Special relativity

The story

The patent office clerk

In 1905 a young man of 26 was working in Bern, Switzerland, as a clerk at the patent office, where people's inventions were checked. His name was Albert Einstein, he had been born in Ulm, Germany, in 1879, and he had not found a job at a university. The work, however, left him time: he wrote down his thoughts and thought about them on trains, at home, and on walks with his friend Michele Besso, talking about physics. That year, which historians call “the miracle year”, he published four papers that changed science.

The first explained that light is made of tiny packets of energy, “quanta”, and for this idea he received the 1921 Nobel Prize in Physics (awarded in 1922). The second explained the zigzag movement of grains of pollen in water by the presence of atoms and molecules, and it helped to convince those who still doubted that atoms existed. The other two papers were about “special relativity”.

One light, two observers

The problem came from a strange fact. If I jump onto a train travelling at a hundred kilometres an hour and throw a ball forwards at twenty kilometres an hour, then for someone standing still on the platform the ball is moving at a hundred and twenty. With light, though, it does not work like that. Experiments at the end of the 19th century showed that light always has the same speed, about three hundred thousand kilometres a second, for anyone who measures it, even if they are moving in the same direction.

Einstein started from two simple ideas: the laws of physics are the same for all observers moving at a steady speed, and the speed of light is the same for everyone. To make the sums work, he had to accept something surprising: space and time are not the same for everybody. Someone moving at very high speed sees time pass more slowly, and distances shrink in the direction of travel. It is as if everyone had their own clock and their own ruler.

These ideas seem absurd because in everyday life we travel at tiny speeds compared with light, and the effects are barely noticeable. But they do exist, and they have been measured: for example, with atomic clocks carried on aeroplanes, and in particles created in the upper atmosphere, which live long enough to reach the ground precisely because they move at almost the speed of light.

E = mc²

At the end of 1905 Einstein wrote a very short paper, just three pages long, with another consequence: mass and energy are two faces of the same thing. The formula has become the most famous in the world: E = mc², where E is energy, m is mass and c is the speed of light. Because c is an enormous number, and it is multiplied by itself, even a tiny amount of mass contains a huge amount of energy: one gram is equivalent to about twenty-one thousand tonnes of TNT. This explains how the Sun shines, turning millions of tonnes of its mass into light and heat every second, and how a nuclear power station works.

Gravity and fame

For ten years Einstein went on thinking, and in 1915 he completed “general relativity”: gravity is not a mysterious force, but a curving of space and time caused by matter, like a heavy ball on a stretched sheet. In 1919, during an eclipse of the Sun, the English astronomer Arthur Eddington measured that the light of some stars bent as it passed close to the Sun, exactly as predicted. The news reached newspapers all over the world, and Einstein became famous. In 1933, faced with the rise of Nazism, he left Germany for the United States, where he stayed until his death in 1955.