it comes out of the time machine, as an angle \(y\) in the vertical plane. All possible \(\langle x,y\rangle\) pairs can thus be visualized as a torus with each \(x\) value picking out a vertical circular cross-section and each \(y\) picking out a point on that cross-section. See figure 1 .

Figure 1 [An extended description of figure 1 is in the supplement.]

Suppose that the dial starts at angle \(i\) which picks out vertical circle \(I\) on the torus. The initial angle \(i\) that the dial is at before it encounters its older self, and the set of all possible final angles that the dial can have when it emerges from the time machine is represented by the circle \(I\) on the torus (see figure 1 ). Given any possible angle of the emerging dial, the dial initially at angle \(i\) will develop to some other angle. One can picture this development by rotating each point on \(I\) in the horizontal direction by the relevant amount. Since the rotation has to depend continuously on the angle of the emerging dial, circle \(I\) during this development will deform into some loop \(L\) on the torus. Loop \(L\) thus represents all possible intermediate angles \(x\) that the dial is at when it is thrown into the time machine, given that it started at angle \(i\) and then encountered a dial (its older self) which was at angle \(y\) when it emerged from the time machine. We therefore have consistency if \(x=y\) for some \(x\) and \(y\) on loop \(L\). Now, let loop \(C\) be the loop which consists of all the points on the torus for which \(x=y\). Ring \(I\) intersects \(C\) at point \(\langle i,i\rangle\). Obviously any continuous deformation of \(I\) must still intersect \(C\) somewhere. So \(L\) must intersect \(C\) somewhere, say at \(\langle j,j\rangle\). But that means that no matter how the development of the dial starting at \(I\) depends on the angle of the emerging dial, there will be some angle for the emerging dial such that the dial will develop exactly into that angle (by the time it enters the time machine) under the influence of that emerging dial. This is so no matter what angle one starts with, and no matter how the development depends on the angle of the emerging dial. Thus even for a circular state-space there are no constraints needed other than continuity.

Unfortunately there are state-spaces that escape even this argument. Consider for instance a pointer that can be set to all values between 0 and 1, where 0 and 1 are not possible values. That is, suppose that we have a state-space that is isomorphic to an open set of real numbers. Now suppose that we have a machine that sets the pointer to half the value that the pointer is set at when it emerges from the time machine.

Figure 2 [An extended description of figure 2 is in the supplement.]

Suppose the pointer starts at value \(I\). As before we can represent the combination of this initial position and all possible final positions by the line \(I\). Under the influence of the pointer coming out of the time machine the pointer value will develop to a value that equals half the value of the final value that it encountered. We can represent this development as the continuous deformation of line \(I\) into line \(L\), which is indicated by the arrows in figure 2 . This development is fully continuous. Points \(\langle x,y\rangle\) on line \(I\) represent the initial position \(x=I\) of the (young) pointer, and the position \(y\) of the older pointer as it emerges from the time machine. Points \(\langle x,y\rangle\) on line \(L\) represent the position \(x\) that the younger pointer should develop into, given that it encountered the older pointer emerging from the time machine set at position \(y\). Since the pointer is designed to develop to half the value of the pointer that it encounters, the line \(L\) corresponds to \(x=1/2 y\). We have consistency if there is some point such that it develops into that point, if it encounters that point. Thus, we have consistency if there is some point \(\langle x,y\rangle\) on line \(L\) such that \(x=y\). However, there is no such point: lines \(L\) and \(C\) do not intersect. Thus there is no consistent solution, despite the fact that the dynamics is fully continuous.

Of course if 0 were a possible value, \(L\) and \(C\) would intersect at 0. This is surprising and strange: adding one point to the set of possible values of a quantity here makes the difference between paradox and peace. One might be tempted to just add the extra point to the state-space in order to avoid problems. After all, one might say, surely no measurements could ever tell us whether the set of possible values includes that exact point or not. Unfortunately there can be good theoretical reasons for supposing that some quantity has a state-space that is open: the set of all possible speeds of massive objects in special relativity surely is an open set, since it includes all speeds up to, but not including, the speed of light. Quantities that have possible values that are not bounded also lead to counter examples to the presented fixed point argument. And it is not obvious to us why one should exclude such possibilities. So the argument that no constraints are needed is not fully general.

An interesting question of course is: exactly for which state-spaces must there be such fixed points? The arguments above depend on a well-known fixed point theorem (due to Schauder) that guarantees the existence of a fixed point for compact, convex state spaces. We do not know what subsequent extensions of this result imply regarding fixed points for a wider variety of systems, or whether there are other general results along these lines. (See Kutach 2003 for more on this issue.)

A further interesting question is whether this line of argument is sufficient to resolve Consistency (see also Dowe 2007). When they apply, these results establish the existence of a solution, such as the shade of uniform gray in the first example. But physicists routinely demand more than merely the existence of a solution, namely that solutions to the equations are stable—such that “small” changes of the initial state lead to “small” changes of the resulting trajectory. (Clarifying the two senses of “small” in this statement requires further work, specifying the relevant topology.) Stability in this sense underwrites the possibility of applying equations to real systems given our inability to fix initial states with indefinite precision. (See Fletcher 2020 for further discussion.) The fixed point theorems guarantee that for an initial state \(S_1\) there is a solution, but this solution may not be “close” to the solution for a nearby initial state, \(S'\). We are not aware of any proofs that the solutions guaranteed to exist by the fixed point theorems are also stable in this sense.

Time travel has recently been discussed quite extensively in the context of general relativity. General relativity places few constraints on the global structure of space and time. This flexibility leads to a possibility first described in print by Hermann Weyl:

Every world-point is the origin of the double-cone of the active future and the passive past [i.e., the two lobes of the light cone]. Whereas in the special theory of relativity these two portions are separated by an intervening region, it is certainly possible in the present case [i.e., general relativity] for the cone of the active future to overlap with that of the passive past; so that, in principle, it is possible to experience events now that will in part be an effect of my future resolves and actions. Moreover, it is not impossible for a world-line (in particular, that of my body), although it has a timelike direction at every point, to return to the neighborhood of a point which it has already once passed through. (Weyl 1918/1920 [1952: 274])

A time-like curve is simply a space-time trajectory such that the speed of light is never equaled or exceeded along this trajectory. Time-like curves represent possible trajectories of ordinary objects. In general relativity a curve that is everywhere timelike locally can nonetheless loop back on itself, forming a CTC. Weyl makes the point vividly in terms of the light cones: along such a curve, the future lobe of the light cone (the “active future”) intersects the past lobe of the light cone (the “passive past”). Traveling along such a curve one would never exceed the speed of light, and yet after a certain amount of (proper) time one would return to a point in space-time that one previously visited. Or, by staying close to such a CTC, one could come arbitrarily close to a point in space-time that one previously visited. General relativity, in a straightforward sense, allows time travel: there appear to be many space-times compatible with the fundamental equations of general relativity in which there are CTC’s. Space-time, for instance, could have a Minkowski metric everywhere, and yet have CTC’s everywhere by having the temporal dimension (topologically) rolled up as a circle. Or, one can have wormhole connections between different parts of space-time which allow one to enter “mouth \(A\)” of such a wormhole connection, travel through the wormhole, exit the wormhole at “mouth \(B\)” and re-enter “mouth \(A\)” again. CTCs can even arise when the spacetime is topologically \(\mathbb{R}^4\), due to the “tilting” of light cones produced by rotating matter (as in Gödel 1949’s spacetime).

General relativity thus appears to provide ample opportunity for time travel. Note that just because there are CTC’s in a space-time, this does not mean that one can get from any point in the space-time to any other point by following some future directed timelike curve—there may be insurmountable practical obstacles. In Gödel’s spacetime, it is the case that there are CTCs passing through every point in the spacetime. Yet these CTCs are not geodesics, so traversing them requires acceleration. Calculations of the minimal fuel required to travel along the appropriate curve should discourage any would-be time travelers (Malament 1984, 1985; Manchak 2011). But more generally CTCs may be confined to smaller regions; some parts of space-time can have CTC’s while other parts do not. Let us call the part of a space-time that has CTC’s the “time travel region” of that space-time, while calling the rest of that space-time the “normal region”. More precisely, the “time travel region” consists of all the space-time points \(p\) such that there exists a (non-zero length) timelike curve that starts at \(p\) and returns to \(p\). Now let us turn to examining space-times with CTC’s a bit more closely for potential problems.

In order to get a feeling for the sorts of implications that closed timelike curves can have, it may be useful to consider two simple models. In space-times with closed timelike curves the traditional initial value problem cannot be framed in the usual way. For it presupposes the existence of Cauchy surfaces, and if there are CTCs then no Cauchy surface exists. (A Cauchy surface is a spacelike surface such that every inextendable timelike curve crosses it exactly once. One normally specifies initial conditions by giving the conditions on such a surface.) Nonetheless, if the topological complexities of the manifold are appropriately localized, we can come quite close. Let us call an edgeless spacelike surface \(S\) a quasi-Cauchy surface if it divides the rest of the manifold into two parts such that

• every point in the manifold can be connected by a timelike curve to \(S\), and
• any timelike curve which connects a point in one region to a point in the other region intersects \(S\) exactly once.

It is obvious that a quasi-Cauchy surface must entirely inhabit the normal region of the space-time; if any point \(p\) of \(S\) is in the time travel region, then any timelike curve which intersects \(p\) can be extended to a timelike curve which intersects \(S\) near \(p\) again. In extreme cases of time travel, a model may have no normal region at all (e.g., Minkowski space-time rolled up like a cylinder in a time-like direction), in which case our usual notions of temporal precedence will not apply. But temporal anomalies like wormholes (and time machines) can be sufficiently localized to permit the existence of quasi-Cauchy surfaces.

Given a timelike orientation, a quasi-Cauchy surface unproblematically divides the manifold into its past (i.e., all points that can be reached by past-directed timelike curves from \(S)\) and its future (ditto mutatis mutandis ). If the whole past of \(S\) is in the normal region of the manifold, then \(S\) is a partial Cauchy surface : every inextendable timelike curve which exists to the past of \(S\) intersects \(S\) exactly once, but (if there is time travel in the future) not every inextendable timelike curve which exists to the future of \(S\) intersects \(S\). Now we can ask a particularly clear question: consider a manifold which contains a time travel region, but also has a partial Cauchy surface \(S\), such that all of the temporal funny business is to the future of \(S\). If all you could see were \(S\) and its past, you would not know that the space-time had any time travel at all. The question is: are there any constraints on the sort of data which can be put on \(S\) and continued to a global solution of the dynamics which are different from the constraints (if any) on the data which can be put on a Cauchy surface in a simply connected manifold and continued to a global solution? If there is time travel to our future, might we we able to tell this now, because of some implied oddity in the arrangement of present things?

It is not at all surprising that there might be constraints on the data which can be put on a locally space-like surface which passes through the time travel region: after all, we never think we can freely specify what happens on a space-like surface and on another such surface to its future, but in this case the surface at issue lies to its own future. But if there were particular constraints for data on a partial Cauchy surface then we would apparently need to have to rule out some sorts of otherwise acceptable states on \(S\) if there is to be time travel to the future of \(S\). We then might be able to establish that there will be no time travel in the future by simple inspection of the present state of the universe. As we will see, there is reason to suspect that such constraints on the partial Cauchy surface are non-generic. But we are getting ahead of ourselves: first let’s consider the effect of time travel on a very simple dynamics.

The simplest possible example is the Newtonian theory of perfectly elastic collisions among equally massive particles in one spatial dimension. The space-time is two-dimensional, so we can represent it initially as the Euclidean plane, and the dynamics is completely specified by two conditions. When particles are traveling freely, their world lines are straight lines in the space-time, and when two particles collide, they exchange momenta, so the collision looks like an “\(X\)” in space-time, with each particle changing its momentum at the impact. [ 2 ] The dynamics is purely local, in that one can check that a set of world-lines constitutes a model of the dynamics by checking that the dynamics is obeyed in every arbitrarily small region. It is also trivial to generate solutions from arbitrary initial data if there are no CTCs: given the initial positions and momenta of a set of particles, one simply draws a straight line from each particle in the appropriate direction and continues it indefinitely. Once all the lines are drawn, the worldline of each particle can be traced from collision to collision. The boundary value problem for this dynamics is obviously well-posed: any set of data at an instant yields a unique global solution, constructed by the method sketched above.

What happens if we change the topology of the space-time by hand to produce CTCs? The simplest way to do this is depicted in figure 3 : we cut and paste the space-time so it is no longer simply connected by identifying the line \(L-\) with the line \(L+\). Particles “going in” to \(L+\) from below “emerge” from \(L-\) , and particles “going in” to \(L-\) from below “emerge” from \(L+\).

Figure 3: Inserting CTCs by Cut and Paste. [An extended description of figure 3 is in the supplement.]

How is the boundary-value problem changed by this alteration in the space-time? Before the cut and paste, we can put arbitrary data on the simultaneity slice \(S\) and continue it to a unique solution. After the change in topology, \(S\) is no longer a Cauchy surface, since a CTC will never intersect it, but it is a partial Cauchy surface. So we can ask two questions. First, can arbitrary data on \(S\) always be continued to a global solution? Second, is that solution unique? If the answer to the first question is \(no\), then we have a backward-temporal constraint: the existence of the region with CTCs places constraints on what can happen on \(S\) even though that region lies completely to the future of \(S\). If the answer to the second question is \(no\), then we have an odd sort of indeterminism, analogous to the unwritten book: the complete physical state on \(S\) does not determine the physical state in the future, even though the local dynamics is perfectly deterministic and even though there is no other past edge to the space-time region in \(S\)’s future (i.e., there is nowhere else for boundary values to come from which could influence the state of the region).

In this case the answer to the first question is yes and to the second is no : there are no constraints on the data which can be put on \(S\), but those data are always consistent with an infinitude of different global solutions. The easy way to see that there always is a solution is to construct the minimal solution in the following way. Start drawing straight lines from \(S\) as required by the initial data. If a line hits \(L-\) from the bottom, just continue it coming out of the top of \(L+\) in the appropriate place, and if a line hits \(L+\) from the bottom, continue it emerging from \(L-\) at the appropriate place. Figure 4 represents the minimal solution for a single particle which enters the time-travel region from the left:

Figure 4: The Minimal Solution. [An extended description of figure 4 is in the supplement.]

The particle “travels back in time” three times. It is obvious that this minimal solution is a global solution, since the particle always travels inertially.

But the same initial state on \(S\) is also consistent with other global solutions. The new requirement imposed by the topology is just that the data going into \(L+\) from the bottom match the data coming out of \(L-\) from the top, and the data going into \(L-\) from the bottom match the data coming out of \(L+\) from the top. So we can add any number of vertical lines connecting \(L-\) and \(L+\) to a solution and still have a solution. For example, adding a few such lines to the minimal solution yields:

Figure 5: A Non-Minimal Solution. [An extended description of figure 5 is in the supplement.]

The particle now collides with itself twice: first before it reaches \(L+\) for the first time, and again shortly before it exits the CTC region. From the particle’s point of view, it is traveling to the right at a constant speed until it hits an older version of itself and comes to rest. It remains at rest until it is hit from the right by a younger version of itself, and then continues moving off, and the same process repeats later. It is clear that this is a global model of the dynamics, and that any number of distinct models could be generating by varying the number and placement of vertical lines.

Knowing the data on \(S\), then, gives us only incomplete information about how things will go for the particle. We know that the particle will enter the CTC region, and will reach \(L+\), we know that it will be the only particle in the universe, we know exactly where and with what speed it will exit the CTC region. But we cannot determine how many collisions the particle will undergo (if any), nor how long (in proper time) it will stay in the CTC region. If the particle were a clock, we could not predict what time it would indicate when exiting the region. Furthermore, the dynamics gives us no handle on what to think of the various possibilities: there are no probabilities assigned to the various distinct possible outcomes.

Changing the topology has changed the mathematics of the situation in two ways, which tend to pull in opposite directions. On the one hand, \(S\) is no longer a Cauchy surface, so it is perhaps not surprising that data on \(S\) do not suffice to fix a unique global solution. But on the other hand, there is an added constraint: data “coming out” of \(L-\) must exactly match data “going in” to \(L+\), even though what comes out of \(L-\) helps to determine what goes into \(L+\). This added consistency constraint tends to cut down on solutions, although in this case the additional constraint is more than outweighed by the freedom to consider various sorts of data on \({L+}/{L-}\).

The fact that the extra freedom outweighs the extra constraint also points up one unexpected way that the supposed paradoxes of time travel may be overcome. Let’s try to set up a paradoxical situation using the little closed time loop above. If we send a single particle into the loop from the left and do nothing else, we know exactly where it will exit the right side of the time travel region. Now suppose we station someone at the other side of the region with the following charge: if the particle should come out on the right side, the person is to do something to prevent the particle from going in on the left in the first place. In fact, this is quite easy to do: if we send a particle in from the right, it seems that it can exit on the left and deflect the incoming left-hand particle.

