Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor

  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 #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 #5


    User Avatar
    Science Advisor

    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 #6


    User Avatar
    Science Advisor

    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.
  8. Jan 23, 2017 #7

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

  9. Jan 27, 2017 #8


    User Avatar
    Science Advisor

    The problem with Lubos is that he is both.
  10. Jan 27, 2017 #9
    How can you tell when he's not mumbling without any credentials? and in which case it's not a triviality?

    I assume that most of the books in physics and maths he didn't even read, how can he judge others' work if he hasn't read them carefully?
  11. Jan 28, 2017 #10
    When he is talking about mainstream particle physics or math, can you provide an examples where he is mumbling? You are probably better able to judge him then I but in matters regarding math and physics I would put his up against most working physicists in a contest of expertise and have him come out on top. In matters regarding string theory and quantum mechanics he seems pretty damn sharp and I am glad at his attempts to simplify such things as I am with the experts here at Physics Forums.

    End ass kissing.
  12. Jan 28, 2017 #11
    One thing that I found, is a post of his in stackexchange where he writes that Source Theory of Schwinger's is nothing but a different interpratation to QFT, while in Schwinger's third volume of "Particles Sources and Fields" Schwinger says that his theory departs from Operator theory and has different predictions; you can check my post in the subforum here in PF of Quantum Physics; for me it's obvious that Motl observation is superficial without him even bothering reading the work of Schwinger, who is by the way a Noble laureate.
  13. Jan 29, 2017 #12
    Thanks, now I'm curious, though Schwinger's source theory work is probably way above me.

    Here I think is Dr. Motl's response to that work,


    With the passing of time, what is the status of Schwinger's Source Theory. Is it something others have spent much time with?

    From my readings of Dr. Motl he usually is very precise in his criticism of others when it comes to physics.

    Again, I am glad the you and Dr. Motl take your valuable time to help others with physics questions. Dr. Motl may be a bit rough around the edges but I overlook that in a master of physics.
  14. Jan 29, 2017 #13
    @Spinnor has Lubos even read Schwinger's books?

    I started reading the first volume in the autumn of 2014, but didn't finish, it's a hard reading indeed.

    How can someone argue for the legitimacy of someone else's work without even reading it, I ask you?
  15. Jan 29, 2017 #14
  16. Jan 30, 2017 #15
    Even an expert who can quickly assimilate complicated material has to wisely pick and choose what to study. A Master is allowed, and can, quickly form an opinion, it is something I think you must do?

    Bottom line, if I had to go into a physics competition with only one assistant I would pick Dr. Motl. His link is right above Physics Forums in my favorites. Have a good week!

  17. Jan 30, 2017 #16
    @Spinnor to form an opinion everyone can do, to back it up you need to understand the work of others, and this takes time.

    If he didn't read Schwinger's work or any other work why should I bother read his criticisms, just because he worked at Harvard doesn't mean a thing, he got fired by them also, I wonder why is that?
  18. Jan 31, 2017 #17
    Dr. Motl's comment on the matter seemed pretty detailed in the link I provided. He is a master, a 10th degree black belt in physics, he formed an opinion of the source theory work of Schwinger and found it lacking, I respect his opinion.

    I will ask you again, did Schwinger's work on source theory go anywhere?

    Lets just agree to disagree, I don't agree with all his points of view but his physics and math seems rock solid.

    Good day!
  19. Jan 31, 2017 #18
    Did he or did he not read Schwinger's work?

  20. Feb 1, 2017 #19
    Judging by his post on the StackExchange I linked he must have at least had some knowledge of it.

    I thought Bugs Bunny always bested Yosemite Sam, my memory is going.

    Last edited: Feb 1, 2017
  21. Feb 1, 2017 #20
    @Spinnor I suspect he didn't really delve into Schwinger's work, since Schwinger himself argues in the third volume that operator theory and his theory come to disagreement.

    As you can read in my post here:

    It's hard reading Schwinger's work and understanding it.

    In the first volume it's written in the beginning volume: "if you can't join them, beat them".
    So I reckon that the mainstream doesn't appreciate this work.

    This volume and Feynman's Gravitation book are on my endless reading list in maths,physics,engineering and logic.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted