The Bohr–Einstein debates were a series of public disputes about quantum mechanics between Albert Einstein and Niels Bohr. Their debates are remembered because of their importance to the philosophy of science, insofar as the disagreements—and the outcome of Bohr's version of quantum mechanics becoming the prevalent view—form the root of the modern understanding of physics. Most of Bohr's version of the events held in the Solvay Conference in 1927 and other places was first written by Bohr decades later in an article titled, "Discussions with Einstein on Epistemological Problems in Atomic Physics". From Albert Einstein: Philosopher-Scientist (1949), publ. Cambridge University Press, 1949. Niels Bohr's report of conversations with Einstein. Based on the article, the philosophical issue of the debate was whether Bohr's Copenhagen interpretation of quantum mechanics, which centered on his belief of complementarity, was valid in explaining nature. Despite their differences of opinion and the succeeding discoveries that helped solidify quantum mechanics, Bohr and Einstein maintained a mutual admiration that was to last the rest of their lives.
Although Bohr and Einstein disagreed, they were great friends all their lives and enjoyed using each other as a foil.Louisa Gilder, The Age of Entanglement, chap. 5, 2008. “ “Not often in my life has a person, by his mere presence, given me such joy as you did,” wrote Einstein in 1920, in that first letter to Bohr. “I now understand why Ehrenfest loves you so. I am now studying your great papers and in doing so—especially when I get stuck somewhere—I have the pleasure of seeing your youthful face before me, smiling and explaining. I have learned much from you, especially also about your attitude regarding scientific matters.” (“What is so marvellously attractive about Bohr as a scientific thinker,” Einstein wrote not long afterward, “is his rare blend of boldness and caution; seldom has anyone possessed such an intuitive grasp of hidden things combined with such a strong critical sense.”) Somewhat awed, Bohr wrote in reply, “To me it was one of the greatest experiences ever to meet you and talk with you…. You cannot know how great a stimulus it was for me to have the long-hoped-for opportunity to hear of your views on the questions that have occupied me. I shall never forget our talks on the way from Dahlem to your home.”
Pre-revolutionary debates Einstein was the first physicist to say that Max Planck's discovery of the energy quanta would require a rewriting of the laws of physics. To support his point, in 1905 Einstein proposed that light sometimes acts as a particle which he called a light quantum (see photon and wave–particle duality). Bohr was one of the most vocal opponents of the photon idea and did not openly embrace it until 1925. The photon appealed to Einstein because he saw it as a physical reality (although a confusing one) behind the numbers presented by Planck mathematically in 1900. Bohr disliked it because it made the choice of mathematical solution arbitrary; Bohr did not like a scientist having to choose between equations. This disagreement was perhaps the first real Bohr-Einstein debate. Einstein had proposed the photon in 1905, and Arthur Compton provided evidence in 1922 with his Compton effect. Bohr, along with Hans Kramers and John C. Slater asserted that conservation of energy only applied to statistical averages in the BKS theory of 1924. However, after the 1925 Bothe–Geiger coincidence experiment, BKS was proved to be wrong and Einstein's position that energy was conserved in individual collisions was shown to be correct.Kumar, Manjit. Quantum: Einstein, Bohr, and the great debate about the nature of reality / Manjit Kumar.—1st American ed., 2008. Chap.5.
The quantum revolution The quantum revolution of the mid-1920s occurred under the direction of both Einstein and Bohr, and their post-revolutionary debates were about making sense of the change. Erwin Schrödinger redeveloped quantum theory in terms of a wave mechanics formulation, leading to the Schrödinger equation. When Schrödinger sent a preprint of his new equation to Einstein, Einstein wrote back hailing his equation as a decisive advance of “true genius.”Kumar, Manjit. Quantum: Einstein, Bohr, and the great debate about the nature of reality / Manjit Kumar.—1st American Edition, "Schrödinger received a letter from Einstein, who told him ‘the idea of your work springs from true genius’. ‘Your approval and Planck’s mean more to me than that of half the world’, Schrödinger wrote back. Einstein was convinced that Schrödinger had made a decisive advance, ‘just as I am convinced that the Heisenberg-Born method is misleading’." In parallel, Werner Heisenberg's 1925 Umdeutung paper reinterpreted old quantum theory in terms of matrix-like operators, removing the Newtonian elements of space and time from any underlying reality. A year later, in 1926, Max Born, collaborating with Heisenberg, proposed that mechanics were to be understood as a probability without any causal explanation.
