What is the maximum speed limit in the universe?

The maximum speed limit in the universe is the speed of light in a vacuum. Usually represented by c, it is exactly 299,792,458 metres per second, or about 300,000 kilometres per second.

This limit is not merely the speed at which light travels. In special relativity, c is the maximum speed at which information, matter, or any causal influence can propagate through spacetime. Light travels at this speed in a vacuum because photons have no rest mass. Light moves more slowly through materials such as glass or water, but that does not change the universal limit.

Massive objects can approach c but cannot reach it. As an object's speed increases, its relativistic energy rises sharply; reaching the speed of light would require unlimited energy. The same limit applies to gravity's propagation and to signals carried by gravitational waves.

A common mistake is to call this simply the fastest speed of light under every circumstance. In a medium, particles can move faster than light moves through that medium, producing Cherenkov radiation, while still remaining slower than c in vacuum. No confirmed observation has shown usable information travelling faster than c.