The physicist betting that space-time isn’t quantum after all

Most experts think we have to tweak general relativity to fit with quantum theory. Physicist Jonathan Oppenheim isn't so sure, which is why he’s made a 5000:1 bet that gravity isn’t a quantum force.

The fundamental nature of space-time remains mysterious

JONATHAN OPPENHEIM likes the occasional flutter, but the object of his interest is a little more rarefied than horse racing or the one-armed bandit. A quantum physicist at University College London, Oppenheim likes to make bets on the fundamental nature of reality – and his latest concerns space-time itself.

The two great theories of physics are fundamentally at odds. In one corner, you have general relativity, which says that gravity is the result of mass warping space-time, envisaged as a kind of stretchy sheet. In the other, there is quantum theory, which explains the subatomic world and holds that all matter and energy comes in tiny, discrete chunks. Put them together and you could describe much of reality. The only problem is that you can’t put them together: the grainy mathematics of quantum theory and the smooth description of space-time don’t mesh.

Most physicists reckon the solution is to “quantise” gravity, or to show how space-time comes in tiny quanta, like the three other forces of nature. In effect, that means tweaking general relativity so it fits into the quantum mould, a task that has occupied researchers for almost a century already. But Oppenheim wonders if this assumption might be mistaken, which is why he made a 5000:1 bet that space-time isn’t ultimately quantum.

New Scientist caught up with him to find out what makes him think conventional wisdom might be misguided here, how the question might be resolved with experiments – and why physicists love a good wager.

Joshua Howgego: Is it fair to say that most physicists think the best route to uniting general relativity and quantum theory is to fiddle with the former?

Jonathan Oppenheim: The smart money is on general relativity being ultimately a quantum theory. But there is a divide between people who study quantum theory and just want to quantise everything, and a smaller number of others. For example, in the relativity community, they think an awful lot about time, and because of this there is more uncertainty. If you try to think about quantising time, you get very confused. So there’s a bit more doubt there.

My perspective is: I don’t really know! I think it’s quite possible that a theory that can, in some sense, describe the subatomic realm, and space and time too, might not be anything like either quantum or classical physics. Then the question is: Will our next theory of gravity be closer to a quantum theory of gravity or a modified classical theory? I think we ought to be more cautious. We could be making a big mistake by putting all our eggs in one basket.

Why is time in particular a sticking point?

We think of quantum theory as describing events in the subatomic world that evolve through time. The theory treats time as a kind of constant background structure, and quantum systems change with respect to this background. The trouble is that in general relativity, space-time itself becomes dynamical: it can warp. If we quantise the speed at which time flows, then we lose that crucial background structure that quantum theory relies on. It is difficult to even talk about an instant of time, because I can’t even say with certainty which “chunks” of space-time lie in the future and which in the past.

It might be possible to get rid of this background structure from quantum theory, but it is very hard. People don’t really know what to do with time.

How did the idea that we need to quantise gravity become dogma?

I think it really solidified in the 1980s, when there was a lot of debate about whether gravity had to be quantised. At the time, people decided that it was inconsistent to keep gravity classical. But it may date back even further. In the late 1950s there was already a lot of debate about the subject. I recently went back and read the proceedings of the Chapel Hill conference, an important meeting in 1957 for which we have a full historical record. There were these debates between luminaries of physics, people like Richard Feynman and John Wheeler, where they debated this question. It makes for a really interesting read. At that conference, I get the impression that many researchers decided that gravity had to be made quantum, mostly based on arguments from Feynman.

But when you revisit the debates, well, our understanding of quantum theory has evolved. We now better understand the role of entanglement, where two particles separated by some distance appear to share information, and the similarity between classical probability distributions and quantum wave-functions, which give you odds on what the properties of a quantum object will be when it is measured. So we now know that it could be consistent not to quantise gravity. However, a certain viewpoint has already been baked in.

There are lots of big questions in physics. How important is this one about quantum gravity?

It’s a pretty big deal. Any questions about cosmology, the standard model of particle physics, dark matter – they are questions about our particular universe. Our universe consists of various particles and forces, but all of these are governed by quantum theories. So quantum theory should be thought of as the framework we use to understand our universe. So, the question of whether the laws of physics are fully quantum, some hybrid or something else is a different order of question. It’s about the framework of natural laws. It’s almost metaphysics.