Carrying on our reflection in this way, we further realize that if the particle comes out on the right, we might as well send it back in order to deflect itself from entering in the first place. So all we really need to do is the following: set up a perfectly reflecting particle mirror on the right-hand side of the time travel region, and launch the particle from the left so that— if nothing interferes with it —it will just barely hit \(L+\). Our paradox is now apparently complete. If, on the one hand, nothing interferes with the particle it will enter the time-travel region on the left, exit on the right, be reflected from the mirror, re-enter from the right, and come out on the left to prevent itself from ever entering. So if it enters, it gets deflected and never enters. On the other hand, if it never enters then nothing goes in on the left, so nothing comes out on the right, so nothing is reflected back, and there is nothing to deflect it from entering. So if it doesn’t enter, then there is nothing to deflect it and it enters. If it enters, then it is deflected and doesn’t enter; if it doesn’t enter then there is nothing to deflect it and it enters: paradox complete.

But at least one solution to the supposed paradox is easy to construct: just follow the recipe for constructing the minimal solution, continuing the initial trajectory of the particle (reflecting it the mirror in the obvious way) and then read of the number and trajectories of the particles from the resulting diagram. We get the result of figure 6 :

Figure 6: Resolving the “Paradox”. [An extended description of figure 6 is in the supplement.]

As we can see, the particle approaching from the left never reaches \(L+\): it is deflected first by a particle which emerges from \(L-\). But it is not deflected by itself , as the paradox suggests, it is deflected by another particle. Indeed, there are now four particles in the diagram: the original particle and three particles which are confined to closed time-like curves. It is not the leftmost particle which is reflected by the mirror, nor even the particle which deflects the leftmost particle; it is another particle altogether.

The paradox gets it traction from an incorrect presupposition. If there is only one particle in the world at \(S\) then there is only one particle which could participate in an interaction in the time travel region: the single particle would have to interact with its earlier (or later) self. But there is no telling what might come out of \(L-\): the only requirement is that whatever comes out must match what goes in at \(L+\). So if you go to the trouble of constructing a working time machine, you should be prepared for a different kind of disappointment when you attempt to go back and kill yourself: you may be prevented from entering the machine in the first place by some completely unpredictable entity which emerges from it. And once again a peculiar sort of indeterminism appears: if there are many self-consistent things which could prevent you from entering, there is no telling which is even likely to materialize. This is just like the case of the unwritten book: the book is never written, so nothing determines what fills its pages.

So when the freedom to put data on \(L-\) outweighs the constraint that the same data go into \(L+\), instead of paradox we get an embarrassment of riches: many solution consistent with the data on \(S\), or many possible books. To see a case where the constraint “outweighs” the freedom, we need to construct a very particular, and frankly artificial, dynamics and topology. Consider the space of all linear dynamics for a scalar field on a lattice. (The lattice can be though of as a simple discrete space-time.) We will depict the space-time lattice as a directed graph. There is to be a scalar field defined at every node of the graph, whose value at a given node depends linearly on the values of the field at nodes which have arrows which lead to it. Each edge of the graph can be assigned a weighting factor which determines how much the field at the input node contributes to the field at the output node. If we name the nodes by the letters a , b , c , etc., and the edges by their endpoints in the obvious way, then we can label the weighting factors by the edges they are associated with in an equally obvious way.

Suppose that the graph of the space-time lattice is acyclic , as in figure 7 . (A graph is Acyclic if one can not travel in the direction of the arrows and go in a loop.)

Figure 7: An Acyclic Lattice. [An extended description of figure 7 is in the supplement.]

It is easy to regard a set of nodes as the analog of a Cauchy surface, e.g., the set \(\{a, b, c\}\), and it is obvious if arbitrary data are put on those nodes the data will generate a unique solution in the future. [ 3 ] If the value of the field at node \(a\) is 3 and at node \(b\) is 7, then its value at node \(d\) will be \(3W_{ad}\) and its value at node \(e\) will be \(3W_{ae} + 7W_{be}\). By varying the weighting factors we can adjust the dynamics, but in an acyclic graph the future evolution of the field will always be unique.

Let us now again artificially alter the topology of the lattice to admit CTCs, so that the graph now is cyclic. One of the simplest such graphs is depicted in figure 8 : there are now paths which lead from \(z\) back to itself, e.g., \(z\) to \(y\) to \(z\).

Figure 8: Time Travel on a Lattice. [An extended description of figure 8 is in the supplement.]

Can we now put arbitrary data on \(v\) and \(w\), and continue that data to a global solution? Will the solution be unique?

In the generic case, there will be a solution and the solution will be unique. The equations for the value of the field at \(x, y\), and \(z\) are:

Solving these equations for \(z\) yields

which gives a unique value for \(z\) in the generic case. But looking at the space of all possible dynamics for this lattice (i.e., the space of all possible weighting factors), we find a singularity in the case where \(1-W_{zx}W_{xz} - W_{zy}W_{yz} = 0\). If we choose weighting factors in just this way, then arbitrary data at \(v\) and \(w\) cannot be continued to a global solution. Indeed, if the scalar field is everywhere non-negative, then this particular choice of dynamics puts ironclad constraints on the value of the field at \(v\) and \(w\): the field there must be zero (assuming \(W_{vx}\) and \(W_{wy}\) to be non-zero), and similarly all nodes in their past must have field value zero. If the field can take negative values, then the values at \(v\) and \(w\) must be so chosen that \(vW_{vx}W_{xz} = -wW_{wy}W_{yz}\). In either case, the field values at \(v\) and \(w\) are severely constrained by the existence of the CTC region even though these nodes lie completely to the past of that region. It is this sort of constraint which we find to be unlike anything which appears in standard physics.

Our toy models suggest three things. The first is that it may be impossible to prove in complete generality that arbitrary data on a partial Cauchy surface can always be continued to a global solution: our artificial case provides an example where it cannot. The second is that such odd constraints are not likely to be generic: we had to delicately fine-tune the dynamics to get a problem. The third is that the opposite problem, namely data on a partial Cauchy surface being consistent with many different global solutions, is likely to be generic: we did not have to do any fine-tuning to get this result.

This third point leads to a peculiar sort of indeterminism, illustrated by the case of the unwritten book: the entire state on \(S\) does not determine what will happen in the future even though the local dynamics is deterministic and there are no other “edges” to space-time from which data could influence the result. What happens in the time travel region is constrained but not determined by what happens on \(S\), and the dynamics does not even supply any probabilities for the various possibilities. The example of the photographic negative discussed in section 2, then, seems likely to be unusual, for in that case there is a unique fixed point for the dynamics, and the set-up plus the dynamical laws determine the outcome. In the generic case one would rather expect multiple fixed points, with no room for anything to influence, even probabilistically, which would be realized. (See the supplement on

Remarks and Limitations on the Toy Models .

It is ironic that time travel should lead generically not to contradictions or to constraints (in the normal region) but to underdetermination of what happens in the time travel region by what happens everywhere else (an underdetermination tied neither to a probabilistic dynamics nor to a free edge to space-time). The traditional objection to time travel is that it leads to contradictions: there is no consistent way to complete an arbitrarily constructed story about how the time traveler intends to act. Instead, though, it appears that the more significant problem is underdetermination: the story can be consistently completed in many different ways.

Echeverria, Klinkhammer, and Thorne (1991) considered the case of 3-dimensional single hard spherical ball that can go through a single time travel wormhole so as to collide with its younger self.

Figure 9 [An extended description of figure 9 is in the supplement.]

The threat of paradox in this case arises in the following form. Consider the initial trajectory of a ball as it approaches the time travel region. For some initial trajectories, the ball does not undergo a collision before reaching mouth 1, but upon exiting mouth 2 it will collide with its earlier self. This leads to a contradiction if the collision is strong enough to knock the ball off its trajectory and deflect it from entering mouth 1. Of course, the Wheeler-Feynman strategy is to look for a “glancing blow” solution: a collision which will produce exactly the (small) deviation in trajectory of the earlier ball that produces exactly that collision. Are there always such solutions? [ 4 ]

Echeverria, Klinkhammer & Thorne found a large class of initial trajectories that have consistent “glancing blow” continuations, and found none that do not (but their search was not completely general). They did not produce a rigorous proof that every initial trajectory has a consistent continuation, but suggested that it is very plausible that every initial trajectory has a consistent continuation. That is to say, they have made it very plausible that, in the billiard ball wormhole case, the time travel structure of such a wormhole space-time does not result in constraints on states on spacelike surfaces in the non-time travel region.

In fact, as one might expect from our discussion in the previous section, they found the opposite problem from that of inconsistency: they found underdetermination. For a large class of initial trajectories there are multiple different consistent “glancing blow” continuations of that trajectory (many of which involve multiple wormhole traversals). For example, if one initially has a ball that is traveling on a trajectory aimed straight between the two mouths, then one obvious solution is that the ball passes between the two mouths and never time travels. But another solution is that the younger ball gets knocked into mouth 1 exactly so as to come out of mouth 2 and produce that collision. Echeverria et al. do not note the possibility (which we pointed out in the previous section) of the existence of additional balls in the time travel region. We conjecture (but have no proof) that for every initial trajectory of \(A\) there are some, and generically many, multiple-ball continuations.

Friedman, Morris, et al. (1990) examined the case of source-free non-self-interacting scalar fields traveling through such a time travel wormhole and found that no constraints on initial conditions in the non-time travel region are imposed by the existence of such time travel wormholes. In general there appear to be no known counter examples to the claim that in “somewhat realistic” time-travel space-times with a partial Cauchy surface there are no constraints imposed on the state on such a partial Cauchy surface by the existence of CTC’s. (See, e.g., Friedman & Morris 1991; Thorne 1994; Earman 1995; Earman, Smeenk, & Wüthrich 2009; and Dowe 2007.)

How about the issue of constraints in the time travel region \(T\)? Prima facie , constraints in such a region would not appear to be surprising. But one might still expect that there should be no constraints on states on a spacelike surface, provided one keeps the surface “small enough”. In the physics literature the following question has been asked: for any point \(p\) in \(T\), and any space-like surface \(S\) that includes \(p\) is there a neighborhood \(E\) of \(p\) in \(S\) such that any solution on \(E\) can be extended to a solution on the whole space-time? With respect to this question, there are some simple models in which one has this kind of extendability of local solutions to global ones, and some simple models in which one does not have such extendability, with no clear general pattern. The technical mathematical problems are amplified by the more conceptual problem of what it might mean to say that one could create a situation which forces the creation of closed timelike curves. (See, e.g., Yurtsever 1990; Friedman, Morris, et al. 1990; Novikov 1992; Earman 1995; and Earman, Smeenk, & Wüthrich 2009). What are we to think of all of this?

The toy models above all treat billiard balls, fields, and other objects propagating through a background spacetime with CTCs. Even if we can show that a consistent solution exists, there is a further question: what kind of matter and dynamics could generate CTCs to begin with? There are various solutions of Einstein’s equations with CTCs, but how do these exotic spacetimes relate to the models actually used in describing the world? In other words, what positive reasons might we have to take CTCs seriously as a feature of the actual universe, rather than an exotic possibility of primarily mathematical interest?

We should distinguish two different kinds of “possibility” that we might have in mind in posing such questions (following Stein 1970). First, we can consider a solution as a candidate cosmological model, describing the (large-scale gravitational degrees of freedom of the) entire universe. The case for ruling out spacetimes with CTCs as potential cosmological models strikes us as, surprisingly, fairly weak. Physicists used to simply rule out solutions with CTCs as unreasonable by fiat, due to the threat of paradoxes, which we have dismantled above. But it is also challenging to make an observational case. Observations tell us very little about global features, such as the existence of CTCs, because signals can only reach an observer from a limited region of spacetime, called the past light cone. Our past light cone—and indeed the collection of all the past light cones for possible observers in a given spacetime—can be embedded in spacetimes with quite different global features (Malament 1977, Manchak 2009). This undercuts the possibility of using observations to constrain global topology, including (among other things) ruling out the existence of CTCs.

Yet the case in favor of taking cosmological models with CTCs seriously is also not particularly strong. Some solutions used to describe black holes, which are clearly relevant in a variety of astrophysical contexts, include CTCs. But the question of whether the CTCs themselves play an essential representational role is subtle: the CTCs arise in the maximal extensions of these solutions, and can plausibly be regarded as extraneous to successful applications. Furthermore, many of the known solutions with CTCs have symmetries, raising the possibility that CTCs are not a stable or robust feature. Slight departures from symmetry may lead to a solution without CTCs, suggesting that the CTCs may be an artifact of an idealized model.

The second sense of possibility regards whether “reasonable” initial conditions can be shown to lead to, or not to lead to, the formation of CTCs. As with the toy models above, suppose that we have a partial Cauchy surface \(S\), such that all the temporal funny business lies to the future. Rather than simply assuming that there is a region with CTCs to the future, we can ask instead whether it is possible to create CTCs by manipulating matter in the initial, well-behaved region—that is, whether it is possible to build a time machine. Several physicists have pursued “chronology protection theorems” aiming to show that the dynamics of general relativity (or some other aspects of physics) rules this out, and to clarify why this is the case. The proof of such a theorem would justify neglecting solutions with CTCs as a source of insight into the nature of time in the actual world. But as of yet there are several partial results that do not fully settle the question. One further intriguing possibility is that even if general relativity by itself does protect chronology, it may not be possible to formulate a sensible theory describing matter and fields in solutions with CTCs. (See SEP entry on Time Machines; Smeenk and Wüthrich 2011 for more.)

There is a different question regarding the limitations of these toy models. The toy models and related examples show that there are consistent solutions for simple systems in the presence of CTCs. As usual we have made the analysis tractable by building toy models, selecting only a few dynamical degrees of freedom and tracking their evolution. But there is a large gap between the systems we have described and the time travel stories they evoke, with Kurt traveling along a CTC with murderous intentions. In particular, many features of the manifest image of time are tied to the thermodynamical properties of macroscopic systems. Rovelli (unpublished) considers a extremely simple system to illustrate the problem: can a clock move along a CTC? A clock consists of something in periodic motion, such as a pendulum bob, and something that counts the oscillations, such as an escapement mechanism. The escapement mechanism cannot work without friction; this requires dissipation and increasing entropy. For a clock that counts oscillations as it moves along a time-like trajectory, the entropy must be a monotonically increasing function. But that is obviously incompatible with the clock returning to precisely the same state at some future time as it completes a loop. The point generalizes, obviously, to imply that anything like a human, with memory and agency, cannot move along a CTC.

Since it is not obvious that one can rid oneself of all constraints in realistic models, let us examine the argument that time travel is implausible, and we should think it unlikely to exist in our world, in so far as it implies such constraints. The argument goes something like the following. In order to satisfy such constraints one needs some pre-established divine harmony between the global (time travel) structure of space-time and the distribution of particles and fields on space-like surfaces in it. But it is not plausible that the actual world, or any world even remotely like ours, is constructed with divine harmony as part of the plan. In fact, one might argue, we have empirical evidence that conditions in any spatial region can vary quite arbitrarily. So we have evidence that such constraints, whatever they are, do not in fact exist in our world. So we have evidence that there are no closed time-like lines in our world or one remotely like it. We will now examine this argument in more detail by presenting four possible responses, with counterresponses, to this argument.

Response 1. There is nothing implausible or new about such constraints. For instance, if the universe is spatially closed, there has to be enough matter to produce the needed curvature, and this puts constraints on the matter distribution on a space-like hypersurface. Thus global space-time structure can quite unproblematically constrain matter distributions on space-like hypersurfaces in it. Moreover we have no realistic idea what these constraints look like, so we hardly can be said to have evidence that they do not obtain.

Counterresponse 1. Of course there are constraining relations between the global structure of space-time and the matter in it. The Einstein equations relate curvature of the manifold to the matter distribution in it. But what is so strange and implausible about the constraints imposed by the existence of closed time-like curves is that these constraints in essence have nothing to do with the Einstein equations. When investigating such constraints one typically treats the particles and/or field in question as test particles and/or fields in a given space-time, i.e., they are assumed not to affect the metric of space-time in any way. In typical space-times without closed time-like curves this means that one has, in essence, complete freedom of matter distribution on a space-like hypersurface. (See response 2 for some more discussion of this issue). The constraints imposed by the possibility of time travel have a quite different origin and are implausible. In the ordinary case there is a causal interaction between matter and space-time that results in relations between global structure of space-time and the matter distribution in it. In the time travel case there is no such causal story to be told: there simply has to be some pre-established harmony between the global space-time structure and the matter distribution on some space-like surfaces. This is implausible.

