Space and time are not fixed

In 1905 this man was a clerk in the patent office in Bern, in Switzerland. In his spare time he was thinking about light. That year Albert Einstein published an idea that changed what people mean by space and by time. It is called special relativity, and it begins with one odd fact.

Throw a ball forward from a moving train and the speeds add up. Somebody beside the track sees the ball going at the speed of the throw plus the speed of the train. Light does not do this. Shine a torch from a ship moving at any speed you like, and everybody who measures that light, on the ship or off it, gets the same answer: 299,792 kilometres every second.

That cannot be true unless something else gives way, and what gives way is time. Picture a clock that ticks by bouncing a flash of light between two mirrors. Set the clock moving and the light has to travel a longer, slanting path between ticks. Since the light cannot speed up, each tick takes longer. A moving clock runs slow. This is true of every kind of clock, and of everything else that happens, a heartbeat included.

At the speeds of daily life the slowing is far too small to notice, but it is real and it has been measured. The satellites that tell your phone where it is go round the Earth at about 14,000 kilometres an hour. Because of that speed their clocks lose about seven millionths of a second every day. If the engineers ignored this and one other effect of the same kind, your position on the map would drift by about ten kilometres a day.

The closer anything gets to the speed of light, the more its clock slows, and the harder it becomes to push it any faster. However much energy you put in, it creeps towards the speed of light and never arrives. Nothing that has mass can reach it. Light itself, which has none, can go at no other speed through empty space.

One more thing fell out of the same idea, and it became the best-known equation there is: E = mc². It says that mass is a very concentrated form of energy. The Sun shines by turning mass into light, and it gets about four million tonnes lighter every second.