what would ftl travel look like

Warp Speed: What Hyperspace Would Really Look Like

The science fiction vision of stars flashing by as streaks when spaceships travel faster than light isn't what the scene would actually look like, a team of physics students says.

Instead, the view out the windows of a vehicle traveling through hyperspace would be more like a centralized bright glow, calculations show.

The finding contradicts the familiar images of stretched out starlight streaking past the windows of the Millennium Falcon in "Star Wars" and the Starship Enterprise in "Star Trek." In those films and television series, as spaceships engage warp drive or hyperdrive and approach the speed of light , stars morph from points of light to long streaks that stretch out past the ship.

This is what University of Leicester physics students suggest hyperspace travel would really look like.

But passengers on the Millennium Falcon or the Enterprise actually wouldn't be able to see stars at all when traveling that fast, found a group of physics Masters students at England's University of Leicester. Rather, a phenomenon called the Doppler Effect, which affects the wavelength of radiation from moving sources, would cause stars' light to shift out of the visible spectrum and into the X-ray range, where human eyes wouldn't be able to see it, the students found. [ How Interstellar Space Travel Works (Infographic) ]

"The resultant effects we worked out were based on Einstein's theory of Special Relativity, so while we may not be used to them in our daily lives, Han Solo and his crew should certainly understand its implications," Leicester student Joshua Argyle said in a statement.

The Doppler Effect is the reason why an ambulance's siren sounds higher pitched when it's coming at you compared to when it's moving away — the sound's frequency becomes higher, making its wavelength shorter, and changing its pitch.

The same thing would happen to the light of stars when a spaceship began to move toward them at significant speed. And other light, such as the pervasive glow of the universe called the cosmic microwave background radiation, which is left over from the Big Bang, would be shifted out of the microwave range and into the visible spectrum, the students found.

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"If the Millennium Falcon existed and really could travel that fast, sunglasses would certainly be advisable," said research team member Riley Connors. "On top of this, the ship would need something to protect the crew from harmful X-ray radiation."

The increased X-ray radiation from shifted starlight would even push back on a spaceship traveling in hyperdrive, the team found, slowing down the vehicle with a pressure similar to the force felt at the bottom of the Pacific Ocean. In fact, such a spacecraft would need to carry extra energy reserves to counter this pressure and press ahead.

Whether the scientific reality of these effects will be taken into consideration on future Star Wars films is still an open question.

"Perhaps Disney should take the physical implications of such high speed travel into account in their forthcoming films," said team member Katie Dexter.

Connors, Dexter, Argyle, and fourth team member Cameron Scoular published their findings in this year's issue of the University of Leicester's Journal of Physics Special Topics.

Editor's Note: This article was updated to correct the following error: As an ambulance moves closer to an observer, its wavelength becomes shorter, not longer.

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What would faster-than-light (hyperspace) travel look like?

Piece of junk or not, the Millennium Falcon looks a lot different when you engage the warp drive.

Dr Alastair Gunn

In the movies, stars stream out into long trails as a spaceship travels through ‘hyperspace’ or uses its ‘warp drive’. Unfortunately, because these concepts are entirely fictional, usually involving alternative universes or extra dimensions, science can say very little about what ‘real’ hyperspace travel might look like.

However, if we regard hyperspace travel as the ability to travel at almost the speed of light, we can categorically dismiss the idea of stars elongating as shown in Star Wars and other movies. In fact, as your speed increased, you would see the stars fade and eventually disappear as their light is redshifted into the X-ray part of the spectrum, which is invisible to the human eye. The starlight would be slowly replaced by a diffuse glow, concentrated towards your direction of travel, caused by the cosmic microwave background (the leftover radiation from the Big Bang which fills the entire sky) being redshifted into the visible part of the spectrum.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.

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5 Faster-Than-Light Travel Methods and Their Plausibility

Science tells us that it is impossible for an object to travel at light speed, let alone faster than that. But so many of our favorite science-fiction movies, games, and TV shows rely on faster-than-light travel to craft their interplanetary adventures.

Let’s take a look at five means of FTL found in sci-fi that don’t break the rules of relativity and examine how plausible they are based on the science behind them.

1. Hyperdrive

Popularized by Star Wars and used extensively in fiction, a hyperdrive enables a spaceship to travel at FTL speeds by entering another dimension known as “hyperspace.” The spaceship isn’t actually traveling faster than the speed of light, but rather is making use of hyperspace as a shortcut, and the hyperdrive is the mechanism that shunts the spaceship into and out of this parallel dimension.

Specific coordinates within hyperspace have corresponding coordinates in normal space, but the distance between those two points will be shorter in hyperspace, allowing for a faster journey. Before making a “hyperspace jump,” calculations must be made to find the matching coordinates between hyperspace and normal space in order to know when and where to exit hyperspace at the desired normal space destination.

Is it plausible?

Physicist Bukrhard Heim proposed a theory in 1977 that FTL travel may be possible by using magnetic fields to enter higher-dimensional space. The theory uses a mathematical model that calls upon six or more dimensions in an attempt to resolve incompatibilities between quantum mechanics and general relativity, but Heim’s ideas have not been accepted in mainstream science. Still, the fact that a theoretical physicist devoted a large portion of his life in pursuit of a theory that could lead to a means of space travel lends the concept of hyperspace a little more credibility than if it were simply the fancy of a sci-fi writer.

2. Jump Drive

Seen in such works as Battlestar Galactica , a jump drive allows for instantaneous teleportation between two points. Similar to a hyperdrive, coordinates must be calculated to ensure a safe jump; the longer the desired travel distance, the more complex the calculation. In theory, there is no limit to how far a jump can take a ship, but an incorrect calculation may result in a catastrophic collision with a planet or space debris.

The Dune universe’s FTL, based on the fictional “Holtzman effect,” can also be considered a jump drive.

Master of hard sci-fi Isaac Asimov was the first to suggest the idea of a jump drive in the Foundation series, which lends some credibility to the idea. However, most fiction doesn’t clearly explain the principles of physics that allow for this teleportation, making it impossible to claim a jump drive as plausible. However, if it functions by opening a wormhole…

3. Wormholes

A wormhole, as seen in the Stargate franchise, allows for near-instantaneous travel across vast distances. Wormholes may be naturally-occurring or man-made, but are almost always temporary and serve as tunnels through spacetime.

Imagine our universe as a piece of paper, and an ant walking on that piece of paper as a spaceship. If the ant wants to walk from one end of that piece of paper to the other, the fastest way to do so would be to travel in a straight line. But paper, like space, bends. If you bend the paper into a U shape, the ant’s journey goes largely undisturbed – it still has to traverse the same distance along that line. However, in 3D space, the two ends of the paper are very close to each other now. Cut off a piece of a drinking straw and let the ant use it as a bridge or tunnel between the two ends of the paper, and the journey is suddenly much shorter.

While we have never directly observed any evidence for one, wormholes are theoretically possible. Albert Einstein and his colleague Nathan Rosen first discovered wormholes in 1935 as solutions to equations within Einstein’s general theory of relativity – the math says they can exist.

Since then, other scientists, including Stephen Hawking, have argued that it may be possible to traverse a wormhole, under the right circumstances. The debate surrounding wormholes isn’t about their plausibility, but rather how they may be created and sustained.

4. Slipstream

The concept of slipstream can be found in such works as Star Trek , Doctor Who , and the Halo video game franchise, but there is no widely-agreed upon definition of what slipstream is or how it works beyond it being a means of FTL. We’ll consider the slipstream seen in Gene Roddenberry’s Andromeda , where it is “not the best way to travel faster than light, it’s just the only way,” as per the show’s protagonist.

Slipstream is a form of interdimensional highway in which ships ride a series of slipstream “strings” – the unseen connections between all objects in the universe. These strings are in constant flux and form a tangled mess of intersections and divergent paths. Any time a pilot reaches a fork in the road, he has to guess which is the correct path to take to continue along toward his desired destination. Before the pilot makes that decision, both paths are simultaneously the correct and incorrect route, and it is the act of choosing a path that forces one to be correct and the other to be incorrect – if this made you think of Shrödinger’s cat, that does seem to be the basis for this concept. A computer selects the “correct” path 50% of the time, but due to intuition, a human picks the correct path 99.9% of the time.

There are no mainstream scientific theories that support this idea of slipstream. Reading the “lore” of this means of FTL evokes fantastical interpretations of string theory, quantum entanglement, and other concepts in modern physics, but the ideas are supported only through their internal consistency rather than actual fact, much like a well-explained magic system that allows fictional wizards to cast spells.

5. Warp Drive

Popularized by Star Trek , a warp drive distorts space around a ship while leaving the ship itself inside a “bubble” of normal space. The space in front of the ship is contracted, while the space behind it is expanded, and the ship “rides” the distortion wave at FTL speeds. Technically, it is not the ship that is moving, but rather space itself, which is how we avoid breaking any laws of physics.

Imagine a surfer slowly paddling back to shore. When a wave comes, it will lower the water level in front of him and raise the water level behind him, and he can ride the downward slope all the way to shore. Relative to the wave, the surfer isn’t moving – he’s staying between the crest and the trough, and it is instead the wave that is moving.

Surfing doesn’t quite work like that, but it’s a simplification that we can all visualize. In a similar manner to how a wave will distort water to propel a surfer, a warp drive will distort space to propel a ship.

In 1994, the Alcubierre drive was proposed as a theoretical means of FTL travel and is based on a mathematical solution to equations within Einstein’s general theory of relativity. Just like a warp drive, the Alcubierre drive would contract space in front of a spaceship and expand space behind it.

NASA has been actively researching this technology since 2012 , and the lead researcher even worked with a 3D artist to develop a model of what a warp-capable ship might look like . As far as real-life FTL goes, warp is the current front-runner to becoming reality.

As far as real-life FTL travel goes, the fictional favorites can be found in Star Trek and Stargate : the warp drive, and wormholes. Both are theoretically possible; however, both require further scientific breakthroughs before practical testing can begin. In either case, we need to discover “exotic matter” – hypothetical particles with negative mass – to get these mechanisms to work. “Element zero” from the Mass Effect series, the rare material that is essential to FTL travel in that universe, doesn’t quite fit the description, but the lore is at least scientifically sound in suggesting that some new, rare form of matter is required to make this technological leap.

The good news is that scientists don’t believe this is a matter of if, but rather when. There will be a time in the future when a stately, bald man in uniform will sit back in a command chair and relay the order, “Engage.”

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Astronomy and space
Research update

Spacecraft in a ‘warp bubble’ could travel faster than light, claims physicist

Albert Einstein’s special theory of relativity famously dictates that no known object can travel faster than the speed of light in vacuum, which is 299,792 km/s. This speed limit makes it unlikely that humans will ever be able to send spacecraft to explore beyond our local area of the Milky Way.

However, new research by Erik Lentz at the University of Göttingen suggests a way beyond this limit. The catch is that his scheme requires vast amounts of energy and it may not be able to propel a spacecraft.

Lentz proposes that conventional energy sources could be capable of arranging the structure of space–time in the form of a soliton – a robust singular wave. This soliton would act like a “warp bubble’”, contracting space in front of it and expanding space behind. Unlike objects within space–time, space–time itself can bend, expand or warp at any speed. Therefore, a spacecraft contained in a hyperfast bubble could arrive at its destination faster than light would in normal space without breaking any physical laws, even Einstein’s cosmic speed limit.

Negative energy

The idea of creating warp bubbles is not new, it was first proposed in 1994 by the Mexican physicist Miguel Alcubierre who dubbed them “warp drives” in homage to the sci-fi series Star Trek . However, until Lentz’s research it was thought that the only way to produce a warp drive was by generating vast amounts of negative energy – perhaps by using some sort of undiscovered exotic matter or by the manipulation of dark energy. To get around this problem, Lentz constructed an unexplored geometric structure of space–time to derive a new family of solutions to Einstein’s general relativity equations called positive-energy solitons.

Though Lentz’s solitons appear to conform to Einstein’s general theory of relativity and remove the need to create negative energy, space agencies will not be building warp drives any time soon, if ever. Part of the reason is that Lentz’s positive-energy warp drive requires a huge amount of energy. A 100 m radius spacecraft would require the energy equivalent to “hundreds of times of the mass of the planet Jupiter”, according to Lentz. He adds that to be practical, this requirement would have to be reduced by about 30 orders of magnitude to be on par with the output of a modern nuclear fission reactor. Lentz is currently exploring existing energy-saving schemes to see if the energy required can be reduced to a practical level.

Any warp drive would also need to overcome several other serious issues. Alcubierre, who regards Lentz’s work as a “significant development”, cites the “horizon problem” as one of the most pernicious. “A warp bubble travelling faster than light cannot be created from inside the bubble, as the leading edge of the bubble would be beyond the reach of a spaceship sitting at its centre,” he explains. “The problem is that you need energy to deform space all the way to the very edge of the bubble, and the ship simply can’t put it there.”

Spacecraft doubts

Lentz describes his calculations in Classical and Quantum Gravity , where other recent research on the topic is outlined in an accepted manuscript from Advanced Propulsion Laboratory researchers Alexey Bobrick and Gianni Martire. The duo describes a general model for a warp drive incorporating all existing positive-energy and negative-energy warp drive schemes, except Lentz’s which they say “likely forms a new class of warp drive space–times”.

However, they argue that a Lentz-type warp drive is like any other type of warp drive in the sense that, at its core, it is a shell of regular material and therefore subject to Einstein’s cosmic speed limit, concluding that “there is no known way of accelerating a warp drive beyond the speed of light”.

Ad astra! To the stars!

Though he recognizes these huge hurdles to building a warp drive, Lentz feels they are not insurmountable. “This work has moved the problem of faster-than-light travel one step away from theoretical research in fundamental physics and closer to engineering,” he says.

After addressing energy requirements, Lentz plans to “devise a means of creating and accelerating (and dissipating and decelerating) the positive-energy solitons from their constituent matter sources”, then confirm the existence of small and slow solitons in a laboratory, and finally address the horizon problem. “This will be important to passing the speed of light with a fully autonomous soliton,” he says.

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What would it be like to travel faster than the speed of light?

Is it even possible?

Physicists at the European Organization for Nuclear Research (CERN) have made a mind-bending — and rule-bending — discovery: They've measured strange subatomic particles called neutrinos traveling faster than the speed of light. "Superluminal travel" may be a common trope in science fiction, but Einstein's theory of special relativity strictly forbids it in the real world, as beating photons in a footrace would seem to require infinite energy.

So either the new data is wrong, or Einstein topples — along with almost every tenet of modern physics.

Imagine the latter scenario. What would a lawless universe, in which particles have free reign to zip around heedless of the light-speed limit , be like? How would your surroundings look and feel if you were that particle?

According to Michael Ibison, a senior research physicist at the Institute for Advanced Studies in Austin, Texas, such a world would be "spooky." First off, it's unclear how you would see light if you were zooming past it. "Thinking about what the world would look like automatically makes you wonder what happens to your ability to see light , period," Ibison, who has studied the possibility of superluminal particles, told Life's Little Mysteries. "You'd be running into [light] that is usually running away from you. I suspect that in order to absorb light, you would have to emit it yourself."

Related: What would happen if the speed of light was much lower?

The concepts of cause and effect — of time flowing in one direction — also shatter in a superluminal world. Imagine riding on a spacecraft made of faster-than-light neutrinos rocketing away from Earth. TV broadcasts playing the day's news are also emanating into space, and those are traveling at light speed. "If you got on a neutrino spacecraft and travelled out to space at neutrino speed, you'd catch up with the TV broadcasts and overtake them, and you would start to see the video of the news running backwards," Ibison said. As the stream of transmissions receded behind you, they would run backward at whatever your excess speed is over and above their speed — the speed of light.

