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Physics World

When Bohr got it wrong

Philip Ball

Published Feb 1, 2025• 0 citations
Publisher

Abstract

Philip Ball peers into the quantum past, and uncovers a little-known paper published by Niels Bohr, Hendrik Kramers and John Slater in 1924, that proposed that the first law of thermodynamics may no longer hold firm. Their idea turned out to be wrong, but in interesting and provocative ways, and it demonstrates the intense turmoil in physics on the brink of quantum mechanics.

DOI
10.1088/2058-7058/38/02/20

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Philip Ball peers into the quantum past, and uncovers a little-known paper published by Niels Bohr, Hendrik Kramers and John Slater in 1924, that proposed that the first law of thermodynamics may no longer hold firm. Their idea turned out to be wrong, but in interesting and provocative ways, and it demonstrates the intense turmoil in physics on the brink of quantum mechanics.
Bohr’s atomic model, one of the very few pieces of physics known to the general public, turned a hundred in 2013: a very good reason to revisit Bohr’s original publications in the Philosophical Magazine, in which he introduced this model. It is indeed rewarding to (re-)discover what ideas and concepts stood behind it, to see not only ‘orbits’, but also ‘rings’ and ‘flat ellipses’ as electron trajectories at work, and, in particular, to admire Bohr’s strong belief in the importance of Planck’s law.
Abstract In January 1924, Niels Bohr, Hendrik Anton Kramers, and John Clarke Slater submitted to the Philosophical Magazine an article [Bl] that contained drastic theoretical proposals concerning the interaction of light and matter. It was written after Compton’s discovery, yet it rejected the photon. It was also written after Einstein and Bohr had met. This chapter on the BKS proposal serves a twofold purpose. It is a postscript to the story of the photon and a prelude to the Bohr Einstein dialogue which will occupy us more fully later on. I have already mentioned that Einstein was immediately and strongly impressed by Bohr’s work of 1913. The two men did not yet know each other at that time. A number of years were to pass before their first encounter; meanwhile, they followed each other’s published work.
This book gives a detailed study of the development and the interpretation given to Niels Bohr's Principle of Correspondence. It also describes the role that this principle played in guiding Bohr's research over the critical period from 1920 to 1927. Quantum mechanics, developed in the 1920s and 1930s by Bohr, Heisenberg, Born, Schrödinger and Dirac, represents one of the most profound turning points in science. This theory required a wholly new kind of physics in which many of the principles, concepts and models representing reality, that had formed the basis of classical physics since Galileo and Newton, had to be abandoned. This book re-examines the birth of quantum mechanics, in particular examining the development of crucial and original insights of Niels Bohr.
Once I tried to teach some quantum mechanics to a class of law students, philosophers, and art historians. As an advertisement for the course I put together the most sensational quotations I could collect from the most authoritative practitioners of the subject. Heisenberg was a goldmine: “The concept of the objective reality of the elementary particles has thus evaporated…”; “the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them … is impossible …” Feynman did his part too: “I think I can safely say that nobody understands quantum mechanics.” But I failed to turn up anything comparable in the writings of Bohr. Others attributed spectacular remarks to him, but he seemed to take pains to avoid any hint of the dramatic in his own writings. You don't pack them into your classroom with “The indivisibility of quantum phenomena finds its consequent expression in the circumstance that every definable subdivision would require a change of the experimental arrangement with the appearance of new individual phenomena,” or “the wider frame of complementarity directly expresses our position as regards the account of fundamental properties of matter presupposed in classical physical description but outside its scope.” I was therefore on the lookout for nuggets when I sat down to review these three volumes – a reissue of Bohr's collected essays on the revolutionary epistemological character of the quantum theory and on the implications of that revolution for other scientific and non-scientific areas of endeavor (the originals first appeared in 1934, 1958, and 1963.)
Abstract The Bohr atom proved to be unworkable for atoms with more than one electron, even when extended with the inclusion of additional quantum numbers. The correspondence principle offered little help. Progress demanded a spinning electron, and Wolfgang Pauli’s ‘exclusion principle’, which Werner Heisenberg welcomed as a record-breaking swindle. Seeking escape from the thicket of atomic contradictions, Bohr sought to slay the ultimate sacred cow. He had resisted the embrace of Einstein’s light quantum, and in a theory devised with Hendrik Kramers and John Slater, he abandoned the conservation of energy in individual atomic events, leaving it to statistics. The question could be settled experimentally, and Bohr was soon obliged to accept the results. He offered to give Bohr–Kramers–Slater (BKS) theory an honourable funeral, and acknowledged the light quantum. Einstein was more convinced than ever that physics must respect causality and allow descriptions of events taking place in space and time.