That’s probably one of the most common questions people ask when thinking about space travel: is it possible to travel faster than light? And if so, will we be able to, someday?

While this could have been part of the article Will we ever travel to other stars?, I chose to write this one separately as there is a lot to say (you could actually write a whole book about that matter), and I surely won’t give an exhaustive list of all the possibilities.

## Theory

First, let’s take a quick look at theory. Faster-than-light (FTL) communications and travel refer to the propagation of information or matter faster than the speed of light. As most of you probably heard, Einstein’s theory of relativity says that nothing can go faster than light in vacuum, which is not quite true. Special relativity gives us the total energy of a particle:

where m is the rest mass of the particle, v its speed and c the speed of light in vacuum.

For a particle with a speed v smaller than that of light c, it appears that an infinite amount of energy is needed to accelerate it to the speed of light, without ever reaching it. However, special relativity does not forbid the existence of particles that travel faster than light at all times, at least mathematically. Such particles are called tachyons.

According to the equation above, this implies an imaginary denominator, and an imaginary rest mass to keep the total energy a real number. At first, an imaginary mass seems to be shocking, but the rest mass of a particle can only be measured in a frame in which it is at rest. In the case of tachyons, such a frame cannot exist: just like ordinary particles, or bradyons, cannot accelerate up to the speed of light, tachyons cannot slow down to the speed of light, as it would require an infinite amount of energy. Thus, rest mass does not have any physical reality in this particular case.

Even if they exist (they haven’t been observed), tachyons are considered to be a sign of pathological behaviour in field theories, making faster than light information transmission and causality violation with tachyons impossible.

In the context of general relativity, where gravity is included, the principle that no object can accelerate to the speed of light in the reference frame of any coincident observer is maintained. However, as we will see later, it permits distortions in spacetime that allow an object to move faster than light from the point of view of a distant observer.

## Trivial FTL phenomena

There are a few examples in which things appear to travel faster than light, but they do not convey energy or information faster than light, so they do not violate special relativity or causality.

1. Shadows and light spots: if a laser is swept across a distant object, the spot of light can easily be made to move at a speed greater than that of light. Similarly, a shadow projected onto a distant object can be made to move faster than c. In neither case does any matter or information travel faster than light.
2. Quantum entanglement: here, two particles are linked in such a way that the quantum state of any of them cannot be adequately described without full mention of the others, even if the individual objects are spatially separated. Apparently, information can be transmitted faster than light. Actually, it does not allow true communication, as there is no control over what is transmitted: it is useless.
3. Expansion of the Universe: distant galaxies recede from us faster than the speed of light. Expansion of the Universe is a growth of spacetime itself; this spacetime may move faster than the speed of light relative to some other location, but the two locations can’t communicate with each other.

## Apparent FTL travel

We will now see that it is theoretically possible to travel faster than light. Except that… you actually wouldn’t travel faster than light! Two of these theoretical possibilities are known as wormholes and warp drives. Both these examples are using spacetime distortions to allow apparent FTL travel.

### Wormholes

A wormhole is a hypothetical “shortcut” through spacetime. Wormholes are predicted by general relativity, and among the various solutions proposed, some would be traversable and allow to travel extremely quickly between two distant regions of the Universe (other solutions would allow time travel or inter-Universe travel, but I won’t describe them here). A wormhole is made up of three parts: two mouths and a throat.

Someone traveling through a wormhole would appear to have traveled faster than light to an external observer. This is actually not the case: a beam of light going through the wormhole would still travel much faster than anything else – the speed of light is not exceeded locally at any time. You can imagine yourself running as fast as you can around to the opposite side of a mountain; this may take much longer than walking through a tunnel crossing it.

For the wormhole to be traversable, some exotic matter with negative mass is required; otherwise, wormholes are unstable and pinch off too quickly for anything to go through. Other models also allow wormholes to remain open without  the need of exotic matter: they require a modification to general relativity involving extra spatial dimensions. Wormholes have never been observed, and these ideas are very speculative: stable wormholes (as well as unstable ones) may simply not exist at all.

### Warp drives

Miguel Alcubierre theorized that it would be possible to create an Alcubierre drive, in which a ship would be enclosed in a “warp bubble” where the space at the front of the bubble is rapidly contracting and the space at the back is rapidly expanding. As a result the bubble can reach a distant destination much faster than a light beam moving outside the bubble, but without objects inside the bubble locally traveling faster than light. The tricky part is that the ship is actually not moving at all; space itself would be moving underneath the spacecraft.

In this case again, for an Alcubierre drive to work, exotic matter is needed. The problem is that unrealistic amounts are needed, and there are other issues: it has been shown that the crew inside the spacecraft would be causally disconnected with the walls of the warp bubble, making it impossible to control, steer or stop the ship.

Recently, another kind of warp drive has been proposed by Gerald Cleaver and Richard Obousy: the warp bubble would be created by manipulating the 11th dimension of the M-theory, an extension of string theory. Shrinking the 11th dimension behind the ship would create a bubble of dark energy, the same dark energy that is causing the expansion of the Universe to accelerate. Expanding the 11th dimension in front of the ship would cause it to decrease. The problem is that the M-theory is purely speculative, and how the 11th dimension would be expanded and shrunk is still unknown. Even assuming that M-theory is right and that such technology could be developed, the amount of energy needed would be insane: it would be equivalent to converting the entire mass of Jupiter into pure energy! It is needless to say that it is far beyond anything we can produce…

Finally, if mankind is ever going to travel at faster than light speeds (well, apparent FTL speeds), which currently seems quite improbable, it looks like it won’t be before a very, very long time.