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In the early 1920s Einstein was the lead figure in a famous weekly physics colloquium at the University of Berlin. On March 30, 1921, Einstein went to New York to give a lecture on his new Theory of Relativity, the same year he was awarded the Nobel Prize. Though he is now most famous for his work on relativity, it was for his earlier work on the photoelectric effect that he was given the Prize, as his work on general relativity was still disputed. The Nobel committee decided that citing his less-contested theory in the Prize would gain more acceptance from the scientific community.

**The "Copenhagen" interpretation**

Einstein and Niels Bohr sparred over quantum theory during the 1920s.

Einstein's relationship with quantum physics was quite remarkable. He was the first to say that quantum theory was revolutionary. His postulation that light can be described not only as a wave with no kinetic energy, but also as massless discrete packets of energy called quanta with measurable kinetic energy (now known as photons) marked a landmark break with the classical physics. In 1909 Einstein presented his first paper on the quantification of light to a gathering of physicists and told them that they must find some way to understand waves and particles together.

In the mid-1920s, as the original quantum theory was replaced with a new theory of quantum mechanics, Einstein balked at the Copenhagen interpretation of the new equations because it settled for a probabilistic, non-visualizable account of physical behaviour. Einstein agreed that the theory was the best available, but he looked for a more "complete" explanation, i.e., more deterministic. He could not abandon the belief that physics described the laws that govern "real things", the belief which had led to his successes with atoms, photons, and gravity.

In a 1926 letter to Max Born, Einstein made a remark that is now famous:

Quantum mechanics is certainly imposing. But an inner voice tells me it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the Old One. I, at any rate, am convinced that He does not throw dice.

To this, Bohr, who sparred with Einstein on quantum theory, retorted, "Stop telling God what He must do!" The Bohr-Einstein debates on foundational aspects of quantum mechanics happened during the Solvay conferences.

Einstein was not rejecting probabilistic theories per se. Einstein himself was a great statistician, using statistical analysis in his works on Brownian motion and photoelectricity and in papers published before the miraculous year 1905; Einstein had even discovered Gibbs ensembles. He believed, however, that at the core reality behaved deterministically. Many physicists argue that experimental evidence contradicting this belief was found much later with the discovery of Bell's Theorem and Bell's inequality. Nonetheless, there is still space for lively discussions about the interpretation of quantum mechanics.

**Bose-Einstein statistics**

In 1924, Einstein received a short paper from a young Indian physicist named Satyendra Nath Bose describing light as a gas of photons and asking for Einstein's assistance in publication. Einstein realized that the same statistics could be applied to atoms, and published an article in German (then the lingua franca of physics) which described Bose's model and explained its implications. Bose-Einstein statistics now describe any assembly of these indistinguishable particles known as bosons. The Bose-Einstein condensate phenomenon was predicted in the 1920s by Bose and Einstein, based on Bose's work on the statistical mechanics of photons, which was then formalized and generalized by Einstein. The first such condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder. Einstein's original sketches on this theory were recovered in August 2005 in the library of Leiden University (see website with original manuscript: [8]).

Einstein also assisted Erwin Schrödinger in the development of the quantum Boltzmann distribution, a mixed classical and quantum mechanical gas model although he realized that this was less significant than the Bose-Einstein model and declined to have his name included on the paper.

**The Einstein refrigerator**

Einstein and Szilárd's refrigerator patent diagram.

Einstein and former student Leó Szilárd co-invented a unique type of refrigerator (usually called the Einstein refrigerator) in 1926. [9] [10] On November 11, 1930, U.S. Patent 1,781,541 was awarded to Albert Einstein and Leó Szilárd. The patent covered a thermodynamic refrigeration cycle providing cooling with no moving parts, at a constant pressure, with only heat as an input. The refrigeration cycle used ammonia, butane, and water.

**World War II**

After Adolf Hitler came to power in 1933, expressions of hatred for Einstein reached new levels. He was accused by the National Socialist regime of creating "Jewish physics" in contrast with Deutsche Physik—"German" or "Aryan physics". Nazi physicists (notably including the Nobel laureates Johannes Stark and Philipp Lenard) continued the attempts to discredit his theories and to blacklist politically those German physicists who taught them (such as Werner Heisenberg). Einstein renounced his German citizenship and fled to the United States, where he was given permanent residency. He accepted a position at the newly founded Institute for Advanced Study in Princeton Township, New Jersey. He became an American citizen in 1940, though he still retained Swiss citizenship.