Response 2. Constraints upon matter distributions are nothing new. For instance, Maxwell’s equations constrain electric fields \(\boldsymbol{E}\) on an initial surface to be related to the (simultaneous) charge density distribution \(\varrho\) by the equation \(\varrho = \text{div}(\boldsymbol{E})\). (If we assume that the \(E\) field is generated solely by the charge distribution, this conditions amounts to requiring that the \(E\) field at any point in space simply be the one generated by the charge distribution according to Coulomb’s inverse square law of electrostatics.) This is not implausible divine harmony. Such constraints can hold as a matter of physical law. Moreover, if we had inferred from the apparent free variation of conditions on spatial regions that there could be no such constraints we would have mistakenly inferred that \(\varrho = \text{div}(\boldsymbol{E})\) could not be a law of nature.

Counterresponse 2. The constraints imposed by the existence of closed time-like lines are of quite a different character from the constraint imposed by \(\varrho = \text{div}(\boldsymbol{E})\). The constraints imposed by \(\varrho = \text{div}(\boldsymbol{E})\) on the state on a space-like hypersurface are:

• local constraints (i.e., to check whether the constraint holds in a region you just need to see whether it holds at each point in the region),
• quite independent of the global space-time structure,
• quite independent of how the space-like surface in question is embedded in a given space-time, and
• very simply and generally stateable.

On the other hand, the consistency constraints imposed by the existence of closed time-like curves (i) are not local, (ii) are dependent on the global structure of space-time, (iii) depend on the location of the space-like surface in question in a given space-time, and (iv) appear not to be simply stateable other than as the demand that the state on that space-like surface embedded in such and such a way in a given space-time, do not lead to inconsistency. On some views of laws (e.g., David Lewis’ view) this plausibly implies that such constraints, even if they hold, could not possibly be laws. But even if one does not accept such a view of laws, one could claim that the bizarre features of such constraints imply that it is implausible that such constraints hold in our world or in any world remotely like ours.

Response 3. It would be strange if there are constraints in the non-time travel region. It is not strange if there are constraints in the time travel region. They should be explained in terms of the strange, self-interactive, character of time travel regions. In this region there are time-like trajectories from points to themselves. Thus the state at such a point, in such a region, will, in a sense, interact with itself. It is a well-known fact that systems that interact with themselves will develop into an equilibrium state, if there is such an equilibrium state, or else will develop towards some singularity. Normally, of course, self-interaction isn’t true instantaneous self-interaction, but consists of a feed-back mechanism that takes time. But in time travel regions something like true instantaneous self-interaction occurs. This explains why constraints on states occur in such time travel regions: the states “ ab initio ” have to be “equilibrium states”. Indeed in a way this also provides some picture of why indeterminism occurs in time travel regions: at the onset of self-interaction states can fork into different equi-possible equilibrium states.

Counterresponse 3. This is explanation by woolly analogy. It all goes to show that time travel leads to such bizarre consequences that it is unlikely that it occurs in a world remotely like ours.

Response 4. All of the previous discussion completely misses the point. So far we have been taking the space-time structure as given, and asked the question whether a given time travel space-time structure imposes constraints on states on (parts of) space-like surfaces. However, space-time and matter interact. Suppose that one is in a space-time with closed time-like lines, such that certain counterfactual distributions of matter on some neighborhood of a point \(p\) are ruled out if one holds that space-time structure fixed. One might then ask

Why does the actual state near \(p\) in fact satisfy these constraints? By what divine luck or plan is this local state compatible with the global space-time structure? What if conditions near \(p\) had been slightly different?

And one might take it that the lack of normal answers to these questions indicates that it is very implausible that our world, or any remotely like it, is such a time travel universe. However the proper response to these question is the following. There are no constraints in any significant sense. If they hold they hold as a matter of accidental fact, not of law. There is no more explanation of them possible than there is of any contingent fact. Had conditions in a neighborhood of \(p\) been otherwise, the global structure of space-time would have been different. So what? The only question relevant to the issue of constraints is whether an arbitrary state on an arbitrary spatial surface \(S\) can always be embedded into a space-time such that that state on \(S\) consistently extends to a solution on the entire space-time.

But we know the answer to that question. A well-known theorem in general relativity says the following: any initial data set on a three dimensional manifold \(S\) with positive definite metric has a unique embedding into a maximal space-time in which \(S\) is a Cauchy surface (see, e.g., Geroch & Horowitz 1979: 284 for more detail), i.e., there is a unique largest space-time which has \(S\) as a Cauchy surface and contains a consistent evolution of the initial value data on \(S\). Now since \(S\) is a Cauchy surface this space-time does not have closed time like curves. But it may have extensions (in which \(S\) is not a Cauchy surface) which include closed timelike curves, indeed it may be that any maximal extension of it would include closed timelike curves. (This appears to be the case for extensions of states on certain surfaces of Taub-NUT space-times. See Earman, Smeenk, & Wüthrich 2009). But these extensions, of course, will be consistent. So properly speaking, there are no constraints on states on space-like surfaces. Nonetheless the space-time in which these are embedded may or may not include closed time-like curves.

Counterresponse 4. This, in essence, is the stonewalling answer which we indicated in section 1. However, whether or not you call the constraints imposed by a given space-time on distributions of matter on certain space-like surfaces “genuine constraints”, whether or not they can be considered lawlike, and whether or not they need to be explained, the existence of such constraints can still be used to argue that time travel worlds are so bizarre that it is implausible that our world or any world remotely like ours is a time travel world.

Suppose that one is in a time travel world. Suppose that given the global space-time structure of this world, there are constraints imposed upon, say, the state of motion of a ball on some space-like surface when it is treated as a test particle, i.e., when it is assumed that the ball does not affect the metric properties of the space-time it is in. (There is lots of other matter that, via the Einstein equation, corresponds exactly to the curvature that there is everywhere in this time travel worlds.) Now a real ball of course does have some effect on the metric of the space-time it is in. But let us consider a ball that is so small that its effect on the metric is negligible. Presumably it will still be the case that certain states of this ball on that space-like surface are not compatible with the global time travel structure of this universe.

This means that the actual distribution of matter on such a space-like surface can be extended into a space-time with closed time-like lines, but that certain counterfactual distributions of matter on this space-like surface can not be extended into the same space-time. But note that the changes made in the matter distribution (when going from the actual to the counterfactual distribution) do not in any non-negligible way affect the metric properties of the space-time. (Recall that the changes only effect test particles.) Thus the reason why the global time travel properties of the counterfactual space-time have to be significantly different from the actual space-time is not that there are problems with metric singularities or alterations in the metric that force significant global changes when we go to the counterfactual matter distribution. The reason that the counterfactual space-time has to be different is that in the counterfactual world the ball’s initial state of motion starting on the space-like surface, could not “meet up” in a consistent way with its earlier self (could not be consistently extended) if we were to let the global structure of the counterfactual space-time be the same as that of the actual space-time. Now, it is not bizarre or implausible that there is a counterfactual dependence of manifold structure, even of its topology, on matter distributions on spacelike surfaces. For instance, certain matter distributions may lead to singularities, others may not. We may indeed in some sense have causal power over the topology of the space-time we live in. But this power normally comes via the Einstein equations. But it is bizarre to think that there could be a counterfactual dependence of global space-time structure on the arrangement of certain tiny bits of matter on some space-like surface, where changes in that arrangement by assumption do not affect the metric anywhere in space-time in any significant way . It is implausible that we live in such a world, or that a world even remotely like ours is like that.

Let us illustrate this argument in a different way by assuming that wormhole time travel imposes constraints upon the states of people prior to such time travel, where the people have so little mass/energy that they have negligible effect, via the Einstein equation, on the local metric properties of space-time. Do you think it more plausible that we live in a world where wormhole time travel occurs but it only occurs when people’s states are such that these local states happen to combine with time travel in such a way that nobody ever succeeds in killing their younger self, or do you think it more plausible that we are not in a wormhole time travel world? [ 5 ]

An alternative approach to time travel (initiated by Deutsch 1991) abstracts away from the idealized toy models described above. [ 6 ] This computational approach considers instead the evolution of bits (simple physical systems with two discrete states) through a network of interactions, which can be represented by a circuit diagram with gates corresponding to the interactions. Motivated by the possibility of CTCs, Deutsch proposed adding a new kind of channel that connects the output of a given gate back to its input —in essence, a backwards-time step. More concretely, given a gate that takes \(n\) bits as input, we can imagine taking some number \(i \lt n\) of these bits through a channel that loops back and then do double-duty as inputs. Consistency requires that the state of these \(i\) bits is the same for output and input. (We will consider an illustration of this kind of system in the next section.) Working through examples of circuit diagrams with a CTC channel leads to similar treatments of Consistency and Underdetermination as the discussion above (see, e.g., Wallace 2012: § 10.6). But the approach offers two new insights (both originally due to Deutsch): the Easy Knowledge paradox, and a particularly clear extension to time travel in quantum mechanics.

A computer equipped with a CTC channel can exploit the need to find consistent evolution to solve remarkably hard problems. (This is quite different than the first idea that comes to mind to enhance computational power: namely to just devote more time to a computation, and then send the result back on the CTC to an earlier state.) The gate in a circuit incorporating a CTC implements a function from the input bits to the output bits, under the constraint that the output and input match the i bits going through the CTC channel. This requires, in effect, finding the fixed point of the relevant function. Given the generality of the model, there are few limits on the functions that could be implemented on the CTC circuit. Nature has to solve a hard computational problem just to ensure consistent evolution. This can then be extended to other complex computational problems—leading, more precisely, to solutions of NP -complete problems in polynomial time (see Aaronson 2013: Chapter 20 for an overview and further references). The limits imposed by computational complexity are an essential part of our epistemic situation, and computers with CTCs would radically change this.

We now turn to the application of the computational approach to the quantum physics of time travel (see Deutsch 1991; Deutsch & Lockwood 1994). By contrast with the earlier discussions of constraints in classical systems, they claim to show that time travel never imposes any constraints on the pre-time travel state of quantum systems. The essence of this account is as follows. [ 7 ]

A quantum system starts in state \(S_1\), interacts with its older self, after the interaction is in state \(S_2\), time travels while developing into state \(S_3\), then interacts with its younger self, and ends in state \(S_4\) (see figure 10 ).

Figure 10 [An extended description of figure 10 is in the supplement.]

Deutsch assumes that the set of possible states of this system are the mixed states, i.e., are represented by the density matrices over the Hilbert space of that system. Deutsch then shows that for any initial state \(S_1\), any unitary interaction between the older and younger self, and any unitary development during time travel, there is a consistent solution, i.e., there is at least one pair of states \(S_2\) and \(S_3\) such that when \(S_1\) interacts with \(S_3\) it will change to state \(S_2\) and \(S_2\) will then develop into \(S_3\). The states \(S_2, S_3\) and \(S_4\) will typically be not be pure states, i.e., will be non-trivial mixed states, even if \(S_1\) is pure. In order to understand how this leads to interpretational problems let us give an example. Consider a system that has a two dimensional Hilbert space with as a basis the states \(\vc{+}\) and \(\vc{-}\). Let us suppose that when state \(\vc{+}\) of the young system encounters state \(\vc{+}\) of the older system, they interact and the young system develops into state \(\vc{-}\) and the old system remains in state \(\vc{+}\). In obvious notation:

Similarly, suppose that:

Let us furthermore assume that there is no development of the state of the system during time travel, i.e., that \(\vc{+}_2\) develops into \(\vc{+}_3\), and that \(\vc{-}_2\) develops into \(\vc{-}_3\).

Now, if the only possible states of the system were \(\vc{+}\) and \(\vc{-}\) (i.e., if there were no superpositions or mixtures of these states), then there is a constraint on initial states: initial state \(\vc{+}_1\) is impossible. For if \(\vc{+}_1\) interacts with \(\vc{+}_3\) then it will develop into \(\vc{-}_2\), which, during time travel, will develop into \(\vc{-}_3\), which inconsistent with the assumed state \(\vc{+}_3\). Similarly if \(\vc{+}_1\) interacts with \(\vc{-}_3\) it will develop into \(\vc{+}_2\), which will then develop into \(\vc{+}_3\) which is also inconsistent. Thus the system can not start in state \(\vc{+}_1\).

But, says Deutsch, in quantum mechanics such a system can also be in any mixture of the states \(\vc{+}\) and \(\vc{-}\). Suppose that the older system, prior to the interaction, is in a state \(S_3\) which is an equal mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\). Then the younger system during the interaction will develop into a mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), which will then develop into a mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\), which is consistent! More generally Deutsch uses a fixed point theorem to show that no matter what the unitary development during interaction is, and no matter what the unitary development during time travel is, for any state \(S_1\) there is always a state \(S_3\) (which typically is not a pure state) which causes \(S_1\) to develop into a state \(S_2\) which develops into that state \(S_3\). Thus quantum mechanics comes to the rescue: it shows in all generality that no constraints on initial states are needed!

One might wonder why Deutsch appeals to mixed states: will superpositions of states \(\vc{+}\) and \(\vc{-}\) not suffice? Unfortunately such an idea does not work. Suppose again that the initial state is \(\vc{+}_1\). One might suggest that that if state \(S_3\) is

one will obtain a consistent development. For one might think that when initial state \(\vc{+}_1\) encounters the superposition

it will develop into superposition

and that this in turn will develop into

as desired. However this is not correct. For initial state \(\vc{+}_1\) when it encounters

will develop into the entangled state

In so far as one can speak of the state of the young system after this interaction, it is in the mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), not in the superposition

So Deutsch does need his recourse to mixed states.

This clarification of why Deutsch needs his mixtures does however indicate a serious worry about the simplifications that are part of Deutsch’s account. After the interaction the old and young system will (typically) be in an entangled state. Although for purposes of a measurement on one of the two systems one can say that this system is in a mixed state, one can not represent the full state of the two systems by specifying the mixed state of each separate part, as there are correlations between observables of the two systems that are not represented by these two mixed states, but are represented in the joint entangled state. But if there really is an entangled state of the old and young systems directly after the interaction, how is one to represent the subsequent development of this entangled state? Will the state of the younger system remain entangled with the state of the older system as the younger system time travels and the older system moves on into the future? On what space-like surfaces are we to imagine this total entangled state to be? At this point it becomes clear that there is no obvious and simple way to extend elementary non-relativistic quantum mechanics to space-times with closed time-like curves: we apparently need to characterize not just the entanglement between two systems, but entanglement relative to specific spacetime descriptions.

How does Deutsch avoid these complications? Deutsch assumes a mixed state \(S_3\) of the older system prior to the interaction with the younger system. He lets it interact with an arbitrary pure state \(S_1\) younger system. After this interaction there is an entangled state \(S'\) of the two systems. Deutsch computes the mixed state \(S_2\) of the younger system which is implied by this entangled state \(S'\). His demand for consistency then is just that this mixed state \(S_2\) develops into the mixed state \(S_3\). Now it is not at all clear that this is a legitimate way to simplify the problem of time travel in quantum mechanics. But even if we grant him this simplification there is a problem: how are we to understand these mixtures?

If we take an ignorance interpretation of mixtures we run into trouble. For suppose that we assume that in each individual case each older system is either in state \(\vc{+}_3\) or in state \(\vc{-}_3\) prior to the interaction. Then we regain our paradox. Deutsch instead recommends the following, many worlds, picture of mixtures. Suppose we start with state \(\vc{+}_1\) in all worlds. In some of the many worlds the older system will be in the \(\vc{+}_3\) state, let us call them A -worlds, and in some worlds, B -worlds, it will be in the \(\vc{-}_3\) state. Thus in A -worlds after interaction we will have state \(\vc{-}_2\) , and in B -worlds we will have state \(\vc{+}_2\). During time travel the \(\vc{-}_2\) state will remain the same, i.e., turn into state \(\vc{-}_3\), but the systems in question will travel from A -worlds to B -worlds. Similarly the \(\vc{+}\) \(_2\) states will travel from the B -worlds to the A -worlds, thus preserving consistency.

Now whatever one thinks of the merits of many worlds interpretations, and of this understanding of it applied to mixtures, in the end one does not obtain genuine time travel in Deutsch’s account. The systems in question travel from one time in one world to another time in another world, but no system travels to an earlier time in the same world. (This is so at least in the normal sense of the word “world”, the sense that one means when, for instance, one says “there was, and will be, only one Elvis Presley in this world.”) Thus, even if it were a reasonable view, it is not quite as interesting as it may have initially seemed. (See Wallace 2012 for a more sympathetic treatment, that explores several further implications of accepting time travel in conjunction with the many worlds interpretation.)

We close by acknowledging that Deutsch’s starting point—the claim that this computational model captures the essential features of quantum systems in a spacetime with CTCs—has been the subject of some debate. Several physicists have pursued a quite different treatment of evolution of quantum systems through CTC’s, based on considering the “post-selected” state (see Lloyd et al. 2011). Their motivations for implementing the consistency condition in terms of the post-selected state reflects a different stance towards quantum foundations. A different line of argument aims to determine whether Deutsch’s treatment holds as an appropriate limiting case of a more rigorous treatment, such as quantum field theory in curved spacetimes. For example, Verch (2020) establishes several results challenging the assumption that Deutsch’s treatment is tied to the presence of CTC’s, or that it is compatible with the entanglement structure of quantum fields.

