When an electron orbiting an atom drops to a lower energy state, an odd thing happens. It shoots off a photon travelling at the speed of light. It’s not like pulling away from the traffic lights because there is no acceleration as we normally know it. At one moment there’s an electron, and the next, there’s an electron with slightly less energy and a photon travelling about 300,000 kilometres per second.

The photon can get to that speed because it has no mass. If it did have mass – even an infinitesimal amount – it could never get that fast. The fastest we’ve been able to get a proton to travel, for example, is about 0.999999990 the speed of light which is still 11km/h slower.

The mass of a proton is tiny, and yet it took a stupendous amount of energy around the 27 kilometre loop at Cern to get it that fast. The problem is that doubling the energy does not double the velocity, and no matter how much energy you add, you’ll never catch up to light.

We know from Einstein’s insight that the faster an object travels, the more massive it becomes and at the speed of light, its mass would be infinite.

Curiously though, it is possible for matter to travel faster than light. That seems like a contradiction, but the trick is to make light slower. When you read ‘‘the speed of light’’, that refers to its velocity in a vacuum. In water, light travels only 0.75 as fast.

This has been observed in Cherenkov radiation, which occurs in the cooling water around nuclear reactors. It emits an eerie blue glow as particles travel faster than light (but less than 300,000km/s). The cause is a bit similar to the shockwave of a sonic boom.

Also strange is that researchers have managed to slow, and even stop, light completely. Their metaphorical photon Zimmer frame uses clever electronics.

There is an imagined particle called the tachyon that travels faster than light, but it’s never been observed and if it does exist, it’d violate the law of consequences by travelling backwards in time. Actually, this column was written by the future version of me.

That was neat, but now I fear the future me will realise he doesn’t need to write it either.

Response: Rod Taylor, Fuzzy Logic, with thanks to Aditya Chopra

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