What if you were standing still in a speed-limitless universe? What would you see then?

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According to Ibison, the situation is analogous to standing on the ground as a supersonic jet passes overhead. Because these jets travel faster than the speed of sound, you see them before you hear them. When the sound does finally hit you, it's in the form of a sonic boom — a shock wave that builds up as sound from the aircraft gets bunched together behind it.

— Can matter travel at light speed?

— What is the smallest particle in the universe? (What about the largest?)

— What if there were no gravity?

Similarly, he said, "If something were traveling faster than the speed of light, such as an airplane made of neutrinos, you wouldn't see it until after it had gone past you. Any light it emitted would be trailing behind in its wake. You would not see the neutrino plane until after it has gone past — and then only if it contained something that reflected or emitted light. And just as a plane passing through the sound barrier emits a sonic boom, a superluminal craft passing through light speed would emit a flash of light."

Again, no one is saying for certain that these scenarios are real. According to Hugh Gallagher, a particle physicist at Tufts University who works on the MINOS neutrino experiment, the CERN result will have to be replicated many times over before he and his colleagues abandon the tenets of special relativity . "But if the results are true, then a lot of the things we don't think of as possible suddenly become open to discussion again," Gallagher said.

Originally published on Live Science.

Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.

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Nasa publishes faster-than-light spaceship design to imagine interstellar exploration

New concept images show a spacecraft with a warp drive that could travel to our nearest star in 4 weeks - current ships would take 80,000 years, article bookmarked.

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Believe it or not, Nasa is serious about creating faster than light travel, and have released a new batch of concept images (see gallery below) showing exactly what a spacecraft equipped with a star-hopping warp drive might look like.

Created in collaboration between Dutch artist Mark Rademaker and Nasa physicist Dr Harold White, the illustrations show a craft powered by an Alcubierre Drive – a hypothetical engine that was first suggested in 1994 as a way of achieving faster than light speeds.

Alcubierre’s device worked by distorting space-time, expanding the space behind a ship and contracting the space in front of it to create a ‘warp bubble’ that essentially moves space and time around the object, rather than actually accelerating the craft to impossible speeds.

For all its craziness, physicists conceded that such a warp drive might be theoretically possible but that it would require staggering amounts of power – something equivalent to the mass-energy of Jupiter, a planet 317 times the mass of the Earth.

Nasa's concept faster-than-light spacecraft: In pictures

However, in 2012, Dr White announced that he had managed to finesse Alcubierre’s original plans to reduce the amount of mass-energy needed from that of a gas giant to a spacecraft the size of the Voyager 1 – and, even more surprisingly, that he was working with a team from Nasa to further develop his ideas.

Unfortunately none of this really brings the idea of a working Alcubierre Drive any closer, not least because the design rests on being able to use something called ‘exotic matter’ as fuel.

This is a hypothetical substance that physicists suggest might have negative mass. In terms of discovering it the best we can say is that we haven’t found a way to rule out its existence and when we compare it to dark matter – another scientific oddity that we’re pretty sure actually exists but that we’ve never directly seen, let alone created - the chances of using exotic matter are slim to none.

This all means that even Dr White – one of the most optimistic scientists when it comes to the warp drive – will only comment on the possibility of faster than light travel by saying “the field equations predict that this is possible, but it remains to be seen if we could ever reduce this to practice."

Nevertheless, it seems that Nasa is happy to fuel people's imaginations instead, and looking at the concept images it's impossible not join in - and hope a little.

With such a spacecraft humanity could travel to nearby stars in just weeks (with current propulsion techology it would take 80,000 years) making it possible to explore entirely new worlds and even contact alien life. We may never create such a craft for many generations - but that doesn't mean we shouldn't dream.

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What If You Traveled Faster Than the Speed of Light?

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When we were kids, we were amazed that Superman could travel "faster than a speeding bullet." We could even picture him, chasing down a projectile fired from a weapon, his right arm outstretched, his cape rippling behind him. If he traveled at half the bullet 's speed, the rate at which the bullet moved away from him would halve. If he did indeed travel faster than the bullet, he would overtake it and lead the way. Go, Superman!

In other words, Superman's aerial antics obeyed Newton's views of space and time : that the positions and motions of objects in space should all be measurable relative to an absolute, nonmoving frame of reference [source: Rynasiewicz ].

In the early 1900s, scientists held firm to the Newtonian view of the world. Then a German-born mathematician and physicist by the name of Albert Einstein came along and changed everything. In 1905, Einstein published his theory of special relativity , which put forth a startling idea: There is no preferred frame of reference. Everything, even time, is relative.

Two important principles underpinned his theory. The first stated that the same laws of physics apply equally in all constantly moving frames of reference. The second said that the speed of light — about 186,000 miles per second (300,000 kilometers per second) — is constant and independent of the observer's motion or the source of light. According to Einstein, if Superman were to chase a light beam at half the speed of light, the beam would continue to move away from him at exactly the same speed [source: Stein , AMNH.org ].

These concepts seem deceptively simple, but they have some mind-bending implications. One of the biggest is represented by Einstein's famous equation, E = mc², where E is energy, m is mass and c is the speed of light.

According to this equation, mass and energy are the same physical entity and can be changed into each other. Because of this equivalence, the energy an object has due to its motion will increase its mass. In other words, the faster an object moves, the greater its mass. This only becomes noticeable when an object moves really quickly. If it moves at 10 percent the speed of light, for example, its mass will only be 0.5 percent more than normal. But if it moves at 90 percent the speed of light, its mass will double [source: LBL.gov ].

As an object approaches the speed of light, its mass rises precipitously. If an object tries to travel 186,000 miles per second, its mass becomes infinite, and so does the energy required to move it. For this reason, no normal object can travel as fast or faster than the speed of light.

That answers our question, but let's have a little fun and modify the question slightly.

Almost As Fast As the Speed of Light?

We covered the original question, but what if we tweaked it to say, "What if you traveled almost as fast as the speed of light?" In that case, you would experience some interesting effects. One famous result is something physicists call time dilation , which describes how time runs more slowly for objects moving very rapidly. If you flew on a rocket traveling 90 percent of light-speed, the passage of time for you would be halved. Your watch would advance only 10 minutes, while more than 20 minutes would pass for an Earthbound observer [source: May ]

You would also experience some strange visual consequences. One such consequence is called aberration , and it refers to how your entire field of view would shrink down to a tiny, tunnel-shaped "window" out in front of your spacecraft. This happens because photons (those exceedingly tiny packets of light) — even photons behind you — appear to come in from the forward direction.

In addition, you would notice an extreme Doppler effect , which would cause light waves from stars in front of you to crowd together, making the objects appear blue. Light waves from stars behind you would spread apart and appear red. The faster you go, the more extreme this phenomenon becomes until all visible light from stars in front of the spacecraft and stars to the rear become completely shifted out of the known visible spectrum (the colors humans can see). When these stars move out of your perceptible wavelength, they simply appear to fade to black or vanish against the background.

Of course, if you want to travel faster than a speeding photon, you'll need more than the same rocket technology we've been using for decades.

In a March 2021 paper published in the journal Classical and Quantum Gravity , astrophysicist Erik Lentz of the University of Göttingen in Germany proposed the idea of rearranging space-time to create a warp bubble, inside which a spacecraft might be able to travel at faster-than-light speeds.

Speed of Light FAQ

Is there anything faster than the speed of light, how fast is the speed of light in miles, why is "c" the speed of light, what is the speed of light on earth, lots more information, related articles.

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American Museum of Natural History. "A Matter of Time. " Amnh.org. (Feb. 16, 2022) https://www.amnh.org/exhibitions/einstein/time/a-matter-of-time
Brandeker, Alexis. "What would a relativistic interstellar traveler see?" Usenet Physics FAQ. May 2002. (Feb. 16, 2022J) http://www.desy.de/user/projects/Physics/Relativity/SR/Spaceship/spaceship.html
Carl Sagan's Cosmos. "Travels in Space and Time." YouTube. Video uploaded Nov. 27, 2006 (Feb. 16, 2022 ) https://www.youtube.com/watch?v=2t8hUaaZVJg
Hawking, Stephen. "The Illustrated Brief History of Time. " Bantam. 1996. (Feb. 16. 2022) https://bit.ly/367UGpZ
EurekAlert! "Breaking the warp barrier for faster-than-light travel. " Eurekalert.org. March 9, 2021. (Feb. 16, 2022) https://www.eurekalert.org/news-releases/642756
Lawrence Berkeley National Laboratory. "Mass, Energy, the Speed of Light – It's Not Intuitive! " Lbl.gov. 1996. (Feb. 16, 2022) https://www2.lbl.gov/MicroWorlds/teachers/massenergy.pdf
Lemonick, Michael D. "Will We Ever Travel at the Speed of Light?" Time. Apr. 10, 2000. (Feb. 16, 2022), 2011) http://content.time.com/time/subscriber/article/0,33009,996616,00.html
May, Andrew. "What is time dilation? " LiveScience. Nov. 17, 2021. (Feb. 16, 2022) https://www.livescience.com/what-is-time-dilation
NOVA Physics + Math. "Carl Sagan Ponders Time Travel." NOVA. Oct. 12, 1999. (Feb. 16, 2022) http://www.pbs.org/wgbh/nova/physics/Sagan-Time-Travel.html
Ptak, Andy. "The Speed of Light in a Rocket." NASA's Imagine the Universe: Ask An Astrophysicist. Jan. 2, 1997. (Feb. 16, 2022) http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970102c.html
Rynasiewicz, Robert, "Newton's Views on Space, Time, and Motion."Stanford Encyclopedia of Philosophy. Summer 2014. (Feb. 16, 2022) https://plato.stanford.edu/cgi-bin/encyclopedia/archinfo.cgi?entry=newton-stm
Stein, Vicky. "Einstein's Theory of Special Relativity. " Space.com. Sept. 20, 2021. (Feb. 16, 2022) https://www.space.com/36273-theory-special-relativity.html
Van Zyl, Miezam (project editor)."Universe: The Definitive Visual Guide." Dorling Kindersley Limited. 2020. (Feb. 16, 2022) https://bit.ly/33q5Mpm.

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U.S. physicist Albert Einstein delivers a lecture at the offices of the Mt. Wilson Observatory, California.

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How Humans Could Go Interstellar, Without Warp Drive

Faster-than-light (ftl) travel is possible..

The field equations of Einstein’s General Relativity theory say that faster-than-light (FTL) travel is possible, so a handful of researchers are working to see whether a Star Trek -style warp drive, or perhaps a kind of artificial wormhole , could be created through our technology.

But even if shown feasible tomorrow, it’s possible that designs for an FTL system could be as far ahead of a functional starship as Leonardo da Vinci’s 16th century drawings of flying machines were ahead of the Wright Flyer of 1903. But this need not be a showstopper against human interstellar flight in the next century or two. Short of FTL travel, there are technologies in the works that could enable human expeditions to planets orbiting some of the nearest stars.

Picking the Target

Certainly, feasibility of such missions will depend on geopolitical-economic factors. But it also will depend on the distance to nearest Earth-like exoplanet. Located roughly 4.37 light years away, Alpha Centauri is the Sun’s closest neighbor; thus science fiction, including Star Trek , has envisioned it as humanity’s first interstellar destination.

In 2012, a planet was identified orbiting closely around Alpha Centauri B, one of three stars comprising the Alpha Centauri system. Three years later, astronomers were unable to find that same planet, but if it exists it would be too hot for life anyway. What we really want to know is whether planets exist further out from the two main stars, or whether their much smaller, dimmer companion star, Proxima Centauri, located just 4.24 light years from Earth, has planets of its own.

Very soon, these questions will be answered by the James Webb Space Telescope (JWST) that NASA will be launching into space in 2018, and by other instruments that will follow, instruments capable of more than merely detecting a planet’s presence. They will also be able to read the chemical composition of planetary atmospheres.

Imagine this: If there’s an Earth-like planet around Alpha Centauri or another nearby star system, astronomers will know about it within a decade or two — certainly long before we can build a ship like the Enterprise .

Maybe we could consider flying under the speed of light.

Propulsion

It is not widely known, but the US government spent real money, tested hardware and employed some of the best minds in late 1950s and early 60s to develop an idea called nuclear pulse propulsion .

Known as Project Orion , the work was classified because the principle was that your engine shoots a series of “nuclear pulse units” — atomic bombs of roughly Hiroshima/Nagasaki power — out the back. Each unit explodes and the shockwave delivers concussive force to an immense, steel pusher plate, which is connected to the most immense shock absorber system that you could imagine.

The researchers calculated that the ship could reach five percent the speed of light (0.05 c ), resulting in roughly a 90-year travel time to Alpha Centauri. The Nuclear Test Ban Treaty of 1963, which forbade nuclear explosions in the atmosphere, and the Outer Space Treaty of 1967, which forbade nuclear explosive devices in space, effectively ended Orion .

In his epic TV series Cosmos, Carl Sagan noted such an engine would be an excellent way to dispose of humanity’s nuclear bombs, but that it would have to be activated far from Earth. But back when Orion was being funded, amazingly, the plan was to use the nuclear pulse engine even for launching the vessel, in one massive piece, from the surface of Earth. Suffice it to say it does not seem likely that we’ll every build a nuclear pulse ship, but it’s something that we already have the technology to build.

A Cleaner System

But what about a less explosive, cleaner propulsion system that could achieve the same end? The British Interplanetary Society took on this goal in the 1970s with Project Daedalus . Named for the inventor from Greek mythology who built wings to escape the island of Crete, the design was based on projected development of inertial confinement fusion (ICF), one of two main strategies for generating nuclear fusion energy on Earth.

The other strategy is magnetic confinement fusion (MCF), and similar to ICF, designs exist for adapting MCF to space propulsion . Like Orion , a Daedalus craft would have to be rather large. But using deuterium and helium-3 (obtained from the lunar surface, or from Jupiter’s atmosphere) as fuel, Daedalus craft could reach 0.12 c , cutting travel time to Alpha Centauri to something like 40 years.

There are other ingenious ideas, such as the Bussard ramjet that could approach the speed of light, but the size of the engines and technological gaps that we must fill become so large that they may not seem easier than warp drive. So let’s limit our discussion to capabilities up to the neighborhood of the 0.12 c of Daedalus as we consider what form a human interstellar voyage might take

The Generation Starship

It has been said that if you want to go fast, go alone, but if you want to go far, go together. This proverb characterizes the strategy of building an interstellar ship so large that you don’t worry so much about the travel time.

Effectively, the ship is a space colony. It contains a large population — current estimates are that a minimum of tens of thousands of colonists are needed for a healthy gene pool — and all that is needed for people to live comfortably, but it follows a trajectory out of the solar system. Ideas for an interstellar ark taking millennia to reach a destination date back to the fathers of the Space Age — Russia’s Konstantin Tsiolkovsky and America’s Robert Goddard— — the idea really set sail with mid 20th century science fiction writers.

In a two-part novel series written in 1941, Robert A. Heinlein wrote of a vessel that took so long to reach its destination that the people aboard had forgotten they were on a ship. Instead, they believed the large craft to be their natural world.

Sending colonists on a voyage lasting centuries or millennia raises social questions, such as whether it is ethical to commit unborn generations to live out their lives in transit between planets.

