<P> Also the deflection of light by massive bodies was predicted . Although the approximation was crude, it allowed him to calculate that the deflection is nonzero . German astronomer Erwin Finlay - Freundlich publicized Einstein's challenge to scientists around the world . This urged astronomers to detect the deflection of light during a solar eclipse, and gave Einstein confidence that the scalar theory of gravity proposed by Gunnar Nordström was incorrect . But the actual value for the deflection that he calculated was too small by a factor of two, because the approximation he used doesn't work well for things moving at near the speed of light . When Einstein finished the full theory of general relativity, he would rectify this error and predict the correct amount of light deflection by the sun . </P> <P> Another of Einstein's notable thought experiments about the nature of the gravitational field is that of the rotating disk (a variant of the Ehrenfest paradox). He imagined an observer performing experiments on a rotating turntable . He noted that such an observer would find a different value for the mathematical constant π than the one predicted by Euclidean geometry . The reason is that the radius of a circle would be measured with an uncontracted ruler, but, according to special relativity, the circumference would seem to be longer because the ruler would be contracted . Since Einstein believed that the laws of physics were local, described by local fields, he concluded from this that spacetime could be locally curved . This led him to study Riemannian geometry, and to formulate general relativity in this language . </P> <P> In 1912, Einstein returned to Switzerland to accept a professorship at his alma mater, the ETH . Once back in Zurich, he immediately visited his old ETH classmate Marcel Grossmann, now a professor of mathematics, who introduced him to Riemannian geometry and, more generally, to differential geometry . On the recommendation of Italian mathematician Tullio Levi - Civita, Einstein began exploring the usefulness of general covariance (essentially the use of tensors) for his gravitational theory . For a while Einstein thought that there were problems with the approach, but he later returned to it and, by late 1915, had published his general theory of relativity in the form in which it is used today . This theory explains gravitation as distortion of the structure of spacetime by matter, affecting the inertial motion of other matter . </P> <P> During World War I, the work of Central Powers scientists was available only to Central Powers academics, for national security reasons . Some of Einstein's work did reach the United Kingdom and the United States through the efforts of the Austrian Paul Ehrenfest and physicists in the Netherlands, especially 1902 Nobel Prize - winner Hendrik Lorentz and Willem de Sitter of Leiden University . After the war ended, Einstein maintained his relationship with Leiden University, accepting a contract as an Extraordinary Professor; for ten years, from 1920 to 1930, he travelled to the Netherlands regularly to lecture . </P>

When was the theory of general relativity published