faster than light travel in water

Tachyons: Facts about these faster-than-light particles

Tachyons are not just the stuff of science fiction.

What is a tachyon?

Tachyons and time travel, tachyons paradoxes, tachyons. could we ever detect them, the power of imagination in science, additional information.

Traveling faster than light and time-travel could be real for tachyons. If one thing science fiction excels at, it's allowing us to marvel at the breaking of the physical laws of the universe. We watch and read in wonder as the warp engines of the starship Enterprise push it to beyond the speed of light, or as Barry or Wally  —  whoever is carrying the name of the Flash at the time  —  does the same in no more than a pair of yellow boots. 

Likewise, we enjoy tales of adventurers like the Doctor, or Doc Brown, using weird seemingly antiquated machinery to violate the laws of causality. What if there was a fundamental particle that could do all these things? Moving faster than light like the Flash, and traveling back through time without the need for a TARDIS or a Delorian or yellow boots. 

That’s a tachyon. But make no mistake, these particles aren’t just the idling's of science fiction writers. Tachyons are the stuff of "hard" science. 

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

• Faster-Than-Light Travel Could Explain Mysterious Signals Beaming Through the Cosmos
• Final Nail? Faster-Than-Light Neutrinos Aren't, Scientists Conclude

Tachyons are one of the most interesting elements arising from Einstein’s theory of special relativity . The 1905 theory is based on two postulates, nothing with mass moves faster than the speed of light ( c ), and physical laws remain the same in all non-inertial reference frames. A significant consequence of special relativity is the fact that space and time are united into a single entity; spacetime. That mean’s a particle’s journey through speed is linked to its journey through time. 

The term "tachyon" first entered scientific literature in 1967, in a paper entitled " Possibility of faster-than-light particles " by Columbia University physicist Gerald Feinberg. Feinberg posited that tachyonic particles would arise from a quantum field with “imaginary mass” explaining why the first populate of special relativity doesn’t restrain their velocity.

This would lead to two types of particles existing in the universe ; bradyons that travel slower than light and compose all the matter we see around us, and tachyons traveling faster than light, according to the University of Pittsburgh . One of the key differences between these particle types is as energy is added to bradyons, they speed up. But, with tachyons, as energy is taken away, their speed increases. 

One of the most important and meaningful results from Einstein’s theory of special relativity is the establishing universal speed limit of c ; the speed of light in a vacuum. 

Einstein suggested that as an object approaches c its mass becomes near-infinite, as does the energy required to accelerate it. This should mean that nothing can travel faster than light. But, imagine an anti-mass particle like a tachyon, its lowest energy state would see it speeding at c . But, why would this lead to backward time travel?

That all hinges on the concept that puts the "relative" into "special relativity."

A common tool used to explain special relativity is the spacetime diagram. 

Spacetime is filled with events ranging from the cosmically powerful and violent, like the supernova explosion of a distant star, or the mundane, such as the cracking of an egg on your kitchen floor. And these are mapped onto the spacetime diagram. This diagram shows as a particle whizzes through spacetime, it traces out a worldline that maps its progress.

Also filling spacetime are observers, each of whom has their own reference frame. These observers may see the events that fill spacetime occurring in different orders. Observer 1 may see event A, the supernova, occur before event B the egg crack. Observer 2 however may see event B happening before event A. 

Each event has a light cone associated with it. If event B falls within the lightcone of event A then the two could be causally linked. The supernova could have knocked the egg off the kitchen counter  —  or maybe the falling breakfast item caused the complete gravitational collapse of a dying star, somehow. That’s because in the light cone a signal traveling slower than light can link the events. The edges of the light cone represent the speed of light. Linking an event outside the light cone with one inside it requires a signal that travels faster than light.

If event A is in the light cone and event B is outside it, then the supernova and egg-related tragedy can't be causally related. But, a tachyon traveling at a speed greater than the speed of light could violate causality by linking these events. 

To see why this is a problem, consider it like this. Image event A is the sending of a signal, and event B is the receiving of that signal. If that signal is traveling at the speed of light, or slower all observers in different reference frames agree that A preceded B.

But, if that signal is carried by a tachyon and thus moves faster than light, there will be reference frames that say the signal was received before it was sent. Thus, to an observer in this frame, the tachyon traveled backward in time.

One of the fundamental postulates of special relativity is that the laws of physics should be the same in all non-accelerating reference frames. That means if tachyons can violate causality and move backward in time in one reference frame, it can do it in them all.  

