Niels Bohr’s Times, in Physics, Philosophy and Polity 
by Abraham Pais.
Oxford, 656 pp., £25, October 1991, 0 19 852049 2
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Two men tower above all other 20th-century physicists. One was lucid, quotable, persuasive and peripatetic; the other, complex, obscure, misunderstood, living and working almost entirely in the land of his birth. One was Einstein. The other was Bohr. While almost everyone has heard of Albert Einstein, few outside the halls of science have heard of Niels Bohr. Yet if we owe our understanding of the Universe in the large to the insight of Einstein, it was Bohr who first untangled the complexities of microscopic matter and wove them into a coherent pattern that revealed the true depths of meaning within the inner space of the atom and its nucleus.

There are laws governing these worlds which prevent us from learning of their state with ever-improving accuracy. No matter how perfect our instruments of observation, there exists an irreducible uncertainty in our simultaneous determination of certain ‘complementary’ pairs of properties. The act of observation produces an inevitable and indeterminable change in their structure that suffices to destroy the Cartesian picture of an ‘observer’ who is separated from the ‘observed’ like a birdwatcher in a perfect hide. We cannot determine at the same time a particle’s location and its velocity.

The indefinite ‘quantum’ reality that holds sway in the microworld of the elementary constituents of matter dictates the workings of many electronic wonders that now ease the burdens of everyday life, and the correspondence between its predictions and our observations of the world are of staggering accuracy. Indeed, quantum mechanics supplies the most successful description of any aspect of the natural world that we possess. As the size of objects increases from the sub-atomic to that of the everyday things around us, so the effects of quantum ambiguity become less and less obvious and the statistical predictions of quantum description converge on the familiar ‘classical’ picture first discovered by Newton and his successors three hundred years ago.

No phenomenon in the Universe has ever been observed which fails to obey the predictions of the quantum mechanics that Bohr established. But this success story was hardwon. Unravelling the mystery of the quantum interpretation of matter involved one of the greatest shifts in thinking that scientists have ever had to come to terms with. The story of how this new perspective emerged and transmogrified the physicists’ world view is at once the story of physics in the first half of the 20th century and the story of Niels Bohr.

Abraham Pais is a physicist who has made fundamental contributions to the quantum picture of reality. But during the last 14 years he has shown himself to be a historian of science who combines deep scientific insight with meticulous scholarship. He is perhaps the unique example in modern times of a world-class physicist turned historian. His first excursion into the history of science was his life of Einstein. The book was widely acclaimed by scientists, but less enthusiastically received by some historians who, while not denying Pais’s mastery of the facts, looked in vain for a deeper interpretation which would make contact, on the one hand, with Einstein’s character and, on the other, with the dramatically changing world which his career spanned. In the present volume, Pais brings his insights to bear on a subject even closer to his heart than Einstein. For Pais was for many years one of Bohr’s young research assistants in Copenhagen, living for long periods in the Bohrs’ family home, and joining in the struggle to uncover the secrets of the atom and the nucleus within it. There is of course a danger here. Some may feel that Pais has been too close to Bohr and his family to provide a full and impartial account of his life.

Niels Bohr was born in Copenhagen in 1885. He grew up in a large and happy family and was throughout his life particularly close to his brother Harald, who was both a world-class mathematician and a footballer of international calibre. Niels, too, was a keen footballer and played in goal for a Danish club side. Their father was a physiologist of no mean achievement, having been nominated for the Nobel Prize for medicine in both 1907 and 1908, for his discoveries concerning the effect of carbon dioxide on human blood. Bohr’s parents and his teachers soon recognised his extraordinary abilities – he displayed extraordinary physical intuition and powers of analysis from his student days.

The influence of his father was important but subtle. For Christian Bohr had interesting philosophical views regarding ‘complementarity’ in science and human enquiry. He had seen that there were indeed questions one could ask of the world which, once asked, render certain other questions meaningless. The biologist must kill an animal in order to discover its internal structure, thereby making it impossible to put other questions concerning the nature of life. Free will and determinism seemed to Christian Bohr to be another pair of complementary opposites. Later, his son would use something akin to this notion of complementarity to crystallise his interpretion of our observations of the quantum world. In the latter part of his life, when his interest broadened to include politics and philosophy, he returned to the philosophical implications of complementarity whenever the analysis of a given situation needed to take into account the entire state of affairs – to include both the observer and the observed – in order to avoid contradiction and paradox. One can even find traces of it in Bohr’s early espousal of an evolutionary epistemology whereby our view of the world is seen as a consequence of a process of natural selection that selects for those ideas which accord with the true nature of things in themselves. On this view, our scientific picture of those aspects of physical reality whose accurate apprehension is a necessary prerequisite for our own evolution may be said to be ‘realistic’.

Pais’s account of Bohr is enriched by his experience of living and working in Bohr’s home. The picture that emerges is confirmed by others who had the same experience: his successful marriage and the pleasure he took in his six children, one of whom was to become a Nobel Prize-winner in physics in his turn, and the shattering double blow he was dealt by the deaths of his oldest son in a boating accident and of the youngest from meningitis.

