The People behind the Quantum

	Book review on Quantum by Kumar, Manjit (2008) Reviewed by Chris Clarke, 2009 published in Network Review No 99
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[Note: single inverted commas are quotations of Kumar's words, while double inverted commas reproduce Kumar's quotations from other authors.]

This is the book about the early days of quantum theory. It is an engrossing read because as each character enters the scene they are introduced with a careful biographical sketch, so that we can identify with them and share their personal grappling with the ideas as they unfold.

The story proper starts when we join Max Planck on the 14th December 1900, 'just after 5.00 pm' as he begins his lecture containing the first mention of the word 'quantum', describing a packet of energy. We hear how, in his own words, this was "a purely formal assumption" to which he "really did not give much thought" - and neither did his audience. Later we meet Einstein, working in the Swiss Patent Office, the job which he said had brought to an end "the annoying business of starving", and learn how while there he read Planck's paper in 1905. Einstein later recalled how "it was as if the ground had been pulled out from under one, with no firm foundation to be seen anywhere", but he grasped the idea of the quantum not as a "formal assumption" but as the clue to a new physical principle. So, one by one, the characters enter. We follow them as they range around Europe looking for universities with the vision to employ them, and as the ideas emerge with their bewildering contradictions, until on page 253 we reach the grand show-down at the 5th Solvay conference in Brussels in 1927, taking place particularly over coffee and croissants in the 'elegant art deco dining room' of the Hotel Metropole between the conference sessions.

Throughout this narrative Kumar describes the key ideas accurately and simply, and when he comes to the Solvay conference he goes into details, with helpful drawings, of the 'thought experiments' that played a large a role in the development of the ideas. Bohr's view was that it was essential to retain the key concepts on which physics was based, concepts like energy, position and momentum. The advantage of this was that it maintained continuity with past physics and allowed physicists to continue to use the common language that had been established over the preceding century. The cost of doing this was that these concepts could not be used to give an account of what was going on at the heart of the experiments in the new physics. Position and momentum worked fine as the experiment was being set up and as the results emerged, but in between there were strictly defined limits to their use, and one had to be guided by an entirely abstract mathematical formalism to connect what went in with what came out. Part of this formalism, moreover, involved an essentially random element. This was a radical departure from the old physics where randomness was used only as a convenient way of summing up the behaviour of complex systems whose real dynamics was regarded as purely mechanical and in principle predictable.

All this was quite contrary to Einstein's basic instincts. His work on relativity 20 years earlier had been based on dropping the old concepts of separate space and time and moving to a united picture of space-time. The limitations on the concepts of position and momentum in quantum theory meant for him that, once again, there was a need for new concepts. The introduction of randomness in quantum theory was just further proof that there was something fundamentally wrong with the theory, though as Einstein said later the problem "is not so much the question of causality but the question of realism." But Einstein had not produced an actual alternative, and so he did not give a presentation at the conference, but limited himself to probing quantum theory with questions that sought to prove that it was  inconsistent in its current form. He maintained this position, and his quest for an alternative, for the rest of his life, resulting in persistent stories that he had become senile and could no longer understand quantum theory.

            The final section of Kumar's book brings the discussion up to date with a brief sketch of more modern developments, particularly those concerning non-locality, in so far as they concern the Bohr-Einstein debate. David Bohm makes his appearance here, introduced with a fascinating account of his relations with Oppenheimer and his struggles with McCarthyism. Bohm was the first to demonstrate a viable alternative to Bohr's approach, arguing that "the mere possibility of such an interpretation proves that it is not necessary for us to give up a precise, rational, and objective description of individual systems at a quantum level of accuracy." And the link with Einstein was underlined by Bohm's thanks, in his first paper on an alternative theory, to Einstein "for several interesting and stimulating discussions." Kumar's focus then shifts to John Bell who, on reading Bohm's papers, said that he "saw the impossible done", and was set on his own course to explore just how far one could get with realising Einstein's vision of an alternative. 

Inevitably this latter part of the book omits a great deal, including much that would be of interest to  SMN readers. There is nothing on the possible role of consciousness (a topic that developed on the fringes of mainstream quantum theory in later years), nor on Bohm's wider vision of reality, within  which his alternative theory to Bohr is only an illustrative example. But the book is unequalled as an exposition of the personalities and ideas on which quantum theory is founded.

Chris Clarke was Professor of Applied Mathematics at the University of Southampton.

 

 

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