Scientists have proposed a new theory that unites two of the biggest, and most contradictory, concepts in physics: Einstein’s theory of general relativity and quantum theory. The theory “challenges our understanding of the fundamental nature of gravity,” according to one scientist who contributed to the work.
Theorist Jonathan Oppenheim, of University College London (UCL), published the proposal on Monday in the journal Physical Review X. He and his colleagues—Carlo Sparaciari and Zachary Weller-Davies from UCL and Barbara Šoda from the Perimeter Institute for Theoretical Physics in Canada—also published another paper in Nature Communications that outlines an experiment to test the new theory.
General relativity is the best explanation we currently have for gravity. It says that mass distorts spacetime and that particles move based on this distortion, like marbles rolling around the well caused by a bowling ball laid on a mattress. Quantum theory, on the other hand, governs how the tiniest particles behave. It explains forces other than gravity, such as electromagnetic force, as an exchange of particles (imagine people throwing a heavy ball back and forth).
But these pillars of physics explain the universe on totally different levels of scale, certainty, and probability. Whereas general relativity explains what happens on a massive scale, as events happening with certainty and continuously over time, quantum theory instead explains the atomic level, where events have some probability of happening and can happen in abrupt leaps. Therein lies the issue: neither theory can explain both what goes on at the atomic level and at the level of the cosmos, and quantum theory struggles to explain gravity as a force.
The main way physicists have tried to get around this problem is by turning general relativity into a quantum explanation, also known as “quantizing;” in other words, breaking some continuous, like time, down into smaller and smaller chunks. This is what string theory and loop quantum gravity try to do. For example, string theory says that the universe is made up of many strings or loops that wiggle and vibrate at different frequencies, which correspond with different particles. Much like the different strings on a guitar vibrate to be different notes.
“It’s important to understand how this contradiction is resolved,” Oppenheim said in a press statement. “Should spacetime be quantised, or should we modify quantum theory, or is it something else entirely?”
The discovery of a new theory to unify these ideas “challenges our understanding of the fundamental nature of gravity but also offers avenues to probe its potential quantum nature,” said Weller-Davies.
In this latest paper, Oppenheim combines the two pillars, suggesting that spacetime is classical (not governed by quantum theory at all) and pairs it with a modified quantum theory. Under this reasoning, spacetime, even at the smallest level, is continuous and not chunked into little leaps. At the same time, spacetime would have random wobbles, similar to how quantum fields can show randomness. While this means there’d be some unpredictability in the universe, it’s vital if the two principles are to work together.
“In this theory the clash between the classical nature of gravity and spacetime and the quantum nature of matter that ‘lives’ on the spacetime is resolved by finding that the spacetime, though classical, exhibits some randomness in its behavior,” Šoda, who’s a postdoctoral researcher at the Perimeter Institute, said in an email to Motherboard. “The randomness in the behavior of spacetime is necessary to make the interaction with quantum fields consistent.”
One way this theory could play out is that objects would weigh slightly different amounts depending on when you weigh them. Since weight is the force of gravity acting on an object (as opposed to mass which is how much “stuff” is in something) these measurements could show spacetime has some randomness to it. But that would only work if there was a way to measure that weight precisely enough.
The second paper, co-authored by Šoda, outlines an experiment to test the theory, doing essentially this. Researchers could, for example, weigh the former international prototype of the kilogram ultra-precisely and see if its weight changes ever-so-subtly from moment to moment.
Not everyone is convinced by the approach: notably physicists Carlo Rovelli, from the Centre de Physique Theorique de Luminy in France, and Geoff Penington, from UC Berkeley. “Speculations are welcome, particularly if they can be experimentally tested. But most speculations turn out to be wrong,” Rovelli told The Guardian. He cheekily added “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.”
It’s worth saying that Rovelli and Penington have some skin in the game, in the form of a 5000:1 bet with Oppenheim. If spacetime turns out to be classical, Rovelli and Penington have to give 5000 crisps, ball-pit balls, or other items of Oppenheim’s choosing.
Sougato Bose, a professor of physics at UCL who wasn’t involved in these latest papers, is hopeful that the theory and future experiments will put an end to the battle for supremacy in a theory to understand the universe. “Experiments to test the nature of spacetime will take a large-scale effort, but they’re of huge importance from the perspective of understanding the fundamental laws of nature,” he said in a press statement. “I believe these experiments are within reach – these things are difficult to predict, but perhaps we’ll know the answer within the next 20 years.”
Šoda, whose calculations helped guide the project, agrees. “I think there is merit in every new experiment. Our experimental colleagues have ingenious ways to test our theories and I’m optimistic this one will give us some interesting answers.”
But it’s likely not worth holding your breath over. Reports have been hinting at a winner emerging for nearly a decade, with no such luck. “Reconciling quantum theory with gravity is one of the great challenges of physics. Thus far we’ve been hopelessly unsuccessful,” Oppenheim has previously said.
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