Neil Turok

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2,693 appearances 2 recordings 1 series first heard Apr 2025 last heard 22 Jun

Neil Turok’s voice in public audio — every appearance, attributed to the second.

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Theories of Everything with Curt Jaimungal · Neil Turok: A Route to Quantum Gravity (Without Strings) · 22 Jun 2026 podcast

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.

And as I say, this is the simple-minded approach.

Now, there are reasons why people more or less abandoned it, though they keep coming back to it.

It's called quadratic gravity because it's quadratic in the curvature.

The action is quadratic in the curvature.

So, yeah, there's actually more and more interest in this possibility, which certainly is the simplest possible theory of quantum gravity.

So it's renormalizable.

And then in the 1980s, Avramidi and Bavinsky showed it's asymptotically free.

So just like QCD, the theory of the strong interactions, when you go to short distances, the coupling constant goes to zero.

And it becomes a trivial theory of just waves which don't interact.

So you can imagine at very short distances, this theory is really extremely simple.

So what's wrong with the theory?

All those statements I made about renormalizability, asymptotic freedom, they are the Euclidean theory.

That is how we do computations of the strong interactions, QCD.

You can put it on the lattice and study the theory, but the only way we really know how to study non-perturbative properties of any field theory

is to work in imaginary time.

The reason is that if you work in real time, then the quantum mechanical path integral is very oscillatory.

It involves the imaginary number, exponentially the imaginary number, it's oscillating like crazy, and if you try to integrate anything, these oscillations are just impossible to control.

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