Physical theories have one downside that physicists are very aware of: they cannot be proven true for always. Instead, they can be considered correct just until someone can demonstrate that they’re wrong. That applies to Einstein’s theories too. His General Theory of Relativity, however, has been amazingly robust so far. Einstein himself proposed three tests for his revolutionary theory, which wasn’t based on experimental findings, but on an almost philosophical line of thinking.
His first test concerned the orbit on which the planet Mercury moved around the Sun. Its point closest to the sun (perihelion) changes in a very specific way, which should be able to be accurately calculated using the General Theory of Relativity. This test confirmed the theory’s predictions back in 1916. The other tests had to wait longer, because technology had to make enough progress to be able to perform them – or because the processes the tests were measuring were rather slow. For example, astronomers had to track the orbit of a star around the black hole, Sagittarius A*, at the heart of the Milky Way for more than 25 years to be able to verify the perihelion precession predicted by the General Theory of Relativity.