Between 1927 and 1936 no one had a greater influence on Bohr's thinking than Einstein. It was Einstein's unremitting criticism which provided the severest test of Bohr's interpretation of quantum physics, and which forced him to clarify and refine his arguments. The debate between Bohr and Einstein concerning the interpretation of quantum physics is generally recognised as one of the great intellectual disputes in the history of science. What was the nature of the disagreement between them, and what were the arguments with which each defended his position? The fifth Solvay Conference, 1927 Einstein was not present at the Volta Centennial Conference at Como in September 1927, where Bohr first presented his theory of complementarity; Einstein first learned of Bohr's interpretation of quantum mechanics at the fifth Solvay Conference of Physics at Brussels in October 1927. In the discussion following Bohr's paper Einstein examined an imaginary experiment in which an electron passes through a slit in a diaphragm and impinges at a point on a hemispherical screen. This experiment, Einstein suggested, could be interpreted in either of two ways: the de Broglie–Schrödinger waves represent either ( a ) an ensemble of electrons spread out in space (but not an individual electron), or ( b ) a wave-packet corresponding to an individual electron. There is, he noted, a difficulty with the latter interpretation.
Between 1927 and 1936 no one had a greater influence on Bohr's thinking than Einstein. It was Einstein's unremitting criticism which provided the severest test of Bohr's interpretation of quantum physics, and which forced him to clarify and refine his arguments. The debate between Bohr and Einstein concerning the interpretation of quantum physics is generally recognised as one of the great intellectual disputes in the history of science. What was the nature of the disagreement between them, and what were the arguments with which each defended his position? The fifth Solvay Conference, 1927 Einstein was not present at the Volta Centennial Conference at Como in September 1927, where Bohr first presented his theory of complementarity; Einstein first learned of Bohr's interpretation of quantum mechanics at the fifth Solvay Conference of Physics at Brussels in October 1927. In the discussion following Bohr's paper Einstein examined an imaginary experiment in which an electron passes through a slit in a diaphragm and impinges at a point on a hemispherical screen. This experiment, Einstein suggested, could be interpreted in either of two ways: the de Broglie–Schrödinger waves represent either ( a ) an ensemble of electrons spread out in space (but not an individual electron), or ( b ) a wave-packet corresponding to an individual electron. There is, he noted, a difficulty with the latter interpretation.
Abstract Einstein was not in the audience at Bohr’s Como lecture, and their debate was joined in earnest at the fifth Solvay conference in Brussels in October 1927. All of quantum theory’s founders were present. Einstein excused himself for not having mastered the theory, and introduced a simple electron diffraction thought experiment for which he offered two conceptions. In the first, the wavefunction does not describe individual events and is purely statistical. In the second, the wavefunction does describe individual events, its modulus-square giving the probability of detecting the next diffracted electron at a specific position. In his view, the second conception could not stand, as it implies a ‘very peculiar mechanism of action at a distance’. Outside the conference room Einstein peppered Bohr with thought experiments devised to reveal the theory’s confusions or contradictions. The exchanges between them went on like a game of chess, with Bohr emerging the victor.
