The ‘speed of light’ (commonly denoted by c) generally refers to the speed at which electromagnetic radiation propagates in a vacuum. Although always referred to as the speed of light, this speed should be more properly (if not so poetically) called the ‘speed of a massless particle’ as it is the speed at which all particles of zero mass (not only photons, but gravitons and massless neutrinos if they exist) travel in a vacuum.

Einstein’s theory of relativity makes several important statements about the speed of light in a vacuum:

1. The speed of light in a vacuum is 299,792,458 metres/second (though it is less in a transparent medium such as air, water or glass, depending on the refraction index).
2. Nothing can travel faster than the speed of light in a vacuum (though some particles can exceed the speed of light in a transparent medium – resulting in Cerenkov radiation).
3. The speed of light in a vacuum is a constant. This means that the speed of light has exactly the same value for observers travelling at different speeds. It is this property that leads to many of the counter-intuitive behaviours accurately predicted by Einstein’s theory of special relativity (e.g. time dilation).