Superluminal communication

Superluminal communication is the hypothetical process by which one might send information at faster-than-light (FTL) speeds. So far, superluminal communication has not been achieved and scientific consensus is that faster-than-light communication is not possible.

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 third, 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.

In standard quantum mechanics, it is generally accepted that the no cloning theorem prevents superluminal communication via quantum entanglement alone, leading to the no-communication theorem. Consider the EPR thought experiment, and suppose quantum states could be cloned. Alice could send bits to Bob in the following way:

If Alice wishes to transmit a '0', she measures the spin of her electron in the z direction, collapsing Bob's state to either |z+>_B or |z->_B. If Alice wishes to transmit a '1', she measures the spin of her electron in the x direction, collapsing Bob's state to either |x+>_B or |x->_B. Bob creates many copies of his electron's state, and measures the spin of each copy in the z direction. If Alice transmitted a '0', all his measurements will produce the same result; otherwise, his measurements will be split evenly between +1/2 and -1/2. This would allow Alice and Bob to communicate across space-like separations.

However, some authors have pointed out that at least some of the no-communication arguments are tautological, having the limitation on superluminal communication built into the starting assumptions.

Although such communication is prohibited in the thought experiment described above, 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.

Birgit Dopfer, a student of Anton Zeilinger's, has performed an experiment which seems to make possible superluminar communication through an unexpected collective beahvior of two beams of entangled photons, one of which passes through a double-slit, in order to to use the creation of a distance interference pattern as bit 0 and the lack of a distance interference pattern as bit 1 (or vice versa). Since it is a collective and probabilistic phenomenon, no quantum information about the single particles is cloned and accordingly the no cloning theorem remains inviolate. Physicist John G. Cramer at the University of Washington is attempting to replicate one of these experiments and demonstrate whether or not it can produce superluminal communication.

See also

• Ansible
• Bell test experiments
• Delayed choice quantum eraser
• Faster-than-light
• Light
• Quantum teleportation
• SETI Institute
• Speed of light
• Synchronicity
• Ultrawave
• Wheeler-Feynman absorber theory