What remains of the grandfather paradox in general relativistic time travel worlds is the fact that in some cases the states on edgeless spacelike surfaces are “overconstrained”, so that one has less than the usual freedom in specifying conditions on such a surface, given the time-travel structure, and in some cases such states are “underconstrained”, so that states on edgeless space-like surfaces do not determine what happens elsewhere in the way that they usually do, given the time travel structure. There can also be mixtures of those two types of cases. The extent to which states are overconstrained and/or underconstrained in realistic models is as yet unclear, though it would be very surprising if neither obtained. The extant literature has primarily focused on the problem of overconstraint, since that, often, either is regarded as a metaphysical obstacle to the possibility time travel, or as an epistemological obstacle to the plausibility of time travel in our world. While it is true that our world would be quite different from the way we normally think it is if states were overconstrained, underconstraint seems at least as bizarre as overconstraint. Nonetheless, neither directly rules out the possibility of time travel.

If time travel entailed contradictions then the issue would be settled. And indeed, most of the stories employing time travel in popular culture are logically incoherent: one cannot “change” the past to be different from what it was, since the past (like the present and the future) only occurs once. But if the only requirement demanded is logical coherence, then it seems all too easy. A clever author can devise a coherent time-travel scenario in which everything happens just once and in a consistent way. This is just too cheap: logical coherence is a very weak condition, and many things we take to be metaphysically impossible are logically coherent. For example, it involves no logical contradiction to suppose that water is not molecular, but if both chemistry and Kripke are right it is a metaphysical impossibility. We have been interested not in logical possibility but in physical possibility. But even so, our conditions have been relatively weak: we have asked only whether time-travel is consistent with the universal validity of certain fundamental physical laws and with the notion that the physical state on a surface prior to the time travel region be unconstrained. It is perfectly possible that the physical laws obey this condition, but still that time travel is not metaphysically possible because of the nature of time itself. Consider an analogy. Aristotle believed that water is homoiomerous and infinitely divisible: any bit of water could be subdivided, in principle, into smaller bits of water. Aristotle’s view contains no logical contradiction. It was certainly consistent with Aristotle’s conception of water that it be homoiomerous, so this was, for him, a conceptual possibility. But if chemistry is right, Aristotle was wrong both about what water is like and what is possible for it. It can’t be infinitely divided, even though no logical or conceptual analysis would reveal that.

Similarly, even if all of our consistency conditions can be met, it does not follow that time travel is physically possible, only that some specific physical considerations cannot rule it out. The only serious proof of the possibility of time travel would be a demonstration of its actuality. For if we agree that there is no actual time travel in our universe, the supposition that there might have been involves postulating a substantial difference from actuality, a difference unlike in kind from anything we could know if firsthand. It is unclear to us exactly what the content of possible would be if one were to either maintain or deny the possibility of time travel in these circumstances, unless one merely meant that the possibility is not ruled out by some delineated set of constraints. As the example of Aristotle’s theory of water shows, conceptual and logical “possibility” do not entail possibility in a full-blooded sense. What exactly such a full-blooded sense would be in case of time travel, and whether one could have reason to believe it to obtain, remain to us obscure.

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How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Adlam, Emily, unpublished, “ Is There Causation in Fundamental Physics? New Insights from Process Matrices and Quantum Causal Modelling ”, 2022, arXiv: 2208.02721. doi:10.48550/ARXIV.2208.02721
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causation: backward | determinism: causal | quantum mechanics | quantum mechanics: retrocausality | space and time: being and becoming in modern physics | time machines | time travel

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A beginner's guide to time travel

Learn exactly how Einstein's theory of relativity works, and discover how there's nothing in science that says time travel is impossible.

Everyone can travel in time . You do it whether you want to or not, at a steady rate of one second per second. You may think there's no similarity to traveling in one of the three spatial dimensions at, say, one foot per second. But according to Einstein 's theory of relativity , we live in a four-dimensional continuum — space-time — in which space and time are interchangeable.

Einstein found that the faster you move through space, the slower you move through time — you age more slowly, in other words. One of the key ideas in relativity is that nothing can travel faster than the speed of light — about 186,000 miles per second (300,000 kilometers per second), or one light-year per year). But you can get very close to it. If a spaceship were to fly at 99% of the speed of light, you'd see it travel a light-year of distance in just over a year of time. 

That's obvious enough, but now comes the weird part. For astronauts onboard that spaceship, the journey would take a mere seven weeks. It's a consequence of relativity called time dilation , and in effect, it means the astronauts have jumped about 10 months into the future. 

Traveling at high speed isn't the only way to produce time dilation. Einstein showed that gravitational fields produce a similar effect — even the relatively weak field here on the surface of Earth . We don't notice it, because we spend all our lives here, but more than 12,400 miles (20,000 kilometers) higher up gravity is measurably weaker— and time passes more quickly, by about 45 microseconds per day. That's more significant than you might think, because it's the altitude at which GPS satellites orbit Earth, and their clocks need to be precisely synchronized with ground-based ones for the system to work properly. 

The satellites have to compensate for time dilation effects due both to their higher altitude and their faster speed. So whenever you use the GPS feature on your smartphone or your car's satnav, there's a tiny element of time travel involved. You and the satellites are traveling into the future at very slightly different rates.

But for more dramatic effects, we need to look at much stronger gravitational fields, such as those around black holes , which can distort space-time so much that it folds back on itself. The result is a so-called wormhole, a concept that's familiar from sci-fi movies, but actually originates in Einstein's theory of relativity. In effect, a wormhole is a shortcut from one point in space-time to another. You enter one black hole, and emerge from another one somewhere else. Unfortunately, it's not as practical a means of transport as Hollywood makes it look. That's because the black hole's gravity would tear you to pieces as you approached it, but it really is possible in theory. And because we're talking about space-time, not just space, the wormhole's exit could be at an earlier time than its entrance; that means you would end up in the past rather than the future.

Trajectories in space-time that loop back into the past are given the technical name "closed timelike curves." If you search through serious academic journals, you'll find plenty of references to them — far more than you'll find to "time travel." But in effect, that's exactly what closed timelike curves are all about — time travel

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There's another way to produce a closed timelike curve that doesn't involve anything quite so exotic as a black hole or wormhole: You just need a simple rotating cylinder made of super-dense material. This so-called Tipler cylinder is the closest that real-world physics can get to an actual, genuine time machine. But it will likely never be built in the real world, so like a wormhole, it's more of an academic curiosity than a viable engineering design.

Yet as far-fetched as these things are in practical terms, there's no fundamental scientific reason — that we currently know of — that says they are impossible. That's a thought-provoking situation, because as the physicist Michio Kaku is fond of saying, "Everything not forbidden is compulsory" (borrowed from T.H. White's novel, "The Once And Future King"). He doesn't mean time travel has to happen everywhere all the time, but Kaku is suggesting that the universe is so vast it ought to happen somewhere at least occasionally. Maybe some super-advanced civilization in another galaxy knows how to build a working time machine, or perhaps closed timelike curves can even occur naturally under certain rare conditions.

This raises problems of a different kind — not in science or engineering, but in basic logic. If time travel is allowed by the laws of physics, then it's possible to envision a whole range of paradoxical scenarios . Some of these appear so illogical that it's difficult to imagine that they could ever occur. But if they can't, what's stopping them? 

Thoughts like these prompted Stephen Hawking , who was always skeptical about the idea of time travel into the past, to come up with his "chronology protection conjecture" — the notion that some as-yet-unknown law of physics prevents closed timelike curves from happening. But that conjecture is only an educated guess, and until it is supported by hard evidence, we can come to only one conclusion: Time travel is possible.

A party for time travelers 

Hawking was skeptical about the feasibility of time travel into the past, not because he had disproved it, but because he was bothered by the logical paradoxes it created. In his chronology protection conjecture, he surmised that physicists would eventually discover a flaw in the theory of closed timelike curves that made them impossible. 

In 2009, he came up with an amusing way to test this conjecture. Hawking held a champagne party (shown in his Discovery Channel program), but he only advertised it after it had happened. His reasoning was that, if time machines eventually become practical, someone in the future might read about the party and travel back to attend it. But no one did — Hawking sat through the whole evening on his own. This doesn't prove time travel is impossible, but it does suggest that it never becomes a commonplace occurrence here on Earth.

The arrow of time 

One of the distinctive things about time is that it has a direction — from past to future. A cup of hot coffee left at room temperature always cools down; it never heats up. Your cellphone loses battery charge when you use it; it never gains charge. These are examples of entropy , essentially a measure of the amount of "useless" as opposed to "useful" energy. The entropy of a closed system always increases, and it's the key factor determining the arrow of time.

It turns out that entropy is the only thing that makes a distinction between past and future. In other branches of physics, like relativity or quantum theory, time doesn't have a preferred direction. No one knows where time's arrow comes from. It may be that it only applies to large, complex systems, in which case subatomic particles may not experience the arrow of time.

Time travel paradox 

If it's possible to travel back into the past — even theoretically — it raises a number of brain-twisting paradoxes — such as the grandfather paradox — that even scientists and philosophers find extremely perplexing.

Killing Hitler

A time traveler might decide to go back and kill him in his infancy. If they succeeded, future history books wouldn't even mention Hitler — so what motivation would the time traveler have for going back in time and killing him?

Killing your grandfather

Instead of killing a young Hitler, you might, by accident, kill one of your own ancestors when they were very young. But then you would never be born, so you couldn't travel back in time to kill them, so you would be born after all, and so on … 

A closed loop

Suppose the plans for a time machine suddenly appear from thin air on your desk. You spend a few days building it, then use it to send the plans back to your earlier self. But where did those plans originate? Nowhere — they are just looping round and round in time.

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Can we time travel? A theoretical physicist provides some answers

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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March 18, 2024

The Great Debate: Could We Ever Travel through Time?

Our space and physics editors go head-to-head over a classic mind-bending question.

By Clara Moskowitz , Lee Billings &

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Clara Moskowitz: Hi, I’m Clara Moskowitz, a space editor here at Scientific American. We’re taking a break this week to look back at some of our favorite podcast episodes. I chose this one about the physics of time travel, because I’m a big sci-fi geek, so I’m fascinated by the topic. But also, it was such a fun debate to have with my colleague and friend, Lee Billings, another space editor here. We each picked a side – I was pro time travel, he was con—and dug our heels in. Check it out!

[Clip: Show theme music]

Moskowitz: We’re here today to talk about time travel. A perennial – dare I say, timeless–topic of science fiction, but is it possible? Is there any chance at all that it could actually happen?

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Lee Billings: No. No, no no no no. (laughs). Well, kinda. Not really. ARGH. I’m Lee Billings.

Moskowitz: I’m Clara Moskowitz, and this is Cosmos, Quickly , the biweekly space podcast from Scientific American . 

Moskowitz: We’re going to have a little friendly debate.

Billings: Really? I came for a throwdown.

Moskowitz: Well, a wrangle. A parley. A confab. Lee, what do you have against time travel?

Billings: So I love the idea of time travel! And in fact I do it all the time—like most everyone else I’m traveling into the future at one second per second. I’m less of a fan, though, of more speculative time travel, which is good fodder for goofy sci-fi stories, but in the real world it’s an implausible distraction.

Moskowitz: But really, we can stay within plausible physics and still see how more extreme versions of time travel are possible. See, Einstein’s special theory of relativity shows that the rate time flows at depends on how fast you’re moving. 

Billings: Einstein strikes again, what a rascal.

Moskowitz: If you’re traveling in a starship at close to the speed of light, you’ll still experience the familiar one-second-per-second ticking of a clock– but an observer back on Earth would see your clock moving glacially slow. To them, you’d be moving through time at a snail’s pace. That means that when you finally got back,  maybe only a year would have passed for you, but a century could have gone by for your friends on Earth. Ergo, you just traveled to the future! 

Billings: Right, right, no one’s disputing any of that! We can even measure this sort of “time dilation” right now on Earth, not with starships, but with subatomic particles. Some of those particles have very short lifetimes, decaying almost instantaneously. But if we drastically speed them up, like in a particle accelerator, we find they endure longer in proportion to how fast they’re going. So riddle me this, though, Clara: How can we travel into the past? That’s something so hard to do–effectively impossible, almost–that it’s scarcely worth thinking about.

[Clip: Back to the Future : “This is what makes time travel possible. The flux capacitor!”]

Moskowitz: I get it—no one has yet conceived of a way to journey to the past. But the crazy thing is it’s not impossible. Time is one of the four dimensions in the universe, along with three dimensions of space. And we move through space in all directions just fine, and according to physics, travel through time should be just as possible.

One way that people have looked into is via a wormhole—a shortcut bridge through spacetime that was predicted by general relativity. Wormholes can connect distant points in spacetime, meaning you could conceivably use one to bridge not just the gap between here and a distant galaxy, but the span between 2023 and 1923. 

[CLIP: Interstellar : “That’s the wormhole.”]

Billings : Ah yes, wormholes—the last refuge of scoundrels and desperate physicists. The trouble with wormholes Clara, is that, unlike a DeLorean, we have no evidence they actually exist—and, even if they did, it seems the only ways to make them traversable and stable involves using negative energy or negative mass  to prop them open. And, guess what, just like wormholes themselves, we have no evidence these weird forms of matter and energy actually exist, either. And let’s just beat this dead horse one more time—even if wormholes exist, as well as the means to make them traversable, to go back in time seems to require anchoring one end in a region of very warped spacetime, like around a black hole, or accelerating it to nearly lightspeed. Are you sensing a theme here, Clara?

Moskowitz: Yeah, yeah. All I can say is that just because there’s no evidence any of these things exist, there’s also no evidence they don’t or can’t exist. Wormholes are real solutions to the equations of general relativity, and even negative energy and mass are concepts that come up in the math and aren’t prohibited.

Billings: Well how about some more practical arguments, then? If time travel were possible, wouldn’t we have met some time travelers by now? Wouldn’t someone have gone back and killed Hitler—or at least prevented me from wearing that ridiculous outfit to my high school prom? You know there’s a famous story about physicist Stephen Hawking, who invited time travelers to come to a party he was holding. The trick was the the party happened in 2009, but the invitation came out in a miniseries that was broadcast in 2010—thus, only time travelers would have been able to attend. 

[CLIP: Stephen Hawking Time Travel Party: “Here is the invitation, giving the exact coordinates in time and space. I am hoping in one form or another it will survive for many thousands of years.”]

Billings: Sadly, the hors d'oeuvres went uneaten and the champagne sat unopened, because, clearly, time travel to the past is impossible! 

Moskowitz: I admit a party with Stephen Hawking should have been pretty alluring to time travelers, if they were out there. But you’re forgetting about the International Clause of Secrecy that all time travelers probably have to swear to, making sure to hide their identities and abilities from those in earlier eras.  

Billings: Hmm, yes the clause of secrecy here. Feels like we’re really veering into science fiction territory special pleading here. And don’t forget all the paradoxes that we have to worry about too. There are lots of good reasons to think time travel might introduce insurmountable paradoxes in physics. The most famous being the grandfather—or grandmother—paradox. If time travel were possible into the past, so the thinking goes, then a person could go back in time and kill their own grandparents, thus making it impossible for them to be born and impossible for them to travel back in time to ever commit the murder, and so on and so on.

Moskowitz: I wonder if it could be like a many-worlds scenario, where each change a time traveler makes to the past spawns a whole new universe that carries on from that point. So if I went back in time and killed one of my forebears, then a new branch universe would begin where that whole line of descendents, including me, never existed. I mean, it sounds crazy, but then again, physics is pretty enamored with multiverses, and they seem to pop up for lots of reasons already. Maybe it’s not impossible? 

Billings: If not impossible, then I’d say, implausible.

Moskowitz: Well, I’m forever an optimist, Lee! Thanks for listening to the Cosmos, Quickly .

Billings: Our show is produced by Jeff DelViscio, Tulika Bose and Kelso Harper.  Our music was composed by Dominic Smith.

Moskowtiz: If you like the show, please consider rating or leaving a review. You can also email feedback, questions, and tips to [email protected] . 

Billings: For more spacetime hijinks and all your science news, head to SciAm.com. This has been Cosmos, Quickly . I’m Lee Billings. 

Moskowitz: I’m Clara Moskowitz. 

Billings: And we’ll see you next time, in the future!

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• grandfather paradox
• time travel

Time Dilation

What Is Time Dilation?

