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The most vexing question in physics right now is how general relativity – the laws that govern gravity and space-time on large scales – meshes with quantum mechanics, the laws that govern very small scales. There are many potential solutions, none proven, and so far very few of them have been conclusively ruled out either – or even rigorously investigated. Now, though, the day is approaching when one of those ideas could be put to the test. If it holds up, it could dramatically change our view of time.

Most of the proposed ideas to combine relativity and quantum mechanics are called theories of quantum gravity, but the creator of this alternative theory, at University College London, calls his post-quantum gravity. Unlike the others, his idea doesn’t attempt to make space-time, and therefore gravity, quantum.

Making a theory quantum, or quantising it, involves breaking it down into its fundamental parts, or quanta. Light is quantum – its quanta are photons, and two of the other three fundamental forces are definitely quantum as well. Gravity is the only one that hasn’t been proven to be quantum, and Oppenheim and his colleagues propose that perhaps it isn’t.

The construction of post-quantum gravity assumes that space-time and gravity aren’t quantum, but rather continuous and fundamental, without constituent building blocks. From there, it is a long chain of complex mathematical calculations and simulations of how this non-quantum space-time would interact with the neatly quantum forces, particles and fields contained within it.

Among the effects that popped out of that chain of calculations is a strange randomness in space-time. When we think of time, we might picture a clock ticking regularly, each tick following the next with equal intervals between them all. In post-quantum gravity, there would be small, random fluctuations in these ticks. It would occur on scales too small for us to notice, but time would become “wobbly”, flowing unpredictably forwards.

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These fluctuations are part of what allows Oppenheim’s theory of gravity to link up with quantum mechanics. When they are included in some basic quantum-mechanical calculations, several fundamental behaviours observed in quantum systems result, including the rules about how a quantum system seems to transform into a classical one when it is observed – the same rule that says that while Schrödinger’s cat may be both alive and dead before the box is opened, once you open the box and take a look, the cat will only be one or the other.

The cause of time becoming wobbly, though, is still uncertain. It arises from the equations, but Oppenheim and his colleagues haven’t yet tied this randomness to any particular source. “Is there something, some specific physical effect, that is causing it to flow in an unpredictable way? It may be, but that’s one level deeper, and at the moment I don’t think we’re ready to go there – scientifically or philosophically,” says Oppenheim. “But if we’re not going to quantise space-time, then it necessarily has to become like this.”

Oppenheim admits that the entire idea is highly controversial among physicists. “I don’t know anyone who thinks it’s more likely to be true than not true – I think I’m probably alone on that one – but I think there are a lot of people who think we ought to test it,” he says.

Testing time

Luckily, the first tests are now becoming possible. Many theories that seek to unite gravity and general relativity are difficult or even impossible to prove or disprove. Becoming testable lends post-quantum gravity a sense of seriousness and scientific potential that some of those other ideas don’t possess, says at the Lawrence Berkeley National Laboratory in California, who is part of a team developing parameters for testing theories of gravity. “I’m sort of agnostic about the theory itself, but as long as it gives some predictions that I can test in the lab, it’s a useful theory.”

The experiments that Oppenheim and others have are to do with measuring the properties of gravity between pairs of objects. Because general relativity inextricably connects space and time, and postulates that the curvature of space-time is the source of gravity, any changes in the properties of space and time will necessarily change the strength of gravity as well. “If the flow of time has this unpredictability to it, then when you measure gravity you will see this unpredictability,” says Oppenheim.

Those experiments are already being built, although it could be decades until they get to the level of precision necessary to actually test post-quantum gravity. It has only just been shown that the tests are even possible – developing the sensors and calculating the parameters necessary to carry them out will be another mammoth task. But even though the theory itself is contentious, many researchers agree with Oppenheim that the tests are worth performing.

“If we were to find some experimental confirmation that post-quantum gravity is accurate, it would be a big deal, first and foremost because it would be very different from all of the other interactions that we’ve analysed throughout the past century,” says Fabiano. Gravity has always diverged in some ways from the other fundamental forces – for one thing, it is much weaker than the rest – but the idea that its form is so radically different from theirs would be an enormous departure from the widely accepted orthodoxy.

It is difficult to imagine just how widespread the effects of a confirmation of post-quantum gravity would be on our understanding of physics. Some problems, such as the combination of general relativity and quantum mechanics, would be solved, but it would doubtless raise many other questions. If time truly is wobbly, it could shift our view of the entire universe.