Faster-than-light

Faster-than-light, or FTL, communications and travel are staples of the science fiction genre. However, according to physics as currently understood, these things are either outright impossible or else well beyond our current technology.

Special relativity makes the speed of light (299,792,458 meters per second in vacuo) an absolute speed limit for the transmission of information. Anything with mass must travel slower. Massless objects, such as photons and the hypothetical gravitons, always travel at exactly the speed of light.

The limit is not quite as absolute in general relativity; in that theory it is possible to arrange for a massive object to move faster than light from the point of view of a distant observer. One such arrangement is the Alcubierre drive metric. Wormholes connecting astronomically distant locations are also possible in general relativity. However, to date all these tricks require exotic matter, infeasible amounts of energy, or both.

General relativity predicts that any technique for faster-than-light travel could also be used for time travel. This raises problems of causality, and therefore many physicists believe that these techniques will eventually prove to be impossible.

Certain phenomena in quantum mechanics, such as entanglement, appear to transmit information faster than light. These phenomena do not allow true communication; they only let two observers in different locations know what the other must see.

It has been postulated that there could exist a class of particles (known as tachyons) which must always travel faster than light, but such particles have never been observed. If they exist and can interact with normal matter, they would also allow causality violations. If they exist but cannot interact with normal matter, their existence cannot be proven, so they might as well not exist.

Note that there is nothing to prevent non-physical things moving faster than light. For example, the point of intersection of a pair of scissor blades as the blades close, or the position of a fast-moving spot of light projected onto a distant object, may exceed c. This is permitted because these processes do not transmit information.