Faster-than-light

However, according to physics as currently understood, the speed of light, 299,792,458 meters per second in vacuo (conventionally notated as c), is in reality an absolute limit for the transmission of information - otherwise, causality could not be preserved. Anything with mass must travel below c. Massless objects (such as the carrier of electromagnetism, the photon) are permitted to attain c, and indeed will always travel at c in a vacuum. It has also been postulated that there could exist a class of particles (known as tachyons) which must always travel faster than c, but to preserve causality such particles could not interact with "conventional" particles and so they would be of no practical use.

Note that there is nothing to prevent non-physical things from exceeding c. 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 no information could be transferred faster than light by such processes.

See Special relativity