Both Einstein and Schrödinger rejected Born's interpretation, with its renunciation of causality which had been a key feature of science still present in general relativity.Kumar, Manjit. Quantum: Einstein, Bohr, and the great debate about the nature of reality / Manjit Kumar.—1st American ed. In a 1926 letter to Max Born, Einstein wrote:
At first, even Heisenberg had heated disputes with Bohr about whether his matrix mechanics were compatible with Schrödinger's wave mechanics.Kumar, Manjit. Quantum: Einstein, Bohr, and the great debate about the nature of reality / Manjit Kumar.—1st American ed. “Heisenberg was totally committed to particles, quantum jumps, and discontinuity. For him the particle aspect was dominant in wave-particle duality. He was not prepared to make room to accommodate anything remotely linked to Schrödinger’s interpretation. To Heisenberg’s horror, Bohr wanted to ‘play with both schemes’.” And Bohr was opposed to Heisenberg's uncertainty principle.Kumar, Manjit. Quantum: Einstein, Bohr, and the great debate about the nature of reality / Manjit Kumar.—1st American ed. “He [Heisenberg] was furious and Bohr upset at the reaction of his young protégé. Living next to door to each other and with their offices on the ground floor of the institute separated only by a staircase, Bohr and Heisenberg did well to avoid one another for a few days before meeting again to discuss the uncertainty paper. Bohr hoped that, having had time to cool down, Heisenberg would see reason and rewrite it. He refused. ‘Bohr tried to explain that it was not right and I shouldn’t publish the paper’, Heisenberg said later.57 ‘I remember that it ended by my breaking out in tears because I just couldn’t stand this pressure from Bohr.’58 ” However, by the fifth Solvay Conference in October 1927, Heisenberg and Born concluded that the revolution was over and nothing further was needed. It was at that last stage that Einstein's skepticism turned to dismay. He believed that much had been accomplished, but the reasons behind the mechanics still needed to be understood.
Einstein's refusal to accept the revolution as complete reflected his desire to see developed a model for the underlying causes from which these apparent random statistical methods resulted. He did not reject the idea that positions in space-time could never be completely known but did not want to allow the uncertainty principle to necessitate a seemingly random, non-deterministic mechanism by which the laws of physics operated. Einstein himself was a statistical thinker but denied that no more needed to be discovered or clarified. Einstein worked the rest of his life to discover a new theory that would make sense of quantum mechanics and return causality to science, what many now call the theory of everything.BBC TV Documentary, September 17, 2014. “But Einstein had a trick up his sleeve. He had already begun a piece of work that he believed would ultimately replace quantum mechanics. It would become later known as his theory of everything – it was his attempt to extend general relativity and unite the known forces in the universe. By completing this theory of everything Einstein hoped he would rid physics of the unpredictability at the heart of quantum mechanics and show that the world was predictable – described by beautiful, elegant mathematics. Just the way he believed God would make the universe. He would show that the way the quantum mechanics community interpreted the world was just plain wrong. It was a project that he would work on for the next 30 years, until the final day of his life.“ https://www.bbc.co.uk/sn/tvradio/programmes/horizon/einstein_symphony_prog_summary.shtml Bohr, meanwhile, was dismayed by none of the elements that troubled Einstein. He made his own peace with the contradictions by proposing a principle of complementarity that assigns properties only as result of measurements.
Post-revolution: First stage As mentioned above, Einstein's position underwent significant modifications over the course of the years. In the first stage, Einstein refused to accept quantum indeterminism and sought to demonstrate that the uncertainty principle could be violated, suggesting ingenious thought experiments which should permit the accurate determination of incompatible variables, such as position and velocity, or to explicitly reveal simultaneously the wave and the particle aspects of the same process. (The main source and substance for these thought experiments is solely from Bohr's account twenty years later.) p.272. (This book contains a translation of the entire authorized proceedings of the 1927 Solvay conference from the original transcripts.)Jammer, M. (1974) The Philosophy of Quantum Mechanics, New York, John Wiley and Sons, p.120. Bohr admits: “As regards the account of the conversations I am of course aware that I am relying only on my own memory, just as I am prepared for the possibility that many features of the development of quantum theory, in which Einstein has played so large a part, may appear to himself in a different light.”Niels Bohr, Original transcript of account of debates by Bohr in 1949, University Institute for Theoretical Physics, Copenhagen Denmark, originally published in “Albert Einstein: Philosopher Scientist, P.A. Schilpp, e., p. 241, The Library of Living Philosophers, Evanston, 1949.
Einstein's argument The first serious attack by Einstein on the "orthodox" conception took place during the Fifth Solvay International Conference on "Electrons and Photons" in 1927. Einstein pointed out how it was possible to take advantage of the (universally accepted) laws of conservation of energy and of impulse (momentum) in order to obtain information on the state of a particle in a process of interference which, according to the principle of indeterminacy or that of complementarity, should not be accessible.
In order to follow his argumentation and to evaluate Bohr's response, it is convenient to refer to the experimental apparatus illustrated in figure A. A beam of light perpendicular to the X axis (here aligned vertically) propagates in the direction z and encounters a screen S1 with a narrow (relative to the wavelength of the ray) slit. After having passed through the slit, the wave function diffracts with an angular opening that causes it to encounter a second screen S2 with two slits. The successive propagation of the wave results in the formation of the interference figure on the final screen F.