Pais’s subtitle reveals the threefold division of Bohr’s life that he sees it in retrospect as having possessed. Bohr’s contribution to philosophy was small and certainly had no effect on leading philosophers either during his life or after his death. Pais has not engaged in any detailed analysis of its content and significance, but instead has devoted himself to the other two sides of Bohr’s activity. In physics Bohr’s story is that of the end of classical physics and the beginnings of quantum theory, with its successes in unravelling the structure and vibrations of atoms, and in the mathematical understanding of the periodic table of the elements and the creation of modern chemistry.

Hand in hand with these successful applications of the new quantum mechanics he was wrestling with the question of the theory’s correct interpretation and with the fact that it gives only probabilistic predictions of atomic behaviour. This drew him into a debate with Einstein regarding the correctness of the quantum description of reality. Einstein saw quantum mechanics as an unsatisfactory but effective description of how things happen, but one whose lack of determinism disqualified it from forming part of the ultimate theory of how things are. Einstein supported his argument by setting up ‘thought-experiments’ which he believed exposed the logical contradictions created by quantum mechanics. But in every instance, as Bohr demonstrated, Einstein was wrong and Bohr was finally able to persuade him of the self-consistency of the quantum description of events when it is correctly and fully applied.

In his dealings with Einstein, and in his discussions with other physicists, one is struck by Bohr’s intuition and speed of thought in unfamiliar territory. Some found it altogether too much; James Franck even left Bohr’s circle to take up a professorship in America, despite his respect for Bohr, because he found working close to him unsettling:

Bohr did not allow me to think through whatever I did to the end. I made some experiments. And when I told Bohr about it, then he said immediately what might be wrong, what might be right. And it was so quick that after a time I felt that I am unable to think at all ... Bohr’s genius was so superior.

Bohr’s next great quest was to understand the nucleus of the atom, a problem that he was led to tackle by George Gamow’s discovery that ‘alpha’ radioactivity could be understood as a quantum mechanical ‘tunnelling’ process that possessed no classical analogue. Gamow visited Bohr’s Copenhagen Institute during the years 1928 to 1931 and used some of that time to write the first book on theoretical nuclear physics.

Bohr’s theoretical research in nuclear physics led inevitably to his involvement in the wartime quest for its military exploitation; and it is a pity that Pais completed his work before the records of the German atom-bomb project and the debriefing of German scientists, including Heisenberg and von Weiszäcker, were released. But neither those records nor Pais’s own account are able to shed very much light on the famous private meeting that took place between Bohr and Heisenberg in Copenhagen when the city was under German control. It is not clear whether Heisenberg was seeking to enrol Bohr in the German bomb project or merely to find out his current interests and intentions. Heisenberg’s own relationship with the German authorities was also ambiguous. Soon afterwards Bohr escaped to Sweden, from where he was flown to Britain under somewhat bizarre circumstances. He made the flight in the bomb-bay of a Mosquito. Before take-off he was shown how to use an oxygen mask, but presumably his thoughts were elsewhere, because when the time came to put on the mask the pilot failed to raise him on the intercom. He landed in Scotland to discover Bohr in good shape, claiming that he had slept soundly all the way – he had evidently been unconscious from the lack of oxygen.

Bohr’s arrival in Britain and his subsequent voyage to the United States to contribute to the Manhattan Project marked the start of the third phase of his life: his political campaigning for international co-operation. He was one of the first potentially influential advocates of openness between East and West over weapons-related scientific expertise. He believed that the sharing of nuclear know-how would create a more stable world than the fast-developing arms race between the superpowers. As far as I am aware, there have been no detailed evaluations of the effectiveness (if any) of Bohr’s interventions by historians of the period. All the commentators seem, like Pais, to have been physicists whose opinion of Bohr is so coloured by their high regard for his other intellectual achievements that one wonders about their objectivity in this matter. Certainly, we know that the meeting between Bohr and Churchill was less than a success: that Churchill’s reaction to Bohr’s obscure and un necessarily labyrinthine arguments was to want ‘this blithering idiot out of here.’ Bohr was subsequently considered a security risk because of his advocacy of information-sharing with the Soviet Union. As a direct result of his interview with Churchill, his attempts to influence Roosevelt were doomed and following a meeting of the two Allied leaders in New York their aide mémoire contains an item to the effect that ‘enquiries should be made regarding the activities of Professor Bohr and steps should be taken to ensure that he is responsible for no leakage of information, particularly to the Russians.’

These are interesting issues that Pais does not dwell on. Perhaps there was no more evidence to be gathered and no more witnesses of these events to question. But I suspect that this was not the case. From Pais’s account of those years one gets the feeling that Bohr was a victim of the very same philosophy that he had stressed so powerfully in his analysis of events in the quantum world – that one must consider the system as a whole and not dichotomise it into the observer and the observed. The ideas that he was promulgating were not in themselves illogical: but they have to be considered in context. In the immediate post-war world Bohr’s idealistic suggestions were inappropriate: today they seem almost uncontroversial.

Niels Bohr was a thinker of enormous depth and subtlety, and Abraham Pais has given us a masterly ordering of the facts. His account of Bohr’s scientific work, of his family life and of his affection for the young physicists who passed through his school is clear and authoritative. The wider range of his interests will repay further study by others less influenced by personal acquaintance and scientific admiration. There is these days much loose journalistic talk about ‘new Einsteins’ but if Pais’s biographies of Einstein and Bohr teach us anything, it is that in achievement and insight no living physicist comes within light years of these two stars in the firmament of 20th-century physics.

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