An accurate clock for one observer may be measured as ticking at a different rate when compared to a second observer’s own equally accurate clock. This effect is not a result of the clocks’ technical properties but of the nature of spacetime itself. [i] Clocks on the International Space Station (ISS), for example, run marginally more slowly than reference clocks back on Earth. This explains why astronauts on the ISS age more slowly, being 0.007 seconds behind for every six months. This is known as time dilation, and it has been frequently confirmed and validated by slight differences between atomic clocks in space and those on Earth, even though all were functioning flawlessly. The laws of nature are such that time itself will bend because of differences in either gravity or velocity, each of which affects time in distinctive ways. This phenomenon will have significant implications for interstellar or intergalactic travel.

What Causes Time Dilation?

Time dilation is triggered by disparities in both gravity and relative velocity. Together these two factors are at constant play in the case of a spacecraft’s crew. When two observers are in relatively uniform motion and not influenced by any gravitational mass, the point of view of each observer will be that the other’s clock is ticking at a slower rate than his or her own. Furthermore, the faster the relative velocity, the larger will be the magnitude of time dilation. This case is occasionally termed special relativistic time dilation.

The Spacecraft Scenario

Two spacecraft moving past each other in space would experience time dilation. If the crew inside each one could somehow have an unobstructed view into the other’s spacecraft, it would see the other craft’s clocks as ticking more slowly than its own. In other words, from Spacecraft A’s frame of reference its clocks are ticking normally, while Spacecraft B’s clocks appear to be ticking more slowly (and vice versa). From a local standpoint, time registered by clocks that are at rest with respect to the local frame of reference always seems to pass at the same rate. For example, if a new spacecraft, Spacecraft C, travels next to Spacecraft A, it is “at rest” relative to Spacecraft A. From Spacecraft A’s point of view, Spacecraft C’s time would also appear normal. Here arises a thought-provoking question. If both Spacecraft A and Spacecraft B think that each other’s clocks are ticking more slowly than the other’s, who’s time is correct, and who would have aged more?

Time Dilation and Interstellar Space Flight

Time dilation would make it conceivable for the crew of a fast-moving interstellar spacecraft to travel further into the future while aging much more slowly, because enormous speed significantly slows down the rate of on-board time’s passage. [ii] That is, the spacecraft’s clock would display less elapsed time than the clocks back on Earth. For extremely high speeds during a journey, the effect would be more dramatic. For example, one year of interstellar travel might correspond to ten years back on Earth. Therefore, constant acceleration at one G would theoretically allow a human crew to travel through the entire known universe in one lifetime. Unfortunately, the crew could return to Earth billions of years in the future. Interstellar travel at high speeds thus would have huge implications from both an anthropological and sociological perspective. The crew volunteering for a mission of this magnitude and speed would have to accept the fact that their loved ones, and perhaps even their home planet or star system, would have died long ago. [iii] Because of this effect, humans might wish to travel to nearby stars without spending their entire lives aboard an interstellar spacecraft.

The Twins Paradox

In this paradox one twin makes an interstellar trip in a fast-moving spacecraft but upon return to Earth finds that the other twin who remained there passed away hundreds or thousands of years ago. [iv] This result appears bewildering because each twin sees the other twin as traveling; therefore, each should find the other to have aged more slowly. The paradox can be resolved, however, within the framework of special relativity. The siblings are not equivalent because the twin on the interstellar trip experienced additional acceleration when switching direction to return back to Earth.

Consider by way of illustration an interstellar spacecraft traveling from Earth to Proxima Centauri, the nearest star system outside our solar system and four light years away. At a speed of 80% of the speed of light, the twins will observe the situation as described in the following paragraphs. To make the math less complicated, the spacecraft is assumed to have reached its full speed instantly upon departure from Earth.

The twin on the interstellar spacecraft would see low-frequency (red-shifted) images for three years. During that portion of the trip he would see his counterpart on Earth in the images grow older by 3/3 = 1 year. On the return trip to Earth, he then sees high-frequency (blue-shifted) images for another three years. During that time he would see his twin on Earth in the images grow older by 3 × 3 = 9 years. When the interstellar trip is completed, the image of the twin on Earth will seem to have aged by 1 + 9 = 10 years.

On the other hand, for nine years the twin back on Earth sees slow (red-shifted) images of the spacecraft twin, during which time the spacecraft twin ages in the images by 9/3 = 3 years. The twin on Earth then sees fast (blue-shifted) images for the remaining one year until the spacecraft returns. In the fast images the spacecraft twin ages by 1 × 3 = 3 years. The total aging of the spacecraft twin in the images received by Earth is 3 + 3 = 6 years, so the spacecraft twin returns a bit younger.

To avoid misunderstanding, note the difference between what each twin actually sees versus what he actually calculates. Each sees an image of his twin that he knows originated at an earlier time and that he knows is Doppler-shifted. He does not take the elapsed time in the image as the age of his twin now. If he wants to estimate when his twin was the age shown in the image, he has to determine how far away his twin was when the signal was emitted. In other words, he has to consider simultaneity for a distant event. If he wants to calculate how fast his twin was aging when the image was transmitted, he tweaks for the Doppler shift. [v]

Time Dilation and Communications with Earth

In theory, time dilation will also affect scheduled meetings between the crew on an interstellar mission and the mission managers back on Earth. For example, the crew would have to set their clocks to count the precise number of years time has passed for them, whereas mission control back on Earth would need to count several years more to allow for time dilation. At the velocities currently possible, however, time dilation is too trivial to be a factor in communications between the ISS and Earth.

Implications for Interstellar Travel

Time dilation will have huge implications for both the crew of a spacecraft and mission managers back on Earth. We must consider, for example, the age of the mission managers for the crew returning to Earth (or for alleged extraterrestrials returning to their home planets) and whether or not an interstellar mission would be sociologically accepted. Consider, for example, a spacecraft traveling at 99% of the speed of light to the center of the Milky Way. If everything goes right, the crew would have aged about 21 years. However, back on Earth over 50,000 years would have passed (as observed from Earth). [vi] Obviously all those involved in the initial planning of the mission, as well as generations thereafter, would have died long ago.

[i] Ashby, Neil (2003). “Relativity in the Global Positioning System.” Living Reviews in Relativity. http://relativity.livingreviews.org/Articles/lrr-2003-1/download/lrr-2003-1Color.pdf.

[ii] Toothman, Jessika (2012). “How Do Humans Age in Space?” HowStuffWorks. Retrieved 2012-04-24.

[iii] Calder, Nigel (2006). Magic Universe: A Grand Tour of Modern Science . Oxford University Press.

[iv] Miller, Arthur I. (1981). “Albert Einstein’s Special Theory of Relativity: Emergence (1905) and Early Interpretation (1905–1911).” SOURCE?

[v] Wheeler, J.; and Taylor, E. (1992). Spacetime Physics . 2nd ed. New York: W. H. Freeman.

[vi] Interstellar Travel Calculator. http://spacetravel.nathangeffen.webfactional.com/spacetravel.php.

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Giant Freakin Robot

Time Travel Equation Solved By Astrophysicist

Posted: March 25, 2024 | Last updated: March 25, 2024

After a lifetime of pursuing the idea, Physics Professor Ronald Mallett at the University of Connecticut has potentially figured out the theoretical aspects of time travel. Professor Mallett believes that black holes, rotating light, and gravitational pulls may hold the key to exploring time, but it’s all theoretical for now. There are still a lot of hurdles and limitations to handle before time travel can have real, practical applications.

A Life Spent Thinking About Time Travel

Love and loss pushed Professor Mallett into an obsession with time and space. When he was 10 years old, his father passed away from a heart attack. It was his father who nourished his love of science, but H.G. Wells’ book The Time Machine pushed him towards a focus on time travel.

He was hooked from the very first paragraph of the book, “Scientific people know very well that Time is only a kind of Space. And why cannot we move in Time as we move about in the other dimensions of Space?”

That paragraph never left him, and the professor let that time travel question guide him through school and into the Professor Emeritus of Physics position at the University of Connecticut.

Einstein And Black Holes

As he grew up, Professor Mallett spent much of his time on Albert Einstein’s theories about black holes. While his interest in time travel only continued to grow, a potential solution never showed itself. At least, not until the professor ended up in a hospital with a heart condition.

There, lying in the hospital bed, inspiration hit him. Black holes and the gravitational fields they created were the answer to time travel. These gravitational fields had the potential to lead to time loops, which then theoretically could allow people and objects to travel back in time.

Black Holes Manipulating Gravity

While this idea offered an ability to manipulate time, the other problem was how to use these time loops for time travel.

Professor Mallett found this time travel solution much easier than the first problem. Strong and continuous beams of light, like a ring of lasers, with a particular rotation could be used to manipulate gravity and mimic the distorting effects of a black hole.

Though the details are rather complicated, the big time travel picture is a lot simpler to grasp. The professor offers a comparison to help people understand. “Let’s say you have a cup of coffee in front of you. Start stirring the coffee with the spoon. It started to spin, right? That’s what a spinning black hole does. In Einstein’s theory, space and time are related to each other. That’s why it’s called space-time. So when the black hole spins, it will actually cause time to shift.”

Much To Figure Out

Professor Mallett may now have a theory on time travel and a machine to use to make it possible, but that doesn’t mean it will be here in the next few decades.

There’s still a lot to figure out to make such travel practical, such as where the insane amount of energy such a machine would require could come from, and how big the machine would need to be.

There’s also a major constraint on the machine. According to his theories, time travel would only be possible to the very beginning of when the machine was first built. In this way, it’s more like a one-way message service. You can potentially go forward quite a distance, but going back in time is limited by the machine’s creation.

Theoretical Aspects Of Time Travel

The professor has made a huge leap in figuring out the theoretical aspects of time travel, but there’s a lot more to discover and quite a few hurdles and paradoxes to figure out before scientists practically start messing around in time.

Still, the theory is a step in the right direction and does suggest that people can push past what science currently considers possible.

Source: Earth.com

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Is Time Travel Actually Possible?

We all know how time travel works, right? You step into a gyroscopic chamber in a secret science research facility, a dude in a white lab coat hits a switch, the chamber spins and makes "bwa wa wa" sounds as arcs of blue "energy" crackle around, and snap: You're face-to-face with Arnold Schwarzenegger circa 200 million B.C.E. for a mission to shoot up dinosaurs with robo gun arms. Pew-pew!

Well, it's time to dash to pieces all of our beloved, sci-fi-conjured time travel visions from "Star Trek," "Back to the Future," "Avengers: Endgame," "The Terminator," and even classy, supposedly more grounded fare like "Interstellar." No, you won't be going back in time to chat up Julius Caesar in English, sire your own grandfather , or save humanity with the power of semi-clever plot contrivances. And yet, sites like Scientific American say that time travel is possible. What gives?

When physicists say that "time travel is possible," they mean that time passes differently for different observers based on speed and distance — i.e., "time is relative," ala Albert Einstein's  famed E = mc 2  theory of special relativity. They also mean that "there's nothing mathematically preventing time from moving in reverse." But we experience time unidirectionally because molecules tend to move from higher to lower states of order, as Quanta Magazine outlines — i.e., "Time's Arrow."  Putting these two together means: 1) No one refutes that forward-moving time travel is possible, and, 2) Backward-moving time travel is impossible.

Space, not time

It sounds like a perfectly typical, baseless, first-year philosophy student proclamation: "You see — time ... doesn't exist!" Cue the smoke bomb and rapturous facial expression. Well, it's time to blow everyone's minds and confirm that, yeah, that statement is kinda-sorta true. Except, it's only true because time isn't time. Time is space, which means time travel is really space travel.

In his 1905 theory of special relativity, Albert Einstein noted that as objects approach the speed of light , they experience time more slowly. As Live Science  overviews, this is referred to as "time dilation." But to that fast-moving object, everyone else is slow. Hence: Time is relative and not the same for all observers. 

Four years later in 1909, German mathematician Hermann Minkowski coined the term "space-time" to refer to the unified spatial and temporal nature of reality. Time, he concluded, is just another aspect — a dimension — of space. You can move through space at the expense of time, or vice-versa, like how you can turn right at the expense of turning left, per Big Think . To the perspective of light particles — the massless, fastest objects in the universe — time doesn't exist, as Forbes explains.

So, if you want to time travel? You've got to travel through space at near-lightspeed. And as the rest of reality zips by, you slow down. Presto: You're in the future. But you can't go back, as Live Science details. Past physical space no longer exists.

Forward, not backward

"You can never go home again," the somewhat cryptic statement goes. But while that statement refers to an inability to recapture the magical fancies of youth, it could also just as easily refer to time travel. 

"Why can't you travel backward in time?" the somewhat consternated headline on Ars Technica asks. Well, because the past doesn't exist in any physical sense with which we could ever interact. It's a psychological construct composed of memories that don't live outside of your head — sorry. The only place the "past" exists is in the far reaches of telescopic sight, like when the James Webb Space Telescope  sees light from the early universe that's just reaching us now. In that moment, we're viewing the remote past.

When considering why we can't travel backward in time, we really have to ask, "Why can't molecules spontaneously rearrange themselves into former compositions?" If that happened, then a cracked egg would reform, or spilled milk would slide back up into a glass. But as the second law of thermodynamics says, entropy in a closed system must always increase. Entropy is the tendency for the arrangement of molecules to grow evermore disordered over time, and the universe is a closed system because nothing exists outside of it. And as Popular Mechanics overviews, entropy has continued to increase since the formation of the cosmos. There's simply no way to slip into any previous molecular arrangement, because remember: Time isn't time. Time is space.

Hitching on a ride with the black hole express

There's one viable time travel method on which physicists can agree: Hitching a ride with a black hole . Or more specifically: Moving past its outer, spinning accretion disc and toward the central event horizon from which nothing can escape. As described by  Royal Museums Greenwich , this process will most certainly kill you by "spaghettifying" your body. So once again, we're talking about a one-way trip. 

Remember we said that as you approach the speed of light, time dilates, i.e., slows down? Well, black holes spin at near-light speed. This is something that one movie — "Interstellar"  — got right. But no, you won't get sucked into a 4D library to push some books off of Murph's bookshelf (it's complicated — watch the movie). As Astronomy says, a person stuck in the event horizon of a black hole would observe the rest of the universe aging much faster. That person, strictly speaking, would be in "the future." Now, if we could escape from a black hole? We'd have a workable solution for future-bound time travel.

And yet, as The Conversation reports, some researchers believe black holes can do the impossible: Send someone backward in time. If a celestial object was so massive that it "dragged" spacetime with it, it could create a closed timelike curve (CTC) that could pull someone into "the past." Still, folks like renowned physicist Stephen Hawking thought this was nonsense.

Feasibly impossible

There are a couple of other weird addendums to the "Is time travel possible?" discussion. Chiefly, as physicist Sabine Hossenfelder says on her YouTube channel: "You need a receiver station for time travel." This is something fictional time travel always leaves out and gets wrong. Earth isn't sitting in space like a stone on the ground. As Scientific American says, it's rotating at about 1,000 miles per hour while revolving around the sun at 67,000 miles per hour. On top of this, our entire solar system is spinning through the Milky Way at 490,000 miles per hour. So if you time traveled just one second forward or backward, but didn't account for planetary motion? You'd teleport right into empty space. And so, like we've said: Time travel is space travel .

And finally, there's quantum entanglement : That thing that you've heard about that no one understands. Quanta Magazine explains how quantum entanglement — the codependence of molecular attributes over distances — gives rise to time's arrow. We'll save you the headache by saying: If it was possible to manipulate the attributes of individual atoms, we could reconstruct "the past." This, however, is whackadoodle fantasy absurdity that necessitates digging into further and further realms of unfounded speculation.

So, is time travel possible? Mathematically, yes. Forward, yes. Backward, no. But is it feasible? Let's put it this way: Best focus on the present, because by the time humanity untangles time travel, our present will be nothing but the memory of entropy's future. 

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Exploring the Reality of Time Travel: Science Fact vs. Science Fiction

By Adi Foord, University of Maryland, Baltimore County November 16, 2023

Time travel, a longstanding fascination in science fiction, remains a complex and unresolved concept in science. The second law of thermodynamics suggests time can only move forward, while Einstein’s theory of relativity shows time’s relativity to speed. Theoretical ideas like wormholes offer potential methods, but practical challenges and paradoxes, such as the “grandfather paradox,” complicate the feasibility of actual time travel.

Will it ever be possible for time travel to occur?

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The Science Behind Time Travel

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time Is Relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Theoretical Possibilities and Challenges

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes, or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and Failed Dinner Parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time, and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Artist’s rendering of the James Webb Space Telescope. Credit: Northrop Grumman

Telescopes Are Time Machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel . As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope , the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang , about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies, and dreams.

Written by Adi Foord, Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County.

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11 comments on "exploring the reality of time travel: science fact vs. science fiction".

Until the problem of the second law of thermodynamics(entropy) is solved, the concept of time travel will remain the subject of science fiction. Since this is a basic law of our universe, there is no conceivable way that we know of to do this. The great thing about our knowledge of the universe is that it continues to grow and with that our view of what is possible continues to change. After all, at one time it was believed we could never leave earth!