10,000 years is a rather long time and means a large number of generations to commit to the interstellar void. But if we’re talking 40 or even 90 years, that’s probably more palatable to many more people. Still, it raises questions as to who would volunteer for such an expedition.

But what about people with shorter attention spans and what if we have no will to build enormous, moving colonies?

Here’s another science fiction strategy: sending cryopreserved human embryos, or gametes (ova and sperm) into deep space. Upon reaching the destination star system, the embryos would be developed. This would require an artificial uterus, which we don’t have yet, but like fusion, here we’re also talking in terms of a matter of decades.

At some point in this century, motherless birth could become a technological reality. Theoretically, we’ll be able to send cryopreserved embryos through space, for centuries if needed due to propulsion limitations, and set them to develop into full-term infants on the new planet.

Then, all you need are robot nannies to raise and educate the infant colonists. And if there’s one area of technological progress that people are supremely confident will keep advancing at warp speed, it’s robots and artificial intelligence.

The egg ship concept is loaded with ethical questions, which can be hashed out in the comments section.

Suspended Animation

As technically ambitious as it may sound, medical science is making incremental progress toward a safe form of human hibernation.

Currently, it’s routine to lower a patient’s body temperature intentionally by a few degrees, thereby inducing a mild hypothermic coma, following cardiac arrest. This enables the brain to recover after oxygen has been cut off, whereas remaining at normal body temperature results in what’s called reperfusion injury.

Not routine yet, but now under clinical trials, trauma surgeons are cooling patients down to just above freezing temperature in cases of severe blood loss. This is true suspended animation . It’s done just for two hours, or possibly three, stalling death so that injuries can be repaired and blood replaced, but the person is basically hibernating during that time.

With incremental progress, the procedure may eventually be extended to time frames of many hours, and eventually days or weeks to treat other conditions. Perhaps, in time, we’ll put people to sleep long enough, and with enough supervision by computers, to slumber away for an entire interstellar voyage the way you now doze off for a transoceanic flight.

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What Does a Faster-Than-Light Object Look Like?

I exchanged a bunch of emails a week or two ago with a journalist who was working on a story involving the possibility of faster-than-light travel. He wanted me to check some statements about the relationship between FTL and causality. FTL creates problems for causality, because if you have an object moving faster than light, there will be pairs of observers who see events involving the FTL object happening in different orders, which means somebody will see an effect happen before its cause.

I talk about this is How to Teach Relativity to Your Dog using the example of a stationary dog, a moving cat, and an alien zipping by at four times the speed of light. Here's a figure showing how this appears to the dog:

A dog and cat interacting with an FTL alien, as seen by the dog.

In this "spacetime diagram," the left-right axis indicates the position along the direction of the cat's motion, while time marches upward into the future. Vertical lines are equally spaced position markers according to the dog, while horizontal lines are equally space instants of time according to the dog. The dashed red lines are rays of light sent out at the instant the cat and dog had the same position, and set the scale for everything.

The dog sees the cat moving left to right at half the speed of light. The alien comes in from the left, passes the dog first (event 1), and then the cat (event 2). Perfectly sensible, and the dog could, for example, hand the passing alien a water balloon which the alien could then use to soak that pesky cat.

This looks very different if you replace the dog's grid of position and time markers with the cat's, though. In that case, you get something that looks like this:

A dog and cat interacting with an FTL alien, as seen by the cat.

Both space and time look different to the cat, due to her high speed. Equally spaced position markers according to the cat have to tip to the right, parallel to the cat's trajectory, while the cat's time instants tip up. The regular squares of the dog's grid become rhombuses in this representation.

According to the cat, then, the alien passes the cat, and only later passes the dog. Which makes the whole water balloon thing kind of problematic-- the alien would appear to the cat to come in from the right empty-handed, soak the cat with a water balloon and then carry that balloon on to the left, and later on hand it to the dog.

This reversal of ordering obviously screws up causality, and is one of the best reasons why FTL travel is impossible. It's even possible to create paradoxes by using FTL communications, so sending information faster than the speed of light is ruled out.

My journalist friend had this basically correct, but we went back and forth a bit about a subtle issue of perception. This is always a tricky business in relativity, as it's very tempting to attribute weird effects to the finite travel time for light to get from one place to another, and say things like "The events according to the cat appear to be in the opposite order." That's not what's really going on, though-- relativity isn't about optical illusions. There is no measurement the cat can do that will tell her anything other than that the alien passed her first, and only later encountered the dog.

I think we got this cleared up-- I don't think the story has appeared yet, which is also why I'm being coy about the identity of the journalist, who is free to identify himself in comments. But it did raise the question in my mind of what this would actually look like from the point of view of the dog. That is, what would the dog see ?

You see, the diagrams above are a sort of "God's eye" view of the scenario, not the kind of thing any of the participants could directly record. The dog could draw this diagram of events, but only after the fact, after either compiling the records of lots of individual observers at the different position markers, or by looking at the light she sees, and working backwards to correct for the travel time of the light emitted by different objects.

So, what would the "dog's eye" view of the scenario really look like? How would the dog perceive her interaction with the alien? Well, we can understand this by adding some extra events to the diagram:

A dog and cat interacting with an FTL alien, as seen by the dog, with extra events added.

Here, I've added events "a," "b," and "c," so there are two events on either side-- while the alien is approaching, and while the alien is heading away. To work out what the dog sees, we need to add lines corresponding to the light emitted by the alien as it passes each of these. Looking at the approaching side first, we see that things are a little weird:

A dog and cat interacting with an FTL alien, as seen by the dog, showing the light from the approaching alien.

The alien, moving at four times the speed of light, arrives well ahead of the light from earlier in its trajectory. Thus, the dog would have absolutely no warning of the alien's approach-- it would just suddenly be there . Then the light from nearby would arrive (event b), and then the light from farther away (event a). If we add in the receding side, we have:

A dog and cat interacting with an FTL alien, as seen by the dog, showing the light from the alien as it approaches and recedes.

Again, the alien outraces its own light, so it's gone just as suddenly as it appears. The light from its departure lags well behind the actual events, with nearby events appearing only after some delay (event c) and more distant events much later (event 2, the soaking of the cat).

So, the answer to the question "What does the dog see ?" is "Some weird stuff." Adding markers for the arrival of the light from each of the events gives you the idea:

A dog and cat interacting with an FTL alien, as seen by the dog, showing the order of events as seen by the dog.

From the dog's point of view, the alien appears without warning (event 1), then seems to move away in both directions simultaneously-- like two identical aliens headed in opposite directions. Light from a given distance on the approaching side will arrive a bit ahead of light from the receding side (event b is seen before event c, though they're the same distance away), so it will look sort of like the alien zipping off to the left is heading away a bit faster than the one heading off to the right. The "dog's eye" sequence of events is not the "a-b-1-c-2" sequence of the "God's eye" view, but "1-b-c-a-2." It's only after the fact, when she's had time to say "What the hell was that?" and do a bit of math that the dog can construct the global picture shown in the diagrams.

So, there's the answer. You could extend this to the cat's scenario by a similar process of event-adding and line-drawing, but I'm not going to. We'll call that homework-- draw and label your diagrams neatly, and send them to Rhett for grading.

And don't even think about moving faster than the speed of light. Seriously, it'll mess with your head.

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I'll come out as the journalist you refer to. I don't think my issue was thinking of relativity as an illusion, but how to explain it without giving this impression. And your post is a great example of how to do so.

Thus, the dog would have absolutely no warning of the alien’s approach– it would just suddenly be there.

This resembles the lies-to-children explanation for why shock waves develop in fluids containing objects moving supersonically: the presence of a moving object in the fluid would normally be transmitted to other locations by sound waves, and of course that can't happen if the object is outrunning its sound waves. Weird things do happen at shocks, like discontinuous changes in density and flow speed (and magnetic field, if the fluid in question is a magnetized plasma). Of course, in fluid mechanics there are other ways to transmit information, so a Mach number greater than one is not a sufficient condition to create a shock. For example, current thinking is that the interface between the Sun's magnetic field and the interstellar medium does not produce a bow shock. That's why this explanation is a lie-to-children in fluid mechanics. But in this scenario, any alternate way of transmitting information would not be any faster than photons, so this explanation might actually be correct, or would be if the scenario were allowed.

I remember doing something similar after learning about Lorentz transforms. Being a total nerd I had a Tardis instantaneously jump to a departing USS Enterprise at 0.9c. Then jump back instantaneously using the Enterprise frame of reference and finding I had traveled back before I left. Just showed up there's no such thing as "at the same time" when FLT is involved.

I wrote an article about faster-than-light travel for a Swedish popular science magazine last year. Unfortunately, it only appeared online in Swedish. (I think I sent an English version to George at some point.) I didn't go into these details though as my point was mainly to say there are different ways to allow faster than light travel. (Hit me over the head. We're all slightly nuts in quantum gravity, I know.) Your post would have made the perfect primer :)

If we observed an object actually crossing close to our path faster than the local speed of light, we would presumably look very closely at whatever happened to space-time close to the object for whatever new physics we might see. Fortunately the dog [and the cat] would see it appear before it passed the cat [the dog], so both get a (fleeting) chance to start the camera to record any weird physical effects. Something moving as you have it doesn't preclude /local/ Lorentz invariance (hopefully in 3+1 dimensions the alien's path doesn't actually intersect with the paths of either dog or cat; I certainly wouldn't want to be too close but please may I have a towel).

i think that is paradox existing only on the einstain TR ground (i mean that on the newton theory ground nothing happen or not??)maybe that vanishing when we found ultimate theory ? but back to story if dog give alien water ballon that he easy found right sequence events no matter how he be tangled that be always way to found right sequence alien come-i give him water ballon-alien gone

The problem getting the causality issues across may be just with saying things like "someone will *see* effects happening before their causes."

To my mind, that's not really the problem at all; the big problem is, as you briefly mentioned, backward-in-time communication: the way that, if Lorentz invariance holds, you must be able to string a couple of these things together to have effects happen inside the cause's own past light cone. That's what induces all the associated causal paradoxes (unless some hackish out intervenes, as some have proposed).

Something similar to your explanation of what the dog sees---an alien appearing out of nowhere, and then basically two copies of the alien flying off in opposite directions---is similar to a picture Feynman invoked when discussing various implications of relativity for quantum mechanics. The combination of relativity and quantum mechanics famously leads to the prediction of antiparticles. I won't go into all the details here, but you can think of the spontaneous creation of a particle/antiparticle pair as essentially what is going on with the alien from your dog's perspective. Let's suppose that instead of flying away, the "two" aliens were to then come back and converge on the dog and then disappear (as spontaneously as they appeared) once they have fully re-converged, you'd basically have the annihilation event. If you draw the spacetime diagram, what you'll see is that the alien worldline is a circle in spacetime! This is the picture Feynman proposed for thinking about particle/antiparticle creation and annihilation.

Naturally, one has to think very carefully about how to make such a picture fully consistent and their are many subtleties.

So, what does it look like from the perspective of the alien?

So the alien is like particle that spontaneously appears and then decays into a pair of particle/antiparticle?

There are some superficial similarities between the appearance of a faster-than-light object as seen in the light arriving from it at the position of a particular observer. It's important to remember the distinction between what you see and what's really happening, though.

That is, in the case of the FTL alien, the dog sees two aliens pop into existence from nowhere and zoom off, one of them moving backwards, but that's an optical illusion. After the fact, she can work backwards and show that it was really a single alien moving at FTL speed.

The cat would see something similar when looking at the light from the alien, but again, would be able to reconstruct the trajectory of a single alien rather than a mysterious pair. she would disagree with the dog about the direction of the alien's motion, though. Neither cat nor dog, however, will ever be able to do a reconstruction showing that there were two aliens present at the same instant at different locations in space.

In the case of a particle-antiparticle pair, though, there's no reconstruction you can do that will show anything other than two particle tracks converging at a single point. Now, for reasons of mathematical convenience, you can choose to view this as a single particle reversing the direction of its motion through time, but there's no ambiguity about the existence of a single vertex, and the fact that both particles are observed at different locations in space at the same time. The dog and the cat will disagree about what constitutes a single instant of time at two different positions, but both will see a particle and its antiparticle existing in different places.

From the perspective of the alien, this isn't that exciting. Both the dog and the cat move from right to left at superluminal speeds, the cat somewhat slower than the dog. The big difference in terms of appearance would be that the alien would be utterly incapable of seeing anything behind it, because the light from events in the wake of its passage could never catch up.

I agree that there is a major difference between the underlying physical reality of the alien speeding by versus the particle/antiparticle pair that spontaneously appears and then annihilates.

I actually think that you're shortchanging the alien-eye-view of the situation. Let me focus on the dog...

If you draw light rays emanating from the time axis (the dog's worldline), you'll notice that for each moment in time, there is pair of light rays being sent out by the dog (or bouncing off of her). Actually, if you want to think about it in 3D space, you should think of it as a spherical shell of light emanating out from the dog at every instant.

When the alien is to the left of the dog's worldline, it will intercept the light rays that are moving out to the left and will see the dog getting older. However, when the alien crosses the dog's worldline something really interesting happens: it starts to catch up to the light rays that were going to the right. So after meeting the dog, the alien should start to see the dog aging in reverse---getting younger.

So the alien does indeed see some weird stuff!

Going with the thought experiment's idea that the metric is constant, there is a frame in which the alien travels at infinite speed from one side of the universe to the other. There isn't a Lorentz invariant property for the alien of moving from left to right or from right to left. In 3+1 dimensions the quotient group O(4)/SO(4) has four discrete components, with time-like forward/backward being invariant under the connected subgroup SO(4), but with no such invariant property for space-like 4-vectors.

But in a space-time in which the metric is locally Lorentzian, in GR it is usually /assumed/ that the manifold is both spatially orientable and temporally orientable ( http://en.wikipedia.org/wiki/Orientability#Lorentzian_geometry: "a space-time is time-orientable if and only if any two observers can agree which of the two meetings preceded the other", Mark J. Hadley.The Orientability of Spacetime. Class Quantum Grav.19(2002)4565-4571 arXiv:gr-qc/0202031v4).

From the perspective of the alien, this isn’t that exciting.

Why not? Let's take the cat out of the scenario for the time being. The situation with just the dog and the alien should be symmetric: in the alien's frame of reference, the dog is moving superluminally. So the alien should see the dog appear and move in two different directions, just as the dog would see the alien appear and move in two different directions. In the alien's frame, the cat would just be a second superluminal object. The alien, if he were recording the scenario, could work out after the fact what happened, and deduce that the dog and cat are moving at different speeds. (What the cat's speed would be, I am not sure, because I'm not sure how to do a double Lorentz boost when one of the boosts is superluminal.)

Also, the alien should be able to see backwards. The speed of light in the alien's frame is the same as the speed of light in the dog's frame. The alien's backward direction may not be backward in the dog's frame, but it would be in his frame.

The only way the scenario with just the dog and the alien could not be symmetric is if there were something to tell us that the dog's frame is preferred. This is exactly what relativity says we can't have, as long as neither dog nor alien is accelerating.

Dumb Layperson Questions dep't:

I'm always skeptical of arguements that go "it can't be, therefore it isn't." In the present case, the well-known arguement that superluminal communication & travel are impossible "because" they would produce logical paradoxes with causality. Therefore:

1) Are there empirical data demonstrating that FTL travel/comms are never observed to occur and for which some alternative hypohesis is not tenable?

2) Are there solid maths that also demonstrate that FTL travel/comms are impossible? In which case how does this math interact with the stuff from kinetics wherein equations work regardless of whether the value for time is set as positive or negative?

