A history of quantum weirdness

Manjit Kumar’s new book charts the historic clash between Einstein and Bohr over quantum mechanics, and the science and philosophy that shaped their arguments.

John Gillott

Topics Books

The science of quantum mechanics underpins much of modern physics and its technological applications. It is phenomenally successful as a predictive theory. But as many readers might know, there is another side to the quantum, which we might call ‘quantum weirdness’.

In popular culture the best-known example is probably Erwin Schrödinger’s famous thought experiment in which a cat in a fiendishly designed box is suspended in a mixed dead/alive state; the cat’s fate is only resolved by the process of observation. Other examples include the idea of wave-particle duality, Heisenberg’s uncertainty principle, and ‘spooky action at a distance’. World Cup-winning rugby player Jonny Wilkinson is only the latest author to claim a link between quantum mechanics and one form or another of Eastern mysticism.

Quantum weirdness is not rooted in popular misconceptions of the ideas of scientists – or not entirely so. The intriguing issue is that it is rooted in a highly influential interpretation of quantum mechanics, known as the Copenhagen Interpretation on account of the fact that the Danish Nobel Laureate Niels Bohr was central to its development. If you weren’t shocked by quantum theory, argued Bohr, you didn’t really understand it. And the most shocking thing for Albert Einstein and some other physicists was that Bohr believed quantum mechanics demanded the abandonment of the idea of an underlying quantum reality existing independently of the observer.

How could anyone not believe in the importance of ‘reality’ in developing a scientific theory? The subtleties involved in that apparently simple question are what make the debate between Bohr and Einstein so fascinating and of such importance, a debate expertly told and analysed in a new book by Manjit Kumar. At stake, he argues, were the soul of physics and the nature of reality.

We are used to thinking in terms of continuity. For example, rapid motion between two points might seem instantaneous, discontinuous, in the ‘blink of an eye’, but if we slow a film of it down we expect to and do see the steps in between. The idea of the quantum, introduced into physics in the first two decades of the twentieth century, challenged this viewpoint. In 1913, Bohr applied the idea to the electron in the atom: it could only possess certain amounts of energy and not others. If an electron absorbed or emitted energy it had to move to a different position in the atom instantaneously, without occupying any place in between. As Kumar notes, ‘This was a phenomenon beyond the ken of classical, non-quantum physics. It was as bizarre as an object mysteriously disappearing in London and an instant later suddenly reappearing in Paris, New York or Moscow.’

Bohr’s quantum atom won the praise of Einstein and others. But everyone agreed that profound difficulties remained, that the introduction of the idea of the quantum into the atomic realm had left the physics of the atom and other phenomena without solid foundations or logical structure. Quantum mechanics, developed in the mid-1920s, provided those foundations and structure. But what did it mean? Bohr more than anyone else was to provide an answer to this that was in keeping with the results of quantum mechanics; one that accepted the strange findings of quantum mechanics, which included not just quantum jumps but also the idea that light and matter were simultaneously wave-like and particle-like. As he did so, it was Einstein more than anyone else that he wanted to convince of the correctness of his interpretation. But even before the formalism of quantum mechanics was developed, Einstein was already expressing grave concerns: ‘I find the idea quite intolerable that an electron exposed to radiation should choose of its own free will, not only its moment to jump off, but also its direction. In that case, I would rather be a cobbler or even an employee in a gaming-house, than a physicist.’ In Einstein’s view, quantum mechanics was clearly powerful but it could not be the final word because it suggested results incompatible with an objective and causal view of the natural world.

The Nobel Prize-winning physicist Murray Gell-Mann believes that ‘Bohr brainwashed a whole generation of physicists into believing that the problem had been solved’. Kumar is in firm agreement with this and, indeed, unlike many writers on the subject, is strongly sympathetic to Einstein’s approach to the issue. In the central section of the book and his somewhat open-ended conclusion, he discusses the different phases of Einstein’s challenge to Bohr’s interpretation of quantum mechanics and the experiments that have been done since their deaths that cast some light on the question. This is one of the novel aspects of the book. But for me this book’s greatest strength is that it outlines the complexity of the scientific and philosophical debate as it unfolded in time. In doing so, Kumar explains the interaction of the science and the philosophy and why it was that Bohr’s approach seemed and seems plausible.

Albert Einstein is regarded as the greatest physicist of the twentieth century, his theory of general relativity in particular is one of the outstanding intellectual achievements not just of the last century but in the history of science. In the aftermath of the First World War, observations seen as vindicating this theory catapulted Einstein to worldwide fame and established him as the successor to Isaac Newton.