The 7 planets are soul pollen in the space @ life has been the world has been prepared @ The pollen of the universe is hidden @ Around the axis of the galaxy, the universe is hidden@@ The pollen of the galaxies is a hidden cluster Itis made that we thought about what wisdom is in God’s work and how it is arranged in the form of words.The verse that is made is as follows @@ John, you are in time @ worlds, planets around the axis of the branches of galaxies @@ 8 Prophets, divine prophets, God-aware witnesses @ God’s words of revelation, they are aware @@ 48 What the words of revelation that every prophet has about @ sometimes Sometimes the message of God has become a verbal cliché in the head @@ 62 The message of God was given to every prophet @ The message was made around the power of God @@ 46 The truth of the religions of the cradle of time is said in the world @ Prophets always came for justice in time @@ 62 A warrior became brave in time @ Delaver Ghahrmani Boud Taarani@@ 43 Omar Noah never died, who knows!@ Imransan Is there an unseen world, immortality!?@@ 49 Men’s rights in the sign @ Human rights, the observance of world justice @@ 40 We have God’s love @ A love that is not patched, separation!!!: @@ 39 Take one word from the end of the first and second stanzas to the bottom of these eight verbal verses, and this sentence is made @@ God-aware, the world is born, you know the sign of God @ Agah,the beginning of time, the world, eternity, the world of separation @@ The meaning of this sentence is That God, who before us humans lived on the earth, formed the earth’s crusts with full knowledge, and we humans know the sign of God, which is on the earth, on the continents and countries, and some names have been shown by God for our knowledge since the end of time.In a later video, if I have a lifetime left, I will explain exactly about these poems, God willing @@ The word of the Prophet 17 is the 17th and Muhammad (PBUH) said that the Bedouin Arabs should pray 17 rakats so that theyperform ablution and be clean and not kill and loot.From the caravans, these were all God’s will, and he is good everywhere, in every nation, God does not like evildoers, sinners, and oppressors.Muhammad (PBUH) was God’s last messenger to the Arab people, he was God’s best prophet, and the third verse is because they do not accept Muhammad (PBUH).Some Iranians who were in contact with me, that’s why in the third verse of this surah, he said that his message was given to every prophet, the message is based on divine power, and the word truth, the first two letters of which istruth, is truth, and truth is the 43rd and forty-third word, and the word is time.It is exactly 46, and this song of the Prophets was revealed at the age of 46 ببخشید اگر قبلآ مطالبی فرستادم که به مذاهب مذاهم ارتباط داد شاید به درد شما نمیخورد من در کامنت های بعدی سعی میکنم از نجوم و حیات زمین سخن بگویم این چند بیت را به خارجی انگلیسی که تبدیل کردم معنی آن حیرت انگیز تعغیر کرد گفتم برای شما بفرستم و بداند که این کلام من نبوده کلام خداوند بوده شما نظرتان در مورد خداوند چیست من میگویم خداوند که پدر ومادر انسان بودند قادر به ساختن ما بودند اما آیا آنها قادر به ساختن ستاره ها هم بودند

Your comment has validity to God. But it surely has no place here, it is only fare if the hole comment were in english and has less of a convincing push in a belief a person either believes or not.

I thank Johnson for saying that my opinion is valid before God.I read Johnson’s other comments about time.Johnson said that time is forward in physics.Time is determined by us.Previously, Einstein said that if an object moves faster than light, time It becomes zero, and it was said that there is a fourth dimension, I should say that they move a thousand times the speed of physical light, time does not become zero,and the world has three dimensions, length, width, height, but we determined and interpreted this length, width, and height, and we must determine the fourth dimension in At the moment, they did not define the fourth dimension and it does not exist.For now, in the case of Johnson, who said that we should see strange changes faster than light, maybe Johnson was right, but how can we see changes ahead of the speed of light, and what will be the use of Johnson’s opinion?The events that are going to happen in the future must be known.I understood from the Lords who were raised by humans that the earth was previously inhabited by the same humans who lived on the earth, continents and seas in the form of four-legged mammals and humans and birds by humans who lived in the millions.They had years of history, continents and seas were formed on the earth, and those who inhabited the earth before us in the past had advanced so much in the science of astronomy, physics and chemistry that they simulated the souls of humans and raised many godsand lords.By the same Lords, I got very, very important information and secrets that I explained in my comments, but because I have been familiar with this very interesting website for a few weeks, I have not yet provided many comments, but soon more information aboutastronomy.and the structure of the solar system and the earth, the issues of belief and the events that are going to happen in the solar system in a few thousand years, I will report to you, those who are interested, the sun is currently located inthe big arm of the galaxy and it is hitting the stars of this arm, the stars that are close to Neptune Uranus, Saturn, Jupiter and eventually hit the sun.There were more than hundreds of stars, all of which were a hundred times smaller than the sun and their mass was several hundred thousand times smaller than the sun, and most of the planets that came with these small stars collided with the objects of the solarsystem.About 240 planets turned into moons and rocky planets of the solar system.I have already said that the sun is slower than the stars of the two arms of the galaxy due to the very heavy mass and weight that it has attached to the stars of the two large arms of the galaxy, but it may not be just becauseof this issue.The objects of the solar system collide with the stars and planets of the arm of the galaxy.Maybe it is because of the very strong gravitational force of several large planets, Jupiter, Saturn, Uranus and Neptune, that the star systems of the arm of the galaxy are pulled towards the solar system.I know for a fact that those who live on earth They had advanced so much in astronomy, chemistry and simulation that they knew about the sad incident of the collision of the stars of the galaxy’s arm with the solar system, that’s why they drew America asa bird that is flying in the sky.God bless you in all the languages ​​of the earth, hoping to meet you dear ones

If the big bang happened, why aren’t there galaxies that are a billion times the size of the Milky Way?I say this theory that when an explosion occurs, the order that galaxies have now could not exist in an explosion in the core of a super black hole, regular materials and elements.They do not spread.I understood from the distance of the stars that the age of the sun is at least a few hundred billion years, and life on earth arose in the last hours and minutes of the sun’s life.It is possible that the earth revolved around the stars of the arm of the galaxy tens of billions of years ago.The galaxy returned to the earth, the moons of the four planets Neptune, Uranus, Saturn, Jupiter, which are about 231 moons, they were separated from the stars that came from the arm of the galaxy towards the sun, from those stars that collidedwith the gas planets, the cause of the hydrogen gas in the gas planets was due to the collision of the stars that I said.Their size is a hundred times smaller than the sun.If those stars had a large size, the earth would never have a temperate climate and it would be so hot that life would never exist. I translate the content that I send to those interested in astronomy and technology on this very interesting website using Google.I retranslate them from Latin to Farsi.Problems occur during translation in dictation, composition and spelling, but because I am very I have to take time to correct them.I sent them to you, dear readers, without any mistakes.I hope you understand the content.If you don’t understand something, call my phone number via text message.Thank you, sorry, and apologies to you, dear doctor.and chosen by the Lords of the world, Mehrdad Kathiri Kasiri time in tehran 10:46 pm

I have to explain a point that there were mistakes in the translation.In the comment at 22:46 that I sent a few minutes ago, I did not say that the stars collided and I did not say that I understood from the distance of the stars how many hundred million years have passed, I said from the distancebetween the stars The arm of the galaxy, I realized that the sun lives not a few years from the age of the sun, I said that since the sun was in the form of a huge cloud and very big and like a nebula, from theage of the formation of the sun to this time, at least a few hundred billion years have passed.

It’s too bad physics can’t come to a complete consensus about time, I would like to add some thought about discoveries it has been proven that time travels in only one direction forward, the experiment dealt with light thru glass and how it reacts in the middle and what change happens after light exits the other side, a simple explanation of this experiment. Brings me to theorize and start that time existed before the big bang and is outside of our universes influence, when time is acted on by gravity the ( Form ) of time is changed until the influence no longer has effect, this could go hand in hand with light photons the photon has a influence in the Form that time has. This can not be a observed difference unless we were to see beyond the speed of light. We do agree that physics changes at a subatomic state and also does some strange changing once the speed of light has been exceeded.

The experiment I referred to was posted on IFL in October 2023 headlined ( Solution to complex light problem shows that time can only go Forward ).

One of the problem with travel time is the one people keep forever. And, that is that the earth is always move through space. Matter of fact, the earth is not in the same place that it was 50 years ago. So you will have to move through space as well as time.

Ironically, the only science fiction that seems to handle this is the original story “The Time Machine”.

Time travel is happening now. It has been done since the 1950s. The method satisfies all the requirements. Traveler can’t change the past, but only observe. You can’t go farther back than when the machine was first invented (1950s). There’s one more limitation, you can only observe what the machine was directed. The time machine, the common video camera, and video tape recorder. Now it’s the camera, and file capture computer. Yes, viewing a video tape is effectively going back in time. It’s more than the video, it’s the sound too. There are working versions of smell, and touch which can be recorded too. If you record, and replicate all the senses, you have effectively complete time travel. The most primitive form is the picture. This technology has been around for thousands of years, and is manually intensive. Later many pictures were strung together to make a film. Using a camera to record film was the first example of complete visual time travel (back to when the film was made). Later sound was added, and we have the movies. A way of going back in time that included sight, and sound. Now we have video systems (YouTub) that can play back past events selected by you. Yes, video systems are virtual time travel machines we have now.

Tom Mariner | December 17, 2023 at 5:26 am | Reply The folks “out there” are just answering the SETI message sent by the Arecibo radar station in Puerto Rico in 1974.

Our neighbors light years away are waiting for another message from that most power radar transmitter ever, but the bright lights have turned the facility into a STEM training facility after other organizations stole the money from Puerto Rico.

dr mehrdad kesiri | December 18, 2023 at 9:06 am | Reply Steve | December 9, 2023 at 6:20 pm | Reply

Would you be able to calculate if the whole multiverse Is spinning and depending on the distance. You are from the center you would be going faster than in another position?

Fixed gravity for you. | December 10, 2023 at 1:20 am | Reply

Kroupa is allowed out there with high profile exposure because he’s flacking for an idea by a new hero of famously top-dog self-victimizing self-absorbed insect opportunists, as Einstein’s gravity theory is becoming known as a closet failure over-celebrated by the generation that bred said famously top-dog self-victimizing self-absorbed insect opportunists.

Fixed gravity for you. | December 10, 2023 at 1:48 am | Reply

The overexposed cat icon seems to be a free-range reddit group run by AI frat boys.

Fixed gravity for you. | December 10, 2023 at 8:35 am | Reply

Replacing “expansion of the universe” with “dilation of the photon” would not be respecting “an establishment of religion” (establishment of a religion-based theory) but science has no bill of rights and gravity experts are prone to shunning reason anyway.

dr mehrdad kesiri | December 17, 2023 at 7:36 am | Reply

I understand to a certain extent, but religion and religions always interfere with progress and old religions should be put aside.I am the representative of your Lords and I am the master of time with hundreds of signs.The Lords of the world tried for hundreds of years until they came to the conclusion that they had to remove the disturbing religions by bringing the new religion and law of God in the Middle East because many people abused religion and came to power.Unfortunately, that person who should appear with his companions I am, but this work will be done because all the signs that the Lords of the world have placed on me in the earth and time are the numbers that are connected to time in the date ofmy birth and the surahs that have been revealed to me.The surahs and the words of the surahs, which are numbered, connect us to the words of signs and time.What we mean is that when the words are counted, they are connected to the time of the events of the words.This issue is very important.I will give you an example.I was injured in the history of 365 and this number belongs to time.This is the kind of owner of time, and God has revealed books, treatises, and surahs to me.This is a religious debate.It has something to do with astronomy, and the destruction of life on Earth, which God said earlier, is the end of time.The sun is hitting the big arm of the galaxy, and it is possible.It is possible that life on earth will be destroyed.This happened more or less a hundred million years ago and humans have raised gods.It is possible that in a few hundred years we will be able to raise gods with the help of artificial intelligence, physics and chemistry.A thousand system stars have joined our solar bodies and collided with Mars and Earth.No stars and planets from the arm of the galaxy that came towards the Sun did not collide with Earth and Mars, but when they collided with gas planets and the Sun, the system The stars of the arm of the galaxy after hitting the sun, theelements of the said planets and stars, like meteorites, their elements hit the earth and the life of the earth was destroyed about a hundred million years ago.This is the true story of Dr. Mehrdad Kathiri. thank you gods by time in in tehran 7:07

I thank Johnson for saying that my opinion is valid before God.I read Johnson’s other comments about time.Johnson said that time is forward in physics.Time is determined by us.Previously, Einstein said that if an object moves faster than light, time It becomes zero, and it was said that there is a fourth dimension, I should say that they move a thousand times the speed of physical light, time does not become zero,and the world has three dimensions, length, width, height, but we determined and interpreted this length, width, and height, and we must determine the fourth dimension in At the moment, they did not define the fourth dimension and it does not exist.For now, in the case of Johnson, who said that we should see strange changes faster than light, maybe Johnson was right, but how can we see changes ahead of the speed of light, and what will be the use of Johnson’s opinion?The events that are going to happen in the future must be known.I understood from the Lords who were raised by humans that the earth was previously inhabited by the same humans who lived on the earth, continents and seas in the form of four-legged mammals and humans and birds by humans who lived in the millions.They had years of history, continents and seas were formed on the earth, and those who inhabited the earth before us in the past had advanced so much in the science of astronomy, physics and chemistry that they simulated the souls of humans and raised many godsand lords.By the same Lords, I got very, very important information and secrets that I explained in my comments, but because I have been familiar with this very interesting website for a few weeks, I have not yet provided many comments, but soon more information aboutastronomy.and the structure of the solar system and the earth, the issues of belief and the events that are going to happen in the solar system in a few thousand years, I will report to you, those who are interested, the sun is currently located inthe big arm of the galaxy and it is hitting the stars of this arm, the stars that are close to Neptune Uranus, Saturn, Jupiter and eventually hit the sun.There were more than hundreds of stars, all of which were a hundred times smaller than the sun and their mass was several hundred thousand times smaller than the sun, and most of the planets that came with these small stars collided with the objects of the solarsystem.About 240 planets turned into moons and rocky planets of the solar system.I have already said that the sun is slower than the stars of the two arms of the galaxy due to the very heavy mass and weight that it has attached to the stars of the two large arms of the galaxy, but it may not be just becauseof this issue.The objects of the solar system collide with the stars and planets of the arm of the galaxy.Maybe it is because of the very strong gravitational force of several large planets, Jupiter, Saturn, Uranus and Neptune, that the star systems of the arm of the galaxy are pulled towards the solar system.I know for a fact that those who live on earth They had advanced so much in astronomy, chemistry and simulation that they knew about the sad incident of the collision of the stars of the galaxy’s arm with the solar system, that’s why they drew America asa bird that is flying in the sky.God bless you in all the languages ​​of the earth, hoping to meet you dear ones

Should America, Britain and its allies be silent, why did they allow Imam Khomeini (RA) to brutally shoot those great chosen ones of the Gods of the Earth?Then God struck the parliament in the year 60 and seventy-two people were killed from those who signed the order to shoot the soldiers.Was it the will of the lords of the gods of the soldiers to avenge the blood of the soldiers that Khomeini and the Ayatollah approved in the Islamic Council of the Islamic Republic?Did they follow the order that Imam Khomeini had given for the execution and shooting of the soldiers?Why do the Gods of Iran send Al-Qaeda to shoot the American passenger planes inside the twin towers on September 11?Maybe these things were done by the Gods of the Earth.It is not known.Before al-Qaeda carried out that brutal suicide operation, in the same way as the United States in 1967, they fired two missiles at the Iranian passenger plane from the launcher, and the missiles proved to the Iranian passenger plane that the two F-14fighters Iran was escorting a passenger plane to reach the destination of Dubai, 290 people were killed in the skies of the Persian Gulf of Iranzmin, 290 innocent people were killed.I don’t know if this was the will of the gods of the earth, that Imam Khomeini should sign Resolution 598, or it was the will of American politicians, why didn’t the big powers stop that horrible and brutal crime of the evilImam Khomeini and Iranian mullahs in 1957?that the gods of Iran bring war in Iran, where the mullahs forbid a morsel, a morsel, a morsel that is not halal, or the mullahs take a morsel from the Islamic Qur’an, God and the people are not pleased that the mullahseat that morsel, the mullahs do not work, most of them and Hafiz The great Iranian poet in 900 years ago, when the Arab mullahs had dominated Iran and the Arabs had attacked the Iranian soil, Muhammad Hafez said in Surah Vaizan that the sheikhs andmullahs eat a morsel of ghosts and hypocrisy, and the mullahs are heartless in the work of God, which means interference.In the case of Iran, the mullahs act as judges and the god of Iran.The mullahs are the judges in Hafez’s poem, which means God.Why did they start the war between Iran and Iraq?The gods of Iran, after the mullahs slaughtered the soldiers unjustly, say that they are stupid and ignorant anklets.In one night, 57 generals and officers of the imperial army Iran was executed and shot, I say Khalkhali is crazy, he didn’t know that the imperial government had spent millions of dollars for a general.Until that general, Khalkhali became a general because of the enemy he made to the Iranian army and destroyed Reza Shah’s grave.People should curse him the worst.But The messenger of the gods of the earth tells me, Mehrdad, you are the representative of the gods of the world of Iran, don’t call anyone a bastard, or if you say a bastard, say that he himself was a bastard,neither his family nor his ancestor, the messenger of the gods of the earth tells me, Mehrdad, because you are the most accurate person, the kindest person, and the smartest.And you are the greatest poet of Iran.Perhaps the gods and lords of the earth chose you, and perhaps they raised you from Nadfa, who entrusted your justice to you.May the gods bring forth the truth and truth of religions until the world is no longer at war over the old religions.As for religion and religions, because religions are a collection of religions, they disturb peoples and nations, like Islam, which is not a religion, there are religions that disturb people.Is it possible to follow all the religions of Islam?Also, know why the word disturbing is the same family as religions.Why is the word disturbing similar to religions?

