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 (presuming blades of infinite rigidness: impossible by currently known physics). Or the position of a fast-moving spot of light projected onto a distant object. This is a tricky one to those without knowledge of the physics: perceived light travels as photons being emitted from a source travelling either directly to the retina or to an object, where the photons are reflected to a varying degree. Swinging a beam of light around is perhaps best likened to waving a garden hose around and spraying water about. It's not a rigid beam. Therefore the velocity may exceed c. These effects are possible, because these processes do not transmit information.

It is quite possible to swing a laser pointer across the face of the moon rapidly enough that the point of intersection travels faster than light by hand, and it is conceivable that this could be done with an industrial laser on a new moon, creating a "faster than light" spot bright enough to be seen (with the help of a telescope) from Earth.

It is also possible for two objects with mass to travel faster than light relative to one another, from the viewpoint of a stationary observer. For example, particles on opposite sides of a cyclotron.