Beyond quantum physics: The search for a more fundamental theory

QUANTUM theory earned its exalted status by providing peerlessly accurate predictions of the behaviour of atoms and molecules, revealing the world of the very small in all its glorious strangeness. But it doesn’t actually make sense of the universe.

For starters, we still don’t understand key tenets of quantum weirdness. Take entanglement: the existence of a telepathic link between spatially separated particles runs counter to all our ideas about how the universe works. Nor do we have a grip on what distils the objective, classical reality we see from the myriad possibilities for what a quantum object might be when it is measured (see “Who or what collapses the wave function?”). That’s a big omission.

It gets worse. Zoom out a tiny bit and you realise that you can’t get the rules of chemistry – how atoms and molecules combine, and the properties of those combinations – from quantum theory. “This has prompted some philosophers to argue against the fundamentality of quantum mechanics,” says Vanessa Seifert at the University of Bristol, UK. Zoom out a lot and it becomes more troubling still, because of the incompatibility of quantum mechanics and general relativity.

All in all, it is clear we need to do better. The hunch is that just as classical physics emerges from quantum physics, there might be a deeper theory from which quantum physics arises. But what would it look like? Ciarán Lee and John Selby, who work at the Perimeter Institute in Ontario, Canada, have suggested that we will have to lose at least one, and possibly two, cherished notions in physics: causality and the idea that information is always conserved. A deeper theory without these can, in the right circumstances, be translated into the quantum theory we know. Alternatively, we could choose to ditch Einstein’s conception of space-time or the notion of human free will. “There are multiple ways quantum theory could be modified and only experiment can decide what is correct,” says Magdalena Zych at the University of Queensland in Australia.

Some researchers hoped, for instance, to see a revealing deviation from the predictions of quantum theory when a third slit was added to the classic double-slit experiment that reveals the wave-particle duality of quantum objects. They hoped in vain.

Or perhaps we need a new version of the Schrödinger’s cat thought experiment to stimulate further progress? Renato Renner and Daniela Frauchinger at the Swiss Federal Institute of Technology in Zurich recently provided just such a thing. In it, a couple of extra observers watch the original experiment observing a potentially dead-and-alive cat. That creates a complicated quantum scenario where no one can agree on the state of the cat, potentially exposing a hidden flaw in quantum theory – although no one can agree what.

Chiara Marletto at the University of Oxford hopes that quantum theory’s problems might force us to approach physics differently altogether. The traditional way of formulating laws of physics says that laws of motion or change, together with some initial conditions, are all that’s available to make sense of the universe. “We are reaching a bottleneck,” she says. “This approach cannot grasp everything when it comes to the physics of information, the physics of life and thermodynamics,” she says. Her new angle of attack, called constructor theory, seeks to reformulate laws of physics in terms of “counterfactual” laws about what can and can’t happen.

For his part, Carlo Rovelli at the University of Aix-Marseille in France argues that no deeper theory will free us from the weirdness of quantum theory. “We might find something that goes behind quantum theory; nothing is definitive and final,” he says. “But I expect that if we do, it will be even more strange to us than quantum theory.”

The other possibility, of course, is that such a deeper theory doesn’t exist. “There is simply no guarantee that any mathematical theory can faithfully and completely represent the universe,” says Zych.

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