I am the owner of time, with dozens of signs in the date of my birth, the gods and lords of Iran, the God of the world, made me the owner of time, that is, the owner of time means the chosen ones.Gods who are connected to time, those who own time on earth in this age, what There may be many people like me on earth, people in Iran should own their time in a different way, because they say that the Imam of the Time doesnot appear except with his companions, but I do not say that I am the Imam of the Time, because of Imam Khomeini who destroyed Iran.I use the word Imam.Before and after 1957, there was Khomeini, whose father was a rebel and was executed by Reza Shah, and Khomeini took revenge from the army for his father Seyyed Mustafa Khomeini, and in 1958, when he came to Iran, he killed hundreds of armytroops.I hate the word imam, but the people of Iran tell me that you are the imam of the time, but I say that I am not the imam of the time and I am the owner of the time with dozens of signs.The earth and in the eight surahs that were revealed to me.I am the owner of time and a human being.I am like all Iranians, but the difference between me and others is that I know what is happening in the unseen world and I will inform you about the future of the earth in time.My date of birth and age with street numbers Tehran and the squares of Tehran are related to time in history when numbers are connected to the year.These things I want to claim have a reason because I am the owner of time and the only representative of the gods of the earth.I know all the secrets of the unseen world and dozens of signs.In the places where I said the street numbers of Tehran and Tehran squares, I was connected to time, for example, how in 1960, when I was 17 years old, our house was on 117th Street, or I was 54 years old.Our house was exactly on 54 Square.At the age of 55. We bought a house at 55 Narmak Square.I was injured in Qasr Shirin war zone in 365, and 365 is the middle of the year, which is the leap year of 366, and I finished my service in the 88th army division in 66 and I did three jobs.88 Zahedan, I did three things.I brought the ammunition to the front line.I brought food to the front line for three companies from the 196th battalion of the 88th army and led the warriors and soldiers to the front line.That is, I was… in the middle of my military career, I went to the army I helped the 88th Armored Division of Zahedan, that is, I helped the army of the 88th Armored Division of Zahedan.I was an IFA driver and I played my role like this, and five times at a distance of a few meters and tens of meters, a Russian IFA truck that was I was driving and was hit by bullets and mortars from a French tank.Our car hit the ground, but maybe the gods of the earth saved me from all the attacks and fire that Saddam Hussein’s military party threw at us soldiers, many fires on the war front.They gave me, but I was lucky to survive in high school.When I was in my first year, in 1957, the guide of the revolution became available, and I was in the seventh grade, and it was the year 1957, and I was connected to time.I belonged to the imperial government and Mohammad Reza Shah was 11 years old.He fed millions of students all over Iran every day, but for several years I was unable to guide people in religion, which direction to go, and I was 17 years old.I should not raise these issues

But there are many issues that I am connected to time, but you should say, well, what is the use of these things that God has placed me as the owner of time on earth, who has chosen me and chosen me as the savior andleader of the world, to the people of Iran.guide and let the elders of the world accept my proxy to express the orders of the Lord according to the order of the new religion and religion that God has revealed to me so that justice is done and we understand that Muslims especially in the Middle East willnot fight over the old religions because Islam It has been everywhere, the Muslim men of those Muslim countries were at war, why should the people of the earth still fight over religious and Islamic issues?Prejudice, prejudices?Why did Israel, Palestine and Iran go to war with Iraq?How am I the owner of time?There is a number in the date of my birth.When I multiply my date of birth from the beginning and the end, it is equal to the speed of the earth.It is obtained in one second and one hour.The speed of the earth and the speed of light are obtained in one second and one hour.And these are all related to time.You can multiply the date, month, year, and decade of my birthday.Then comes the number 90, and the number ninety is related to the right angle, and Qam has several meanings.Qam means a ninety-degree angle.Qam means firm.Unfortunately, Mehrdad is the ruler of time and being the representative of God is not interesting to me.Now how am I the owner of time when ninety multiplied by 40, which is forty, is ten Kan. On my date of birth, ninety multiplied by forty equals three thousand six hundred three thousand six hundred seconds in one hour, and This is howI am the ruler of time in the combination of my father’s birthday, which is Muhammad, and he was born on the 17th, and I was born on the 43rd, together with my father’s birthday, which is the 17th, and my birthday, whichis the 43rd.60 and sixty seconds in one minute and sixty minutes in One hour should be 360 ​​and I was 17 years old in the year 360. And from a school in Vahidiya called Hefdeh Azar, that school was called when I was 17 years old andwe went to 17th Street in Tehran, Pars.You should know that there is a very strange issue that you You must know that I know for sure that God has chosen me in such a way that England and America, who are the most versatile people in the world, should know the secret of whoI am, Mehrdad, the owner of time, and why God has entrusted all the secrets of the earth to me.See, words are not like that.pass by them, there is a secret in the words of the Lord of the Earth, that not everyone is privy to the secrets of God, that God will reveal this secret to a human being, if the capacity and tolerance of that person is notperfect and he does not have the capacity to hear it, with whom should the secret be kept explain the world of the unseen and how they can help the justice of God to be implemented by his savior who unfortunately this very heavy responsibility is institutionalized on Mehrdad’sshoulders.

This very, very heavy responsibility on Mehrdad is institutionalized in me.This issue is so complex, sensitive and precise that it has no limits.Maybe I know the entire secret of the creation of the earth and the creation of the universe, but this is not an interesting thing for me, a person who knows everything.He can’t have a peaceful life.These issues in the date of my son’s birth become more complicated by combining my own date of birth, my wife’s date of birth with birth certificate number and my date.My birthday with the date and the time I was in my mother’s womb with the addition of the day I was born will be the number of 114 surahs of the Qur’an, the number of 30 parts of the Qur’an and the number of60 parts of the Qur’an, all in the combination of the dates that I said at the time of my birth with the date of birth I and the number 17 of the Muslim prayer and the number 72, which is related to the seventy-two people who were killed, are related to the story of the Qur’an.By combining the dates of birth of my paternal family members with mine and dozens of these more complicated issues, it made me wonder how they could do God’s work.So, compared to the work of the gods and lords of the world, it is very strange and amazing and immeasurable.It is not possible for anyone to learn from me all the experiences that I have and have experienced about the unseen world, and now I have learned all my knowledge in one night, unless you have experienced what I have experienced in practice, the same problemthat I have said, but I I may not be a happy person.I have not saved anything from the world and I suffered very, very hard.I was forced to go to camps for 11 years and sat in 900 sessions in Narcotics Anonymous and collected the experiences of others.If he has worked the twelve steps and sat for ninety sessions and listened to the words, he will become a saint.A saint means, for example, he will become a professor and perhaps a philosopher, but I sat for 900 sessions and listened to the words, and later I understood that all of this was a divine test.I will tell you later.I am sending a message that I have a book and a treatise, and the great powers of the world should help me bring my treatise to the fore, so that the old religions become new and the disturbing religions are abandoned, and the law of thegods and the masters of the world gradually take the place of religion and religions.He revealed and said that the time has come for the Muslims of the world to follow the law instead of religion, and the treatise that I bring from the Lords and Masters of the world separated religion and religion from politics and did not make the choice ofreligion mandatory, our Lord God said that time It has come that the law works instead of religion, and God and our reason said that mankind has progressed so much in terms of establishing security in countries that there is no longer a need for the clergy toforbid people from sinning and history has shown that in order to implement the orders of the Qur’an The old religions are the most abuses and killings of Muslims because of disturbing religions and the time has come for God to bring a new religion and religion thatrepresents God from among the Shiites who should be from those who agree with the Islamic principles of the Qur’an and the correct verses of the Qur’an and the perspective of the Qur’an.I have to reveal the book of God’s law that I have written

to bring to light so that old religions become new and disturbing religions are abandoned and the law of God and the masters of the world gradually take the place of religion and religions.God revealed to me and said that the time has come for the Muslims of the world to practice law instead of religion which I bring from the Gods and the masters of the world, they separated religion from politics and did not force the choice of religion.The idea of ​​establishing security in countries where there is no longer a need for clerics to forbid people from sinning and history has shown that due to the implementation of the false orders of the old books of religions, the most abuses and killings of Muslimsare due to disturbing religions and it is time that God By bringing a new religion and religion that is the representative of God from among the Shiites, he must be one of those who agree with the Islamic principles of the Qur’an and the correct versesof the Qur’an and the view of the Qur’an.He set me up with the Quran and other times when the numbers of the times are related to the verses of the holy books and to the times when I am connected in the numbers of the squares of Tehran to the numbers of the verses of the Quranin the times when I was residing in Mehrdad and I was living in the numbers The fields that are related to the numbers of the verses of the Quran, none of the Muslims, especially the clergy of Iran, can not accept Mehrdad’s being chosen and theowner of time, who is the vicegerent of the time, the vicegerent, the savior, and the savior of the human world, but I have nothing to do with ordinary people.I have no difference from a human point of view, only my difference is that the Gods and the masters of the world chose me to put aside the old religions and wars and conflicts because of religious, ethnic, racial differences and other issues, etc., which caused war, and no religion over another There is no superiority.All religions lived in order to guide mankind to the right and true path.They came so that people should have good deeds and good behavior.Whether these principles came from God or from earthly people, we must be righteous and righteous.I was 17 years old.We went to Haq Azar Middle School, Tehran, Pars Khazarin, 117. I shouldn’t bring up these issues, but there are many issues that I am tied to time, but you say, well, what is the use of these things thatGod has given me time on earth to to be chosen by the world as a savior and executor to guide the people of Iran and let the elders of the world accept my proxy so that I can put forward the divine orders through the orders of the religionand the new religion that God has revealed to me.Lord, so that justice is done and realized, and Muslims, especially in the Middle East, do not fight over old religions, because wherever Islam was, the Muslim men of those Muslim countries were fighting, why should people still fight over religious issues andIslamic prejudices, Israel and Palestine.And why did Iran and Iraq go to war?How do I own time?Now there are signs in my birth date.When I multiply my date of birth from beginning to end, it is equal to the speed of the earth.It is obtained in one second and one hour.The speed of the earth and the speed of light are obtained in one second and one hour

And these are all related to time.From the day, month, year, to the decade of my birth, multiply it and get the time ruler.In the combination of the birth of my father who is Muhammad and he was born on the 17th and I was born on the 43rd with the birth My father, who is 17 years old, and my birthday, which is 43, are combined.60 and sixty seconds in a minute and sixty minutes in an hour.It may be 360 ​​and I was 17 in 360. These issues in the date of my son’s birth are combined with my own date of birth., my wife’s date of birth is complicated with birth certificate number and my date.Birthday with my wife and son, combining the date of birth of my paternal family members with me and dozens of such things.He thought how to do God’s work.Work May the gods of the world be so precise that all this is shown to me, compared to the work of the gods and lords of the world, it is very strange and amazing and immeasurable, this is how Iran is connected to the earth.It was connected and has a strange meaning.The old word Iran-earth I remember 52 years.Before the start of the nationwide program of the Iranian television network, which started working, the logo of the program was Iranzmin, the name of the program of the first channel of Iranzmin and Iran was a message to all the countries of the earth.In terms of the language of the gods and lords of the earth, there are many words.I have to say because I have to tell and tell the truths of the gods of the earth.Why am I the owner of time, because I know from the future what will happen in the world and what will come to the earth and what will be the order of the gods of the earth for the superpowers of the world and my religions.I am saying important things, but those who were from the Iranian army were shot in 1958 and 1957 by Imam Khomeini (RA) and the leadership of the Assembly of Experts.The execution and shooting of generals were done by the Ayatollahs, but why America and England?did not stop them from being executed, know that the structure of the gods made by humans is so detailed that I believe there are forty of your gods.It still appears in various forms.I could not understand the structure of your gods and my gods, but you The superpowers of the world, with all that progress, you still haven’t been able to understand the secrets of God’s creation, how many million years of history and civilization didhumans have?When did the earth get destroyed?It has been burning for trillions of years and the earth is a planet that has been formed for twenty billion years, but it was not habitable because it has thousands of active volcanoes.Scientists said that the life of the universe is about thirteen billion years.A year has passed, but now that the James Webb telescope The most distant galaxies arrived, those galaxies said that they are ninety-seven billion light-years away from us earthlings.Now they should say that ninety-seven billion years have passed since the age of the universe, but this is not the case.The distance of the galaxies has nothing to do with the age of the universe.No bang.Consider that two hundred billion galaxies are placed in a black hole, in that big black hole there will be no healthy hydrogen atoms left and after the explosion the stars will be reborn and in a very, very big explosion, it’s great.Materials Chemicals, proteins and vitamins, which are the necessary ingredients for life.They are not healthy enough to create life on a planet next to the stars after the big bang.I said thirty years ago that Herschel and Kepler’s theory could not be true.

Of course, I read the book Asterism and the Galaxy, which the two American scientists used a radio telescope to emit microwave waves from the galaxies, and it took billions of years for them to reach the earth.They were able to imagine the world with that very large radio telescope and I saw the images of billions of galaxies opened my mind and I read that astronomy book when we were in Tehran Pars Street 117 and I was 17 years old, that is forty-two years ago I had read that book and at that time I also believed in the Big Bang theory I had my doubts about the two great American scientists, but I would not reject it.However, I accept America, which is very supportive.Its astronaut scientists are the first to speak in the world, but the general policy of America is in the hands of the big American capitalists, and they, unfortunately, determine the final decisions of the Parliament and the Assembly.They are the enemies of America, and this has caused the United States to implement policies for the countries of the Middle East region in which the economic interests of the United States are considered, and this policy has caused a lot of damage to Iranians, whichincludes many other social and cultural issues.The clerics have been left open in Iran and this policy of American politicians has caused the power in Iran to be in the hands of a handful of religious sheikhs, mullahs and extreme Shia ayatollahs in Iran and the situation of the unfortunate people of Iran and theMuslim people of the Middle East since the time when Christians The priest and the pope held power in Europe and committed heinous crimes against the people of Europe.The situation of the people of Iran is no better than during the Middle Ages and the Renaissance, it is even worse.And the institution, headquarters, and courts that the clerics had at their disposal have already been shown in history that they oppressed the people through both Islamic and Christian religions.And they came to power according to the Qur’an, they were the cruelest religious government and they committed so much cruelty and murder.Just refer to the history books of Tabari and Sana’i.It is not necessary for me to explain anymore.I only ask America and England, which are the great powers of the world, to Help me so that I can reveal the instructions of our Gods, whose language is the Gods and masters of the world, most of them are Farsi, perhaps all theGods of the earth, those who previously had millions of years of history on the earth came to the conclusion that the alphabets of the Gods who create should be It should be Farsi because the most accurate alphabet and words are in Farsi and the signs I sawon the countries and seas are related to Farsi.This issue is so important in the work of the authors of the earth and you should not easily pass it by.Regarding this issue, it is very, very important.I can’t explain it in a few pages, but over time, I will send you a message in this regard, the great servers of the world, and let me also say that when I translate Persian into English, you know the text,dictation, and spelling will change, but you will understand the meaning of the text in general.May you understand Mehrdad, chosen by God, the land of Iran, may he protect you in all stages of your life

Sorry thank you dr aghakesiri ghaeem sahebzaman in iran and universal

How Stellar Cannibalism Illuminates Cosmic Evolution

جزایر فیلیپین دایناسوری که توسط انسان خلق شده در بیش ده ها میلیون سال و بخاطر ریخته شدن دوهزار متر خاک غرق شده بخاطر بالا آمدن آب اقیانوس اما جزایر فیلیپین شبیه دایناسوریست

The address of the above comment on the site about a thief who was trained by a dinosaur bird who was trained by humans tens of millions of years ago and who arrested murderers and robbers.The police were arrested by big birds.don’t the scientists of the world think about this?They were buried in the bed of important cities, they were buried with all the tools and machines, the traces of humans tens of millions of years ago, they had a civilization and a history of hundreds of thousands of years, they built a base underthe earth, from the meteorites that explode from the planets when they hit the sun, and they knew that several thousand meters of soil is poured on the surface of the seas and islands, and they knew that the shapes they made of the islands in thecountry of Papua may go under the ocean, of course, the Philippine islands.The picture is of a baby dinosaur that went under the ocean, but the northern island of Australia, which is Papua, is quite clear.It is a big dinosaur whose tail is towards the east and its mouth is open towards the west.There is the Philippines, but it was more difficult to take the soil to the Philippine islands to create a dinosaur than the island of Papua, that is why the height of the soil in the Philippines is lower, and when the meteorites fell a few kilometerson the surface of the ocean, the image created by humans under the ocean in the shape of a dinosaur is hidden in the American continent The picture is of a bird in the shape of a dinosaur that is flying, and this bird was made to flyby the Indians of the tribe, that bird was talking to people, but its spirit might have heard something from the police because a thief while in the bird’s mouth He was handcuffed by the police and the weapon, which is a machine gun, is fromthe east of the American continent on the coast

The country of Florida is a machine gun.When you continue to New York City, you will reach the mouth of the dinosaur, where a thief is trapped in the mouth of a bird, and the little finger of the police handcuffed the thief’s hand, and a small colt is in the hand ofthe thief, who the police caught in the mouth of the dinosaur.put in the mouth of the bird dinosaur, you can clearly see that the thief fell on the ground and was shot in the head, and it is clear that his forehead was pierced, the bird’s mouth is open in flight, the head of the birdis from the east of the American continent, and a fish is placed in the bird’s mouth in the water of the ocean.The stretched glove of the police, which is in the form of a fist, with a handcuff attached to the left hand of the thief who fell on the ground in the sea and the mouth of the bird, the head of the thief and the killer, is located towards the southwest and west coast of America.All these images were created from the American continent and islands by Humans were created, but they didn’t have enough fuel and time to create more accurate images and meteorites ruined the beauty of the images, but it is clear that all these changes were createdby humans, but you have to consider that two thousand meters of soil from meteorites are fish.And they buried the whales under the beaches, and after a very long time, the bodies of the whales turned into oil under the two thousand meters of soil on the beaches, and on the other hand, the presence of two thousand meters of soil onthe surface of the seas and droughts could not make the created images disappear.

