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A Can asymptotic safety in quantum gravity be right?

  1. Jan 6, 2017 #1
    Asymptotic safety in quantum gravity is a local QFT. According to many people, local quantum field theories cannot be correct in terms of being a quantum gravity theory.
    Lubos Motl outlines 4 reasons why they can't be right.
    "Quantum gravity cannot be described as a local field theory in the bulk because of many reasons, including​

    1. the infinitely many types of terms that could be added; Weinberg discussed some partial successes but I don't think that there exists any known sensible UV fixed point for gravity; after all, its non-existence was the reason why so many people began to look at Hořava's non-relativistic extension of it recently;
    2. the wrong scaling of the entropy: scale-invariant field theories always have a volume-extensive entropy density and it seems impossible to guarantee that the entropy bounds will be imposed, i.e. that the black hole with its area-extensive entropy remains the record-holder for the total entropy in a volume (and therefore the ultimate stage of a collapse);
    3. the information preservation during the Hawking radiation that implies that physics of quantum gravity must allow for some kind of nonlocal effects that are able to get the information out of the black hole; these effects are impossible if the causal structure dictated by a metric tensor (quantum field) strictly holds;
    4. wrong trans-Planckian, very high-energy scattering amplitudes; the probability to create two particles in such a collision should exponentially decrease, as seen from general black hole thermodynamics, but that won't happen in a local theory in the bulk that is scale-invariant in the UV; the latter would lead to power laws."
    What do you guys think? Can Asymptotic safety be right?
  2. jcsd
  3. Jan 6, 2017 #2


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  4. Jan 9, 2017 #3
    It is well known that Lubos Motl is a crank. Trying to make sense of his "arguments" is usually a waste of time. He has his own very limited world view and always tries to make up arguments against anything that doesn't fit into his world view.
  5. Jan 15, 2017 at 5:27 AM #4
    On the contrary, Lubos Motl's physical reasoning is usually very substantial and highly informed. Anyway, the propositions about why gravity can't be a local field theory aren't original to him, I'm sure you could find them in the physics literature on quantum gravity, and if a quantum gravity theorist wants to deny them, the propositions ought to be answered somehow.

    I am not a quantum gravity theorist. My philosophy crudely summarized is, field theory is OK for quantum gravity only as an approximation; for fundamental theory, anything with strings or twistors might be OK, anything else is doubtful.

    Regarding asymptotic safety for gravity, I am therefore apriori skeptical. In its favor, the critics were apparently wrong when they said that the fixed point would go away when the next counterterm was taken into account (atyy's citation 1).

    On the other hand, I am very skeptical about the claimed "effective reduction in spacetime dimension" (atyy's citation 2, and references therein). It has the feel of an imposed interpretation, rather than an actual result.

    The biggest point in favor of the AS research program, is that it managed to predict the mass of the Higgs boson. Perhaps that reasoning can be reproduced without gravity, but it's still impressive and deserving of study.


    However, the real reason I decided to comment on this thread, was that I thought of another issue: How does AS deal with the gravitational contribution that should be coming from vacuum condensates, like those in QCD or the Higgs sector? Along with the zero-point energy of all quantum fields, they should be contributing to the effective cosmological constant.

    The way that string theory currently deals with this, is by the anthropic principle. The total vacuum energy has to be close to zero, if the universe is going to allow the formation of complex structures, so out of the many possible worlds of string theory, we had to find ourselves in a world where the vacuum energy contributions from all the braneworlds and fluxes and so on, almost cancel out exactly, as if by coincidence.

    It is a logical argument, and it is an argument that allowed Steven Weinberg to predict the approximate size of dark energy before it was even detected. Still, so long as we don't actually have a final theory, we might want to remain alert to the possibility that deeper reasons are at work, especially because of various "cosmic coincidences" connecting dark energy to the other parts of the universe's mass-energy budget.

    But how does AS propose to deal with this problem of the cosmological constant? I know that the cosmological constant is (alongside Newton's constant) part of the fixed point in their models. But is their renormalized cosmological constant capable of absorbing the vacuum energy contribution that comes from condensates? A demonstration of that would be very impressive to me.
  6. Jan 15, 2017 at 8:56 AM #5


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    I think Shaposhnikov and Wetterich https://arxiv.org/abs/0912.0208 are among the few that really try to do Asymptotic Safety of gravity and the standard model, so in principle, they should be able to answer your question, but I don't see it in their paper.

    I think in most cases, the work is at the level of whether Asymptotically Safe gravity exists at all, so that it could be a theory of quantum gravity in some universe - somewhat like most work on AdS/CFT in which there is no attempt to match the matter content of our universe.
  7. Jan 17, 2017 at 8:31 AM #6


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    I found Oliver Rosten's https://arxiv.org/abs/1106.2544 which agrees with you that one of the usual arguments about "effective reduction in spacetime dimension" has some problems.
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