The strange behaviour of the velocity of light

As we have seen, Roemer showed as long ago as I 676 that the velocity of light was not infinite. Subsequent measurements by Michelson and others now agree on a value for the speed of light of some 299 792 kilometres/second. This applies not only to the visible part of the electromagnetic spectrum but also to much longer-wavelength radio waves and much shorter-wavelength gamma rays, as expected from Maxwell's equations. Now, according to Newton's laws of motion, there is nothing special about the speed of light. There is nothing, in principle, to stop one accelerating an object – or indeed oneself – to any speed whatsoever. It was the problem of what one would see in a mirror if both observer and mirror were moving at the speed of light that set Einstein on his path to relativity.

It is sometimes said that Einstein showed little exceptional talent when he was at school. This may be true, but it is certain that few schoolboys could have formulated the key paradox of the mirror at the age of sixteen.