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

Artist’s illustration of Lense-Thirring frame-dragging resulting from a rotating white dwarf in the PSR J1141-6545 binary star system. (Image credit: Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav))

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.

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.

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