Can asymptotic safety in quantum gravity be right?

[Nickyv2423]

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?

 

[atyy]

For point 1, there's work suggestive that UV fixed points can exist: https://arxiv.org/abs/1612.07315

For the rest of the points, the unusual nature of the fixed point may lead to an effective reduction in spacetime dimension: https://arxiv.org/abs/1401.4452

 

[jakob1111]

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?

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.

 

[mitchell porter]

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.

 

[atyy]

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.

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.

 

[atyy]

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.

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.

 

[A. Neumaier]

related:

R.P. Woodard, Perturbative Quantum Gravity Comes of Age, Int. J. Modern Physics D 23 (2014), 1430020.
http://arxiv.org/abs/1407.4748.

H. Gies et al., The Gravitational Two-Loop Counterterm is Asymptotically Safe, Phys. Rev. Lett. 116 (2016), 211302.
https://arxiv.org/abs/1601.01800

Gravity as a non renormalizable theory,
https://www.physicsforums.com/threads/gravity-as-a-non-renormalizable-theory.849632

 

[Demystifier]

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.

On the contrary, Lubos Motl's physical reasoning is usually very substantial and highly informed.

The problem with Lubos is that he is both.

 

[MathematicalPhysicist]

The problem with Lubos is that he is both.

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?

 

[Spinnor]

How can you tell when he's not mumbling without any credentials? and in which case it's not a triviality?

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.

 

[MathematicalPhysicist]

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.

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.

 

[Spinnor]

Motl observation is superficial without him even bothering reading the work of Schwinger, who is by the way a Noble laureate.

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,

http://physics.stackexchange.com/questions/98694/source-theory-alternative-to-qft

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.

 

[MathematicalPhysicist]

@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?

 

[MathematicalPhysicist]

BTW, even if you have the degree of PhD doesn't make you immune from being a crank.

https://en.wikipedia.org/wiki/Igor_and_Grichka_Bogdanoff

 

[Spinnor]

@Spinnor has Lubos even read Schwinger's books?

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!

 

[MathematicalPhysicist]

@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?

 

[Spinnor]

How can someone argue for the legitimacy of someone else's work without even reading it, I ask you?

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!

 

[MathematicalPhysicist]

Did he or did he not read Schwinger's work?


:-)

 

[Spinnor]

Did he or did he not read Schwinger's work?

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.

 

[MathematicalPhysicist]

@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:
https://www.physicsforums.com/threads/a-question-regarding-schwingers-sources-theory.896575/

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.

 

[Spinnor]

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

I smell an Insights article in the making, "The idiots guide to understanding Schwinger's source theory, all the good stuff!". I look forward to such a post from you when when you understand Schwinger work to your satisfaction,
though maybe it is not possible for idiots to understand?

Thanks for your help here at Physics Forums!

 

[MathematicalPhysicist]

I smell an Insights article in the making, "The idiots guide to understanding Schwinger's source theory, all the good stuff!". I look forward to such a post from you when when you understand Schwinger work to your satisfaction,
though maybe it is not possible for idiots to understand?

Thanks for your help here at Physics Forums!

Yes, it would be suitable for idiots like yourself.
:-D

 

[Spinnor]

"However, instead of being the path of the future, the theory he dubbed "source theory" became the path not taken."

From http://escholarship.org/uc/item/70m7r3r8#page-2

From a Google search https://www.google.com/search?q=Schwinger's+source+theory&rlz=1C1AOHY_enUS709US709&oq=Schwinger's+source+theory&aqs=chrome..69i57j69i60&sourceid=chrome&ie=UTF-8

Still would like to read your idiots guide to source theory.

 

[MathematicalPhysicist]

We can learn from any theory, even those that are wrong, cause eventually every scientific theory is wrong, you can ask Popper for clarification.

I'll be wary of theories that are not even wrong...
More a religion than everything.

 

[MathematicalPhysicist]

Unless you of course have a theory that predicts everything with 100% accuracy without any errors.

Do tell us of such a theory? How is it called nowadays?

A-B-C theory.
:-D

 

[julian]

Unless you of course have a theory that predicts everything with 100% accuracy without any errors.

Do tell us of such a theory? How is it called nowadays?

A-B-C theory.
:-D

What if you are right but computational methods aren't good enough to confirm the correctness? What if there is no computational algorithm that can reproduce reality? What does it mean to have 100% accuracy without exact experimentation results? What does it mean to have 100% accuracy with non-deterministic quantum theories?

 

[julian]

The dream of God?

 

[PAllen]

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,

http://physics.stackexchange.com/questions/98694/source-theory-alternative-to-qft

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.

Hmm. In my experience, even when I agree with physics aspects of one of his critiques, his critiques always include high volume ad hominem personal attacks, so as to be very unpleasant to read. It is only when he is not focused on critique that I can stand reading him.

 

[MathematicalPhysicist]

The dream of God?

One side of me wishes it were possible, the other half reckons that if it were possible and will be achieved then that game of physics, of theorizing toy models and so on will be over.

I don't think this game will ever be over.(which is a good thing, I love this game).

 

[Spinnor]

More a religion than everything.

I think there is a bit of room for "faith" in physics. Schwinger had a bit of faith that his path was the right one in regards to his source theory?

Happy hunting!