Faster-than-light (also superluminal or FTL) communications and travel refer to the propagation of information or matter faster than the speed of light. This concept is a staple of the science fiction genre, and is also the subject of ongoing scientific study.
Faster than light travel
In the context of this article, FTL refers to transmitting information or matter faster than a constant equal to the speed of light in a vacuum, roughly 300,000 kilometres per second, or 186,000 miles per second. This is not quite the same as travelling faster than light, since:
Some processes propagate faster than c, but cannot carry information
Some processes propagate faster than c, but cannot carry information
Light travels at speed c/n when not in a vacuum but travelling through a medium with refractive index = n (causing refraction), and in some materials other particles can travel faster than c/n (but still slower than c), leading to Cherenkov radiation.
Neither of these phenomena violates special relativity or creates problems with causality, and thus neither qualifies as FTL as described here.
Neither of these phenomena violates special relativity or creates problems with causality, and thus neither qualifies as FTL as described here.
Possibility of FTL
Faster-Than-Light travel or communication is problematic in a universe that is consistent with Einstein's theory of relativity. In a hypothetical universe where Newton's laws of motion and the Galilean transformations are exact, rather than approximate, the following would be true:
- The laws of physics are the same in every frame of reference, although some laws would have to include terms containing the velocity of the frame of reference
- Quantities measured in different reference frames are related by Galilean transformations, although for some quantities the transformation under the Galilean group is complicated
- Velocities add linearly
- A fixed point x in one reference frame corresponds to the trajectory x-vt in a frame moving with relative velocity v to the first.
- There is nothing fundamental about the wave velocity of light
- All observers agree on the time, up to an overall shift
- Simultaneity is a well-defined concept in that all observers agree on whether any two events are simultaneous
However, according to Einstein's theory of special relativity, what we measure as the speed of light in a vacuum is actually the fundamental physical constant c. This means that all observers, regardless of their acceleration or relative velocity, will always measure zero-mass particles (e.g., gravitons as well as photons) naturally traveling at c. This result means that measurements of time and velocity in different frames are no longer related simply by constant shifts, but are instead related by Poincaré transformations. These transformations have important implications:
- To accelerate an object of non-zero rest mass to c would require infinite time with any finite acceleration, or infinite acceleration for a finite amount of time
- Either way, such acceleration requires infinite energy. Going beyond the speed of light in a homogeneous space would hence require more than infinite energy, which is not a sensible notion.
- Observers with relative motion will disagree which occurs first of any two events that are separated by a space-like interval. In other words, any travel that is faster-than-light in any inertial frame of reference will be travel backwards in time in other, equally valid, frames of reference.
Because of this, there appear to be only a limited number of ways to justify Faster-Than-Light behavior:
Option A: Ignore special relativity
This is the simplest solution, and is particularly popular in science fiction. Empirical evidence unanimously affirms that the universe obeys Einstein's laws rather than Newton's where they disagree. However general relativity is only an approximation due to its incompatibility with quantum mechanics. Special relativity is easily incorporated into nongravitational quantum field theories, however it only applies to a flat Minkowski universe. In particular our expanding universe contains stress-energy which curves the ambient space time and perhaps even has a cosmological constant and so is not Minkowski and in particular is not invariant under Poincaré transformations. However even in the broader context of general relativity, acceleration from subluminal to superluminal speeds does not appear to be possible.
Option B: Get light to go faster (Casimir vacuum)
Einstein's equations of special relativity posit that the speed of light is invariant in inertial frames. That is, it will be the same from any frame of reference moving at a constant speed. The equations do not specify any particular value for the speed of the light itself. That is an experimentally determined quantity, though it has an exact value because the units of length are defined using the speed of light.
The experimental determination has been made in vacuum. However the vacuum we know is not the only possible vacuum which can exist. The vacuum has energy associated with it, called the vacuum energy. This vacuum energy can be changed in certain cases. When vacuum energy is lowered, light itself can go faster than the standard value 'c'. Such a vacuum can be produced by bringing two perfectly smooth metal plates together at near atomic diameter spacing. It is called a Casimir vacuum. Calculations imply light will go faster in such a vacuum. However, there has been no experimental verification, since the technology to detect the change isn't yet available.
