Quantum mechanics
Certain phenomena in
quantum mechanics, such as
quantum entanglement, appear to transmit information faster than light. According to the
no-communication theorem these phenomena do not allow true communication; they only let two observers in different locations see the same event simultaneously, without any way of controlling what either sees.
Wavefunction collapse can be viewed as an
epiphenomenon of quantum decoherence, which in turn is nothing more than an effect of the underlying local time evolution of the wavefunction of a system and
all of its environment. Since the
underlying behaviour doesn't violate local causality or allow FTL it follows that neither does the additional effect of wavefunction collapse, whether real
or apparent.
The
uncertainty principle implies that individual photons may travel for short distances at speeds somewhat faster (or slower) than
c, even in a vacuum; this possibility must be taken into account when enumerating
Feynman diagrams for a particle interaction.
[22] In quantum mechanics,
virtual particles may travel faster than light, and this phenomenon is related to the fact that static field effects (which are mediated by virtual particles in quantum terms) may travel faster than light (see section on static fields above). However, macroscopically these fluctuations average out, so that photons do travel in straight lines over long (i.e., non-quantum) distances, and they do travel at the speed of light on average. Therefore, this does not imply the possibility of superluminal information transmission.
There have been various reports in the popular press of experiments on faster-than-light transmission in optics—most often in the context of a kind of
quantum tunneling phenomenon. Usually, such reports deal with a
phase velocity or
group velocity faster than the vacuum velocity of light. But, recall from above, that a superluminal
phase velocity cannot be used for faster-than-light transmission of information. There has sometimes been confusion concerning the latter point. Additionally a channel that permits such propagation cannot be laid out faster then the speed of light.
Quantum teleportation transmits quantum information at whatever speed is used to transmit the same amount of classical information, likely the speed of light. This quantum information may theoretically be used in ways that classical information can not, such as in quantum computations involving quantum information only available to the recipient. In science fiction, quantum teleportation is either used as a basis for teleportation of physical objects at the speed of light, presumably preserving some important aspect of the entanglement between the particles of the object, or else is misrepresented as allowing faster-than-light communication.
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