John Donoghue: We Have Already Quantized Gravity (And It Works)
episodePreviously titled “John Donoghue: The Physicist Who Says We've Already Quantized Gravity” — renamed by the publisher on Aug 3, 2026
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Why do quantum physics and gravity work together despite common claims?
I think the popular phrasing is totally wrong. Quantum physics and gravity go perfectly well, as well as any other theory we know about. One of the biases of the field is that things unify, and we don't really have any evidence for that. I'm actually a champion of a crazier theory.
You've heard it before, quantum theory and general relativity are fundamentally incompatible. But is that actually true? Or is it something we just say so often we start to believe it? Professor John Donahue thinks this entire framing is misleading. Gravity is a field, the metric, so you quantize it like QCD. In fact, Feynman and DeWitt did exactly that several decades ago. So what's the actual problem? Donoghue argues it's hidden assumptions. Perhaps something like causality, supersymmetry, or grand unification, even so-called naturalists, could be a human bias rather than an objective law. Today, we delve into quadratic gravity theory, and another more speculative theory called random dynamics. On this channel, my name's Kurt Jaimungal and I interview researchers about their theories of reality, most often in physics.
And today's a particularly technical talk, so be prepared. I'm excited because you'll see why John Donoghue is a legend in the subject of gravity and its quantization. We'll delve into effective field theory and learn why this professor's judicious restraint unnerves his colleagues. Professor Donahue, you're known, in a sense, for being radical, for not being radical. Explain that.
Well, I am, in a way, quite conservative, because I grew up as a phenomenologist, where I learned to listen to what nature is telling us. And nature has told us, gradually over time, that the fundamental interactions are gauge theories, and the gauge theories are... composed in particular ways. And we've learned to understand quantum mechanics and the fundamental interactions through interactions with experiment. And so I tend not to deviate from that very much.
You mentioned that quantum mechanics may fail at some point. And when we're thinking of quantum gravity, we're assuming that something about quantum mechanics, whether it's the Dirac-Von Neumann axioms or something is held sacred and then gravity has to bow in a sense to those. But you said when we were speaking off air that you don't think that's necessarily true. And I'd like you to comment more about that.
Okay, so the point is that all our theories have limits. We've tested them in some range of energies and conditions, and we're used to thinking of the various interactions as having limits, like the standard model we expect to be supplanted by other interactions. But the same should hold true, we should hold the same standard to quantum mechanics. The standard assumption is is that quantum mechanics is valid at all energies, all scales, all everything. Because at present, we don't really need deviations. But nevertheless, there could be deviations both at high energies or on macroscopic scales. And so there are experiments testing quantum mechanics on macroscopic scales, and I think they're very interesting because that's a frontier for the limits of quantum mechanics.
And perhaps the theory as we know it now will be changed at some scale, some macroscopic scale. The connection to gravity is that gravity may be the best place to test this, Because you can get macroscopic bodies out of gravity, whereas it's hard to get very macroscopic charges, for example. If you're trying to get large amounts of charge, it's hard. Large amounts of mass is easy.
So, sir, when popularizers talk about quantum mechanics and GR are incompatible, are they getting something wrong? And also, I just said quantum mechanics and not quantum theory. People talk about quantum mechanics, quantum theory, and quantum field theory. I don't know if you see those as different when speaking about general relativity and the combination of those
two. Actually, I think the popular... phrasing is totally wrong, that quantum physics and gravity go perfectly well, as well as any other theory that we know about.
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