It is 100 years since the publication of Einstein's great theory, and arguably one of the greatest scientific theories of all time. To mark the occasion, Brian Cox takes Robin Ince on a guided tour of General Relativity. With the help of some of the world's leading cosmologists, and a comedian or two, they explore the notions of space time, falling elevators, trampolines and bowling balls, and what was wrong with Newton's apple. It is a whistle stop tour of all you will ever need to know about gravity and how a mathematical equation written 100 years ago predicted everything from black holes to the Big Bang, to our expanding universe, long before there was any proof that these extraordinary phenomena existed.They try to give some short simplified descriptions of general relativity, but they seem unsatisfactory to me. So I will try.
General relativity is a way of reconciling gravity with the causal structure of spacetime. Newtonian gravity involves masses exerting a force over a large distance, without any intervening mechanism. Maxwell devised a causal theory of electromagnetism, where charges generate local fields that propagate to interact with other charges. Relativity interprets this in terms of a geometry of space and time, and describes gravity as a similar curvature field that propagates to other masses.
They said that GR was developed from theory, not experiment. That is partially true, but I would not say it that way. Special relativity was driven by trying to reconcile the Michelson-Morley experiment with Maxwell's equations. The result was spacetime non-Euclidean geometry. Was that theory or experiment? Some of each.
A commenter notes that Einstein's work did not directly use the Michelson-Morley experiment. That is true, but even Einstein conceded that the experiment was crucial for the discovery of special relativity. It just was not needed for Einstein's recapitulation of Lorentz's work.
Solar system observations were the experimental data for gravitational theory. Newton and Laplace showed that gravity must be instantaneous or propagate much faster than light to explain planetary orbits. Poincare used relativity to show that gravity could propagate at the speed of light, and also to partially explain anomalies in Mercury's orbit. Einstein showed that a Ricci-flat spacetime could fully explain the Mercury anomaly, and that convinced him that he had the correct relativistic gravity theory.
Other aspects of GR were not properly tested until decades later, but experiment was still crucial for its original development.
In the last month there have been a lot of rumors about a pending announcement of the discovery of gravity waves, and the news stories always say that this would be a vindication of Einstein's theory of general relativity. As a reader points out, Einstein himself did not believe in gravity waves, and tried to publish a paper proving that they did not exist. It was Einstein's only refereed paper, and the referee said that it was wrong.
Poincare proposed a theory of relativistic gravity waves in 1905. He was decades ahead of Einstein on this point.