The Bohr-Einstein debate, the most interesting intellectual contest in science of this century, began in the late afternoon of October 24,1927, at the Fifth Solvay Conference in Brussels, Belgium. 50 (Einstein had not attended the meeting in Como a month before, at which Bohr first introduced complementarity.) The theorists Bohr, Born, de Broglie, Brillouin, Dirac, Einstein, Ehrenfest, Heisenberg, Pauli, Planck, and Schrödinger were in attendance, as well as the experimentalists Bragg, Compton, Madame Curie, and Debye. The eminent Dutch theorist H.A. Lorentz, the grandfather of relativity (after whom the theory’s space-time transformations were named), chaired the sessions. The organizers had chosen “Electrons and Photons” as the official title of the conference, but a better one might have been: “Quantum Mechanics: What Does it Mean?” As Einstein’s opinion had become widely known, fireworks were expected. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
ASPECTS OF THE DEBATE The debate between Niels Bohr and Albert Einstein concerning the interpretation of quantum mechanics extended from the fifth Solvay Conference in 1927 until the end of Einstein's life. The most dramatic exchange occurred in 1935, when Einstein, in collaboration with B. Podolsky and N. Rosen, published a paper in Physical Review entitled “Can quantum-mechanical description of physical reality be considered complete?”, concluding with a negative answer, and Bohr replied in the same journal, with a paper of the same title but giving a positive answer. Their arguments were restated, in some respects with greater clarity, in the Library of Living Philosophers volume on Einstein in 1949. The disagreements between Bohr and Einstein concerned not only the physical question expressed in the common title of their papers, but also philosophical questions about physical reality and human knowledge. Much light was thrown upon the Bohr–Einstein debate by the theorem of J. S. Bell (1964) and the experiments which it inspired. Bell showed that any physical theory which applies to a pair of spatially separated systems (as considered in the thought experiment of Einstein, Podolsky, and Rosen) and satisfies a certain locality condition will, in certain circumstances, disagree statistically with quantum mechanics. Consequently, the supplementation of the quantum mechanical description envisaged by Einstein and his collaborators (usually referred to as a “hidden variables theory”) must either violate the locality condition or clash with quantum mechanical predictions.
The Bohr--Einstein debate is one of the more remarkable protracted intellectual exchanges in the history of physics. Its influence has been lasting: One of the few clear patterns in a 2013 survey about quantum foundations was that the physicists who believed Bohr to be correct were apt to say that Einstein had been wrong. The exchanges began when Bohr and Einstein first met in 1920, continued at the Solvay conferences of the following decade, reached a dramatic crescendo with the EPR paradox in 1935, and continued thereafter. Not every episode in this long story has been investigated equally. In particular, one late statement attributed to Bohr has received much more intense examination than Einstein's equally pithy reply.
The fifth Solvay conference is usually remembered for the clash that took place between Bohr and Einstein, supposedly concerning in particular the possibility of breaking the uncertainty relations. It might be assumed that this clash took the form of an official debate that was the centrepiece of the conference. However, no record of any such debate appears in the published proceedings, where both Bohr and Einstein are in fact relatively silent. The available evidence shows that in 1927 the famous exchanges between Bohr and Einstein actually consisted of informal discussions, which took place semiprivately (mainly over breakfast and dinner), and which were overheard by just a few of the participants, in particular Heisenberg and Ehrenfest. The historical sources for this consist, in fact, entirely of accounts given by Bohr, Heisenberg and Ehrenfest. These accounts essentially ignore the extensive formal discussions appearing in the published proceedings. As a result of relying on these sources, the perception of the conference by posterity has been skewed on two counts. First, at the fifth Solvay conference there occurred much more that was memorable and important besides the Bohr–Einstein clash. Second, as shown in detail by Howard (1990), the real nature of Einstein's objections was in fact misunderstood by Bohr, Heisenberg and Ehrenfest – for Einstein's main target was not the uncertainty relations, but what he saw as the non-separability of quantum theory.
Now that my account of Bohr's philosophy of physics has been completed, it remains for me to give a general assessment of it. What value should we assign to Bohr's main ideas – the notions of correspondence and complementarity? How does his interpretation of quantum physics fare in comparison with some of its rivals? What should our assessment of the Bohr–Einstein debate be? I shall take up these questions in the reverse order. Einstein or Bohr? The final verdict The Bohr–Einstein debate is primarily a dispute about the philosophical foundations in terms of which the interpretation of quantum mechanics should be based: specifically it is a dispute about the intrinsic-properties theory, i.e. about the scope of a realist interpretation of quantum mechanics. Einstein wished to maintain the two realist theses, the intrinsic-properties theory and the objective-values theory, whereas Bohr wished to hold only the latter. My own philosophical leanings are in the direction of realism. To my mind the notion of a physical reality the existence and constitution of which are independent of human thought and perception is coherent and plausible: what physical objects there are, and what properties they have, is not determined by human thought or perception, or by human practical interests. I have sympathy, therefore, with Einstein's position concerning the indefinability thesis and the question of the completeness of quantum mechanics.