Soon after completing the general theory, Einstein turned his attention to the quantum theory of electromagnetic radiation and postulated the existence of stimulated emission, the process that now underpins the operation of lasers. However, in 1917 he became seriously ill. He was nursed back to health by his cousin Elsa, whom he married in 1919. His second marriage seems to have been reasonably happy, but he was not, by his own admission, a good husband, and is known to have had a roving eye.

By the early 1920s Einstein's best scientific work was done: he wrote in 1921 ‘Discovery in the grand manner is for young people... and hence for me is a thing of the past’. He was none the less extremely influential in the physics community and he did much to prepare the ground for many later developments. He travelled a lot, and became increasingly active in social and political causes, particularly in support of Zionism. (Many years later he was offered the presidency of Israel, which he declined.)

In 1932, Einstein and his wife left Germany for good, mainly in response to growing anti-Semitism, and moved to the USA where Einstein settled as a professor at the Institute for Advanced Study in Princeton, New Jersey. Einstein eventually became an American citizen, though he also retained the Swiss citizenship he had held since his twenties. Although Einstein was a believer in peace and harmony, and eventually argued for a world government, he also recognized the dangers of German militarism and the potential power of atomic science. As a result, in 1939, he was persuaded to co-sign a letter to the American President, Franklin D Roosevelt, warning of the possibility of atomic weapons. This is widely thought to have had a decisive effect in prompting the US government to undertake the development of its own atomic bomb, though Einstein himself played no part in the project.

Although Einstein had been deeply involved in the birth of quantum physics, he became increasingly dissatisfied with the way the subject developed after the mid-1920s. He did not believe that it gave a truly fundamental account of natural phenomena. His last major contribution to the field was the development of Bose-Einstein statistics in 1925. However his name is also recalled in the Einstein-Podolsky-Rosen experiment  a ‘thought experiment’ proposed in 1935 in an attempt to show that quantum physics was seriously flawed. The attempt was unconvincing, but it did emphasise the gulf that separated quantum physics from the ‘classical’ physics that preceded it. The other project of Einstein’s later years that continues to be remembered is his search for a unified field theory that would bring together gravity and electromagnetism. Einstein continued to work on this up to the time of his death, often with great ingenuity, but little of that work is regarded as being of enduring value. Einstein died in Princeton in 1955.

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