Asked by: Alan Edgington, Ramsgate
It isn't. When it passes through some mediums, such as water, it slows down considerably. In the case of diamond, its speed is cut by over 50 per cent. But according to Einstein's Special Theory of Relativity, the speed of light in the vaccum of empty space is said to be the same for all observers, at just short of 300,000km/s.
This is undoubtedly weird, as every other speed is measured relative to something else. For example, a train can move at 150km/h relative to someone on a platform, but to the train's passengers its speed is pretty much zero. The speed of light is no ordinary speed, however: it's a universal constant that emerges from the laws of physics.
Specifically, it's the speed at which electromagnetic waves travel through the vacuum of space - and its value can be predicted by equations unifying our understanding of electricity and magnetism, as discovered over 150 years ago by the Scottish physicist and mathematician James Clerk Maxwell.
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