Arthur EddingtonEinstein's Theory of General Relativity, proposed in 1915, predicted that light would bend around a massive object, such as the Sun. Newton's laws of gravity, made in the 1680s, also predicted this would happen. But General Relativity said that light would bend twice as much as predicted by Newton. Measuring how much light bent around the Sun would show which of these great scientists was right. But how was it possible to check whether the Sun was bending light from distant stars, when it is too bright to allow the stars to be seen? The answer was to run the experiment during a total solar eclipse when the sky went temporarily dark. The British astronomer Sir Arthur Eddington (1882–1944) set out to observe a solar eclipse occurring on 29th May 1919—exactly 100 years ago—and so prove whether Einstein's theory was correct or not.
Albert Einstein in 1921
General Theory of Relativity
The German-born physicist Albert Einstein (1879–1955) completed his General Theory of Relativity in 1915. He wanted to extend his earlier theory of relativity, put forward in 1905, to include acceleration, the gathering of speed that is due to the effect of gravity. In his General Theory, he showed that gravity, instead of being viewed as the pull exerted by another object, could be seen as the bending of space around that object—exactly in the same way that a person can bend a trampoline when standing on it—causing passing bodies to fall in towards it. A star in space could be thought of as a ball on a rubber sheet. A massive object like a star would “bend” space, and anything close to it would fall, or curve, in towards it—including light.
A heavy object bending a rubber sheet
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