The most challenging unresolved question in physics today is how general relativity—the framework governing gravity and spacetime on large scales—integrates with quantum mechanics, the principles that regulate the smallest scales. Numerous potential solutions exist, none of which have been proven, and only a handful have been definitively ruled out or even thoroughly explored. However, a moment is approaching when one of these concepts could be put to the test. If confirmed, it might fundamentally transform our understanding of time. A new study has been published in the journal Physical Review X.
Most proposed ideas for reconciling relativity and quantum mechanics are referred to as theories of quantum gravity, but the creator of this alternative approach, Jonathan Oppenheim from University College London, terms it post-quantum gravity. Unlike other theories, his concept does not aim to make spacetime—and by extension, gravity—quantum in nature.
Quantizing a theory involves breaking it down into fundamental units, or quanta. Light is a quantum force, with photons as its quanta, and two of the other three fundamental forces are also definitively quantum. Gravity is the sole force whose quantum nature remains unproven, and Oppenheim and his colleagues suggest that it might not be quantum at all.
Developing post-quantum gravity assumes that spacetime and gravity are not quantum but rather continuous and fundamental, lacking any constituent building blocks. This leads to an extensive chain of complex mathematical calculations and modeling to describe how this non-quantum spacetime would interact with the quantum forces, particles, and fields within it.
Among the effects emerging from this computational chain is a peculiar randomness in spacetime. When we envision time, we often picture a clock ticking steadily, each second following the next with uniform intervals. In post-quantum gravity, these ticks would exhibit slight random fluctuations. They would occur on scales too minute for us to perceive, yet time would become “jittery,” flowing forward in an unpredictable manner.
These fluctuations are part of what enables Oppenheim’s theory of gravity to connect with quantum mechanics. Incorporating them into basic quantum-mechanical calculations reveals several fundamental phenomena observed in quantum systems, including the rule that a quantum system typically transitions into a classical state upon observation—the same principle stating that although Schrödinger’s cat can be both alive and dead until the box is opened, after opening and inspection, the cat is one or the other.
However, the source of time’s instability remains unclear. It emerges from the equations, but Oppenheim and his team have yet to link this randomness to any specific origin. “Is there something, some concrete physical effect, causing it to flow unpredictably? Possibly, but that’s a level deeper, and at this point, I don’t think we’re ready for it—neither scientifically nor philosophically,” says Oppenheim. “But if we’re not going to quantize spacetime, then it inevitably has to become like this.”
Oppenheim acknowledges that this idea is highly controversial among physicists. “I don’t know anyone who thinks it’s more likely than not—I’m probably the only one who holds that view—but I believe many think we should test it,” he says.
Fortunately, initial tests are now becoming feasible. Many theories aiming to unify gravity and general relativity are difficult or impossible to prove or disprove. The testability of post-quantum gravity gives it a seriousness and scientific potential that some other ideas lack, says Giuseppe Fabiano from the Lawrence Berkeley National Laboratory in California, who is part of a group developing parameters for testing gravity theories. “I’m somewhat agnostic about the theory itself, but as long as it produces predictions I can test in the lab, it’s a useful theory.”
The experiments proposed by Oppenheim and others involve measuring the properties of gravity between pairs of objects. Since general relativity inextricably links space and time and posits that spacetime curvature is the source of gravity, any changes to the properties of space and time would inevitably alter the strength of gravity as well. “If time’s flow has this unpredictability, then when measuring gravity, you’d see that unpredictability,” Oppenheim states.
These experiments are already underway, though it may take decades to reach the precision needed to actually test post-quantum gravity. It has only recently been proven that such tests are even possible—developing sensors and calculating the necessary parameters for conducting them will be another monumental challenge. Yet, despite the theory’s controversial nature, many researchers agree with Oppenheim that these tests are worth pursuing.
“If we found experimental evidence supporting the accuracy of post-quantum gravity, it would be a monumental event, primarily because it would be strikingly different from all other interactions we’ve analyzed over the past century,” says Fabiano. Gravity has always been distinct from other fundamental forces—for instance, it is far weaker than the rest—but the notion that its form is so radically different from them would represent a major departure from widely accepted orthodoxy.
It is difficult to envision the extensive impact that confirming post-quantum gravity would have on our understanding of physics. Some problems, like reconciling general relativity and quantum mechanics, would be resolved, but this would undoubtedly generate many other questions. If time is indeed unstable, it could reshape our conception of the entire universe.
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