Tell me about this bet. What led you to make it?

In 2020, I gave an online talk about classical gravity and quantum theory. I was arguing that, as I mentioned, things have changed and it’s not so unreasonable for gravity to look like a classical theory – in other words, to not quantise it. Carlo Rovelli, a theorist at Aix-Marseille University in France and one of the founders of a quantum gravity hypothesis called loop quantum gravity, was in the audience and we had a rather robust exchange of ideas. He certainly thinks that gravity has to be quantised.

At some point later, I wrote about this on Twitter and a string theorist at the University of California, Berkeley, named Geoff Penington chimed in and said he would be prepared to bet me a crisp that gravity is quantum. We signed the fine print of the bet a couple of weeks ago. The final agreement says that, if I lose, I have to buy Carlo and Geoff some item worth 20p, like a handful of crisps, but, if they lose, they buy me some item worth £1000.

Hang on, why did crisps come into this?

I once had another bet with theoretical physicist Adrian Kent at the University of Cambridge where, if I won, I would get a packet of crisps. If he won, he would get £1000. This was a related bet about Bell’s theorem, one of the most important results in physics. It states that certain outcomes of measurements performed on entangled particles can’t be explained by any local classical model. At the time, experiments had shown this to be true, with just a few tiny loopholes remaining. I bet that one of these loopholes would be closed.

Did you win?

I won that bet, yes, thanks to an experiment carried out in 2008. I got the packet of crisps, they were delicious. Salt and vinegar, as I recall.

What makes you so sure that you will win your current bet about space-time?

I’m not. I have no idea whether I will win this bet. The point is that I think we have no idea what the theory of gravity will look like, and whether it will be a quantum theory of gravity or something else entirely. And so I think I was given great odds: 5000:1. I took the bet because there’s no way we can be that certain that gravity should be quantised.

If it isn’t quantum gravity, what do you think a unification of quantum theory and general relativity could look like?

Well, I’ve proposed an alternative. I’ve called it a post-quantum theory of classical gravity, where quantum theory is modified a tiny bit, and classical general relativity is modified a little bit, so that the two become mathematically consistent. But we’re all just constructing our best models of gravity, and most likely these are all just approximations. Maybe our next theory will be neither quantum nor classical nor hybrid, but something else entirely.

Will it be possible to settle the bet any time soon?

I hope so, yes. There are a number of proposed experiments that could do it. There is one that my research group has proposed, which looks for diffusion in measurements of the mass of a 1-kilogram object. If gravity has a classical flavour, then experiments that measure this precisely will get different answers. The fluctuations in the answers will be very tiny, so we need precise measurements.

Other researchers, such as Sougato Bose and colleagues at University College London and Chiara Marletto and Vlatko Vedral at the University of Oxford, have proposed different kinds of experiments that are to do with entanglement, in other words quantum correlations between particles that have been prepared in a certain way. These experiments basically look to see if gravity can be used to create entanglement. If so, then this would be convincing evidence that gravity has a quantum nature or, at least, doesn’t have a classical nature.

Why do you like betting so much?

Historically, physicists have always been into betting. There was a famous bet between Stephen Hawking and Kip Thorne against John Preskill to do with a riddle called the black hole information paradox, where the winner would receive an encyclopaedia of their choice. So it’s not just me. And there is a serious side to it, because writing the wager helps you precisely formulate the question and what it would take to change your mind. And it asks how strong your degree of belief is in something. That is what determines the odds you are prepared to accept.

If space-time isn’t quantum after all, what would be the immediate impact?

Just about all our efforts are focused on trying to quantise gravity at the moment. So I think it would have far-reaching and radical implications. It would change the direction of physics. The two big quantum gravity efforts, string theory and loop quantum gravity, which are currently regarded as the only two games in town, would have to be dropped and we would need to find some alternative.

Our framework for physics – quantum theory – which we use to understand all the other forces of nature, would need to be modified. Perhaps only a little bit, but perhaps radically. Either way, it would be a monumental shift in our basic conceptual framework for understanding the universe.