Letting the quantum cat out of the bag

Einstein complained that younger physicists had left reality behind – and he may have had a point

352pp, OUP, T £19.99 (0808 196 6794), RRP£25, ebook £14.99 ÌÌÌÌÌ

Viewers of Christopher Nolan’s Oscar-hoovering biopic about the physicist J Robert Oppenheimer will have learnt a lot about the race to build nuclear weapons, but not much about the hero’s involvement in arguably a much more important debate: what science ought to be able to tell us about reality. Oppenheimer has a walk-on part in this account of that epic struggle, introduced as “psychologically tender” and quoted as declaring that Albert Einstein (impersonated in the movie by Tom Conti) had “no understanding of or interest in modern physics”.

It was indeed once the popular history that by the time the Danish genius Niels Bohr and his comrades had built the startling edifice of quantum mechanics, Albert Einstein was already too old and set in his ways to accept it, and spent the rest of his life kvetching impotently from the sidelines. In fact, and as this hugely detailed narrative of the 20th-century battle for the meaning of physics emphasises, Einstein’s challenges to the theory were tremendously powerful and inspired decades of fruitful research into questions that are still not definitively settled.

Bohr’s base of theoretical operations lent its name to what became known as the “Copenhagen interpretation” of quantum mechanics, according to which – to put it very crudely – subatomic particles and maybe bigger things don’t exist anywhere in particular until you look at them. Bohr was unperturbed by the implications, trusting in the equations.

But Einstein would not accept a style of physics that, as he saw it, abandoned its duty to describe the real world. Nor, completely, would his friend Schrödinger, whose famous cat – which is somehow both alive and dead in its box until someone opens it – was intended as a reductio ad absurdum.

This book is at the crunchily technical end of pop science, but also of its highest-quality peak. It helps to be able to read an equation or three, but the style is humane and interesting, and also blessed with a fantastically dry sense of humour. Among the story’s teeming cast we meet “a co-operative experimentalist who did indeed find the desired result and could have found any result desired, for he faked his experiments”, and a physics-curious English theologian, “author of too many books”. We can no longer stand on the hill where Heisenberg had an

important insight, “for the British blew it up, not from opposition to quantum mechanics, but because it capped a fortress the Germans had used during World War II”.

The authors also trace how apparently hermetic notions leaked out into the wider culture, particularly during the late-1960s and 1970s, when hippies, some scientists among them, sought to unify quantum mechanics with Eastern philosophy, resulting in such bestselling, if not altogether scientifically reliable, treatises as The Tao of Physics.

The Austrian physicist Wolfgang Pauli, one of the strongest of Einstein’s

critics, had himself long been a fascinated adept of Jungian psychoanalysis. He acquired more grievances to analyse when “his wife, a former cabaret dancer, went off with a chemist, and not a very good one”.

Following laboratory experiments of exquisitely sensitive design from the 1970s to the present, it now seems that there can be no normal system of deterministic cause and effect, of the kind Einstein desired, somehow underlying the mysteries of quantum observations. To measure a particle in front of you can affect another particle light years away that is “entangled” with it. It might even be possible for macroscopic objects – ones visible to the naked eye, such as a cat or a person – to exist in different states simultaneously.

But the struggle to understand what quantum mechanics tells us about the world we live in goes on. As our authors relate wonderingly near the book’s end, it can seem to imply that objects do not even exist until we look at them: modern physics has in some sense “wiped the universe clean of things”.

There remains a tribe of heterodox hold-outs, however, cleaving to the conviction that science must speak of a fundamental reality and that quantum theory must therefore as yet be incomplete. It is still too soon to say, perhaps, that old Einstein was wrong.

Meeting of minds: Niels Bohr, left, with his rival Albert Einstein, c1920