Steven Weinberg was visiting CERN recently and gave a talk entitled The Quantum Theory of Fields: Effective or Fundamental? He discussed the ups and downs of the “market price” of quantum field theory, showing a decline since a peak in 1984, followed by a conjectured increase in the future. He also described the history of his work that led to the modern point of view on the role of QFT as an effective theory.

He ended with comments on the “asymptotic safety” approach to quantum gravity, noting that it is quite possible that string theory is not needed, that the world can just be described at a fundamental level by quantum field theory (and thus his conjecture that QFT may come back into fashion as a fundamental theory):

I don’t want to discourage string theorists, but there’s just the possibility that maybe that isn’t the way the world is, that the world is much more like we’ve always known, that is, the Standard Model and General Relativity.

**Update**: Weinberg has a new paper on the arXiv, covering much the same material as this talk.

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The talk is somewhat confused. If the world is really described by the standard model plus general relativity, why do we need to discuss asymptotic safety at all? This is not made clear by Weinberg.

Charles,

This is all about the renormalizability problem of gravity. The asymptotic safety idea allows you to make sense of GR as a quantum theory, despite the renormalizability problem. It’s a matter of some controversy whether this actually works and does what one would like.

My own interpretation of what Weinberg is saying (which he might not agree with…) is something like: “unifying the SM and quantum gravity via string theory may very well be the way to go, but given that this hasn’t been a success so far, it’s a good idea to keep in mind that there’s another possibility: QFT by itself might be all that is needed, with asymptotic safety one way the gravity renormalizability problem could get solved”.

Lubos Motl wrote interesting comment on this talk, pointing out that quantum gravity as local quantum field theory is not compatible with the results of black hole thermodynamics. Are there any loopholes to solve this problem?

V_NO,

What Weinberg is talking about is the standard claim for string theory that it is needed to deal with the perturbative renormalizability problems of quantum gravity. That may not be true, either because of asymptotic safety, or also because of possible perturbative finiteness of some supergravity theories.

Non-perturbatively, you don’t really even know what string theory is, I don’t see how one can claim it’s the only possible way to deal with non-perturbative problems in quantum gravity. The problem is more complicated than just invoking black hole thermodynamics, which was originally discovered as a semi-classical phenomenon in QFT.

Sorry, but the Lubos rants on this subject tend to be more ideology than science. It’s quite clear that he would equally vociferously make the opposite argument if he felt that was the “pro-string theory” position.

Getting ready to get on a plane to China. I’m not going to be able to deal with comments here for a while, so will shut off comments for now.

If you want to discuss Lubos’s claims that Weinberg is wrong on this point of QFT, you can do that at his blog.