Einstein's equations of special relativity have an implicit assumption of homogeneity. Space is assumed to be the same everywhere. In the case of the Casimir vacuum, this assumption is clearly violated. Inside the Casimir vacuum, we have homogeneous space, and outside it, we have homogeneous space as well. Inside the Casimir vacuum, the equations of special relativity will apply with the increased value of the speed of light. Outside it, the equations of special relativity will apply with the normal 'c'. However, when considering two frames of reference, one inside the vacuum, and one outside, the equations of special relativity can no longer be applied, since the assumption of homogeneity has been broken. In other words, the Casimir effect breaks up space into distinct homogeneous regions, each of which obey the special relativity laws separately.
While this may technically qualify as 'faster-than-light', that is only true relative to two disconnected regions of space. It is unclear whether (and unlikely that) a Casimir vacuum is stable under quantum mechanics, and whether non-trivial communication is possible between two such regions.
Option C: Give up causality
Another approach is to accept special relativity, but to posit that mechanisms allowed by general relativity (e.g., wormholes) will allow traveling between two points without going through the intervening space. While this gets around the infinite acceleration problem, it still would lead to closed timelike curves (i.e., time travel) and causality violations. Causality is not required by special or general relativity, but is nonetheless considered a basic property of the universe that should not be abandoned. Because of this, most physicists expect (or perhaps hope) that quantum gravity effects will preclude this option. An alternative is to conjecture that, while time travel is possible, it never leads to paradoxes; this is the Novikov self-consistency principle.
Option D: Give up (absolute) relativity
Due to the strong empirical support for special relativity, any modifications to it must necessarily be quite subtle and difficult to measure. The most well-known attempt is doubly-special relativity, which posits that the Planck length is also the same in all reference frames, and is associated with the work of Giovanni Amelino-Camelia and João Magueijo. One consequence of this theory is a variable speed of light, where photon speed would vary with energy, and some zero-mass particles might possibly travel faster than c. While recent evidence casts doubt on this theory, some physicists still consider it viable. However, even if this theory is true, it is still very unclear that it would allow information to be communicated, and appears not in any case to allow massive particles to exceed c.
There are speculative theories that claim inertia is produced by the combined mass of the universe (e.g., Mach's principle), which implies that the rest frame of the universe might be preferred by conventional measurements of natural law. If confirmed, this would imply special relativity is an approximation to a more general theory, but since the relevant comparison would (by definition) be outside the observable universe, it is difficult to imagine (much less construct) experiments to test this hypothesis.
Option E: Go somewhere where special relativity does not apply
A very popular option taken in science fiction novels, movies, television programs, and computer games is to assume the existence of some other realm (typically called hyperspace or subspace) which is accessible from this universe, in which the laws of relativity are usually distorted, bent, or nonexistent, facilitating rapid transport between distant points in this universe, sometimes with acceleration differences - that is, not requiring as much energy or thrust to go faster. To accomplish rapid transport between points in hyperspace/subspace, special relativity is often assumed not to apply in this other realm. An alternative solution sometimes used is to posit that distant points in the mundane universe correspond to points that are close together in hyperspace.
This method of faster-than-light travel does not correspond to anything seriously proposed by mainstream science, although there are also no arguments precluding its existence.
Option F: Become faster without acceleration
An often-implicit assumption about getting something past light speed is that one must get it to light speed as an intermediate step, thus encountering the infinite energy problem. Similarly to the idea of using wormholes to instantly change location, there might be a method to instantly change velocity, rather than having to accelerate through all intermediate velocities. The energy required for acceleration hits an asymptote as one approaches light speed. Thus, an object going much faster than light speed might only need energy comparable to an object going much slower than light; the difficulty lies in figuring out how to "convince" particles to move faster than light without resorting to acceleration. (This also gets around the problem of including a human being; inertia is related to acceleration, not velocity, so it would not occur.)
As of yet, no method is known of instantly changing the velocity of matter.
Option G: SpaceTime Fabric
Contrary to popular belief, Einstein never claimed that it was impossible to go faster than light, it was assumed from his equations. He however has no objections to accepting that spacetime fabric can travel faster than light. It is hypothesized that at the creation of the universe, spacetime fabric travelled faster than light. Therefore, if we could bend spacetime, we could travel faster than light. Miguel Alcubierre theorized that it would be possible to "warp" spacetime by shrinking spacetime in front of you and expanding it behind you. Unfortunately, such warping would require the emission of negative energy, which has not been discovered or created yet.
