Review by Andrew Crumey

Published in the Telegraph

Manjit Kumar
Icon Books, £20

Even popular science is not safe from Bond mania: Manjit Kumar describes how Einstein, appointed to Prague University in 1911, found the pay-check “a quantum of solace against the creeping sense of isolation.” His book may not be high on thrills but it is certainly an adventure, as he traces the history of quantum theory.

Kumar has taken on a formidable task: modern physics is famously incomprehensible, even to people who work on it. As Richard Feynman supposedly said, “If you think you understand quantum mechanics, you probably don’t.” But did he ever really say that? That’s another problem for Kumar: weighing up the evidence. It would be a rare author who could fully address both the philosophical and the historical issues – an even rarer one who could make it all palatable and entertaining to a general audience. If Kumar scores less than full marks it is only because of the admirably ambitious scale of his endeavour.

His chapters take us through the work of Planck, Bohr, Einstein and other luminaries, mixing biographical facts with an outline of what they did. The section on Heisenberg illustrates both the strength and weakness of the approach. Trying to make sense of the energy levels of atoms – and suffering atrociously from hay-fever – the young Heisenberg went to the North Sea island of Helgoland and had his eureka moment. Measuring first the position and then the momentum of an electron, he realised, would yield a different answer from doing the measurements the other way round. Thus was born his famous uncertainty principle, which Heisenberg interpreted as saying that any observation changes what is being observed.

Kumar accurately and succinctly retells this famous story, but a historian will immediately question the sources, and a philosopher will question the meaning. The source in this case is Heisenberg himself, writing many years later, after he had led the German atom bomb project. No one would take at face value Heisenberg’s claims that he was a reluctant Nazi who wanted the bomb to fail; we should surely be equally cautious about his recollection of conversations with Einstein that supposedly led him to his great discovery. Yet these are reported as historical fact.

Niels Bohr, as Kumar points out, quickly objected to Heisenberg’s interpretation of the principle; but Kumar’s explanation is clumsy. “What prohibits the precise measurement of the momentum of the electron is not the discontinuous and uncontrollable nature of the mometum change, Bohr argued, but the impossibility of measuring that change exactly.” Bohr was notoriously obscure but never quite so circular: Kumar eventually arrives at Bohr’s real point. Saying that measurement alters what is being measured implies a pre-existing “real” value we can never know; yet for Bohr, as Kumar says, “an unobserved electron does not exist”.

It led to a debate between Bohr and Einstein about the meaning of reality itself, in which Heisenberg (and nearly everyone else) sided with Bohr. That great debate forms the heart of Kumar’s book, and is its most enjoyable part. Kumar is a trained physicist, and his explanation of the ingenious “thought experiments” with which the two giants tried to outwit each other is clear and precise. It is perhaps a pity that the whole book was not geared around this episode (as the cover subtitle implies), since it brings into clearest focus the theory’s puzzling core. Most general readers, I suspect, will be more interested in such basic questions than in black-body radiation or electron spin, which get almost equal coverage. Some may also be disappointed that although the narrative takes us up to recent experimental vindications of Bohr’s “Copenhagen interpretation”, there is very little on Everett’s “many worlds” alternative, and nothing at all on quantum field theory, whose predictions are to be tested by the Large Hadron Collider.

Still, although I would not recommend this book to anyone hoping to learn quantum theory without tears, for those who already know at least some of the physics and want a brief, non-academic but thorough history of the subject’s golden age, it is hard to beat, even if nits can be picked. Most interesting to me is the light this book sheds on minor figures whose names are never mentioned in lecture courses, though they made crucial contributions that were taken up by the bigger stars. In physics as in everything else, history gets written by the winners. Kumar thankfully gives some runners-up their due.