3) What ever happened to tachyons, that are supposed to move backward across time?

4) What about the idea that any sort of causal paradox that might be produced, is instead isolated in a manner similar to that of a local universe: each element in the paradox becomes separated from the other so they both occur but cannot interact locally? (You can visualize this as each element of the paradoxical events getting trapped in vacuoles;-) (Or have I just come up with an inelegant way of describing the same situation you've already described but I failed to understand?)

In the superluminal dark, no cats are grey because the gamma rays from those receding are red-shifted into the blue , and the photons from those approching would torpedo any retina made by God or man

@comment #16: I imagine that different people will approach your questions differently. Here are some of my takes:

(1) We have *never* observed any signal or physical phenomenon that can travel faster than light. That doesn't mean that we won't ever do so, just that it's never happened. Furthermore, the aether hypothesis suggested that we should be able to observe light moving at different speeds and was soundly put to rest by the famous Michelson-Morley. experiments.

There are alternative hypotheses---Lorentz and Poincare derived the transformations at the heart of relativity by trying to save the aether theory. They proposed that physical bodies moving in the aether deform and their clocks run slow precisely so as to make it *look* like light has a universal speed. However, I would say that Einstein's more parsimonious approach is the more attractive one.

(2) Special relativity begins with the assumption that light moves at a speed that all inertial observers (observers who don't have any external forces applied to them) agree on. Remarkably, from that one can show that (a) if an object has a real mass (you know, the kind that we all have), then it is impossible for it to ever reach the speed of light. As you accelerate the object, more of the energy is effectively diverted into an increasing mass rather than an increasing speed. This means you need more force to maintain the same amount of acceleration. To get to the speed of light, you will need to expend an infinite amount of energy.

And yes, the maths here is quite solid!

(3) Tachyons in the context of special relativity are objects that move faster than light (incidentally, whether you take them to go backwards or forwards in time is a matter of perspective. The alien in Chad's example would be a tachyon). The problem with the naive view of tachyons is that they would require imaginary masses which is typically taken as a sign that they aren't physical.

On a more sophisticated level, there are situations that can fruitfully be described as tachyonic. The existence of a tachyon in your model typically means that your model is unstable or that you are approximating around an unstable equilibrium. The tachyon isn't really an object in this situation, it's more a sign that tells you that some sort of decay or change is going to occur that takes you to a different "sector" of your theory. I won't elaborate on this here.

(4) I'm not sure I follow here. There'd need to be a clear physical mechanism for this and I don't know of any plausible ones.

David @ 17:

Thanks! (And another question at the end of this.)

1) Agreed, parsimony wins over the aetheric equivalent of epicycles;-) If it was necessary to add a bunch of needless bits to save the idea of a variable speed of light, that doesn't work. (Though, I assume that the observation that photons slow slightly in water is accounted for without going back into the proverbial aether.)

2) Aha! You just supplied me with a big missing piece. (Here I should mention that I'm not a dumb-dumb but I'm dyslexic so I can get this stuff conceptually but I've never been able to get the maths.)

I've known that in the current body of theory, acceleration of objects toward c produces an increase in mass that theoretically becomes infinite (for which reason, when I speculate about interstellar migration, I assume the best we'll be able to do is a small single-digit percentage of c and I use .01c as an example). But I've always taken it "on faith" and never had an idea of the mechanism before (blush).

What you said was "more of the energy is effectively diverted into an increasing mass rather than an increasing speed." Keyword "diverted." Bingo!, that would seem to be the mechanism. Presumably Einstein's equations govern that diversion. That's the piece I was missing: the mechanism for the increasing mass approaching c.

Question: Would it be correct to consider that a form of "energy conversion," in a manner that's very roughly analogous to other types of energy conversion (e.g. sunlight reaches planet's surface and is converted to heat, or electrons in an LED are converted to light, etc.)? If not, then what?

3) Tachyons: from what you said, they appear to be a creation of theory in order to deal with mathematical errors or instabilities that shouldn't exist. Something like this: "if you get tachyons here, you made a mistake somewhere." Alternately, "a different sector of your theory," which suggests a different reference frame (yes I'd like to know what you meant by that but you said you weren't going into it further here, so I'll park that question for the moment and look for some other opportunity to ask someone;-)

4) Apologies for not marking that item off with "the following isn't science, it's a wild speculation." What I was looking for were ways the "paradox" might be "solved." Something like Everett's many-worlds theory: "the universe splits here" so each branch of the paradox exists in a separate universe.

For example (using the nonviolent version of the Grandfather Paradox), you call your grandfather on the tele-time-phone and ask him to use condoms for a month. You don't ever find out if he actually does so. But if he does, then something similar to your local universe splits, whereby your existence continues in the universe where he does not use condoms (or a condom fails), and another timeline occurs into a different universe where the condoms work and you don't exist: but you never get to observe the second universe.

To my mind that's approximately as counterintuitive as the idea that the universe splits at every wavefunction collapse.

And while we're on that subject, another question that's been bugging me for years:

Where does Everett get the energy needed to create multiple universes at every wavefunction collapse?, or, are the splits in his theory confined to the immediate locality of each particle? (I'm highly skeptical of "the whole universe splits" but I'm agnostic about "the immediate locality of the particle splits," and in any case I don't see where the energy would come from to duplicate a universe either generally or locally.)

1) Light does propagate more slowly in a medium other than vacuum. This is essentially due to the fact that light---thought of as an electromagnetic wave---interacts with the atoms or molecules making up the material. This interaction is often characterized as the photons of the light being absorbed and re-emitted by the electrons in the medium, but that isn't quite right. The actual story is more complicated. But the gist of it is as I said, an interaction between the light and the medium.

(and no aether is needed! We have a real medium in this case.)

2) I'm glad that this explanation helps. Let me try to be a bit more precise instead of speaking about "effective" increases in mass. Einstein derived a formula for the total energy of a freely moving object. This formula combines the rest energy and the kinetic energy of the object as the two contributions. However, the kinetic energy is different in form from the non-relativistic one that we learn in school. In particular, it is strongly modified at very high speeds so that it goes to infinity as the speed approaches that of light.

The kinetic energy and rest energy (the energy inherent in the mass you'd measure when the object is at rest) combine in a rather natural way to give you the following for the total energy:

m c^2 / sqrt(1-v^2/c^2)

where m is the mass as measured at rest. Some people take this total energy and divide by c^2 to get a quantity that is called "relativistic mass". You can think of this as a generalization of the rest mass which includes the effects of it being in motion. If you look at the formula above you'll notice that the total energy, and thus, the relativistic mass increases to infinity as the speed v approaches the speed of light c.

Some people prefer not to describe things in terms of relativistic mass since this type of mass is completely equivalent to the total energy of the object. Instead, you can say that since energy and mass are interconvertible, energy itself also resists being accelerated. So if the energy stored by an object (say its kinetic energy) goes up, then it becomes harder to accelerate, so you have to push it harder.

So the answer to your follow up question is that this is not exactly the same thing as transferring energy from one store of energy to another, such as solar to chemical. Rather, the kinetic energy has an in-built resistance to acceleration that is basically the same as the resistance that the Newtonian concept of mass captures. As the kinetic energy goes up, the overall inertia of the object goes up.

That said, the energy stored up in the rest mass of an object can indeed be unlocked. In fact, turning this story around, we find that the majority of the mass in our bodies is actually in the form of the binding energy! The nuclei of the atoms in our bodies are made of protons and neutrons. These protons and neutrons are themselves bound states of subatomic particles called quarks and gluons. The quarks themselves have tiny masses, but it is the *energy* that binds them together that is responsible for most of the mass in a proton or a neutron.

3) I'd put this a bit differently. There is nothing in special relativity that mathematically forbids one from asking what the properties of an object would be if it travels faster than light. However, when you do this, you discover the bizzaro result that the mass would have to be described by an imaginary number. So at first blush you might say, well the theory is saying GIGO---garbage in, garbage out. The assumption that this thing can be going faster than light is garbage, so I'm getting garbage results.

However, it turns out that there is a utility to the notion of imaginary mass or energy. This will likely be cryptic and will probably set off more questions than answers since I don't have time to get into all the details, but here goes!

Consider the following mathematical functions:

where g and E are some constants chosen in units of inverse time (let's say in Hertz). The first function is just an exponentially decaying function. As time gets big, the function approaches zero exponentially fast.

The second function is an oscillatory function. Yes, it is imaginary, but you can take its real part and you get a cosine that oscillates with a frequency determined by the constant E.

In quantum theory, a free particle is described by a wavefunction that looks like this sort of persistent oscillatory function. The constant E really is related to the free particle's energy in this case.

A particle that decays isn't actually free, but its wavefunction will look like a damped oscillatory function, that is, it will look like a product of the two functions above. The constant E will again be related to the energy, and the constant g will set the time scale over which this particle decays. So you end up with a wavefunction that is sort of like this:

e^{i E t} e^{-gt}

but this can be rewritten as

e^{iEt - gt}

Now let me introduce a new notation. Let the complex number

be the "complexified energy" of this particle. Notice that the real part is just the ordinary energy, while the imaginary part is related to the decay constant. We can rewrite the "wavefunction" as

this is the same information as before, but its a nice notation. Intriguingly, you can see that giving the energy an imaginary part yields a description of something unstable---something that decays away. This is a hint that tachyons are somehow connected to instabilities.

The analogy can be pushed further. Again, you won't find that tachyons can be observed as real, physical particles. Rather, they represent fields that are unstable.

I'll end with what may be an enigmatic note: the version of string theory that is used to introduce the subject---the so-called bosonic string theory---has a tachyon as its lowest energy field. This is taken to imply that bosonic string theory is unstable and requires completion by embedding into some other type of theory where the theory may evolve into a stable regime (getting rid of the tachyon). This is still an area of active research in string theory.

4) Ah, I think I see---the idea here is that paradox is avoided by thinking of the universe as branching off so that if you go back in time and do something, your time line ends up going to a different branch, not intersecting itself in a potentially contradictory manner.

That's certainly a valid way of trying to solve time-travel paradoxes, but there aren't really physical mechanisms that suggest that such a thing actually can happen. I suppose that if you are a many worldser (someone who subscribes to the many worlds interpretation of quantum theory) then you might concoct some explanation of this as interference between different branches of the universal wavefunction...but color-me-skeptical! (I'm actually rather skeptical of the whole many worlds approach in general. It's actually a much less straightforward interpretation than meets the eye. Much of its messiness is ignored or obfuscated in popular discussions).

I've got to run, but I'll try to answer your last question when I have some time.

Alright G, let's see if we can tackle that last question.

You asked about where all that energy comes from in Everett's many worlds interpretation for the creation of new universes or at least the "unzipping" of a universe at certain events in time.

Well, first one needs a more precise model of what many worlds is and what these universes are. The simplest approach is to start with the assumption that the state of the entire universe is described by a wavefunction that I'll call Psi. Let me point out from the get-go that this is actually a major assumption---something that often is glossed over in discussions about many worlds. It is not at all clear that our universe and its evolution can indeed be described by the assumption that its dynamical state is captured by a wavefunction evolving via the Schrodinger equation. What if the universe is fundamentally an "open" quantum system? In that case, the dynamics can be much harder to grasp...

But, taking the plunge, assume that Psi describes the universe and that there is a quantum law (a Hamiltonian) that gives us the evolution of Psi via the Schrodinger equation. According to many worlds, that's all there is.

So, where are the many branches of the universe (or multiverse)? They come from how you can decompose Psi into different terms. This is similar to how you can decompose a position vector into x, y, or z components. The idea is that at any given time, there is a decomposition of Psi into (probably infinitely) many components. If all goes well with the interpretation, our universe is described by one such component or branch, while other universes correspond to the different branches. As time evolves, macroscopic interactions with microscopic objects leads to decoherence, a process whereby initially "quantum" looking things tend toward looking more classical. It's thought that this describes how universes split into new branches.

So what about energy? Well, the simplest answer to write down is that the universe may not have a well-defined total energy. The components of Psi that describe the different universe-branches also don't have to have a well-defined energy. So there's no sense in which "splitting" would have a well defined energy either.

But suppose that it just so happened that the branches that describe macroscopically familiar universes do have precise energies associated with them (technically we'd say that these branches are "eigenstates" of the universe's Hamiltonian---it's energy operator). If that's the case, then each branch has a total energy of its own that won't change as it evolves, so within a universe-branch, there will be conservation of the total energy.

Actually, I'm not sure how to make the second picture consistent with the idea of branching. It seems to me that in such a scenario (where the energies are well defined for each branch) then the universal wavefunction would have to come with a predefined branching structure that is already in place. That's rather unlikely.

So the first answer is probably a better one: total energy is probably not a well defined property of the various branches that make up the universal wavefunction. That precludes any problems with the "creation" of new energy when a split occurs.

As a parting note, I'll point out that the total energy of the universe is actually not a generally well defined concept in general relativity either. For example, our universe probably does not have a well-defined total energy. This occurs because space itself is able to evolve in non-trivial ways in general relativity, and the dynamics of space ends up potentially messing up any general definition of total energy you might try to dream up.

Relativity and FTL Travel

By jason w. hinson ( [email protected] ), part i: special relativity, edition: 5.1 last modified: april 8, 2003 url: http://www.physicsguy.com/ftl/ ftp (text version): ftp://ftp.cc.umanitoba.ca/startrek/relativity/.

[Physics FAQ] - [Copyright]

By Philip Gibbs, 1997, 1998.

It might be thought that special relativity provides a short negative answer to this question. In actual fact, there are many trivial ways in which things can be going faster than light (FTL) in a sense, and there may be other more genuine possibilities. On the other hand, there are also good reasons to believe that real FTL travel and communication will always be unachievable. This article is not a full answer to the question (which no doubt will continue to be discussed in the newsgroups for the foreseeable future), but it does cover some of the more common points that are repeatedly made.

It is sometimes objected that "they said no-one would ever go faster than sound and they were wrong. Now they say no-one will ever go faster than light..." Actually it is probably not true that anybody said it was impossible to go faster than sound. It was known that rifle bullets go faster than sound long before an aircraft did. The truth is that some engineers once said that controlled flight faster than sound might be impossible, and they were wrong about that. FTL travel is a very different matter. It was inevitable that someone would one day succeed in flying faster than sound, once technology got around the problems. It is not inevitable that one day technology will enable us to go faster than light. Relativity has a lot to say about this. If FTL travel or FTL communication were possible, then causality would probably be violated and some very strange situations would arise.

First we will cover the trivial ways in which things can go FTL. These points are mentioned not because they are interesting, but because they come up time and time again when FTL is being discussed, and so they are necessary to deal with. Then we will think about what we mean by non-trivial FTL travel/communication and examine some of the arguments against it. Finally, we will look at some of the more serious proposals for real FTL. Many of these things are discussed in more detail elsewhere in the FAQ and hyper-links are provided. The sections are numbered so that they can be referred to individually.

Trivial FTL Travel

1. cherenkov effect.

One way to go faster than light is to make the light slow down! Light in vacuum travels at a speed c which is a universal constant (see the FAQ entry Is the speed of light constant? ), but in a dense medium such as water or glass, light slows down to c/n where n is the refractive index of the medium (1.0003 for air, 1.4 for water). It is certainly possible for particles to travel through air or water faster than light travels in that medium, and Cherenkov radiation is produced as a result. See the FAQ entry Is there an equivalent of the sonic boom for light? .

When we discuss moving faster than light, we are really talking about exceeding the speed of light in vacuum c (299,792,458 m/s). The Cherenkov effect is thus not considered to be a real example of FTL travel.

