What If Gravity Isn't Quantum? New Experiments Examine

**Matt O'Dowd** (0:00)
Thank you to Brilliant for supporting PBS. If we discover how to connect quantum mechanics with general relativity, we'd pretty much win physics. There are multiple theories that claim to do this, but it's notoriously difficult to test them. They seem to require absurd experiments like particle colliders the size of galaxies. Or we could try to physics smarter instead of physicsing harder. Let's talk about some of the ideas for quantum gravity experiments that can be done on a non-galaxy-sized lab bench, and in some cases already have been done.
It's been almost exactly 100 years since quantum mechanics was discovered, and 110 since general relativity. Together, these theories explain almost everything. Quantum gives us the behavior of the subatomic world, and so the building blocks of all matter and their interactions via the quantum forces, and GR gives us the background in which that matter lives, space and time along with the force of gravity. Despite their incredible successes in their own domains, the two theories seem to contradict each other in deep ways. Now, there must be some master theory that encapsulates both and resolves these contradictions. But in the century following the discovery of general relativity and quantum mechanics, our efforts to find that theory have come up short. The common approach to reconciling general relativity with quantum mechanics has been to quantize gravity, to make the gravitational field and so the geometry of space-time obey the strange rules of quantum mechanics. And why not? It's worked for everything else. For example, we made the electromagnetic field quantum, showing that it was mediated by force carrying photons. And that was a stunning success. But quantizing gravity proved extraordinarily difficult. The hypothetical force carrier for gravity, the graviton, has an energy so tiny that it's hard to even conceive of an experiment that would detect it directly. And then there's the fact that the field of gravity is the fabric of space time itself. We quantize the other forces of nature with respect to a continuous background of space and time. But so far it's been exceptionally challenging to quantize that background itself. The most prominent approaches like string theory and loop quantum gravity may have made progress, but they struggle to make testable predictions.
So what if we've been looking at it the wrong way around? What if instead of quantizing gravity we should be trying to gravitate the quantum? That is, instead of trying to give quantum properties to gravity, we should accept that gravity is fundamentally classical and try to understand how truly quantum matter can result in truly classical gravity. Now, we talked about approaches in which gravity stays classical in the last episode, when we explored the new post-quantum gravity hypothesis of Jonathan Oppenheim, in which random fluctuations in the gravitational field act on the quantum wavefunction causing it to collapse. We also talked about the Deyosi-Penrose model a while ago, in which a superposition of different spatial distributions of mass and energy is ultimately forced to choose one such distribution consistent with a singular gravitational field. There are compelling reasons on both sides of the discussion, but which is it? Is gravity quantized or is the quantum gravitized? Unlike many questions about quantum gravity, this one may be possible to answer with actual doable experiments. Let's consider the two scenarios and see which predictions they make that we might be able to test. One, gravity is quantum. This means gravity and by extension, the fabric of space time, exhibit the various weird properties of the quantum world like superposition and entanglement. If we can experimentally demonstrate one of these things, then we know gravity is quantum. Two, the quantum is gravitized. This can be tested in the context of specific scenarios like the DOC Penrose model or Oppenheimer's post quantum gravity. Now I'm going to talk about gravitizing the quantum first, but then I'll come back to tests of actual quantum gravity. In both the DOC Penrose and Oppenheimer approaches, the gravitational field is responsible for collapsing the quantum wavefunction. In these ideas, there's only one gravitational field, only one space time, and quantum systems do quantum stuff within that classical background. One of the things quantum systems can do is to be in superpositions of states. They can exist simultaneously in multiple states at once, like Schrödinger's alive and dead cat. The job of the quantum wavefunction is to describe this range of possible states. Measurement or observation collapses the wavefunction. It forces the system to choose one state from the possibilities. If gravity is classical, then the superposition of its contents shouldn't cause a superposition of the gravitational field. A superposition might involve different distributions of mass, which normally would mean different space-time curvatures, perhaps a superposition of space-times. But if gravity is classical, then there's only one space-time regardless of its contents. This could lead to a tension between the gravitational field and the matter producing that field. The Dioce-Penrose approach says that when the tension gets too large, the wavefunction collapses. Another way to think about it is that the fuzziness in the matter distribution is within a certain range of matching a single space-time curvature, and in that case it can stay in superposition. But if that distribution of matter strays outside that range, then its superposition collapses into a well-defined matter distribution.