“If I could tell you in two sentences,” said Richard Feynman, “I wouldn’t have won the Nobel Prize.” It was October 1965 and a reporter had asked him to explain in potted form what he had done to deserve the accolade. Feynman was being rewarded for his part in developing quantum electrodynamics, a theory that deals with the interactions of light and matter. Yet later, this celebrated physicist admitted: “I think I can safely say that nobody understands quantum mechanics.” He went on to caution: “Do not keep asking yourself, if you can possibly avoid it, ‘But how can it be like that?’ Nobody knows how it can be like that.”
As the science journalist and writer Philip Ball explains in Beyond Weird, what troubled Feynman was the sort of reality the theory seemed to be describing. No one has ever disputed the correctness of the equations of quantum mechanics, just how to interpret them. It was this that Albert Einstein and Niels Bohr debated on and off for the best part of 30 years: what does quantum mechanics actually mean? It was Bohr’s position, later called the Copenhagen interpretation, which so bothered Feynman even though it was and remains popular among those who don’t ask, “How can it be like that?”
The Copenhagen interpretation claims that the act of measurement constructs the reality that is measured. An electron with position just does not exist until a measurement is performed to determine its position. If that’s the case, then there seems little point in asking what lies beyond the observation. According to this view, explains Ball, the notion of an objective, pre-existing reality must be abandoned and we have to accept that measurement and observation bring specific realities into being from a “palette of possibility”. As strange as it sounds, the way nature behaves seems to depend on how, or even if, we choose to observe it.
For Bohr, Erwin Schrödinger’s mythical cat trapped in a box with a vial of poison was neither dead nor alive until the box was opened. It was the act of measurement – opening the box – that decided the fate of the cat. Einstein thought this was absurd: he believed that the cat was either dead or alive and to find out which all one had to do was look in the box.
Fifty-odd years after Feynman issued his warning, Ball explores the progress made in getting to grips with the central concepts of quantum mechanics and testing their limits. A subtle unpacking of Heisenberg’s famous uncertainty principle, that we cannot know both the precise position of a particle and its momentum at the same time because the greater the knowledge we have of one, the greater our ignorance of the other, is alone worth the price of the book. And you’ll want to re-read it because from discussing that the wave function encodes all that can be known about a quantum object, Ball takes us on a whirlwind tour through the quantum realm that includes Bell’s theorem, cryptography, teleportation, information, non-locality and entanglement, in which two particles are inexorably linked no matter how far apart they are.
To understand this last point, imagine possessing a collection of pairs of quantum dice. You roll the first two dice and get double six. You roll the next set and get double three. Each individual die on its own behaves as expected, yet its entangled partner somehow always gives exactly the same number. Entanglement has been demonstrated in the lab with pairs of atoms and photons and tells us that a quantum object may have properties that are not entirely located in that object. That might be difficult to understand, but as Ball points out, we lack words to convey such concepts as entanglement with precision and clarity, and so we need different ways of looking at them before we can begin to grasp them fully.
What is often described as the weirdness of quantum physics is the result of our inability to find pictures for visualising it. If in the end we can’t say what quantum mechanics means, argues Ball, we can after the developments of the past two or three decades say that it is less about particles and waves, uncertainty and fuzziness, than a theory about information: about what can be known and how. The question of interpretation may simply come down to taste.
The many-worlds interpretation (MWI) is the most extraordinary, alluring and thought-provoking of all the ways in which quantum mechanics has been understood, claims Ball, after discussing some of the alternatives. In its most familiar guise, MWI treats each outcome of every possible quantum event as existing in a real world. For Schrödinger’s cat this would mean that the universe would divide into two; one in which the box is opened and the cat is dead, and one in which the cat is alive.
Woody Allen, meanwhile, has an interpretation of his own: “There is no question that there is an unseen world. The problem is, how far is it from midtown and how late is it open?”
Beyond Weird: Why Everything You Thought You Knew About Quantum Physics is Different
Bodley Head, 370pp, £17.99
Manjit Kumar is the author of “Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality” (Icon).
This article appears in the 25 Apr 2018 issue of the New Statesman, The Corbyn ultimatum