2. Third-Party Observers

If a rocket A is travelling away from me at 0.6c in a westerly direction, and another B is travelling away from me at 0.6c in an easterly direction, then the total distance between A and B as seen in my frame of reference is increasing at 1.2c . An apparent relative speed greater than c can be observed by a third person in this way.

But this is not what is normally meant by relative speeds. The true speed of rocket A relative to rocket B is the speed at which an observer in rocket B observes his distance from A to be increasing. The two speeds must be added using the relativistic formula for addition of velocities. (See the FAQ entry How do You Add Velocities in Special Relativity? ) In this case the relative speed is actually about 0.88c , so this is not an example of FTL travel.

3. Shadows and Light Spots

Think about how fast a shadow can move. If you project the shadow of your finger using a nearby lamp onto a distant wall and then wag your finger, the shadow will move much faster than your finger. If your finger moves parallel to the wall, the shadow's speed will be multiplied by a factor D/d where d is the distance from the lamp to your finger, and D is the distance from the lamp to the wall. The speed can even be much faster than this if the wall is at an angle to your finger's motion. If the wall is very far away, the movement of the shadow will be delayed because of the time it takes light to get there, but the shadow's speed is still increased by the same ratio. The speed of a shadow is therefore not restricted to be less than the speed of light.

This behaviour of a shadow is all about the arrival of successive "pieces of light" (photons, if you will) at a screen. It is really no different to the faster-than-light speed of a spot on the Moon's surface caused by a laser that has been aimed at that surface and is being waved around on Earth. Given that the distance to the Moon is 385,000 km, try working out the speed of that spot if you wave the laser at a gentle speed. You might also like to think about a water wave arriving obliquely at a long straight beach. How fast can the point at which the wave is breaking travel along the beach?

This sort of thing turns up in Nature; for example, the beam of light from a pulsar can sweep across a dust cloud. A bright explosion emits an expanding spherical shell of light or other radiation. When this shell intersects a surface, it creates a circle of light which expands faster than light. A natural example of this has been observed when an electromagnetic pulse from a lightning flash hits an upper layer of the atmosphere.

These are all examples of "things" that seem to be moving faster than light. In reality, no object or signal is moving faster that light here. For a more prosaic example, imagine squirting water from a garden hose at a fence, and moving your aim from one end of the fence to the other. The intersection point of water stream and fence moves quickly, but of course no thing or signal is really moving along the fence. A succession of water molecules strikes the fence, but their speed of travel has nothing to do with how quickly you move the hose. It is a kind of optical illusion for us to think that the wet spot advancing along the fence is a moving object or signal. The ban in relativity against faster-than-light travel actually concerns the speed of signals (which includes material objects and waves): in a vacuum, no signal is allowed to move faster than light moves in its vicinity. Neither a moving shadow, nor a laser spot, nor a wet spot on a fence, constitute a signal that is being sent from the initial position of those spots to the final position. Since these moving spots don't constitute a signal, they are all allowed to move faster than light. This is not really what we mean by faster-than-light travel, although it shows how difficult it is to define what we really do mean by faster-than-light travel. See also the FAQ The Superluminal Scissors .

4. Rigid Bodies

If you have a long rigid stick and you hit one end, wouldn't the other end have to move immediately? Would this not provide a means of FTL communication?

Well, it would if there were such things as perfectly rigid bodies. In practice the effect of hitting one end of the stick propagates along it at the speed of sound in the material; this speed depends on the stick's elasticity and density. Relativity places an absolute limit on material rigidity in such a way that the speed of sound in the material will not be greater than c .

The same principle applies if you hold a long string or rod vertically in a gravitational field and let go of the top end. The point at which you let go will start to move immediately, but the lower end cannot move until the effect has propagated down the length. That speed of propagation depends on the nature of the material and the strength of the gravitational field.

It is difficult to formulate a general theory of elastic materials in relativity, but the general principle can be illustrated with newtonian mechanics. The equation for longitudinal motion in an ideal elastic body can be derived from Hooke's law. In terms of the mass per unit length p and Young's modulus of elasticity Y , the longitudinal displacement X satisfies a wave equation (see for example Goldstein's "Classical Mechanics"):

Plane wave solutions travel at the speed of sound s where s 2 = Y/p . This wave equation does not allow any causal effect to propagate faster than s . Relativity therefore imposes a limit on elasticity: Y < pc 2 . In practice, no known material comes anywhere near this limit. Note that even if the speed of sound is near c , the matter does not necessarily move at relativistic speeds. But how can we know that no material can possibly exceed this limit? The answer is that all materials are made of particles whose interaction are governed by the standard model of particle physics, and no influence faster than light can propagate in that model (see the section on Quantum Field Theory below).

So although there is no such thing as a rigid body, there is such a thing as rigid body motion; but this is another example in the same category as the shadows and light spots described above which do not give FTL communication. (See also the FAQ articles The Superluminal Scissors and The Rigid Rotating Disk in Relativity ).

5. Phase, Group, and Signal Velocities

Look at this wave equation:

This has solutions of the form:

These solutions are sine waves propagating with a speed

But this is faster than light, so is this the equation for a tachyon field? (See the paragraph on tachyons below ). No, it is the usual relativistic equation for an ordinary particle with mass!

Superluminal speeds such as this present no problem once we recognise three types of speed associated with wave motion: phase velocity , group velocity , and signal velocity . Phase velocity is the velocity of waves that have well-defined wavelengths, and it often varies as a function of this wavelength. We can combine ("superpose") waves of different wavelengths to build a wave packet , a blob of some specified extent over which the wave disturbance is not small. This packet does not have a well-defined wavelength, and because it usually spreads out as it travels, it doesn't have a well-defined velocity either; but it does have representative velocity, and this is called its group velocity, which will usually be less than c . Each of the packet's constituent wave trains travels with its own individual phase velocity, which in some instances will be greater than c . But it is only possible to send information with such a wave packet at the group velocity (the velocity of the blob), so the phase velocity is yet another example of a speed faster than light that cannot carry a message.

In some situations, we can build a fairly exotic wave packet whose group velocity is greater than c . Does this then constitute an example of information being sent at a speed faster than light? It turns out that for these packets, information does not travel at the group velocity; instead, it travels at the signal velocity , which has to do with the time of arrival of the initial rise of the wave front as it reaches its destination. You might not now be surprised to learn that the signal velocity turns out always to be less than c .

6. Superluminal Galaxies

If something is coming towards you at nearly the speed of light and you measure its apparent speed without taking into account the diminishing time it takes light to reach you from the object, you can get an answer that is faster than light. This is an optical illusion, and is not due to the object's moving at FTL. See the FAQ Apparent Superluminal Velocity of Galaxies .

7. Relativistic Rocket

A controller based on Earth is monitoring a space ship moving away at a speed 0.8c . According to the theory of relativity, he will observe a time dilation that slows the ship's clocks by a factor of 5/3, even after he has taken into account the Doppler shift of signals coming from the space ship. If he works out the distance moved by the ship divided by the time elapsed as measured by the onboard clocks, he will get an answer of 4/3 c . He infers from this that the ship's occupants determine themselves to be traversing the distances between stars at speeds greater than the speed of light when measured with their clocks. From the point of view of the occupants their clocks undergo no slowing; rather, they maintain that it is the distance between the stars which has contracted by a factor of 5/3. So they also agree that they are covering the known distances between stars at 4/3 c .

This is a real effect which in principle could be used by space travellers to cover very large distances in their lifetimes. If they accelerate at a constant acceleration equal to the acceleration due to gravity on Earth, they would not only have a perfect artificial gravity on their ship, but would also be able to cross the galaxy in only about 12 years of their own "proper time": that is, they would age 12 years during the journey. See the FAQ What are the Equations for the Relativistic Rocket?

Nevertheless, this is not true FTL travel. The effective speed calculated used the distance in one reference frame and the time in another. This is no way to calculate a speed. Only the occupants of the ship benefit from this effective speed. The controller will not measure them to be travelling large distances in his own lifetime.

8. Speed of Gravity

Some people have argued that the speed of gravity in a gravitationally bound system is much greater than c or even infinite. In fact, gravitational effects and gravitational waves travel at the speed of light c . See the articles Does Gravity Travel at the Speed of Light? and What is Gravitational Radiation? for the explanation.

9. EPR Paradox

In 1935 Einstein, Podolsky, and Rosen published a thought experiment that seemed to produce a paradox in quantum mechanics, as well as demonstrating that it was incomplete. Their argument used the fact that there can be an apparent instantaneous interaction in the measurement of two separated particles that have been prepared in a certain "entangled" manner. Einstein called it "spooky action at a distance". It has been shown by Eberhard that no information can be passed using this effect; so there is no FTL communication, but the paradox is still very controversial. See the FAQ article The EPR Paradox and Bell's Inequality for more details.

10. Virtual Photons

In quantum field theory forces are mediated by "virtual particles". The Heisenberg Uncertainty Principle allows these virtual particles to move faster than light. But virtual particles are not called "virtual" for nothing. They are only part of a convenient mathematical notation, and once again, no real FTL travel or communication is possible. See the FAQ Virtual Particles .

11. Quantum Tunnelling

Quantum Tunnelling is the quantum mechanical effect that permits a particle to pass through a barrier when it does not have enough energy to do so classically. You can do a calculation of the time it takes a particle to tunnel through such a barrier. The answer you get can come out less than the time it takes light to cover the distance at speed c . Does this provide a means of FTL communication? Ref: T. E. Hartman, J. Appl. Phys. 33 , 3427 (1962).

The answer must surely be "No!"—otherwise our understanding of QED is very suspect. Yet a group of physicists have performed experiments that seem to suggest that FTL communication by quantum tunneling is possible. They claim to have transmitted Mozart's 40th Symphony through a barrier 11.4cm wide at a speed of 4.7 c . Their interpretation is, of course, very controversial. Most physicists say this is a quantum effect where no information can actually be passed at FTL speeds. If the effect is real it is difficult to see why it should not be possible to transmit signals into the past by placing the apparatus in a fast-moving frame of reference. Refs: W. Heitmann and G. Nimtz, Phys. Lett. A196 , 154 (1994); A. Enders and G. Nimtz, Phys. Rev. E48 , 632 (1993).

Terence Tao has pointed out that apparent FTL transmission of an audio signal over such a short distance is not very impressive. The signal takes less than 0.4 ns to travel the 11.4 cm at light speed, but it is quite easy to anticipate an audio signal ahead of time by up to 1000 ns simply by extrapolating the signal waveform. Although this is not what is being done in the above experiments, it does illustrate that the experimenters will need to use a much higher frequency random signal, or transmit over much larger distances, if they are to demonstrate FTL information transfer convincingly.

The likely conclusion is that there is no real FTL communication taking place, and that the effect is another manifestation of the Heisenberg Uncertainty Principle.

12. Casimir Effect

The Casimir Effect describes the fact that a very small but measurable force exists between two uncharged conducting plates when they are very close together. It is due to the existence of vacuum energy (see the FAQ article on the Casimir Effect ). A surprising calculation by Scharnhorst suggests that photons travelling across the gap between the plates in the Casimir Effect must go faster than c by a very very small amount (at best 1 part in 10 24 for a 1 nanometre gap.) It has been suggested that in certain cosmological situations, such as in the vicinity of cosmic strings if they exist, the effect could be much more pronounced. Even so, further theoretical investigations have shown that, once again, there is no possibility of FTL communication using this effect. Refs: K. Scharnhorst, Physics Letters B236 , 354 (1990) S. Ben-Menahem, Physics Letters B250 , 133 (1990) Andrew Gould (Princeton, Inst. Advanced Study). IASSNS-AST-90-25 Barton & Scharnhorst, J. Phys. A26 , 2037 (1993).

13. Expansion of the Universe

According to Hubble's Law, two galaxies that are a distance D apart are moving away from each other at a speed HD , where H is Hubble's constant. So this interpretation of Hubble's Law implies that two galaxies separated by a distance greater than c/H must be moving away from each other faster than light. Actually, the modern viewpoint describes this situation differently: general relativity takes the galaxies as being at rest relative to one another, while the space between them is expanding. In that sense, the galaxies are not moving away from each other faster than light; they are not moving away from each other at all! This change of viewpoint is not arbitrary; rather, it's in accord with the different but very fruitful view of the universe that general relativity provides. So the distance between two objects can be increasing faster than light because of the expansion of the universe, but this does not mean, in fact, that their relative speed is faster than light.

As was mentioned above, in special relativity it is possible for two objects to be moving apart by speeds up to twice the speed of light as measured by an observer in a third frame of reference. In general relativity even this limit can be surpassed, but it will not then be possible to observe both objects at the same time. Again, this is not real faster-than-light travel; it will not help anyone to travel across the galaxy faster than light. All that is happening is that the distance between two objects is increasing faster when taken in some cosmological reference frame.

14. The Moon revolves round my head faster than light!

Stand up in a clear space and spin round. It is not too difficult to turn at one revolution each two seconds. Suppose the Moon is on the horizon. How fast is it spinning round your head? It is about 385,000 km away, so the answer is 1.21 million km/s, which is more than four times the speed of light! It might sound ridiculous to say that the Moon is going round your head when really it is you who is turning, but according to general relativity all co-ordinate systems are equally valid, including rotating ones. So isn't the Moon going faster than light?

What it comes down to is the fact that velocities in different places cannot be compared directly in general relativity. Notice that the Moon is not overtaking any light in its own locality. The speed of the Moon can only be compared to the speeds of other objects in its own locality. Indeed, the concept of speed is not a very useful one in general relativity, and this makes it difficult to define what "faster than light" means. Even the statement that "the speed of light is constant" is open to interpretation in general relativity. Einstein himself, on page 76 of his book "Relativity: the Special and the General Theory", wrote that the statement cannot claim unlimited validity. When there is no absolute definition of time and distance it is not so clear how speeds should be determined.

Nevertheless, the modern interpretation is that the speed of light is constant in general relativity and this statement is a tautology given that standard units of distance and time are tied together using the speed of light. The Moon is given to be moving slower than light because it remains within the "future light cone" propagating from its position at any instant.

Relativity Arguments Against FTL Travel

15. what does "faster than light" mean.

The cases given so far only demonstrate how difficult it is to pin down exactly what we mean by FTL travel or communication. If we do not include things such as moving shadows, then what exactly do we mean by FTL?

In relativity there is no such thing as absolute velocity, only relative velocity; but there is a clear distinction between "world lines" that are "timelike", "lightlike", and "spacelike". By "world line" we mean a curve traced out in the four dimensions of space-time. Such a curve is the set of all events that make up the history of a particle. If a world line is spacelike then it describes something moving faster than light. So there is a clear meaning of what is meant by a "faster-than-light" speed which does not depend on the existence of third-party observers.

But what do we mean by an "object" if we don't want to include shadows? We could define an object to be anything that carries energy, charge, spin, or information; or perhaps just that it must be made of atoms, but there are technical problems in each case. In general relativity energy cannot be localised, so we had better avoid using energy in our definition. Charge and spin can be localised, but not every object need have charge or spin. Using the concept of information is better but tricky to define, and sending information faster than light is really just FTL communication—not FTL travel. Another difficulty is knowing whether an object seen at A is the same as the one that was earlier seen at B when we claim that it has travelled across the gap faster than light. Could it not be a duplicate? It could even be argued that FTL communication makes FTL travel possible, because you can send the blueprint for an object FTL as advance information, and then reconstruct the object—although not everyone would accept such teleportation as FTL travel.

