Albert Einstein (14 March 1879 – 18 April 1955) was according to many the greatest of all physicists. He was the creator of relativity theory and a founding father of quantum mechanics. In 1904 he independently laid the foundations of statistical mechanics, in the manner of Gibbs rather than Boltzmann; in 1905 he created relativity theory, the theory of Brownian motion, and the theory of quantization of energy, including the concept of light quanta (photons). It was for the photoelectric effect that he was eventually awarded the Nobel Prize, in 1921.
Relativity theory effectively demolished the foundations of classical mechanics and Newton’s Theory of Gravity, and from 1906 Einstein sought a relativistic theory of gravity to replace it. He found it in 1916, the General Theory of Relativity (GTR), the foundation of all gravitational physics and of modern cosmology. This theory and quantum mechanics are the two pillars of modern physics: the struggle to unite or to otherwise reconcile them remains the principal challenge of theoretical physics.
The original field equations of GTR did not permit a homogeneous, static solution. Einstein, believing such a solution was required, at least as an idealisation of the actual universe, introduced an additional scalar coupling to the metric field, the ‘cosmological constant’ Λ. The amended equations did admit such a solution (the Einstein static universe), a spatially closed universe of uniform density ρ, with positive cosmological constant Λ=4πGρ/c2. With this Einstein missed out on what would have been a truly spectacular prediction of GTR. The solution was, moreover, unstable; it is probably for this reason that Einstein referred to this episode as the ‘greatest blunder of my life’.
Einstein continued to work on quantum mechanics from 1905 until 1924, where he made his last substantive contribution to physics, the theory of Bose-Einstein statistics. In that period his work on specific heats and his derivation of the Planck distribution from his theory of A and B coefficients, which first introduced probability theory into the theory of radiative transitions, was crucial to the final formulation of the theory by Heisenberg and others in 1925-1926.
Several of the founding fathers of quantum mechanics were unable to accept that the theory was a fundamental one. Einstein was its principal critic. In 1935 he, in collaboration with Podolsky and Rosen, devised a paradox (the ‘EPR paradox’) aimed to establish that quantum mechanics was either incomplete or non-local. The thought experiment on which the paradox was based was subsequently reworked by Bohm in 1951 and by Bohm and Aharonov in 1957; in this form it was studied by Bell, who showed that any attempt to supplement quantum mechanics with so-called ‘hidden variables’, subject to a locality constraint, would lead to predictions contrary to quantum mechanics. Others have subsequently argued that Bell’s results show that quantum mechanics is itself non-local, so that the argument has no bearing on whether or not it should be supplemented with hidden variables.
Einstein made no further contribution to quantum theory, nuclear physics, or particle physics: he remained convinced that the road to a satisfactory and more fundamental theory lay in the unification of the theory of gravity with the theory of electromagnetism. But his work on unified field theory proved fruitless. Whilst his early work in relativity and quantum theory had been marked by the use of the operational analysis of concepts, his discovery of general relativity marked a shift to a reliance on mathematical aesthetics and a more mathematical method of discovery.
Einstein was born in Ulm in Germany, but shortly after his family moved to Munich, where he attended the Luitpold Gymnasium. At the age of 16 he applied, unsuccessfully, to the Swiss Federal Polytechnic in Zurich, but he was admitted to the Aargau Cantonal School in Aarau. In 1896 he renounced German citizenship to avoid military service in Germany. At the age of 17 he began a four-year teaching diploma course at the Polytechnic. He became a Swiss citizen in 1901, at which time he began work at the Swiss Patent Office, where he remained until 1909 following his appointment to a lectureship at the University of Berne. He subsequently moved to the Charles-Ferdinand University in Prague in 1911; in 1914 he was appointed director of the Kaiser Wilhelm Institute for Physics. There he remained until 1932, when, with the rise to power of Adolf Hitler, he found himself as a Jew considered an assassination target. After a brief sojourn in England he took up a position at the newly created Institute of Advanced Study in Princeton, where he remained to the end of his life. He never returned to Europe.