Superluminal communication is a hypothetical process in which information is sent at faster-than-light (FTL) speeds. The current scientific consensus is that faster-than-light communication is not possible, and to date it has not been achieved in any experiment.
Superluminal communication is believed to be impossible because, in a Lorentz-invariant theory, it could be used to transmit information into the past. This contradicts causality and leads to logical paradoxes.
A number of theories and phenomena related to superluminal communication have been proposed or studied, including group velocities greater than the speed of light, tachyons, quantum nonlocality, and wormholes.
Group velocity > speed of light
The group velocity measures the speed of wave packets, or groups of waves. There are real systems with superluminal group velocity, some of which have been experimentally realized and studied in the lab. However, it is not the group velocity but the signal velocity that determines the communication speed, and superluminal group velocity does not imply superluminal signal velocity.
Tachyonic particles are hypothetical particles that travel faster than light. These would allow superluminal communication, and for this reason are widely believed not to exist. By contrast, tachyonic fields - quantum fields with imaginary mass - certainly do exist, and exhibit superluminal group velocity under some circumstances. However, such fields have luminal signal velocity and do not allow superluminal communication.
Quantum mechanics is non-local in the sense that distant systems can be entangled. Entangled states lead to correlations in the results of otherwise random measurements, even when the measurements are made nearly simultaneously and at far distant points. The impossibility of superluminal communication lead Einstein, Podolsky, and Rosen to propose that quantum mechanics must be incomplete (see EPR paradox).
However, it is now well-understood that quantum entanglement does not allow any influence or information to propagate superluminally. Technically, the microscopic causality postulate of axiomatic quantum field theory implies the impossibility of superluminal communication using any phenomena whose behavior can be described by orthodox quantum field theory. A special case of this is the no-communication theorem, which prevents communication using the quantum entanglement of a composite system shared between two spacelike-separated observers. Some authors have argued that using the no-communication theorem to deduce the impossibility of superluminal communication is circular, since the no-communication theorem assumes that the system is composite.
If wormholes are possible, then ordinary subluminal methods of communication could be sent through them to achieve superluminal transmission speeds. Considering the immense energy that current theories suggest would be required to open a wormhole large enough to pass spacecraft through it may be that only atomic-scale wormholes would be practical to build, limiting their use solely to information transmission. Some theories of wormhole formation would prevent them from ever becoming "timeholes", allowing superluminal communication without the additional complication of allowing communication with the past.
- Bell test experiments
- Delayed choice quantum eraser
- Near and far field
- Quantum teleportation
- SETI Institute
- Wheeler–Feynman absorber theory
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... so existence of particles v > c ... Called tachyons ... would present relativity with serious ... problems of infinite creation energies and causality paradoxes.
- Lisa Randall, Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions, p.286: "People initially thought of tachyons as particles travelling faster than the speed of light...But we now know that a tachyon indicates an instability in a theory that contains it. Regrettably for science fiction fans, tachyons are not real physical particles that appear in nature."
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