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There’s No Way to Make Space Travel Good for Planet Earth Right Now

S paceX has never been reluctant to brag, especially when it comes to its celebrated Falcon 9 rocket. Since 2010, as a company toteboard shows, 217 Falcon 9s have flown, with 61 launches in 2022 alone, making it the workhorse of the current global space fleet. So what’s not to like? Plenty, actually—at least if you care about the environment.

The Falcon 9 uses a fuel mixture of liquid oxygen and simple kerosene, and while the oxygen does not do any harm to the skies, the black soot created by the burning kerosene is injected directly into the stratosphere—the layer of air ranging from 12 km (7.5 mi.) to 50 km (31 mi.) above the Earth. There the soot lingers for up to five years , absorbing heat, contributing to climate change, and damaging the ozone layer, which exposes the planet to dangerous ultraviolet (UV) radiation. And SpaceX is not remotely alone.

According to a study by the National Oceanic and Atmospheric Administration (NOAA), global rocket launches (of which there were 180 last year, the study notes) inject about 1,000 tons of soot into the upper atmosphere per year. That will only get worse, NOAA warns, as the industry continues to expand. “The bottom line is projected increases in rocket launches could expose people in the Northern Hemisphere [where most rocket launches take place] to increased harmful UV radiation,” environmental scientist Christopher Maloney, the study’s lead author, said in a statement .

By themselves, rocket launches are small contributors to overall atmospheric pollutants. The aviation industry burns 100 times more fuel each year than all of the rockets launched globally combined. But there is a key atmospheric difference: airplanes fly in the troposphere about 11 km (6.6 mi.) above the ground. Soot precipitates quickly from this range compared to stratospheric soot which sticks around much longer. Indeed, according to the NOAA report, a single passenger aboard a rocket is responsible for 100 times more climate-changing pollution than a passenger aboard an airplane.

Not only does all of this warm the planet and damage the ozone, the NOAA scientists warn, but the change in temperatures can also slow subtropical jet streams , worsening summer monsoons in Africa and India. “We need to learn more about the potential impact of hydrocarbon-burning engines on the stratosphere and on the climate at the surface of the Earth,” said Maloney.

A version of this story also appears in the Climate is Everything newsletter. To sign up, click here .

The type of fuel used in the rockets can make a difference. SpaceX’s massive, 33-engine Starship spacecraft , for example, uses methane in place of kerosene. While methane is a powerful greenhouse gas by itself, it does burn cleaner than kerosene, putting out less black soot. Blue Origin’s New Shepard rocket is cleaner still, burning liquid oxygen and liquid hydrogen, and producing only water vapor as an exhaust—water vapor in the upper atmosphere still traps and retains heat, but not nearly as much as black soot, methane, or carbon dioxide do.

None of this means that the private rocket industry or growing space powers like China, India, and the United Arab Emirates—to say nothing of the U.S.—will be slowing down their launch schedules or becoming less pollution-intensive any time soon. Indeed, NASA’s new Space Launch System moon rocket, which first launched in November 2022, is an especially dirty machine. While it uses a liquid oxygen-hydrogen mix in its four main engines, its two attached solid fuel engines, which account for most of the vehicle’s thrust, produce the ozone-damaging pollutant chlorine.

The thriving space industry is typically seen as a boon for both the economy and for human exploration—and it is. But the launching of a monster rocket—with monster exhaust—like SpaceX’s anticipated Starship is a reminder that there can be too much of a good thing. If we keep increasing not just the size of rockets but the number of launches, we do so at a price; and as with so many other things, it is the climate that pays.

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Write to Jeffrey Kluger at [email protected]

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White House directs NASA to create unified time standard for the Moon and other celestial bodies

NASA will establish a unified standard of time for the Moon and other celestial bodies, as the White House aims to set international norms in space. 

The directive comes amid a growing lunar race among nations and private companies.

The head of the White House Office of Science and Technology Policy (OSTP) instructed the space agency to work with other parts of the US government to devise a plan by the end of 2026,  in a memo seen by Reuters.

The setting would be called Coordinated Lunar Time (LTC). 

The differing gravitational force on the Moon and on other celestial bodies change how time unfolds relative to how it is perceived on Earth. 

The LTC would provide a time-keeping benchmark for lunar spacecraft and satellites that require extreme precision for their missions, NASA's space communications and navigation chief Kevin Coggins says. 

"The same clock that we have on Earth would move at a different rate on the Moon," Mr Coggins said. 

"Think of the atomic clocks at the US Naval Observatory [in Washington]. They're the heartbeat of the nation, synchronising everything," Mr Coggins said.

"You're going to want a heartbeat on the Moon."

Under its Artemis program, NASA is aiming to send astronaut missions to the Moon in the coming years and establish a scientific lunar base that could help set the stage for future missions to Mars.

Dozens of companies, spacecraft and countries are involved in the effort.

An OSTP official said without a unified lunar time standard it would be challenging to ensure the data that transfers between spacecraft are secure and that communications between Earth, lunar satellites, bases and astronauts are synchronised.

Discrepancies in time also could lead to errors in mapping and locating positions on or orbiting the Moon, the official said.

On Earth, most clocks and time zones are based on Coordinated Universal Time, or UTC.

This internationally recognised standard relies on a vast global network of atomic clocks placed in different locations around the world.

They measure changes in the state of atoms and generate an average that ultimately makes up a precise time.

Deployment of atomic clocks on the lunar surface may be needed, according to the OSTP official.

While the US is the only country to have put astronauts on the Moon, others have lunar ambitions.

Countries have their eyes on potential mineral resources on the Moon, and lunar bases could help support future crewed missions to Mars and elsewhere.

In 2023, China said it aims to put its first astronauts on the Moon by 2030.

In January, Japan became the fifth country to put a spacecraft on the Moon.

India last year became the first country to land a spacecraft near the unexplored lunar south pole, and it has announced plans to send an astronaut to the Moon by 2040.

"US leadership in defining a suitable standard — one that achieves the accuracy and resilience required for operating in the challenging lunar environment — will benefit all space faring nations," the OSTP memo stated.

Defining how to implement Coordinated Lunar Time will require international agreements, the memo added.

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White   House Office of Science and Technology Policy Releases Celestial Time Standardization Policy

Knowledge of time in distant space operating regimes is fundamental to the scientific discovery, economic development, and international collaboration that form the basis of U.S. leadership in space. Today, the White House Office of Science and Technology Policy (OSTP) is releasing the first-ever U.S. government policy memorandum on time standards at and around celestial bodies other than Earth, building upon the Biden-Harris Administration’s National Cislunar Science and Technology Strategy .

“As NASA, private companies, and space agencies around the world launch missions to the Moon, Mars, and beyond, it’s important that we establish celestial time standards for safety and accuracy,” said OSTP Deputy Director for National Security Steve Welby . “Time passes differently in different parts of space—for example, time appears to pass more slowly where gravity is stronger, like near celestial bodies—and as a result the length of a second on Earth is different to an observer under different gravitational conditions, such as on the Moon. A consistent definition of time among operators in space is critical to successful space situational awareness capabilities, navigation, and communications, all of which are foundational to enable interoperability across the U.S. government and with international partners.”

A unified time standard—Coordinated Lunar Time (LTC)—will act as the established standard to enable cislunar operations and can be tied to Coordinated Universal Time (UTC), the primary time standard globally used to regulate clocks and time on Earth. This policy directs NASA to work with the Departments of Commerce, Defense, State, and Transportation to deliver a strategy for the implementation of LTC no later than December 31, 2026. NASA will also coordinate with other federal agencies as appropriate and international partners through existing international forums, including Artemis Accords partner nations.

More information and the full policy are available here .

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The 2024 total solar eclipse is 1 week away. Here's what you need to know.

We're ready, are you? The countdown to the total solar eclipse has begun!

Last minute preparations

Eclipse weather.

There's only one week left until the total solar eclipse 2024 is visible across North America! Are you ready? 

Memories will be made when the moon crosses in front of the sun and turns the daytime sky dark. The total solar eclipse will travel through Mexico, 15 U.S. States and Canada and will be one of the most-watched eclipses ever. You can view the entire path of totality including start and end times for different stages of the solar ellipse at each location in this helpful interactive map from NASA . 

If you cannot watch the eclipse in person you can watch the total solar eclipse live here on Space.com courtesy of NASA. Coverage will begin at 1 p.m. EDT (1700 GMT) . You can also keep up with all the actions with our total solar eclipse 2024 live updates blog.

And if you capture a great photo of the solar eclipse and would like to share it with us and our readers, please email it to [email protected] .  

Related: Solar eclipse viewing through history: A roundup of some of the best photos  

Our how to read and understand a solar eclipse map will help you get the most out of your eclipse viewing venture!

By now you've most likely decided on a viewing location and have all the supplies needed for a successful eclipse viewing experience (don't forget those eclipse glasses !). 

But if you're still scrambling for some solar-safe viewing equipment and haven't been able to get hold of a pair of eclipse glasses don't worry, we've got some alternative ways to view the eclipse with items from around the home . 

Get the Space.com Newsletter

Breaking space news, the latest updates on rocket launches, skywatching events and more!

If you're looking for a way to entertain the little ones before, during and after the eclipse we've got a great guide on how to organize an eclipse event for kids . 

As we get closer to April 8, more reliable meteorological weather forecasts will become available. NOAA's Weather Prediction Center is a great place to find increasingly reliable forecasts, which can help you decide on a viewing location where the probability of cloud cover is low. While we are all wishing for clear skies, we can't help but wonder how clouds could impact the viewing experience. It turns out a cloudy forecast might not be as bad as you would initially think, as it all depends on the type, thickness and extent of the cloud cover. You can read more about what happens if it's cloudy during the eclipse and how to give yourself the best chance of clear skies in our helpful guides 

With so many people flocking to watch the eclipse safety is the top priority. Here we've compiled a couple of guides on How to stay safe during the eclipse and also how to avoid getting stuck in traffic on the big day.

Everyone observing the partial phases of this eclipse — and for those outside the path of totality, that's the entire event — will need to wear solar eclipse glasses while cameras, telescopes and binoculars will need solar filters placed in front of their lenses. 

Only those in the path of totality will be able to remove them briefly to see the sun's corona with their naked eyes. Those not in the path of totality must keep them on the entire time. Our how to observe the sun safely guide tells you everything you need to know about safe solar observations.

Solar eclipse glasses are crucial for most to safely observe the eclipse, but with such high demand for the vital piece of kit, fake eclipse glasses are rife. The American Astronomical Society (AAS) is warning people about the risks of counterfeit and knock-off solar glasses so we have come up with a guide to how to check yours are safe . 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Daisy Dobrijevic joined Space.com in February 2022 having previously worked for our sister publication All About Space magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the National Space Centre in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master's in Environmental Science, she is currently based in Nottingham, U.K. Daisy is passionate about all things space, with a penchant for solar activity and space weather. She has a strong interest in astrotourism and loves nothing more than a good northern lights chase! 

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How the Key Bridge Collapsed in Baltimore: Maps and Photos

By Weiyi Cai ,  Agnes Chang ,  Lauren Leatherby ,  Lazaro Gamio ,  Leanne Abraham and Scott Reinhard

On Tuesday, a major bridge in Baltimore collapsed into the water seconds after it was struck by a cargo ship, sending vehicles on the bridge into the river below. The ship lost power and issued a mayday call shortly before it hit the bridge.

The ship, a 948-foot-long cargo vessel called Dali, was about a half hour into its journey toward Colombo, Sri Lanka, when it hit a main pillar of the bridge. All crew members are safe, according to the ship’s owners.

Follow our live coverage .

A mayday call from the ship gave officials enough time to stop traffic at both ends of the bridge. The waters where the bridge collapsed are about 50 feet deep. By Tuesday morning, six construction workers who had been fixing potholes on the bridge remained missing as divers and other emergency workers on boats and helicopters continued to search for them. Two others had been rescued, and one was in the hospital.

Francis Scott

Patapsco River

The ship left the Port

of Baltimore around

1 a.m. on Tuesday.

Where impact occurred

Direction of the ship

The ship hit the

bridge at 1:28 a.m.

The ship hit the bridge at 1:28 a.m.

Where impact

Source: Spire Global

The New York Times; satellite image by Google Earth

The lights of the ship flickered on and off as it lost power in the minutes before the ship changed bearing and hit the bridge.

Ship approached from

the Port of Baltimore

Road repair crews

Ship changed heading

as it neared pillar

Ship hit pillar

Southern and central spans

of bridge began to collapse within

seconds of impact

Northern span began to

collapse seconds later

Within 30 seconds of impact,

the central part of bridge had

entirely collapsed.

Source: StreamTime Live via YouTube

Timestamps are from StreamTime Live video.

The New York Times

The Francis Scott Key Bridge was opened in 1977 and carried more than 12.4 million vehicles last year. The bridge was one of the three major ways to cross the Patapsco River and formed part of Baltimore’s beltway.

The Port of Baltimore is a major trade hub that handled a record amount of foreign cargo last year. It is an especially important destination — the nation’s largest by volume last year — for deliveries of cars and light trucks.

Ship impact

To Chesapeake Bay

Sources: Maryland Port Administration, OpenStreetMap, MarineTraffic

Note: Ship positions are as of 2:46 p.m. Eastern time.

Overall, Baltimore was the 17th biggest port in the United States in 2021, ranked by total tons, according to the Bureau of Transportation Statistics. The bridge collapse brought marine traffic there to a standstill, with seven cargo or tanker ships stranded in the harbor as of Tuesday afternoon.

Gov. Wes Moore declared a state of emergency for Maryland and said that his office was in close communication with Pete Buttigieg, the U.S. transportation secretary. The White House issued a statement saying that President Biden had been briefed on the collapse.

Erin Schaff/The New York Times

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US President Biden will travel to Baltimore on Friday after bridge collapse

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Reporting By Steve Holland and Jarrett Renshaw; editing by Jonathan Oatis

Our Standards: The Thomson Reuters Trust Principles. , opens new tab

Small uncrewed Ukrainian plane likely used in attack deep inside Russia - experts

A Ukrainian uncrewed aerial vehicle (UAV) that hit Russia's Tatarstan region this week was likely a modified Aeroprakt A-22 Ukrainian-made light aircraft, several experts said, offering insight into one of Kyiv's deepest drone strikes to date.

An activated but faulty missile launcher on a Danish navy vessel triggered a closure of airspace and shipping traffic in the Great Belt strait on Thursday, the Danish armed forces said.