Neil Turok: A Route to Quantum Gravity (Without Strings)
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What is the main topic discussed in this episode?
What happened at the Big Bang? What goes on in black holes? These kind of questions have not been solved by these very complex frameworks for quantum gravity.
You don't believe this, and you have some recent results.
I used to believe it, that quantizing gravity required extra dimensions, strings, membranes. What are the assumptions that go into this theory? One of them is that the theory lives in a Hilbert space. We have an example of a theory which doesn't require that assumption,
For some background for the viewers, I was sent this paper last night. Professor Neil Turok is the inaugural Higgs chair at Edinburgh, former director of Perimeter, and a 2026 Fellow of the Royal Society.
States of negative norm are called ghosts. A state with negative norm corresponds to a negative probability. That's just not true. You can't observe the norm of a quantum state. Provided the S-matrix or Hamiltonian of this theory has this symmetry, the answers you get are always positive, and the probabilities always add up to one. We claim we understand quantum gravity in a certain limit, The trick we used to make sense of it may or may not apply to the full thing.
On this channel, I, Kurt Jeimungal, interview researchers regarding their theories of reality with rigor and technical depth.
Sam says, I think I know how it works. And all it took is a slight tweak of the Born Rule.
Today, ghosts, the Born Rule, why strings may not be forced in nature, and why simplicity still matters. Why is simplicity so important? Simplicity leads to understanding. Quantum gravity in four dimensions is usually said to require strings or require some other extra structure. You have some new interesting results. Right. Which we're premiering today.
Okay. So, I used to believe it, that quantizing gravity was this... required this huge amount of extra paraphernalia, extra dimensions, strings, membranes. The whole story has become more and more complex as time progressed without actually solving any real problem. And what I mean by real problem is what happened at the Big Bang. What goes on in black holes? Is there information loss? These kind of questions have not been solved by these very complex frameworks for quantum gravity. So what we've recently realized is that there's rather a simple-minded approach to quantum gravity, which actually has been around since the 1970s. It was begun by somebody called Kelly Stell, who unfortunately passed away recently.
But he wrote a paper arguing that if you include terms in the gravitational action, so you generalize Einstein's action, Einstein's action involves the curvature of spacetime and also a length scale. which is called a Planck mass or Planck length or Newton's constant. It's all the same thing. So there's a scale in Einstein's theory of gravity. If you include terms in the action, which are the square of the curvature. In addition to the regular? In addition to the regular Einstein action and the cosmological constant, which has no derivatives. Then you have Einstein's term, which has two derivatives because it's a curvature. And then you can include curvature squared terms. And that makes gravity much more like a gauge theory.
Because in a gauge theory, the action is an integral of the curvature, the field strength squared. Maxwell's theory, QCD, they all work the same way. So in gravity, you can put the curvature squared into the action. And then there's almost a trivial argument that tells you that that theory, which includes Einstein, but also these four derivative terms, is renormalizable. Now, renormalizable means that when you do quantum field theory and you calculate things, it is possible to, although you get infinities in various calculations, you can absorb these into redefinitions of the coupling constants. And so basically you're led to a sensible theory with what we call a continuum limit, namely at short distances, the theory is completely under control.
So there is a renormalizable theory of quantum gravity, which has been known since the 1970s.
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Chapters
6 chapters
1
What is the main topic discussed in this episode?
0:00–15:20
2
What is quadratic gravity and why revisit it for quantum gravity?
15:20–26:58
3
How does asymptotic freedom and renormalization make quadratic gravity UV-complete?
26:58–38:28
4
What is the Ostrogradsky instability and how can it be reinterpreted in gravity?
38:28–52:40
5
How can negative-norm 'ghost' states be made physically acceptable?
52:40–1:07:49
6
What slight generalization of the Born rule resolves negative probabilities?
1:07:49–1:53:38
Speakers
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