Superluminal communication

Superluminal communication is the hypothetical process by which one might send information at faster-than-light (FTL) speeds. The current scientific consensus is that faster-than-light communication is not possible and to date superluminal communication has not been achieved in any experiment.

Some theories and experiments include:

• Group velocity > c experiments
• Evanescent wave coupling
• Tachyons
• Quantum nonlocality

According to the currently accepted theory, three of those four phenomena do not produce superluminal communication, even though they may give that appearance under some conditions. The fourth, tachyons, arguably do not exist as their existence is hypothetical; even if their existence were to be proven, attempts to quantize them appear to indicate that they may not be used for superluminal communication, because experiments to produce or absorb tachyons cannot be fully controlled.

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.

The microscopic causality postulate of axiomatic quantum field theory implies the impossibility of superluminal communication using 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.

However, some argue that superluminal communication could be achieved via quantum entanglement using other methods that don't rely on cloning a quantum system. One suggested method would use an ensemble of entangled particles to transmit information, similar to a type of quantum eraser experiments where the observation of an interference pattern on half of an ensemble of entangled pairs is determined by the type of measurement performed on the other half. In these cases, though, the interference pattern only emerges with coincident measurements which requires a classical, subluminal communication channel between the two detectors. Physicist John G. Cramer at the University of Washington is attempting to perform one type of these experiment and demonstrate whether or not it can produce superluminal communication.

See also

• Ansible
• Bell test experiments
• Delayed choice quantum eraser
• Light
• Quantum teleportation
• SETI Institute
• Synchronicity
• Ultrawave
• Wheeler–Feynman absorber theory