In 1915, Einstein provides an explanation for the deviation of the planet Mercury’s orbit from the predictions of classical physics - a first success for General Theory of Relativity. Yet other explanations are also possible for this perihelion rotation of Mercury. Einstein needs additional evidence.

Einstein has been striving to find such astronomical evidence since before 1911. He concentrates on two effects in particular: the bending of light in the gravitational field of the sun, and the red shift of light in gravitational fields. Yet it is not certain whether either effect can be detected at all using the equipment available at the time. The astronomers show little interest, with the exception of the young assistant Erwin Freundlich. A solar eclipse expedition he initiates is thwarted by the outbreak of World War I.

Triumph must wait until 1919, when an English expedition observing a solar eclipse proves that the light of the stars is indeed deflected by the gravitational field of the sun. This dramatic refutation of Newtonian theory, combined with the international collaboration so soon after World War I, make Einstein famous throughout the world.