Beyond the Quantum review: A remarkable book on quantum mechanics reveals a really big idea

Behind the quantum
Antony Valentini, Oxford University Press

It’s fair to say that physics didn’t go as planned. After decades of hopeful searching, dark matter has still not been directly detected. We found the Higgs boson, but nothing to pave the way forward. And string theory, the much-vaunted theory of everything, has yet to make a clear, testable prediction. Confidence is low. where do we go from here?

In recent years, many physicist-writers of popular science have avoided this question. Where they once boldly pointed to the next great discovery, they are now often seen retreating into philosophical musings or re-explaining what we already know. Not so Antony Valentini of Imperial College London. IN Beyond the Quantum: The Search for the Origins and Hidden Meaning of Quantum Mechanics, it represents something almost obsolete in its rarity: the big idea.

As the name suggests, its central focus is quantum mechanics, which has been the foundation of physics for a century. This depends on the wave function, a mathematical expression that can, the textbooks say, specify the complete state of any system, from an elementary particle to a cat or even you and me.

What’s special about the wave function is that it usually doesn’t describe regular, localized objects at all, but rather spread out, fuzzy, wave versions of things. Never mind. When we look at an object – again, as the textbooks say – the wave function “collapses” into a known but random result, with a probability given by Born’s rule (named after the physicist Max Born). Only now we have an object with defined properties in a defined place.

Although mainstream physics tries to ignore it, the interpretation of the wave function has always been a mystery – and there are basically only two realistic answers. One is that the wave function really does describe reality—that electrons, cats, and people really exist in many states at once, spread across space and possibilities. This is the many-worlds interpretation with its far-reaching metaphysical implications.

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Pilot wave theory has long been known to reproduce all the predictions of quantum mechanics
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The second answer is that the wave function is not the whole story. The dominant theory that Valentini developed a lot is the pilot wave theory, first proposed by theorist Louis de Broglie in 1927 and later revived by physicist David Bohm.

Pilot wave theory considers the wave function to be real but incomplete. He suggests that it acts as a guiding structure for individual particles, rather like the waves that drive plastic bottles floating in the sea. The particles themselves are never spread out or indeterminate – their wave behavior comes from the pilot wave and where they sit on it.

Pilot wave theory has long been known to reproduce all the predictions of quantum mechanics without fundamental randomness. But as Valentini points out, this agreement rests on an assumption: that the particles are in equilibrium with the wave and are distributed in the right way. This assumption is consistent with today’s experimental data—the results are virtually undeniable—but it may not always be true.

Valentini suggests that in the early universe, particles were distributed far from quantum equilibrium before “relaxing” into their current state, rather as a cup of coffee cools to match its surroundings. In this view, Born’s rule and its randomness are no longer fundamental features of nature, but historical accidents—they are by-products of cosmology.

This remarkable reward is not the only one. Quantum randomness also prevents any practical use of non-locality, the immediate interaction of objects separated by time and space. If the Born rule did not apply in the early universe, Valentini argues, instantaneous communication over vast distances would have been possible, perhaps leaving subtle imprints in the cosmic microwave background. If any relics from this era still exist, such superluminal signaling might now be attainable.

Given the lack of evidence, this might sound a bit wild if it weren’t for Valentini’s careful analysis of how orthodox quantum mechanics got its hands on it (which alone makes the book worth reading). If there is one weakness, it is the lack of a clear description of the pilot wave. Still, whether this turns out to be true or not, Valentini’s work at least shows us that in a field with a lack of confidence, this is what a truly big idea looks like.

Jon Cartwright is a writer based in Bristol, UK

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