# 'Wobbly spacetime' is latest stab at unifying physics

## Grudge match between quantum mechanics and general relativity attracts new effort to find harmony

Since the early 20th century, physicists have struggled to marry theories governing the very big with those for the very small.

Despite the staggering achievements in modern science, the conflict between Einstein's general theory of relativity and quantum mechanics has become a stumbling block in developing a consistent, reliable theory explaining everything.

University College London professor Jonathan Oppenheim proposes to overcome the barrier with the idea of "wobbly spacetime."

Earlier efforts to unify the two main columns of modern physics had gone with the idea that gravity – the nature of which is reliably explained by General Relativity – should somehow be quantized. That means divided into discrete blocks of magnitude rather than distributed along a continuum that can always be subdivided. Two prominent exponents of the idea are string theory and loop quantum gravity.

But the professor of quantum theory argues that making quantum theory fit relativity would be more fruitful.

In a paper published this week in the journal *Physical Review X*, Oppenheim proposes to retain the classical nature of gravity but allow for the probabilistic nature of quantum mechanics by inserting certain unpredictable but continuous "wobbles" into spacetime itself.

His approach relies on two separate statistical approaches for the quantum and classical aspects of a system. "In the statistical description of the quantum side, states are described using density operators that evolve as if the system were open – that is, susceptible to uncontrolled influences from the environment," an accompanying article explains.

"In the statistical description of the classical side, states are probability distributions on phase space – a framework that is often used to model large numbers of particles, where one does not know the individual position and momentum of each particle."

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"The rate at which time flows is changing randomly and fluctuating in time," Oppenheim told The Guardian. "It's quite mathematical. Picturing it in your head is quite difficult."

Another paper published in Nature Communications, written by Oppenheim's colleague, PhD student Zach Weller-Davies, proposes approaches to verify or disprove the theory experimentally.

"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," he told website Physics.org.

"If spacetime is classical, the fluctuations have to be larger than a certain scale, and this scale can be determined by another experiment where we test how long we can put a heavy atom in superposition of being in two different locations."

But this being theoretical physics, not everyone is convinced. Loop theory proponent Carlo Rovelli, an Italian theoretical physicist, told The Guardian: "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."

Rovelli has signed a 5,000-to-one bet with Oppenheim against the theory being proven correct. So much for unity in physics. ®

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