To see how this leads to problems called paradoxes, consider two observers, Stella aboard a spacecraft orbiting Earth, and Terra based on the surface of the planet. The two are communicating by sending messages with tachyons. 

This means that if Stella sends a signal to Terra which moves faster than light in Stella’s frame but backward in time in Terra’s frame. Terra then sends a reply as ordered which moves faster than light in her frame but backward in time in Stella’s frame, Stella could receive the reply before sending the original signal. 

What if this response signal from Terra says "do not send any signals"? Then Stella does not send the original signal, and Terra then has nothing to respond to and never sends the tachyon signal that says "don’t send any signals." 

So not only do tachyons violate causality in every frame they open the door to severe logical paradoxes.

There are suggestions as to how these paradoxes could be avoided. Of course, the most simple solution is that tachyons don’t exist. 

A less draconian suggestion is that observers in different reference frames can’t tell the difference between the emission and absorption of tachyons. 

That means a tachyon traveling back in time could always be interpreted as a tachyon moving forward in time because receiving a tachyon from the future always creates the same tachyon and sends it forwards in time.

Another suggestion is that tachyons aren’t like any other particle we know of, in that they don't interact and can never be detected or observed. Meaning that the tachyon communication system used by Stella and Terra in the above example can’t exist. 

Along similar lines, other researchers say that tachyons can’t be controlled. The receipt and emission of tachyons just happen at random. Thus, there’s no way to send a tachyon with a causality violating message. 

Aside from the fact that like other particles, they are likely incomprehensibly tiny, because tachyons always travel faster than light it isn’t possible to detect one on its approach. That’s because it’s moving faster than any associated photons. 

After it passes, an observer would see the image of the tachyon split into two distinct images. These would show it simultaneously arriving in one direction and disappearing in the opposite direction.

If detecting tachyons, at least of their approach, with light is out of the picture, is there another way we could detect these faster than light particles?

Possibly. Tachyons are proposed to have an "anti-mass" but this still constitutes mass energy. That means these particles should still have some gravitational effect. It’s possible highly sensitive detectors could spot this effect.

An alternative detection method could arise from their faster-than-light nature.

While the speed of light in a vacuum c is a universal speed limit, particles have been made to travel faster than light in other mediums. When electrically charged particles are accelerated up to and beyond the speed of light in certain mediums like water, they release a form of radiation called Cherenkov radiation, according to the International Atomic Energy Agency .

That means that if tachyons are electrically charged, one way of detecting them would be measuring Cherenkov radiation in the near-vacuum of space. 

What tachyons really demonstrate is the importance of imagination in our ongoing quest to understand the universe. They may not exist, and if they do we may have no hope of ever measuring one.

But what our technology can’t capture, our minds can. We can consider the possibility of a particle that journeys back through time and what that says about the nature of time, and the Universe, and the events that fill them.

In an interview with George Sylvester Viereck published in " The Saturday Evening Post " in 1929, Albert Einstein is believed to have said: "Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world."

•  Discover more about Tachyons with this informative YouTube video . 
•  Explore the possible experimental evidence for the existence of tachyons with George Mason University .  
•  Find out how Cherenkov radiation works with this video from Fermi Lab .  

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

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Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

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• Quantum Mechanics

When objects travel faster than the speed of sound, they produce a sonic boom. So, in theory, if something travels quicker than the speed of light, it should yield something like a “luminal boom.”

In fact, this light boom occurs on a daily basis in facilities around the world — you can see it with your own eyes. It’s called Cherenkov radiation, and it shows up as a blue glow inside of nuclear reactors, like in the Advanced Test Reactor at the Idaho National Laboratory in the image to the right. Cherenkov radiation is named for Soviet researcher Pavel Alekseyevich Cherenkov, who first measured it in 1934 and was awarded the Nobel Physics Prize in 1958 for his discovery.

Cherenkov radiation glows as the core of the Advanced Test Reactor is submerged in water to keep it cool. In water, light travels at 75 % the speed it would in the vacuum of outer space, but the electrons produced by the reaction inside of the core travel through the water faster than the light does.

Particles, like these electrons, that exceed the speed of light in water, or some other medium such as glass, create a shock wave like to the shock wave from a sonic boom. When a rocket, for example, travels through air, it produces pressure waves in front that move away from it at the speed of sound, and the nearer the rocket reaches that sound barrier, the less time the waves have to move out of the object’s path. Once it extents to the speed of sound, the waves bunch up generating a shock front that forms a loud sonic boom.

