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In time it was discovered that the energy of an electron inside an atom was 'quantised'; it could possess only certain amounts of energy and not others. The same was true of other physical properties, as the microscopic realm was found to be lumpy and discontinuous and not some shrunken version of the large-scale world that humans inhabit, where physical properties vary smoothly and continuously, where going from A to C means passing through B. Quantum physics, however, revealed that an electron in an atom can be in one place, and then, as if by magic, reappear in another without ever being anywhere in between, by emitting or absorbing a quantum of energy. 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.

By the early 1920s it had long been apparent that the advance of quantum physics on an ad hoc, piecemeal basis had left it without solid foundations or a logical structure. Out of this state of confusion and crisis emerged a bold new theory known as quantum mechanics. The picture of the atom as a tiny solar system with electrons orbiting a nucleus, still taught in schools today, was abandoned and replaced with an atom that was impossible to visualise. Then, in 1927, Werner Heisenberg made a discovery that was so at odds with common sense that even he, the German wunderkind of quantum mechanics, initially struggled to grasp its significance. The uncertainty principle said that if you want to know the exact velocity of a particle, then you cannot know its exact location, and vice versa.

No one knew how to interpret the equations of quantum mechanics, what the theory was saying about the nature of reality at the quantum level. Questions about cause and effect, or whether the moon exists when no one is looking at it, had been the preserve of philosophers since the time of Plato and Aristotle, but after the emergence of quantum mechanics they were being discussed by the twentieth century's greatest physicists.

With all the basic components of quantum physics in place, the fifth Solvay conference opened a new chapter in the story of the quantum. For the debate that the conference sparked between Einstein and Bohr raised issues that continue to preoccupy many eminent physicists and philosophers to this day: what is the nature of reality, and what kind of description of reality should be regarded as meaningful? 'No more profound intellectual debate has ever been conducted', claimed the scientist and novelist C.P.Snow. 'It is a pity that the debate, because of its nature, can't be common currency.'6

Of the two main protagonists, Einstein is a twentieth-century icon. He was once asked to stage his own three-week show at the London Palladium. Women fainted in his presence. Young girls mobbed him in Geneva. Today this sort of adulation is reserved for pop singers and movie stars. But in the aftermath of the First World War, Einstein became the first superstar of science when in 1919 the bending of light predicted by his theory of general relativity was confirmed. Little had changed when in January 1931, during a lecture tour of America, Einstein attended the premiere of Charlie Chaplin's movie City Limits in Los Angeles. A large crowd cheered wildly when they saw Chaplin and Einstein. 'They cheer me because they all understand me,' Chaplin told Einstein, 'and they cheer you because no one understands you.'7

Whereas the name Einstein is a byword for scientific genius, Niels Bohr was, and remains, less well known. Yet to his contemporaries he was every inch the scientific giant. In 1923 Max Born, who played a pivotal part in the development of quantum mechanics, wrote that Bohr's 'influence on theoretical and experimental research of our time is greater than that of any other physicist'.8 Forty years later, in 1963, Werner Heisenberg maintained that 'Bohr's influence on the physics and the physicists