The problems of specifying just what we mean by FTL are more difficult in general relativity. A valid form of FTL travel may mean distorting space-time (e.g. making a worm hole) to get from A to B without going on a spacelike curve locally. There is a distinction between going faster than light locally and getting from A to B faster than light globally . When a gravitational lens bends the light coming from a distant galaxy asymmetrically, the light coming round the galaxy on one side reaches us later than light that left at the same time and went round the other side. We must avoid a definition of FTL travel that says a particle going from A to B gets there before light that has made the same journey along a lightlike geodesic. This makes it very difficult, perhaps impossible, to define global FTL travel unambiguously.

If you were expecting me to finish this section with a precise definition of what is meant by FTL travel and FTL communication, I am afraid I must disappoint you! The above difficulties are insurmountable. Nonetheless, you will probably recognise the real thing when confronted with it now that I have given some examples of what would not be FTL travel.

16. The Infinite-Energy Argument

When Einstein wrote down his postulates for special relativity, he did not include the statement that you cannot travel faster than light. There is a misconception that it is possible to derive it as a consequence of the postulates he did give. Incidentally, it was Henri Poincare who said "Perhaps we must construct a new mechanics [...] in which the speed of light would become an impassable limit." That was in an address to the International Congress of Arts and Science in 1904—before Einstein announced special relativity in 1905.

It is a consequence of relativity that the energy of a particle of rest mass m moving with speed v is given by

As the speed approaches the speed of light, the particle's energy approaches infinity. Hence it should be impossible to accelerate an object with rest mass to the speed of light; also, particles with zero rest mass must always move at exactly the speed of light, since otherwise they would have no energy. This is sometimes called the "light speed barrier", but it is very different from the "sound speed barrier". As an aircraft approaches the speed of sound it starts to feel pressure waves which indicate that it is moving close to the speed of sound, and before the existence and effects of these waves were well understood, they destroyed several aircraft in the mid 20th century; hence the old name of sound "barrier". In fact, with more thrust and the right aerodynamics, an aircraft can certainly pass through the sound barrier.

The situation is different for light. As the light speed barrier is approached (in a perfect vacuum) there are no such waves according to relativity (destructive or otherwise). Moving at 0.999 c is just like standing still with everything rushing past you at −0.999 c . Particles are routinely pushed to these speeds and beyond in accelerators, so the theory is well established. Trying to attain the speed of light in this way is a matter of chasing something that is forever just out of your reach.

This explains why it is not possible to exceed the speed of light by ordinary mechanical means. But it does not in itself rule out FTL travel. It is really just one way in which things cannot be made to go faster than light, rather than a proof that there is no way to do so. Particles are known to decay instantly into other particles which fly off at high speed. It is not necessary to think in terms of the particles' having been accelerated, so how could we say that they could not go faster than light? What about the possibility of particles that might always have been moving faster than light, and which might be used to send information if they can be detected without ever slowing down to less than the speed of light? Even if such "tachyons" don't exist (and we don't believe that they do exist), there may be ways of moving matter from A to B faster than light is able to travel from A to B by the usual route, but without anything having to go at a FTL speed locally. See the paragraph on tachyons below .

17. Quantum Field Theory

Except for gravity, all physical phenomena are observed to comply with the "Standard Model" of particle physics. The Standard Model is a relativistic quantum field theory which incorporates the nuclear and electromagnetic forces as well as all the observed particles. In this theory, any pair of operators corresponding to physical observables at space-time events separated by a spacelike interval "commute" (i.e. their order can be reversed). In principle, this implies that effects cannot propagate faster than light in the standard model, and it can be regarded as the quantum field theory equivalent of the infinite energy argument.

But no completely rigorous proofs of anything exist in the quantum field theory of the Standard Model, since no one has yet succeeded in showing that the theory is completely self consistent; and in fact, most likely it is not! In any case, there is no guarantee that there are not other undiscovered particles and forces that disobey the no-FTL rule. Nor is there any generalisation that takes gravity and general relativity into account. Many physicists working on quantum gravity doubt that such simplistic expressions of causality and locality will be generalised. All told, there is no guarantee that light speed will be meaningful as a speed limit in a more complete theory that might arise in the future.

18. Grandfather Paradox

A better argument against FTL travel is the Grandfather Paradox. In special relativity, a particle moving FTL in one frame of reference will be travelling back in time in another. FTL travel or communication should therefore also give the possibility of travelling back in time or sending messages into the past. If such time travel is possible, you would be able to go back in time and change the course of history by killing your own grandfather. This is a very strong argument against FTL travel, but it leaves open the perhaps-unlikely possibility that we may be able to make limited journeys at FTL speed that did not allow us to come back. Or it may be that time travel is possible and causality breaks down in some consistent fashion when FTL travel is achieved. That is not very likely either, but if we are discussing FTL then we had better keep an open mind.

Conversely, if we could travel back in time we might also claim the ability to travel FTL, because we can go back in time and then travel at a slow speed to arrive somewhere before light got there by the usual route. See the FAQ article on Time Travel for more on this subject.

Open Possibilities for FTL Travel

In this last section I give a few of the speculative but serious suggestions for possible faster-than-light travel. These are not the kinds of thing usually included in the FAQ because they raise more questions than answers. They are included merely to make the point that serious research is being done in this direction. Only a brief introduction to each topic is given; more information can be found all over the Internet (and should, like almost everything on the Internet, be taken with a huge grain of salt!).

19. Tachyons

Tachyons are hypothetical particles that travel faster than light locally. Their mass must take on imaginary values (i.e. to do with the square root of −1) to be able to do so, but they have real-valued energy and momentum. Sometimes people imagine that such FTL particles would be impossible to detect, but there is no reason to think so. Shadows and spotlights suffice to show that there is no logic in this suggestion, because they can certainly go FTL and still be seen.

No tachyons have definitely been found and most physicists doubt their existence. There has been a claim that experiments to measure neutrino mass in tritium beta decay indicated that the neutrinos were tachyonic. ; while this is very doubtful, it is not entirely ruled out. Tachyon theories have problems because, apart from the possibility of causality violations, they destabilise the vacuum. It may be possible to get around such difficulties—but then we would not be able to use tachyons for the kind of FTL communication that we would like.

The truth is that most physicists consider tachyons to be a sign of pathological behaviour in field theories, and the interest in them among the wider public stems mostly from the fact that they are used so often in science fiction. See the FAQ article on Tachyons .

20. Worm Holes

A famous proposition for global FTL travel is to use "worm holes". Worm holes are shortcuts through space-time from one place in the universe to another which would permit you to go from one end to the other in a shorter time than it would take light passing by the usual route. Worm holes are a feature of classical general relativity, but to create them you have to change the topology of space-time. That might be possible within a theory of quantum gravity.

To keep a worm hole open, regions of negative energy would be needed. Misner and Thorne have suggested using the Casimir Effect on a grand scale to generate the negative energy, while Visser has proposed a solution involving cosmic strings. These are very speculative ideas which may simply not be possible. Exotic matter with negative energy may not exist in the form required.

Thorne has found that if worm holes can be created, then they can be used to construct closed timelike loops in space-time which would imply the possibility of time travel. It has been suggested that the "multiverse" interpretation of quantum mechanics (many universes co-existing) gets you out of trouble by allowing time to evolve differently if you succeed in going back to a previous time. But multiverses are entirely out of keeping with the Ockham's Razor approach to doing science, and constitute more of a popular interpretation of quantum mechanics than a serious physical theory. Hawking says that worm holes would simply be unstable and therefore unusable. The subject remains a fertile area for thought experiments that help clarify what is and what is not possible according to known and suggested laws of physics. Refs: W. G. Morris and K. S. Thorne, American Journal of Physics 56 , 395–412 (1988) W. G. Morris, K. S. Thorne, and U. Yurtsever, Phys. Rev. Letters 61 , 1446–9 (1988) Matt Visser, Physical Review D39 , 3182–4 (1989) See also "Black Holes and Time Warps", Kip Thorne, Norton & co. (1994) For an explanation of the multiverse see "The Fabric of Reality" David Deutsch, Penguin Press.

21. Warp Drives

A "warp drive" such as used in the Star Trek science fiction series would be a mechanism for warping space-time in such a way that an object could move faster than light. Miguel Alcubierre made himself famous by working out a space-time geometry which describes such a warp drive. The warp in space-time makes it possible for an object to go FTL while remaining on a timelike curve. The main catch is the same one that may stop us making large worm holes. To make such a warp, you would need exotic matter with negative energy density. Even if such exotic matter can exist, it is not clear how it could be deployed to make the warp drive work. Ref. M. Alcubierre, Classical and Quantum Gravity, 11 , L73–L77, (1994). Ref. S. Finazzi, S. Liberati, C. Barcel�, Semiclassical instability of dynamical warp drives at arxiv.org.

To begin with, it is rather difficult to define exactly what is really meant by FTL travel and FTL communication. Many things such as shadows can go FTL, but not in a useful way that can carry information.
There are several serious possibilities for real FTL which have been proposed in the scientific literature, but these always come with technical difficulties.
The Heisenberg Uncertainty Principle tends to stop the use of apparent FTL quantum effects for sending information or matter.
In general relativity there are potential means of FTL travel, but they may be impossible to make work. It is thought highly unlikely that engineers will be building space ships with FTL drives in the foreseeable future, if ever, but it is curious that theoretical physics as we presently understand it seems to leave the door open to the possibility.
FTL travel of the sort science fiction writers would like is almost certainly impossible. For physicists the interesting question is "why is it impossible and what can we learn from that?"

What Would a Starship Actually Look Like?

Imagine a starship —a vessel capable of ferrying human beings from one solar system to another. Would it have wings and a cockpit? Or would it look like an aircraft carrier hauled out into the void and fitted with flame-belching rockets and glowing ion drives?

Science fiction has offered us all sorts of visions of interstellar spacecraft, from avian-inspired Klingon birds of prey to hulking masses such as the Borg cube. In general, sci-fi leans toward sleek designs with lines borrowed from planes or cars, since those are the kinds of looks we've been conditioned to think of as "fast." But if there's no air in space, why make things aerodynamic? Does it matter what a spacecraft looks like?

Yes, it turns out, and it depends upon what kind of space travel you're looking to undertake. The reality of starship design is more complex than anything Hollywood has dreamed up and implanted in our collective unconsciousness.

While a manned interstellar mission isn't exactly on NASA's upcoming schedule, researchers haven't abandoned the topic to science fiction. In fact, the 100 Year Starship initiative—which began as a DARPA-funded contest to lay the foundations for a flight across the stars, gathering physicists, entrepreneurs, and anyone seriously interested in long-distance space travel—just finished its annual symposium this past weekend.

We asked Millis, who once led NASA's Breakthrough Propulsion Physics Project, to take us through the basics of starship design.

Starships Aren't Spaceplanes

One look at the Icarus design—or its predecessor, the Daedalus—and it's clear what starships don't need: wings. The only real-world spacecraft that bother with wings are ones designed to make regular landings on runways, such as the retired Space Shuttle, the upcoming Lynx (a suborbital two-seater from XCOR) or the Dream Chaser, an in-development orbital craft from Sierra Nevada. And wings aren't even required for landings. Like the Russian Soyuz capsule, SpaceX's Dragon currently splashes down in the ocean (though SpaceX plans to move toward rocket-powered launchpad landings).

In both the near and far-term future, experts such as Millis imagine interstellar vessels won't spend much of their time in an atmosphere. Perhaps the small ships that carry people from surface to starship will remain winged, but truly interstellar vehicles can scrap aerodynamics and all of the design principles that are beholden to reducing wind resistance. A starship doesn't need to be sleek or have a pointy nose—even the stocky Battlestar Galactica is pointlessly aircraft-shaped. If anything, the equivalent Cylon ships in the rebooted TV series are more rational interstellar travelers, with their spindly arms and flagrant disregard for the entire air-centric history of aerospace.

Surviving Sublight

Predicting what the first unmanned starships might look like is relatively simple. In the case of Icarus, for example, the entire structure is devoted to propulsion. It's a colossal rocket, albeit a weird fusion-powered one.

Millis says the first person-carrying starships, however, will be dominated by the technologies that keep those passengers alive. Consider gravity, a necessity on long-distance spaceflights. In prolonged zero-g, the human body erodes, losing bone and muscle density. "With the physics we know, you create gravity with a giant centrifuge, a rotating cabin, basically," Millis says. The spinning disc on the Jupiter-bound Discovery One in 2001: A Space Odyssey illustrated this concept well, but Millis says that to better simulate Earth gravity, the real thing would actually have to be much larger. The smaller the centrifuge, the less consistent the centrifugal force is across a crew member's body—the head, in other words, will feel lighter than the feet. Aside from being disoriented by chronic light-headedness, if the goal is to re-create the way blood circulates under the influence of gravity, consistency is key.

Discovery One, from 2001: A Space Odyssey

Of course, mankind can't survive on gravity alone. A starship designed to keep its occupants alive for years, decades, or even centuries, would require systems unheard of in current spacecraft. Sections for growing crops or livestock, for example, could dwarf more traditional compartments. And spacious recreational facilities, with enough room and resources to support vast interior parks, might be crucial for fighting off the existential crisis of spending an entire lifetime crammed inside a spacecraft. What might seem laughable today, and a colossal waste of mass, could become the most defining feature of a vessel filled not with astronauts, but a wider swath of humanity—including, quite possibly, children born en route. Suddenly, a giant, rotating playground bisecting your vessel isn't such a bad idea.

The look of your starship depends a lot on your method of transportation, too, and all of the proposed methods of interstellar propulsion carry their own problems. Anything that requires the ship to have a massive surface area—such as using a sail propelled by the sun's photons or onboard lasers—would have to contend with intergalactic dust. There isn't much material out there in space, but even tiny particles are a hazard to vessels moving at some significant fraction of the speed of light. Those dust particles could cut through a solar sail; perhaps the crew would have to replace or repair the sail when it comes too perforated.

Ikaros, Japan's solar sail project. Credit: JAXA

Perforated sails might be replaceable, but all fast-moving starships will need to worry about dust. Forget the layouts of Firefly 's Serenity or the more recent eponymous vessel from Prometheus , with their swooping birdlike profiles and aircraft-style front-mounted cockpits. The risk of dust impacts probably means turning crew compartments into bunkers, and sticking people and any essential systems behind redundant layers of physical shielding. The result would seem ugly by sci-fi standards closer to the Icarus from 2007's Sunshine (not to be confused with the work-in-progress concept), with its solar shield making it look more like a giant umbrella than a bird of prey.

The more you think about it, the less inherently sexy the starship becomes. Even Star Trek 's Enterprise is a star-hopping Ferrari compared to the industrial monstrosities that might actually make the trip survivable, based on our current grasp of physics. The Federation's mastery of time, space, and everything in between allows writers to ignore the dangers of galactic cosmic ray bombardment, using various kinds of force fields to ward off disaster as opposed to the staggeringly thick hulls filled with water that a real starship would probably need. And while impulse drives apparently dump 100 percent of their energy into thrust, a real vessel, beholden to the laws of thermodynamics, would likely be bristling with a dizzying array of panels to radiate the excess heat generated by propulsion systems.

Even if humanity manages to upend physics and learns to directly manipulate gravity, matter, and the properties of time and space, Millis says, "the geometry of the ship will be heavily dependent on those technological breakthroughs." For example, gravity-producing plates might call for wide halls with extremely low ceilings, whereas a field that generates gravity in a long cylinder might lead to a skyscraper-shaped starship. Both concepts, says Millis, "are in the realm of playful speculation," but whatever shapes those miraculous devices take, the end-result is likely to be stranger than fiction.

