Faster-than-light

Faster-than-light (also superluminal or FTL) communications and travel are staples of the science fiction genre. However, according to currently understood physics, these concepts require exotic conditions that current technology can't establish, and that may be directly forbidden by more complete models of the universe's physical laws. Current theories of physics suggest that FTL travel or communication would necessarily involve time travel and thus would almost certainly cause problems with causality.

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:

Related Topics:
Theory of Relativity - Newton's laws of motion - Galilean transformations

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• space and time measurements always give the same results in every 'frame of reference'
• velocities add linearly
• there is nothing fundamental about the wave velocity of light
• Simultaneity is a well-defined concept

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 space, time, and velocity are not consistent between different reference frames, but are instead related by the Lorentz transformations. These transformations have two important implications:

Related Topics:
Special Relativity - Speed of light - Acceleration - Velocity - Graviton - Photon - Lorentz transformations

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• 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
• transmitting information faster than c can violate causality; more precisely, doing this produces situations where observers in different reference frames will disagree on the order of causally-linked events.

Because of this, there appear to be only three ways to justify Faster-Than-Light behavior:

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Option A: Ignore Special Relativity

This is the simplest solution, and is particularly popular in science fiction. Alas, empirical evidence unanimously affirms that the universe obeys Einstein's laws rather than Newton's where they disagree. And while physicists consider General Relativity only an approximation (due to its incompatibility with quantum mechanics), virtually all consider special relativity exact, and there appear to be no serious theoretical challenges to its supremacy.

Related Topics:
Science fiction - General Relativity - Quantum mechanics

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Option B: Give up causality

The other 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 somehow never leads to paradoxes; this is the Novikov self-consistency principle.

Related Topics:
Wormholes - Closed timelike curve - Quantum gravity - Novikov self-consistency principle

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Option C: 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 double 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.

Related Topics:
Planck length - Giovanni Amelino-Camelia - João Magueijo - Variable speed of light

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More generally, any theory which allows FTL yet preserves causality would appear to imply that some observers measurements of physical laws (i.e., those who would observe causality violations in their reference frame) are not legitimate, thus violating one of the central tenets of special relativity. For example, 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.

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