Likewise, when electrons travel through water at speeds faster than light speed in water, they produce a shock wave of light that sometimes shines as blue light, but can also shine in ultraviolet. While these particles are roaming faster than light does in water, they’re not really breaking the cosmic speed limit of 670,616,629 miles per hour.

When the rules don’t apply

A 3D map of the cosmic web at a distance of 10.8 billion light years from Earth. Keep in mind that Einstein’s Special Theory of Relativity states that nothing with mass can go faster than the speed of light, and as far as physicists can tell, the cosmos follow by that rule. But what about something without mass?

Photons, by their very nature, cannot surpass the speed of light, but particles of light are not the only massless entity in the cosmos. Empty space comprises of no material substance and consequently, by definition, has no mass.

“Since nothing is just empty space or vacuum, it can enlarge faster than light speed since no material object is breaking the light barrier,” said theoretical astrophysicist Michio Kaku on Big Think. “Therefore, empty space can surely expand faster than light.”

This is precisely what physicists think occurred instantaneously after the Big Bang during the epoch called inflation, which was first hypothesized by physicists Alan Guth and Andrei Linde in the 1980s. Within a trillionth of a trillionth of a second, the cosmos constantly doubled in size and as a result, the outer edge of the cosmos expanded very quickly, much faster than the speed of light.

Quantum entanglement makes the cut

Quantum entanglement sounds complex and scary but at a rudimentary level entanglement is just the way subatomic particles communicate with each other.

“If I have two electrons close together, they can vibrate in unison, according to the quantum theory,” Kaku explains on Big Think.

Now, isolate those two electrons so that they’re hundreds or even thousands of light years apart, and they will keep this instant communication bridge open.

“If I jiggle one electron, the other electron ‘senses’ this vibration instantaneously, faster than the speed of light. Einstein believed that this therefore negated the quantum theory, since nothing can go faster than light,” Kaku wrote.

In fact, in 1935, Einstein, Boris Podolsky and Nathan Rosen, tried to disprove quantum theory with a thought experiment on what Einstein referred to as “spooky actions at a distance.”

Ironically, their paper laid the foundation for what today is called the EPR (Einstein-Podolsky-Rosen) paradox, a paradox that defines this immediate communication of quantum entanglement — an integral part of some of the world’s most cutting-edge technologies, like quantum cryptography.

Dreaming of wormholes

Since nothing with mass can travel faster than light, you can kiss interstellar travel goodbye — at least, in the classical sense of rocket ships and flying.

However, Einstein trampled over our aspirations of deep-space road trips with his Theory of Special Relativity, he gave us a new hope for intergalactic travel with his General Theory of Relativity in 1915. While Special Relativity wed mass and energy, General Relativity knitted space and time together.

“The only viable way of breaking the light barrier may be through General Relativity and the warping of space time,” Kaku writes.

This warping is what we colloquially call a “wormhole,” which theoretically would let something travel vast distances rapidly, fundamentally enabling us to break the cosmic speed limit by travelling unlimited distances in a very short amount of time.

In 1988, theoretical physicist Kip Thorne — the science consultant and executive producer for the recent film “Interstellar” — used Einstein’s equations of General Relativity to forecast the likelihood of wormholes that would forever be open for space travel. But in order to be traversable, these wormholes need some strange, exotic matter holding them open.

“Now it is an amazing fact that exotic matter can exist, thanks to weirdnesses in the laws of quantum physics,” Thorne writes in his book “The Science of Interstellar.”

And this exotic matter has even been prepared in laboratories here on Earth, but in very tiny amounts. When Thorne proposed his theory of stable wormholes in 1988 he called upon the physics community to help him define if enough exotic matter could exist in the cosmos to support the likelihood of a wormhole.

“This generated a lot of research by a lot of physicists; but today, nearly thirty years later, the answer is still unknown.” Thorne writes. At the moment, it’s not looking good, “But we are still far from a final answer,” he concludes.

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How Fast is the Speed of Light?

With our current understanding of motion, it seems that the speed of light is the highest of all, being 874,030 times faster than the speed of sound.

The speed of sound travels at around 343 m/s, while the speed of light travels at 299,792,458 m/s. In miles per hour/mph, the speed of light is at around 670,616,629, while in kilometers per hour, light travels at 1,079,252,848.

In terms of seconds, light travels at around 300,000 kilometers per second or 186,000 miles per second in a vacuum.

In water, the speed of light is slower, at 225,000 km / 139,808 mi per second, and 200,000 km / 124,274 mi per second in glass. It seems that nothing can be faster than the speed of light.