What Color Is Your Time Machine?

Strangest of all is the faster-than-light (FTL) starship. All his life, Millis has been running the number crazy propulsion theories, including in his 2009 book The Frontiers of Propulsion Science , which he's trying to adapt into a more mainstream version through a Kickstarter project . Since the 1930s, he says sci-fi has remained fixated on concepts like warp drives and hyperspace, when it's the crazier-sounding technologies that might actually realize FTL travel.

Take, for example, the old soap-boat experiment : Put a drop of soap behind a toy boat, and watch it scoot across the surface of the bath. In 1996 physicist Miguel Alcubierre proposed a warp drive that works much the same way—it focuses not on the notional vessel and its own built-in propulsion, but on the distortion of space-time into a ship-propelling wave.

Millis says the only equations that support FTL involve space-time sleight-of-hand, such as wormholes or Alcubierre's warp effect . And a vessel that can cheat its way between the stars isn't exactly a "starship" in the way we think of them. It isn't going fast, so there's no real risk of explosive dust impacts. By its very nature, it isn't traveling for very long durations, so never mind the cavernous hydroponic farms. It might even be a single-stage craft, designed to be towed before and after its bizarre shortcut and with almost no traditional propulsion of its own.

In other words: The vessel look like just about anything, so there's no reason to assume it will look like something that flies. The FTL starship is more of a time machine than a rocket, a device capable of impossibly high-energy physics, none of which involves thermodynamic thrust. If a sub-light interstellar spacecraft has all the sex appeal of a nuclear powerplant, the warp-class version is likely to have the swooping curves of 10 daisy-chained Large Hadron Colliders.

Science Fact?

Millis isn't the kind of physicist who has to be baited into discussing warp drives and wormholes, and considering that his foundation is named after Tau Zero , Poul Anderson's 1970 novel about an interstellar colonization mission gone awry, he doesn't dismiss the role of science fiction in his life's work. "It gives you starting points to picture these capabilities, to imagine and list them all," says Millis. "Then you can distill them down, extract what the workable questions are. That's where you can transition to science investigation."

[youtube]http://www.youtube.com/v/scBY3cVyeyA?version=3&hl=en_US[/youtube]

He says the closest that Hollywood has come to capturing the nonspaceship weirdness of a notional FTL vehicle is 1997's Contact . If you've seen the movie you know "vehicle" isn't the right term; the character call their FTL device the "machine." It entails both a single-seat spherical pod and the mass of rotating, overlapping rings that the pod (with Jodi Foster inside) drops into.

As for slower-than-light starships, Millis was impressed by the design featured (briefly) in the beginning of James Cameron's Avatar , because of its massive heat radiators, bigger than any ship he's seen on the large or small screen. "If you have excess energy, which is usually in the form of low-grade heat, you need huge radiators to keep the vehicle from destroying itself," he says.

Millis doesn't expect Hollywood to nail the details of space travel. Too much realism could undercut what science fiction does best—inspire new generations of pioneers to tackle problems that can't be solved in their lifetimes. "We need those pioneers. People don't want to start solving a problem till it looks like it can be solved," Millis says. "But if we're dealing with the survival of humanity, do we really want to procrastinate? If it's going to take a couple centuries to figure it out, shouldn't we start now, instead of when the asteroid is spotted, and we have three years to evacuate?"

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What Would FTL Combat Look Like?

Thread starter Friendlysociopath
Start date Jan 11, 2019

Friendlysociopath

Jan 11, 2019

Minus the massive explosion of plasma and the like of other environmental death factors to you and everything around you. Putting those inconvenient factors aside for a moment- what would it look like if say one character performed a series of FTL movements to cut down a man in front of a normal observer? For example: Would the FTL character seem to disappear for the entire sequence and only reappear at the end when they stopped moving? Would the movements and their results (the man cut down) happen and then the observer would see the FTL person move? Would the FTL character seem to not move at all but simply went from one stance to the next while the cut man starts spurting blood for no discernible reason?

Jan 12, 2019

It wouldn't look like anything because by the time you see the other guy, they're literally already gone. Seriously though, it depends entirely on how the FTL works and what you can cram it into. Ships using nBSG style teleports to jump around to evade fire and get into advantageous attack positions will look VERY different from ships trading shots while maneuvering around each other using Trek style warp drives. And of course FTL beam weapons will look very different from FTL missiles. Or hell, FTL missiles using nBSG teleport drives vs Trek warp drives.

I want to kill the lampreys

The poor guy being targeted dies before you actually see the attack. The guy will just suddenly die and then his assailant would mysteriously manifest out of thin air. FTL=Time Travel. Never forget.

Okay, rereading the OP, I suddenly realized he's talking about person on person combat, not combat between starships. So, what would it look like for one man to cut the other down at FTL speed? Instant. Literally, the FTL guy would be moving so fast that the casual HUMAN observer would see no movement at all. The entire maneuver would be done literally before his brain would even register anything at all. Even if moving at "only" light speed, the victim would be dead before the light from his killing reaches the observer's eyes, and looooong before the nerve impulses created by that light (~119 m/s according to Google) reaches the observer's brain. So what the observer would see is that one moment the victim is fine, and the next instant, the victim is toppling over dead due to whatever injuries the attacker did. The attacker himself will have appeared to teleport in that same instant.

Friendlysociopath said: Minus the massive explosion of plasma and the like of other environmental death factors to you and everything around you. Putting those inconvenient factors aside for a moment- what would it look like if say one character performed a series of FTL movements to cut down a man in front of a normal observer? For example: Would the FTL character seem to disappear for the entire sequence and only reappear at the end when they stopped moving? Would the movements and their results (the man cut down) happen and then the observer would see the FTL person move? Would the FTL character seem to not move at all but simply went from one stance to the next while the cut man starts spurting blood for no discernible reason? Click to expand... Click to shrink...

Geomax said: From the perspective of the person being attacked the person would appear instantly and then (assume they can perceive time fast enough) an afterimage would move in reverse to the start location before disappearing. Click to expand... Click to shrink...

Friendlysociopath said: Move in reverse? Click to expand... Click to shrink...

Jan 13, 2019

Most of the combatants explode before anyone can start fighting, pardoxically preventing any of the combatants from going back in time to blow up the enemy combatants as a result of FTL allowing time travel. Alternatively both sides use the their FTL to diplomatically resolve conflicts centuries before they escalate.

Jan 14, 2019

mulazer said: Most of the combatants explode before anyone can start fighting, pardoxically preventing any of the combatants from going back in time to blow up the enemy combatants as a result of FTL allowing time travel. Alternatively both sides use the their FTL to diplomatically resolve conflicts centuries before they escalate. Click to expand... Click to shrink...

Second mover

evilauthor said: OP specifically says that such things aren't happening. The attacker is moving FTL and not creating any of the secondary effects that moving that fast in an atmosphere would cause. Click to expand... Click to shrink...

Has anyone else read Steve White's Prince of Sunset books? They have a fairly good go at a reasonable depiction.

UnwrittenGuy

Jan 15, 2019

if we asume the combatants had some method for detecting each other at light speed for simplictys sake then it would be quite intresting. You would know what your aponent was doing at a given location after the number of light seconds distance from the loctation they were in had passed and have no way of detecting what they are doing other than that. Both combatents would be operating blind and would have to guess there oponents next move baced on what they had already done. The closer they got there would be less lag between seeing there oponent.

Jan 16, 2019

Abrupt explosions from nowhere basically.

Dormammu's got the power every second of the hour

Jan 19, 2019

Realistically speaking. If an object with moves at FTL its mass becomes infinite, and so does the energy required to move it. it would effectively be universe shattering.

Dormammu said: Realistically speaking. If an object with moves at FTL its mass becomes infinite, and so does the energy required to move it. it would effectively be universe shattering. Click to expand... Click to shrink...

UnwrittenGuy said: if we asume the combatants had some method for detecting each other at light speed for simplictys sake then it would be quite intresting. You would know what your aponent was doing at a given location after the number of light seconds distance from the loctation they were in had passed and have no way of detecting what they are doing other than that. Both combatents would be operating blind and would have to guess there oponents next move baced on what they had already done. The closer they got there would be less lag between seeing there oponent. Click to expand... Click to shrink...

Friendlysociopath said: This sound interesting so can you reword that in a way that's easier to understand? It sounds like the farther away they are the harder it will be for them to keep track of one another but the closer they get the easier it will be. Click to expand... Click to shrink...

Bo_Lo Knight

Heliostorm said: No it wouldn't, objects at FTL have imaginary mass, not infinite mass. Click to expand... Click to shrink...

Bo_Lo Knight said: That's for objects already at FTL speeds, objects accelerating to those speeds(either the object itself or those it interacts with) do however have to deal with relativistic mass exponentially rising to infinity. Click to expand... Click to shrink...

Jan 20, 2019

Heliostorm said: You will never get to FTL speeds by accelerating, so the only way it would work is if you just magically jump to FTL speeds. Click to expand... Click to shrink...

Bo_Lo Knight said: But that would require zero interaction with any matter as any atoms in your path would upon contact be accelerated to extremely high relativistic speeds as they are "pushed" aside by your FTL object whereby their R.Mass would result in the damn things ripping what ever is pushing them. Click to expand... Click to shrink...

Heliostorm said: Because there's no classical interpretation of imaginary mass, no one knows what would happen in an FTL collision. But the math suggests that any real-mass objects exchanging momentum with the FTL object would be forced onto an imaginary vector, thereby leaving the universe . Click to expand... Click to shrink...

Jan 21, 2019

Heliostorm said: Because there's no classical interpretation of imaginary mass, no one knows what would happen in an FTL collision. But the math suggests that any real-mass objects exchanging momentum with the FTL object would be forced onto an imaginary vector, thereby leaving the universe. Click to expand... Click to shrink...

Here's What 14 Famous Landmarks Would Look Like If They Were Built In A Different Era

S ome landmarks are so iconic that there's a picture of them in your core memory. but what if that landmark looked different what if it was built in a different time when a distinct architectural style was all the rage well, i used ai to answer that very question, and here are the strange and beautiful results, 1. here's what the eiffel tower in paris looks like in real life:, this is what ai thinks the eiffel tower would look like in a "midcentury modern" architectural style., 2. here's what big ben in london looks like in real life:, here's what ai thinks big ben would look like in a "deconstructivism" architectural style., 3. here's what sagrada familia in spain looks like in real life:, ai thinks sagrada familia would look like this in an art deco architectural style., 4. here's what the golden gate bridge in san francisco looks like in real life:, this is what ai thinks the golden gate bridge would look like in a gothic architectural style., 5. here's what the burj khalifa in dubai looks like in real life:, this is what ai thinks the burj khalifa would look like in a byzantine architectural style., 6. here's what the leaning tower of pisa in italy looks like in real life:, this is what ai thinks the leaning tower of pisa would look like in a california craftsman architectural style., 7. here's what the colosseum in rome looks like in real life:, this is what ai thinks the colosseum would look like in a brutalist architectural style., 8. here's what the space needle in seattle looks like in real life:, here's what ai thinks the space needle would look like in a sustainability architectural style., 9. here's what cinderella's castle in orlando looks like in real life:, here's what ai thinks cinderella's castle would look like in a pueblo architecture style., 10. here's what the sydney opera house in australia looks like in real life:, here's what ai thinks the sydney opera house would look like in a greek architectural style., 11. here's what petra in jordan looks like in real life:, here's what ai thinks petra would look like in a victorian architectural style., 12. here's what the taj mahal in india looks like in real life:, here's what ai thinks the taj mahal would look like in a modernist architectural style., 13. here's what the empire state building in new york city looks like in real life:, here's what ai thinks the empire state building would look like in a tudor architectural style., 14. finally, here's what notre dame cathedral in paris looks like in real life:, and here's what ai thinks notre dame cathedral would look like in a romanesque architectural style., which landmarks did you like which do you think should stay the same sound off in the comments below.

10 safest countries to travel to in 2024!

Apr 12, 2024

Understanding Global Peace Index (GPI)

GPI has ranked the safest and most peaceful countries in the world for 2024. Here's a look at 10 of the safest nations in the world which are based on factors such as low crime rates, political stability, healthcare, and safety:

Topping the chart is Iceland! The country is known for its natural beauty but what makes it safest is the low crime rates, close community, strong social welfare system, and effective law.

New Zealand

New Zealand is on number two in the list. The country is known for its gorgeous natural beauty, friendly people, low crime rates and stable governance.

Ireland, on number three in the list, boasts warm and welcoming people, low crime rates, strong law and peaceful political climate.

Denmark is on the number four spot in the list. The country is famous for its high standard of living, good public services, and low crime rates.

Austria is all about beautiful cities, neat streets, and low crime rates which make it a safe place to explore.

Czech Republic

What makes the Czech Republic a safe nation are the low crime rates, efficient healthcare system and a developed infrastructure.

Singapore is on the number nine spot in the list and is famous for its law-abiding citizens, strict laws, streets that are so clean, low crime rates, and efficient public services.

The number 10 spot is taken by Japan. The country, with its low crime rates, efficient public transportation, and strong sense of community, is one of the safest places in the world.

Thanks For Reading!

Next: Incredible things on Earth that can be seen from space; one is in India

This diagram shows what happens during a total solar eclipse

A total solar eclipse will be visible from Texas to Maine on Monday.
This cosmic event occurs when the Earth, sun, and moon align perfectly.
One diagram shows how a total solar eclipse works, and why it darkens the sky in the middle of the day.

A total solar eclipse will turn afternoon skies dark from Texas to Maine on Monday.

During the eclipse, the moon will cross between the Earth and the sun, completely blocking out the sun's light. If you're in the moon's shadow, the sky will go dark for about three to four minutes, depending on your location.

It's the climax of a cosmic dance between our planet , the moon, and the sun.

What causes a total solar eclipse

During a total solar eclipse, three key conditions happen at the same time: The moon is in the "new moon" phase; the moon crosses the plane of the Earth's orbit ; and the moon is at its closest point to Earth in its orbit.

When those conditions are just right, the Earth, sun, and moon line up. This diagram shows how that looks:

Then, if you're in the path of totality — which is basically the center of the moon's shadow, called the umbra — the moon appears to obscure the sun.

If you're in the penumbra — the outer region of the moon's shadow — you'll see a partial solar eclipse , where the moon appears to partially overlap the sun.

A total solar eclipse happens somewhere on Earth about every 18 months on average. It's rare for one to occur in any single place, though, because of the complex movements of the Earth and moon.

The moon orbits Earth every 29.5 days, while Earth has its own orbit around the sun. The moon's orbit is tilted about five degrees, which is large enough to keep its shadow off the Earth and the Earth's shadow off the moon most of the time.

There are two points — called nodes — where the moon's orbit crosses the Earth's plane. In the diagram above, the moon is lined up on a node.

What the total solar eclipse will look like

In the path of totality on Monday, where the moon's umbra falls over Earth, the total solar eclipse will have 10 distinct phases , each with different amounts of the sun visible from the ground.

The phenomenon kicks off with what's called first contact, when the moon starts to pass across the sun. After about an hour, the moon will almost completely mask the sun, and you'll start to see a bright light radiate out of the sliver of remaining sun, known as the "diamond ring."

Then the moon will fully eclipse the sun, turning the sky dark in the middle of the day.

During totality only the sun's outermost atmosphere, called the corona, will be visible glowing around the dark disc of the moon.