Einstein was also one of the first to embrace and develop, in a radical way that shocked his contemporaries, the idea of the quantum in physics. Indeed, it was for his pioneering work on the quantum rather than relativity theory that he was awarded the Nobel Prize. But he did not simply embrace and develop the quantum, he also harboured doubts about it, doubts that were to crystallise through one of the most intense and fascinating debates in the history and philosophy of science. Despite the many hard years of work he put into developing relativity theory, the quantum and its interpretation took up far more of Einstein’s time and thoughts. The verdict of history and indeed of many of his biographers is that while Einstein was a revolutionary when it came to relativity theory, he was, in the end, a conservative when it came to the quantum, in particular the all-encompassing theory of quantum mechanics and its interpretation developed by Niels Bohr and the Young Turks of physics he inspired in the mid to late 1920s.

Einstein’s challenge to the Copenhagen Interpretation of quantum mechanics went through two phases. In the first, from 1927 to around 1930, he tried to prove that quantum mechanics was inconsistent and contradictory. He came up with a number of ingenious thought experiments that he sprang on Bohr and his supporters at the conferences they attended together, designed to highlight a flaw in the theory. Bohr was sometimes plunged into doubt and even despair, but he always came out smiling as he was able to rebut each challenge in turn. His victories alongside the success of quantum mechanics as both a research programme and a predictive theory persuaded most scientists to settle down to using the theory rather than worrying about its philosophical claims. Einstein was in no doubt by the end of this period that quantum mechanics had proved its worth: ‘I know this business is free of contradictions’, he said. However, he went on to add: ‘yet, in my view it contains a certain unreasonableness.’

This set the stage for the second phase of his challenge, contained in a paper written in 1935 in collaboration with Boris Podolsky and Nathan Rosen. The EPR paper, as it became known, was titled Can [the] Quantum Mechanical Description of Physical Reality Be Considered Complete? EPR’s answer was ‘no’. The novelty and importance of Einstein’s approach was that while he still sought to highlight scientific problems and contradictions, his guiding principle was philosophical (realist), and on this basis he made a judgement about the incompleteness of the theory as a description of reality. Bohr, by contrast, fused philosophy and theory together, and on the basis of an essentially positivist approach declared quantum mechanics to be a complete theory.

The wider significance of Einstein’s challenge was that it was Bohr and not Einstein who was the scientific conservative. It was Bohr who was declaring existing concepts and categories to be the final word in physics. Prior to the EPR paper Einstein had already noted this feature of the debate in a letter to Schrödinger: ‘The ‘Heisenberg-Bohr tranquilizing philosophy – or religion? – is so delicately contrived that for the time being, it provides a gentle pillow for the true believer from which he can not very easily be aroused. So let him lie there.’

It would be foolish of course to believe that any one book could provide the lay reader with a deep understanding of the history, philosophy and science of quantum mechanics. Nevertheless, the virtue of Kumar’s book is that it takes us deeper than many and in doing so gives an insight into what we don’t know.

Readers should not expect a brisk popular introduction to the science as a prelude to the debate about its meaning and interpretation. Rather, starting at the beginning with Max Planck in the late nineteenth century, Kumar tells the fascinating story of how the quantum – in contrast to the idea of continuity – was introduced into physics, sometimes as an act of desperation, sometimes as a heuristic device and often in a wild and ad hoc way. By the early 1920s there was a crisis in physics, a crisis crying out for a new and comprehensive approach. This was provided by two distinct but equally powerful mathematical approaches developed by Werner Heisenberg and Erwin Schrödinger. But what did their solutions mean, physically speaking? Kumar’s discussion of Heisenberg’s and Schrödinger’s work sets the scene for the core of the book; the debate between Einstein and Bohr over the interpretation of the new quantum mechanics.

There is a place for something close to ‘Whiggish’ histories when it comes to natural science (more so than in the history of society perhaps), and many books, this one included, in telling a historical story will quite naturally tend to focus on the scientific ideas that led on to what became the key ideas. However, Kumar tempers this, making sure that it doesn’t tip over into reading history backwards, by ensuring that, at each point in the development of quantum physics, we get to see the leading scientists struggling to understand problems in the science and what it might mean using the categories available to them at the time; we get to see, in other words, science as it really happened.

Kumar’s book in this regard captures the spirit of the recollection of Robert Oppenheimer, quoted in the prologue: ‘It was a period of patient work in the laboratory, of crucial experiments and daring action, of many false starts and many untenable conjectures. It was a time of earnest correspondence and hurried conferences, of debate, criticism and brilliant mathematical improvisation. For those who participated it was a time of creation.’

John Gillott is co-author, with Manjit Kumar, of Science and the Retreat from Reason (Merlin, 1995 & Monthly Review Press, 1997).

Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality, by Manjit Kumar, is published by Icon Books Ltd. (Buy this book from Amazon(UK).)

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