asymptotic safety not so safe?

A bit more than a week ago, Ethan wrote a pop. sci. piece about quantum gravity and explained why he thinks the asymptotic safety scenario is more convincing to him than string theory.
Obviously, Lubos had to respond in his trademark style, but he also linked to an old blog post of Jacques, who argued in 2008 that "the first nontrivial test of the asymptotic safety proposal" has yet to be calculated.

I used this opportunity to post a comment there to ask if somebody had performed this non-trivial calculation by now, seven years later (*). Jacques actually responded that "the situation looks considerably worse to me than it did at the time"; the reason being that all of those calculations break BRST invariance. (I recommend to push the 'view chronologically' button if you want to read the latest comments.)

I wonder what is going on in theoretical physics nowadays - is this just another breakdown of the peer review system or something worse?
I assume Reuter et al. have their arguments to pursue a line of research which Jacques et al. thinks is "completely unreliable". But it seems the field is falling apart into different tribes, strings, a.s., lqg, cdt, etc., who no longer discuss and argue with each other and gave up on the basic notion of right vs wrong some time ago.

I think if this continues for another seven years it can only get worse and to me this is all very sad.


(*) As far as I am aware the answer is no.


added later: Jacques wrote a new blog post about his problems with asymptotic safety for quantum gravity.

added much later: I wrote another short entry about this a while later.

10 comments:

Lee said...

>> But it seems the field is falling apart into different tribes,

I think maybe the only difference with what is going on now between competing quantum gravity models and what was going on between the S-matrix bootstrap vs QFT camps in the 50's and 60's is that data became available in the 70's which allowed QFT to lead to the standard model. If that hadn't happened physicists would still be arguing about bootstrap vs QFT.

Physics and the rest of science is just a human endeavor and is subject to all the characteristics of the human condition. IMO physics is still in a lot better shape ethically than any of the social sciences or in medicine. More than a little of what is published in those areas seems to be downright fraudulent. There are a lot of reasons I don't consider social sciences as science and that is one of them.

wolfgang said...

>> physicists would still be arguing about bootstrap vs QFT.
But it seems to me that this is the difference, they are not even arguing with each other anymore.

If string theorists think that a.s. is "completely unreliable" and lqg is
"obviously flawed" etc. then why don't they write a paper which lists these
problems and forces a discussion?
On the other hand, why does Reuter et. al. not follow up on his own remark (in one of the first papers) and explain in more detail why one can ignore loss of BRST invariance?
The blog post Jacques wrote 7 years ago was read by at least one a.s. author, why did nobody bother to calculate the non-trivial test Jacques proposed or at least comment on why it can be ignored?

I would think it should be more important to Reuter et al. to convince Jacques et al. rather than writing more and more papers it which do not address any of the open questions.

wolfgang said...

Btw this phenomenon is of course not new in fields like religion, politics, economics etc.
but this is physics and when I was young(er) there was only one tribe and now there are several and I don't like it.

Lee said...

>> when I was young(er) there was only one tribe

Yeah, but with quantum gravity models physicists are essentially wandering in the dark. That wasn't the case in the development of the standard model. During its development new data were coming in from SLAC, Brookhaven, and Fermilab as well as CERN and ideas could be tested against those data. If that hadn't happened it's very possible that in the intervening 40 years physicists would have broken into tribes who ignore legitimate criticisms of their competing ideas just as the quantum gravity people seem to do now. It's a very normal human thing to do and even physicists are incapable of removing themselves from the human condition. I still think elementary particle physics in in good shape, but needs new data that clearly violate the standard model. That may be a long time in coming though. I hope not.

Jacques Distler said...

Two quick points.

1) Life is too short for me to be publishing papers about AS, LQG, CDT, ... A blog post every several years is already more effort than is warranted (at least, that's what some of my prominent colleagues chide me).

2) I disagree with Lee about what motivated the abandonment of S-matrix theory in favour of QFT, which took place in mid 1970s. It was NOT some new experimental data (Bjorken scaling in deep inelastic scattering had been known for half a decade). It was a pair of theoretical developments: a) the discovery of asymptotic freedom and b) the proof of renormalizability of nonabelian gauge theories.

True, the discovery of weak neutral currents clinched the deal for QFT (and what would later become known as the Standard Model), but the tide had already turned by that time.

wolfgang said...

>> A blog post every several years
How about one per year? 8-)

I am really curious if/how Reuter et al. will respond to your recent blog post.

Anonymous said...

Distler has a good point about the death of the boostrap, but I think that quarks and partons held the bloody dagger even before that - nuclear democracy no longer made sense once nucleons were clearly not fundamental.

OmarZ said...

>> But it seems the field is falling apart into different tribes

To the extent of my knowledge of the community, the AS people do not perceive themselves as a "tribe". Rather they are happy to discuss about other models and approaches. Also, they are perfectly aware of the problems raised here (and in Distler's blog) and are trying to address them.

Some of the issues raised back in 2008 have been discussed and solved. Some others are still open. I do agree that some more effort could've spent on the open ones, but time was used to develop other directions for what I assume were good reasons.

I really think that an expert's opinion on the issue of BRST invariance in background FRG computations would help making this entry less biased. I suggest to advertise it to whom it might concern, next time.

I am no expert, but I can try to spend a couple of sentences. In short, BRST invariance is preserved in a deformed way due to the presence of the cutoff. Using this deformed BRST to constraint the space of couplings is complicated for YM and very complicated for gravity. That is why it is taking so long for the latter.

wolfgang said...

OmarZ,

thank you for your comment.

>> an expert's opinion on the issue of BRST invariance in background FRG computations

I am certainly not an expert and I can only hope that one of them finds the time to respond to J.Distler's blog post(s).

>> they are perfectly aware of the problems raised here

I don't want to start a polemic but my impression is that the questions and problems J. Distler raised were not exactly emphasized in the available literature.

OmarZ said...

Problems are most certainly not emphasized, but the same goes for various fields and for several reasons. Good luck getting a research proposal that goes in this direction financed these days. Well, wish me luck! :)

Do these "problems" admit a solution? I honestly think so. On the other hand, the issues have to some extent been treated quite pragmatically by the community with methods coming from, say, the study of critical phenomena.

I suggest the reading of
arxiv.org/abs/1508.06244
to clarify the status of BRST/split invariance in FRG computations, and
arxiv.org/abs/1502.07396
for a relatively recent discussion of background dependence in asymptotically safe gravity. An example of "pragmatic" approach could be
arxiv.org/abs/1507.08859