June 16, 2024

At the heart of modern physics is a gulf that scientists have spent more than a century trying to bridge. Quantum mechanics gives an apparently flawless description of the forces that dominate at the atomic scale. Albert Einstein’s theory of general relativity has never been proven wrong in its predictions of how gravity shapes cosmic events. But the two theories are fundamentally incompatible.

Now, scientists have proposed a framework that they say could unify these two pillars of physics, through a radical rethink of the nature of spacetime. Instead of time ticking away predictably, under the “postquantum theory of classical gravity”, the rate at which time flows would wobble randomly, like the ebb and flow of a stream.

“Quantum theory and Einstein’s theory of general relativity are mathematically incompatible with each other, so it’s important to understand how this contradiction is resolved,” said Prof Jonathan Oppenheim, a physicist at University College London, who is behind the theory.

Not everyone is convinced by the proposition, including the theoretical physicist and author Prof Carlo Rovelli, who has signed a 5,000:1 odds bet with Oppenheim against the theory being proven correct.

Until now, the prevailing assumption has been that Einstein’s theory of gravity must be modified, or “quantised”, in order to fit within quantum theory. This is the approach of string theory, which advances the view that spacetime comprises 10, 11 or possibly 26 dimensions. Another leading candidate, advanced by Rovelli and others, is loop quantum gravity, in which spacetime is composed of finite loops woven into an extremely fine fabric.

Oppenheim’s theory, published in the journal Physical Review X, challenges the consensus by suggesting that spacetime may be classical and not governed by quantum theory at all. This means spacetime, however closely you zoomed in on it, would be smooth and continuous rather than “quantised” into discrete units. However, Oppenheim introduces the idea that spacetime is also inherently wobbly, subject to random fluctuations that create an intrinsic breakdown in predictability.

“The rate at which time flows is changing randomly and fluctuating in time,” said Oppenheim, although he clarifies that time would never actually go into reverse. “It’s quite mathematical,” he added. “Picturing it in your head is quite difficult.”

This proposed “wobbliness” would result in a breakdown of predictability, which, Oppenheim says, “many physicists don’t like”.

These include Rovelli and Dr Geoff Penington, a string theory advocate at the University of California, Berkeley, who also has a 5,000:1 stake on the question.

“Speculations are welcome, particularly if they can be experimentally tested,” said Rovelli. “But most speculations turn out to be wrong. I think it is good that Oppenheim explores this possibility, even if not very plausible, but big claims about a ‘New theory unites Einstein’s gravity with quantum mechanics’ sounds a bit overblown to me.”

Ultimately, whether the theory is correct is not an aesthetic preference, but a question of whether it is a faithful representation of reality. A second paper, published simultaneously in Nature Communications by Dr Zach Weller-Davies, formerly of UCL and now at Canada’s Perimeter Institute, proposes an experiment designed to uncover “wobbles” in spacetime through tiny fluctuations in the weight of an object.

For example, the International Bureau of Weights and Measures in France routinely weigh a 1kg mass which used to be the 1kg standard. If the fluctuations in measurements of this 1kg mass are smaller than a certain threshold, the theory can be ruled out.

“We have shown that if spacetime doesn’t have a quantum nature, then there must be random fluctuations in the curvature of spacetime which have a particular signature that can be verified experimentally,” said Weller-Davies.

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