If you want an example of how fast the speed of light is, think about this, if we were to launch an imaginary spacecraft from Earth that would travel at around 153,454 mi / 246,960 km per hour constantly, it would reach the Sun in 606 hours, or 25 days. 

However, if our spacecraft would be traveling at the speed of light, we would reach the Sun in only 8.3 minutes. If you traveled around the Earth with the speed of light, you would make a complete tour of our planet 7.5 times in just one second.

In theory, it seems that nothing is faster than the speed of light, or is there? Let’s find out.

Is There Anything Faster Than the Speed of Light?

It appears that nothing is faster than the speed of light, but the Universe , as always, eludes our perception once again. Scientists have demonstrated that the Universe is expanding, and this expansion is even faster than the speed of light.

Since space is theoretically “nothing,” it isn’t susceptible to the laws of physics. If you were to hold a torch and run with it, the speed of its light would still travel at the same rate.

Some galaxies are moving away from our Milky Way faster than the speed of light, and this is happening because space itself is moving along with them.

If there were something more efficient than traveling with the speed of light, it would be traveling through wormholes. Wormholes are hypothetical, but their mechanism is quite intriguing, and in a way, if it were possible, they are supposedly faster than the speed of light.

This is because a wormhole connects two distant points, and, in theory, if you were to travel from point a to b, regardless of its distance, you would reach your destination extremely fast.

How Fast is the Speed of Dark?

Many consider that the speed of darkness is simply a poetic metaphor and wouldn’t have any legitimate scientific basis, since dark is simply the absence of light.

However, this may seem a bit more complicated. If we were to put a dark spot in a beam of light, darkness would theoretically move at the same speed as light.

The same holds true if we would illuminate a dark corner. It is uncertain if darkness itself has a speed, but when it comes to dark matter, things start to unfold.

Dark matter is hypothetical energy, which makes up more than 80% of our Universe. In some studies, scientists estimated that this mysterious element might travel at around 54 m/s, to equate for its existence, but this is quite slow when compared to the speed of light.

Things get complicated if we look at black holes as part of the definition of darkness. Black holes are devoid of light, and if anything gets near their event horizon, not even light can escape from them.

Some black holes are fast-spinner as well, with some of them being recorded with having a spinning speed of around 84% of the speed of light. Darkness or the speed of dark is quite a fascinating subject, but it remains elusive to our current understanding.

What is the Fastest Thing in the Universe?

The fastest thing in the Universe, based on our current knowledge, is light. If you want to play dirty, you could say that the Universe/space is the fastest thing in existence, since it expands with a speed even faster than the speed of light.

If, in the future, we will understand how black holes can capture even light, maybe some of their mechanisms are the fastest thing in the Universe.

What Would Happen if You Would Travel Faster Than the Speed of Light?

The theory of special relativity states that nothing should travel faster than the speed of light, and if something does so, it will move backward in time.

Traveling faster than the speed of light might simply mean time travel. However, is this were true, in some ways, you might as well achieve immortality, as no cause could affect you, not even time, especially if, hypothetically speaking, you wouldn’t even be subjected to the impacts of the objects you would travel through.

Our current understanding of light speed is minimal, and even more so when it comes to surpassing it. We, as a species, with our current technology, have only just reached small percentages of the speed of light. We aren’t even halfway there.

What is the 2 nd Fastest Thing in the Universe?

Blobs of hot gas embedded in streams of material ejected from blazars, which are highly active galaxies , travel at around 99.9% of the speed of light.

Thus, the physical processes that occur at the cores of blazars are so energetic that they can propel matter quite close to light speed, and as such, they are probably the second fastest thing in the Universe. 

Did you know?

The fastest speed reached by a land vehicle is the ThrustSSC supersonic car. This vehicle reached 1,227 km / 772 mi/h, and it maintains its title as the most rapid land vehicle since 1997.

The fastest plane/aircraft in the world is the Lockheed SR-71 Black Bird. It achieved this title in 1976, and it reached a speed of 3,529.6 km/ 2,192 mi per hour.

The Parker Solar Probe is currently the fastest spacecraft ever designed by man. It reached 153,454 miles / 246,960 kilometers per hour.

