# A Brief History of Relativity

## What is it? How does it work? Why does it change everything? An easy primer by the world's most famous living physicist

Toward the end of the 19th century scientists believed they were close to a complete description of the universe. They imagined that space was filled everywhere by a continuous medium called the ether. Light rays and radio signals were waves in this ether just as sound is pressure waves in air. All that was needed to complete the theory was careful measurements of the elastic properties of the ether; once they had those nailed down, everything else would fall into place.

Soon, however, discrepancies with the idea of an all-pervading ether began to appear. You would expect light to travel at a fixed speed through the ether. So if you were traveling in the same direction as the light, you would expect that its speed would appear to be lower, and if you were traveling in the opposite direction to the light, that its speed would appear to be higher. Yet a series of experiments failed to find any evidence for differences in speed due to motion through the ether.

The most careful and accurate of these experiments was carried out by Albert Michelson and Edward Morley at the Case Institute in Cleveland, Ohio, in 1887. They compared the speed of light in two beams at right angles to each other. As the earth rotates on its axis and orbits the sun, they reasoned, it will move through the ether, and the speed of light in these two beams should diverge. But Michelson and Morley found no daily or yearly differences between the two beams of light. It was as if light always traveled at the same speed relative to you, no matter how you were moving.

The Irish physicist George FitzGerald and the Dutch physicist Hendrik Lorentz were the first to suggest that bodies moving through the ether would contract and that clocks would slow. This shrinking and slowing would be such that everyone would measure the same speed for light no matter how they were moving with respect to the ether, which FitzGerald and Lorentz regarded as a real substance.

But it was a young clerk named Albert Einstein, working in the Swiss Patent Office in Bern, who cut through the ether and solved the speed-of-light problem once and for all. In June 1905 he wrote one of three papers that would establish him as one of the world's leading scientists--and in the process start two conceptual revolutions that changed our understanding of time, space and reality.

In that 1905 paper, Einstein pointed out that because you could not detect whether or not you were moving through the ether, the whole notion of an ether was redundant. Instead, Einstein started from the postulate that the laws of science should appear the same to all freely moving observers. In particular, observers should all measure the same speed for light, no matter how they were moving.

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