# Definition:Speed of Light

## Physical Law

The speed of light (in a vacuum) is a physical constant.

Information cannot travel faster.

It is usually denoted $c$, and its value is given as:

$c = 299 \, 792 \, 458 \text { m s}^{-1}$

exactly.

The metre is in fact defined in terms of the speed of light and the definition of the second.

As the speed of light is at base a velocity, it has dimension $L T^{-1}$.