Image Sources:

• https://images.immediate.co.uk/production/volatile/sites/4/2018/08/GettyImages-524396835-bca79f7.jpg?quality=90&resize=960%2C408
• https://cdn.britannica.com/s:800×450,c:crop/83/179683-138-D3C80B7C/Scientists-speed-of-light.jpg
• https://cdn.hswstatic.com/gif/speed-of-darkness-orig.jpg
• https://4.bp.blogspot.com/-7qK2eGoouKI/TprUhQT6hcI/AAAAAAAAEww/BHe3uF_S-rU/s1600/Time-travel-through-a-wormhole-thumb-550xauto-38205.jpg
• https://i.insider.com/5b47524e744a9820008b4838?width=1100&format=jpeg&auto=webp

Speed of Light Calculator

Table of contents

With this speed of light calculator, we aim to help you calculate the distance light can travel in a fixed time . As the speed of light is the fastest speed in the universe, it would be fascinating to know just how far it can travel in a short amount of time.

We have written this article to help you understand what the speed of light is , how fast the speed of light is , and how to calculate the speed of light . We will also demonstrate some examples to help you understand the computation of the speed of light.

What is the speed of light? How fast is the speed of light?

The speed of light is scientifically proven to be the universe's maximum speed. This means no matter how hard you try, you can never exceed this speed in this universe. Hence, there are also some theories on getting into another universe by breaking this limit. You can understand this more using our speed calculator and distance calculator .

So, how fast is the speed of light? The speed of light is 299,792,458 m/s in a vacuum. The speed of light in mph is 670,616,629 mph . With this speed, one can go around the globe more than 400,000 times in a minute!

One thing to note is that the speed of light slows down when it goes through different mediums. Light travels faster in air than in water, for instance. This phenomenon causes the refraction of light.

Now, let's look at how to calculate the speed of light.

How to calculate the speed of light?

As the speed of light is constant, calculating the speed of light usually falls on calculating the distance that light can travel in a certain time period. Hence, let's have a look at the following example:

• Source: Light
• Speed of light: 299,792,458 m/s
• Time traveled: 100 seconds

You can perform the calculation in three steps:

Determine the speed of light.

As mentioned, the speed of light is the fastest speed in the universe, and it is always a constant in a vacuum. Hence, the speed of light is 299,792,458 m/s .

Determine the time that the light has traveled.

The next step is to know how much time the light has traveled. Unlike looking at the speed of a sports car or a train, the speed of light is extremely fast, so the time interval that we look at is usually measured in seconds instead of minutes and hours. You can use our time lapse calculator to help you with this calculation.

For this example, the time that the light has traveled is 100 seconds .

Calculate the distance that the light has traveled.

The final step is to calculate the total distance that the light has traveled within the time . You can calculate this answer using the speed of light formula:

distance = speed of light × time

Thus, the distance that the light can travel in 100 seconds is 299,792,458 m/s × 100 seconds = 29,979,245,800 m

What is the speed of light in mph when it is in a vacuum?

The speed of light in a vacuum is 670,616,629 mph . This is equivalent to 299,792,458 m/s or 1,079,252,849 km/h. This is the fastest speed in the universe.

Is the speed of light always constant?

Yes , the speed of light is always constant for a given medium. The speed of light changes when going through different mediums. For example, light travels slower in water than in air.

How can I calculate the speed of light?

You can calculate the speed of light in three steps:

Determine the distance the light has traveled.

Apply the speed of light formula :

speed of light = distance / time

How far can the speed of light travel in 1 minute?

Light can travel 17,987,547,480 m in 1 minute . This means that light can travel around the earth more than 448 times in a minute.

Speed of light

The speed of light in the medium. In a vacuum, the speed of light is 299,792,458 m/s.

[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?"

Can anything travel faster than the speed of light?

Does it matter if it's in a vacuum?

In 1676, by studying the motion of Jupiter's moon Io, Danish astronomer Ole Rømer calculated that light travels at a finite speed. Two years later, building on data gathered by Rømer, Dutch mathematician and scientist Christiaan Huygens became the first person to attempt to determine the actual speed of light, according to the American Museum of Natural History in New York City. Huygens came up with a figure of 131,000 miles per second (211,000 kilometers per second), a number that isn't accurate by today's standards — we now know that the speed of light in the "vacuum" of empty space is about 186,282 miles per second (299,792 km per second) — but his assessment showcased that light travels at an incredible speed.

According to Albert Einstein 's theory of special relativity , light travels so fast that, in a vacuum, nothing in the universe is capable of moving faster. 

"We cannot move through the vacuum of space faster than the speed of light," confirmed Jason Cassibry, an associate professor of aerospace engineering at the Propulsion Research Center, University of Alabama in Huntsville.

Question answered, right? Maybe not. When light is not in a vacuum, does the rule still apply?

