Item #111 Discussion with Einstein on Epistemological Problems in Atomic Physics. NIELS BOHR, ALBERT EINSTEIN.
Discussion with Einstein on Epistemological Problems in Atomic Physics
Discussion with Einstein on Epistemological Problems in Atomic Physics
Discussion with Einstein on Epistemological Problems in Atomic Physics
Discussion with Einstein on Epistemological Problems in Atomic Physics

Discussion with Einstein on Epistemological Problems in Atomic Physics

“The discussions with Einstein which have formed the theme of this article have extended over many years which have witnessed great progress in the field of atomic physics. Whether our actual meetings have been of short or long duration, they have always left a deep and lasting impression on my mind, and when writing this report I have, so-to-say, been arguing with Einstein all the time even when entering on topics apparently far removed from the special problems under debate at our meetings. As regards the account of the conversation I am, of course, aware that I am relying 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. I trust, however, that I have not failed in conveying a proper impression of how much it has meant to me to be able to benefit from the inspiration which we all derive from every contact with Einstein.”


Bohr’s decades-long debate with Einstein on the foundations of quantum physics is “one of the great scientific debates in the history of physics, comparable, perhaps, only to the Newton-Leibniz controversy of the early eighteenth century... In both cases it was a clash of diametrically opposed philosophical views... a clash between two of the greatest minds of their time” (Jammer Philosophy of Quantum Mechanics). “Bohr’s account of his discussion with Einstein has been called ‘one of the great masterpieces of modern scientific reporting.’ According to Abraham Pais ‘nowhere in the literature can a better access to [Bohr’s] thinking be found’” (Jammer).

“It is generally agreed that Bohr’s article of 1949 is his finest exposé on complementarity (Pais, Niels Bohr’s Times). The highlight of the paper, and of the Bohr-Einstein debate itself, was Bohr’s refutation of Einstein’s famous ‘photon in a box’ Gedankenexperiment. The Bohr-Einstein debate began in 1920, when Bohr met Einstein in Berlin, and continued in 1927 at the fifth Solvay conference, where Einstein presented arguments to show that quantum mechanics was an inconsistent theory, arguments which Bohr refuted. At their next encounter, in 1930 at the following Solvay meeting in Brussels, Einstein informed Bohr that he had found a counterexample to the uncertainty relation between energy and time.

“The argument was quite ingenious. Consider a box having in one of its walls a hole that can be opened or closed by a shutter controlled by a clock inside the box. The box is filled with radiation. Weigh the box. Open the shutter for a brief interval during which a single photon escapes. Weigh the box again, some time later. From the weight difference one can deduce the energy of the photon by using E = mc2. Thus (in principle) one has found to arbitrary accuracy both the photon energy and its time of passage, in conflict with the energy-time uncertainty relation.

“Rosenfeld who was in Brussels at that time has recalled: ‘It was quite a shock for Bohr... he did not see the solution at once. During the whole evening he was extremely unhappy, going from one to the other and trying to persuade them that it couldn’t be true, that it would be the end of physics if Einstein were right; but he couldn’t produce any refutation. I shall never forget the sight of the two antagonists leaving [the Fondation Universitaire], Einstein a tall majestic figure, walking quietly, with a somewhat ironic smile, and Bohr trotting near him, very excited … The next morning came Bohr’s triumph.’

“The flaw Bohr had found was that Einstein had not taken into account that the weighing process amounts to observing the displacement of the box in a gravitational field... The imprecision in the displacement of the box generates an uncertainty in the determination of the mass, and hence of the energy, of the photon. When the box is displaced then so is the clock inside which therefore ticks in a gravitational field slightly different from the one in the initial box position. Now Bohr noted the well-known fact that the rate of a clock depends in a specific way on its position in a gravitational field. Correspondingly the imprecision in the box displacement generates an uncertainty in the determination of time. Using the position-momentum uncertainty relation as input, Bohr could show that the uncertainties in energy and in time are just linked by the energy-time uncertainty relation. All was well.

“Bohr’s version of the clock-in-the-box experiment was not published until 1949
when it appeared in a volume planned to be presented to Einstein on his 70th birthday” (Pais). “This episode was one of the highlights of the Bohr-Einstein debate – and this not only because of the dramatic features involved. It was also a turning point in Einstein’s attitude toward quantum mechanics. Accepting Bohr’s counterargument – for what could have been nearer to his heart than his own red-shift formula – he gave up any hope of refuting the quantum theory on the grounds of an internal inconsistency. Instead... after the 1930 Solvay Congress Einstein concentrated on demonstrating the incompleteness, rather than the inconsistency, of quantum mechanics” (Jammer).

Offprint from: Albert Einstein: Philosopher-Scientist. Evanston, IL: Library of Living Philosophers, 1949. Octavo, (232 x 155 mm), pp. 201-241. Original printed wrappers. Some very light sunning to wrappers, otherwise a very fine copy. RARE.

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