The speed of light is constant

The central mystery is light. It is, first of all, astonishingly fast. With a

flick of a switch, light floods a room. Before the rise of modern

science, it was sometimes thought that light leapt magically across

space without taking any time at all. This changed, however, after

Galileo first turned the telescope toward the skies in 1609. Clever

astronomers realized they could use the regular orbits of Jupiter’s

moons as giant clocks, and were able to measure the speed of light

with surprising accuracy. The numbers they produced shocked

people. Who could conceive of a speed of 186,000 miles per second or

300,000 kilometres per second?

But another, more perplexing, surprise lay in wait: the speed of

light is constant. That is, all observers who measure the speed of light

in empty space will find the same number no matter how fast they are

moving. An observer standing still will find starlight racing by at

300,000 kilometres per second. A spaceship cruising at 200,000

kilometres per second and chasing a light beam will still find that the

beam races away from the nose of the ship at 300,000 kilometres per

second. This means, for example, that no one can race fast enough to

catch a light beam. No matter how fast someone is moving, light will

be faster by 300,000 kilometres per second.

This is very peculiar. By way of contrast, consider a speeding

motorist being chased along a road by the police. At the start, with the

police car at a standstill at the side of the road, the speeding car zips

away at 150 kilometres per hour. As the police car reaches 30

kilometres per hour, the speeding car travels only 120 kilometres per

hour faster. As they accelerate, the relative speed of the fugitive drops

down further and further, and finally dwindles to zero as the police

catch up and race alongside flashing their lights. This is common

sense. If the speeding car goes at 150 kilometres per hour and the

police are chasing at 130 kilometres per hour, then their relative

speed is 20 kilometres per hour.

But light is not commonsensical. Light races away from any

standing or moving body at the same speed. The speed of light relative

to any moving body is a constant.

This fact was discovered experimentally in the late 1800s. It was so

strange there was no agreement about what it meant, or even whether

the experiments could be correct. Even today we have no deep

explanation of why the speed of light is constant. Many have derived

the fact, but only by making other, equally mysterious assumptions. It

was Einstein’s great achievement to see this bizarre fact as a clue. He

was able to place it at the centre of a powerful new theory, and thus

opened up a new vision of our universe.

The constancy of the relative speed of light is an experimental

fact. Even today, there is no agreement about why this should be

true. Einstein simply assumed it was and drew some surprising

consequences.