Related: How many atoms are in the observable universe?

"Technically, the statement 'nothing can travel faster than the speed of light' isn't quite correct by itself," at least in a non-vacuum setting, Claudia de Rham, a theoretical physicist at Imperial College London, told Live Science in an email. But there are certain caveats to consider, she said. Light exhibits both particle-like and wave-like characteristics, and can therefore be regarded as both a particle (a photon ) and a wave. This is known as wave-particle duality.

If we look at light as a wave, then there are "multiple reasons" why certain waves can travel faster than white (or colorless) light in a medium, de Rham said. One such reason, she said, is that "as light travels through a medium — for instance, glass or water droplets — the different frequencies or colors of light travel at different speeds." The most obvious visual example of this occurs in rainbows, which typically have the long, faster red wavelengths at the top and the short, slower violet wavelengths at the bottom, according to a post by the University of Wisconsin-Madison . 

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When light travels through a vacuum, however, the same is not true. "All light is a type of electromagnetic wave, and they all have the same speed in a vacuum (3 x 10^8 meters per second). This means both radio waves and gamma rays have the same speed," Rhett Allain, a physics professor at Southeastern Louisiana University, told Live Science in an email.

So, according to de Rham, the only thing capable of traveling faster than the speed of light is, somewhat paradoxically, light itself, though only when not in the vacuum of space. Of note, regardless of the medium, light will never exceed its maximum speed of 186,282 miles per second.

Universal look

According to Cassibry, however, there is something else to consider when discussing things moving faster than the speed of light.

"There are parts of the universe that are expanding away from us faster than the speed of light, because space-time is expanding," he said. For example, the Hubble Space Telescope recently spotted 12.9 billion year-old light from a distant star known as Earendel. But, because the universe is expanding at every point, Earendel is moving away from Earth and has been since its formation, so the galaxy is now 28 billion light years away from Earth.

In this case, space-time is expanding, but the material in space-time is still traveling within the bounds of light speed.

Related: Why is space a vacuum?

So, it's clear that nothing travels faster than light that we know of, but is there any situation where it might be possible? Einstein's theory of special relativity, and his subsequent theory of general relativity, is "built under the principle that the notions of space and time are relative," de Rham said. But what does this mean? "If someone [were] able to travel faster than light and carry information with them, their notion of time would be twisted as compared to ours," de Rham said. "There could be situations where the future could affect our past, and then the whole structure of reality would stop making sense."

This would indicate that it would probably not be desirable to make a human travel faster than the speed of light. But could it ever be possible? Will there ever be a time when we are capable of creating craft that could propel materials — and ultimately humans — through space at a pace that outstrips light speed? "Theorists have proposed various types of warp bubbles that could enable faster-than-light travel," Cassibry said.

But is de Rham convinced?

"We can imagine being able to communicate at the speed of light with systems outside our solar system ," de Rham said. "But sending actual physical humans at the speed of light is simply impossible, because we cannot accelerate ourselves to such speed.

"Even in a very idealistic situation where we imagine we could keep accelerating ourselves at a constant rate — ignoring how we could even reach a technology that could keep accelerating us continuously — we would never actually reach the speed of light," she added. "We could get close, but never quite reach it."

Related: How long is a galactic year?

This is a point confirmed by Cassibry. "Neglecting relativity, if you were to accelerate with a rate of 1G [Earth gravity], it would take you a year to reach the speed of light. However, you would never really reach that velocity because as you start to approach lightspeed, your mass energy increases, approaching infinite. "One of the few known possible 'cheat codes' for this limitation is to expand and contract spacetime, thereby pulling your destination closer to you. There seems to be no fundamental limit on the rate at which spacetime can expand or contract, meaning we might be able to get around this velocity limit someday."

— What would happen if the speed of light were much lower?

— What if the speed of sound were as fast as the speed of light?

— How does the rubber pencil illusion work?

Allain is similarly confident that going faster than light is far from likely, but, like Cassibry, noted that if humans want to explore distant planets, it may not actually be necessary to reach such speeds. "The only way we could understand going faster than light would be to use some type of wormhole in space," Allain said. "This wouldn't actually make us go faster than light, but instead give us a shortcut to some other location in space."

Cassibry, however, is unsure if wormholes will ever be a realistic option.

"Wormholes are theorized to be possible based on a special solution to Einstein's field equations," he said. "Basically, wormholes, if possible, would give you a shortcut from one destination to another. I have no idea if it's possible to construct one, or how we would even go about doing it." Originally published on Live Science.

Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester. 

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