After that, the moon will continue to travel across the sky to form another crescent. The eclipse ends when the moon ceases to cover the sun.

Types of solar eclipses

There are three types of solar eclipses .

Total solar eclipses, like this one, occur when the moon appears to completely cover the sun. If the moon only somewhat covers the sun, that's a partial eclipse . Many people who are near the path of totality, but not in it, on Monday will see a partial eclipse.

The third type, an annular eclipse , occurs when the moon is too far from Earth to fully block out the sun from our perspective. The outer edge of the sun remains visible as a bright ring around the moon.

A total solar eclipse is considered the most spectacular. Globally, only about a third of all solar eclipses are total.

The next total solar eclipse in the contiguous US will be in 2044.

How to watch the eclipse

If you plan to watch the eclipse, make sure you are wearing ISO-certified eclipse glasses . These are 1,000 times darker than regular sunglasses. Without them, staring at the sun could damage your eyes.

The only safe time to look at the eclipse without glasses is during totality.

Leanna Garfield and Anaele Pelisson contributed to an earlier version of this post .

Watch: Why the sun has two giant holes, and what that means for Earth

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Solar eclipse 2024: Follow the path of totality

Solar eclipse, what you need to know to watch monday's total solar eclipse.

The NPR Network

A stunning celestial event is visible across the country Monday, when the moon crosses directly in front of the sun: a total solar eclipse. For those in the path of totality, there will be a few brief moments when the moon completely covers the sun and the world becomes dark.

Traveling for totality? Skip ahead.

This will be the last chance to catch a total solar eclipse in the continental U.S. for about 20 years, so here's what you need to know to safely enjoy!

When is the eclipse?

April 8, 2024 there will be a total solar eclipse that crosses from the Pacific coast of Mexico through the United States.

What is totality and why it matters

According to NASA , totality will start around 11:07 a.m. PDT/1:07 EDT in Mexico and leave Maine at around 1:30 pm PDT/3:30 pm EDT.

Here's what time the eclipse will be visible in your region

Check out this table for when the partial eclipse and totality are visible in each region or check by zip code here.

A partial solar eclipse will be visible across the contiguous United States, so even if you're not directly in the path, you should be able to see something special, weather permitting.

Unable to get to totality? We'll be sharing highlights here from across the NPR Network throughout the day Monday if you can't see it in real time.

Where to see totality?

More than 30 million people live in the path of totality for Monday's eclipse, and many more in nearby areas.

Here's what we know about Monday's weather forecast.

Why totality matters

As NPR's Neil Greenfieldboyce explains , "During a total eclipse, the sky darkens suddenly and dramatically . The temperature drops. Stars come out. Beautiful colors appear around the horizon. And the once-familiar sun becomes a black void in the sky surrounded by the glowing corona — that's the ghostly white ring that is the sun's atmosphere."

Eclipse Science

For april's eclipse, going from 'meh' to 'omg' might mean just driving across town.

A partial eclipse, while still a fun experience, is hardly as dramatic. Those with a view of the partial eclipse will see crescent-shaped shadows like those seen here in 2017.

How to watch safely

If you plan to look directly at the eclipse (partial or totality), you're going to need eclipse glasses handy because looking directly at the sun without proper protection ( traditional sunglasses don't count! ) can be harmful to your eyes.

The perfect celestial soundtrack to the total solar eclipse

As NPR's Joe Hernandez explains, "Proper eye protection must be worn throughout a total solar eclipse — except for the roughly 3 1/2 to 4 minutes when the moon fully obscures the sun, a brief period known as 'totality.' (You will need to take your glasses off during totality to actually see it.)"

If you don't have access to eclipse glasses, you can get crafty with things you have around the house ( like some of us did back in 2017!) More on that here.

Traveling for totality?

The celestial event is driving a ton of domestic travel to the path of totality. If you're headed out of town to view the eclipse, here are some NPR Network resources for areas in the path of totality:

Texas The path of totality crosses through the Lone Star State, with some areas expecting a possible influx of visitors in the hundreds of thousands to catch prime viewing. Our member stations across the state have gathered local resources to help you navigate the region and the eclipse!

San Antonio: Check out the latest from Texas Public Radio
Dallas: Explore KERA's coverage for the latest
Austin: Head to KUT for the best local resources

Arkansas The eclipse will be cutting through the state, putting Little Rock in the path of totality. Check out Little Rock Public Radio for local resources.

The southwestern edge of the state will be well-positioned to witness the total solar eclipse this year. Kentucky Public Radio is covering the eclipse throughout the region, from Kentuckiana eclipse mania to the University of Louisville's free class about the celestial event. Keep an eye on WKMS for the latest local updates.

Missouri The southeastern corner of the state will be in the path of totality, crossing across towns like Whitewater and Ste. Genevieve. Head to St. Louis Public Radio for local coverage and resources. Illinois Carbondale seems to have won the eclipse lottery, being in the path of totality both in 2017 and for this year's eclipse . For resources from across the state, check out Illinois Public Media .

Indiana A huge portion of the state will be within the path of totality, giving cities across Indiana, including Bloomington and Indianapolis, prime viewing of the eclipse.

Bloomington: Check out Indiana Public Media
Indianapolis: Head to WFYI for the latest
Fort Wayne: Just north of the path of totality, WBOI has resources for the Allen County area

Ohio The Buckeye State is getting bisected by this year's path of totality, plunging a number of the state's most populous areas into darkness for a few minutes on Monday.

Cleveland: Head to Ideastream Public Media for the latest.
Columbus: With the capital city just south of totality, head to WOSU for regional resources.
Cincinnati: Totality will just miss the border town. Here are some tips from WVXU on how to navigate the eclipse in the region.

Pennsylvania Only the northwestern-most corner of the state will catch totality, with views from the lakeside in Erie being particularly well-positioned for a stunning viewing experience. WESA has more from across the region.

Plan to watch the eclipse from a wild mountain summit? Be ready for harsh conditions

New York Buffalo, Rochester, Syracuse and Plattsburgh will fall under the path of totality on Monday. If you're planning to travel to the region for the best views, here are some local resources to stay safe and informed:

Buffalo: Head to WBFO for the latest
Syracuse: WAER has more on plans in the Salt City
North Country: NCPR has the latest from across the region, as well as information on local viewing events to check out

Vermont The Green Mountain State will see totality across its most populous region, including Burlington and Montpelier, as well as the Northeast Kingdom on the Canadian border. Vermont Public has everything you need to know to navigate your time in the region to enjoy the eclipse safely. New Hampshire The northernmost region of the Granite State will be in the path of totality, providing prime viewing to those in Coos County. NHPR has info on local events, travel updates as well as special coverage with New Hampshire Public Television. Maine The last state in the path of totality in the U.S., much of Northern Maine will be positioned for prime viewing. The rural region is preparing for an influx of visitors, and safety officials are encouraging visitors and locals alike to be prepared. Maine Public will be covering the eclipse and has everything you need to know to navigate the region safely.

How to document the eclipse safely

With the ease of cell photography , it can be tempting to reach for your phone to document the eclipse and the moments of totality, but make sure to do so safely.

As NPR's Scott Neuman explains , "For starters, you'll need to wear eclipse glasses or similar protective eye gear while aiming your camera or even just observing the eclipse."

Feeling ambitious? Here are a few more tips.

Or if you're not inclined to capture the moment visually, you lean into some other forms of creative expression. Indiana, for example, has named Linda Neal Reising the official poet in the state for this year's eclipse.

As former NPR reporter and eclipse superfan David Baron shared with Life Kit , viewing totality "[is] like you've left the solar system and are looking back from some other world."

So consider focusing on being present in the moment to enjoy the celestial spectacle.

More resources to enjoy the eclipse

Sharing the eclipse with tiny humans? Check out these kid-friendly total solar eclipse learning guides from Vermont Public's But Why, and this great explainer from KERA Kids on the difference between a solar and a lunar eclipse.
Want to see how a solar eclipse alters colors? Wear red and green on Monday
Plan to wander into the wild for the best view? Here are some tips from outdoor experts.
Tips from Bill Nye on the best ways to enjoy the eclipse.

NPR will be sharing highlights here from across the NPR Network throughout the day Monday if you're unable to get out and see it in real time. NPR's Emily Alfin Johnson compiled these resources.

2024 eclipse

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COMMENTS

Warp Speed: What Hyperspace Would Really Look Like
The science fiction vision of stars flashing by as streaks when spaceships travel faster than light isn't what the scene would actually look like, a team of physics students says.
What would faster-than-light (hyperspace) travel look like?
Piece of junk or not, the Millennium Falcon looks a lot different when you engage the warp drive.
5 Faster-Than-Light Travel Methods and Their Plausibility
Let's take a look at five means of FTL found in sci-fi that don't break the rules of relativity and examine how plausible they are based on the science behind them. 1. Hyperdrive
New theoretical warp drive design clears "negative energy" barrier
Faster-than-light (FTL) travel is a staple of sci-fi, hand-waving away multi-millennia journeys between stars. Such a technology would of course be incredibly handy to us in the real world, and ...
This NASA Animation Shows What It's Really Like to Travel Close to The
Watch on. For the sake of this video, titled "NASA's Guide to Near-light-speed Travel" (shown above), it is assumed that the interstellar traveler (who appears to be an alien creature) has built a spacecraft that is capable at traveling at 90 percent the speed of light (0.9 c). The video is presented as an information video for an interstellar ...
Scientists Are Starting to Take Warp Drives Seriously ...
In layman's terms, the Alcubierre Drive achieves FTL travel by stretching the fabric of space-time in a wave, causing the space ahead of it to contract while the space behind it expands. In theory, a spacecraft inside this wave would be able to ride this "warp bubble" and achieve velocities beyond the speed of light.
Spacecraft in a 'warp bubble' could travel faster than light, claims
Albert Einstein's special theory of relativity famously dictates that no known object can travel faster than the speed of light in vacuum, which is 299,792 km/s. This speed limit makes it unlikely that humans will ever be able to send spacecraft to explore beyond our local area of the Milky Way. However, new research by Erik Lentz at the ...
The Impossible Physics of Faster-Than-Light Travel
From the Earth, it looks like one ship leaves and travels a good distance before another ship moving four times as fast overtakes it. From the perspective of the slower ship, things look a little ...
Faster-than-light
Faster-than-light ( superluminal or supercausal) travel and communication are the conjectural propagation of matter or information faster than the speed of light ( c ). The special theory of relativity implies that only particles with zero rest mass (i.e., photons) may travel at the speed of light, and that nothing may travel faster.
What Would It Be Like to Travel Faster than the Speed of Light?
Similarly, he said, "If something were traveling faster than the speed of light, such as an airplane made of neutrinos, you wouldn't see it until after it had gone past you. Any light it emitted ...
Nasa publishes faster-than-light spaceship design to imagine
A zoomed out illustration of the faster-than-light craft. The donut-like bands are what would be used to create the warp bubble in which it would travel. Credit: Marl Rademaker
What would a warp-drive ship actually look like?
The idea comes from the work published by Miguel Alcubierre in 1994. His version of a warp drive is based on the observation that, though light can only travel at a maximum speed of 186,000 miles ...
What If You Traveled Faster Than the Speed of Light?
As an object approaches the speed of light, its mass rises precipitously. If an object tries to travel 186,000 miles per second, its mass becomes infinite, and so does the energy required to move it. For this reason, no normal object can travel as fast or faster than the speed of light. That answers our question, but let's have a little fun and ...
How Humans Could Go Interstellar, Without Warp Drive
The field equations of Einstein's General Relativity theory say that faster-than-light (FTL) travel is possible, so a handful of researchers are working to see whether a Star Trek-style warp drive, or perhaps a kind of artificial wormhole, could be created through our technology. But even if shown feasible tomorrow, it's possible that designs for an FTL system could be as far ahead of a ...
What Does a Faster-Than-Light Object Look Like?
Both space and time look different to the cat, due to her high speed. Equally spaced position markers according to the cat have to tip to the right, parallel to the cat's trajectory, while the cat ...
Why FTL implies time travel
And relativity is true, because we all measure light to travel at the same speed (also, you need relativity for electromagnetism to work, which you probably do want). If you could travel or communicate FTL, you can time travel, or at least communicate backwards in time. And that would be troubling - and doesn't seem to be the Universe we live ...
outside observers on object traveling FTL: what COULD they see?
I dont think its a super well defined question to ask what something going ftl would "look" like. A traveller moving faster than light would have imaginary redshift, so from your perspective the ship would be impossible to visually describe. The entire concept of "light" sort of falls apart.
Relativity and FTL Travel: Part I
This is Part I of the "Relativity and FTL Travel" FAQ. It contains basic information about the theory of special relativity. ... its "observed mass" approaches infinity. However, this does not mean that the object will eventually look like a black hole predicted by general relativity (as it would if the same object really did have a huge mass ...
How would FTL travel appear through window of ship?
How would faster-than-light travel appear through window of a space vessel? All the movies and TV shows like Star Trek and Star Wars seem unrealistic: ... It would probably be very subtle to the naked eye as stars mostly look like white dots and as the higher frequency light was shifted beyond the visible spectrum, infra-red light would be ...
Is Faster-Than-Light Travel or Communication Possible?
1. Cherenkov Effect. One way to go faster than light is to make the light slow down! Light in vacuum travels at a speed c which is a universal constant (see the FAQ entry Is the speed of light constant? ), but in a dense medium such as water or glass, light slows down to c/n where n is the refractive index of the medium (1.0003 for air, 1.4 for ...
What Would a Starship Actually Look Like?
In other words: The vessel look like just about anything, so there's no reason to assume it will look like something that flies. The FTL starship is more of a time machine than a rocket, a device ...
UCSB Science Line
This spares us from worrying about all the paradoxes (such as time-travel and all the sci-fi related paradoxes) if things can travel above the speed of light. Answer 3: So far as we know in the practical sense, time travel is impossible, and so is faster-than-light (FTL) travel. In fact, under our current theories, time travel and FTL travel ...
What Would FTL Combat Look Like?
With an FTL object the light from its closest approach reaches you first, with light from its more distant points coming later. So if you're in its direction of travel, you would see the object suddenly appear before you, then an image of it go backwards along its path to its start point, as the light catches up.
Here's What The Eclipse Will Look Like In 'The Path Of Nope'
Your local forecast, plus daily trivia, stunning photos and our meteorologists' top picks. All in one place, every weekday morning. The eclipse's so-called "path of nope" is still really ...
Here's What 14 Famous Landmarks Would Look Like If They Were ...
Here's what AI thinks the Taj Mahal would look like in a modernist architectural style. 13.Here's what the Empire State Building in New York City looks like in real life: Here's what AI thinks the ...
10 safest countries to travel to in 2024!
Here's a look at 10 of the safest nations in the world which are based on factors such as low crime rates, political stability, healthcare, and safety: 10 safest countries to travel to in 2024!
Diagram Shows What Happens During Solar Eclipse
During a total solar eclipse, three key conditions happen at the same time: The moon is in the "new moon" phase; the moon crosses the plane of the Earth's orbit; and the moon is at its closest ...
What you need to know to watch Monday's total solar eclipse
Over 30 million people will be within the path of totality for Monday's solar eclipse as it crosses the U.S. from Texas to Maine. Here's what you need to know to safely enjoy the celestial spectacle.
2024 solar eclipse: What time is it and what will the weather be?
People across North America will be treated to the view of a lifetime later on Monday when a total solar eclipse spans across the continent, from Mexico to the very eastern tip of Canada. Hundreds ...