What Are the Empirical Challenges Facing Quantum Gravity Theories?

[ohwilleke]

TL;DR Summary

One of the leading researchers in Loop Quantum Gravity discusses existing observations that should guide future quantum gravity research in a three page paper.

Rovelli points to three pieces of existing observational evidence that should guide future quantum gravity research. Bottom line:

* abandon Lorentz invariance violating quantum gravity theories,
* abandon supergravity and string theory,
and
* stop working on the anti-deSitter/conformal field theory relationship in gravity and cosmology work (leave it to the condensed matter physicists who have legitimate uses for it).

I tend to agree but would welcome other views.

What other experimental data discredit large volumes of research papers?

Stop working on models of dark matter that predict NFW dark matter particle distributions would be a good start as they have repeatedly and convincingly been shown to be at odds with the observational data. If your theory predicts this, it is wrong. See, e.g., Pengfei Li, Federico Lelli, Stacy McGaugh, James Schombert, "A comprehensive catalog of dark matter halo models for SPARC galaxies" (January 28, 2020). arXiv 2001.10538; Marie Korsaga, et al., "GHASP: an Hα kinematics survey of spiral galaxies - XII. Distribution of luminous and dark matter in spiral and irregular nearby galaxies using Rc-band photometry" (September 17, 2018) Kyriakos Grammatikos, Vasiliki Pavlidou, "Getting the tiger by the tail: Probing the turnaround radius of structures with outer halo density profiles" (September 17, 2018); Antonino Del Popolo et al., "Correlations between the Dark Matter and Baryonic Properties of CLASH Galaxy Clusters" (August 6, 2018), Lin Wang, Da-Ming Chen, Ran Li "The total density profile of DM halos fitted from strong lensing" (July 31, 2017); here (2017), and here (2016), and here (2011). Since an NFW flows analytically (almost trivially) from a simple one component collisionless dark matter particle model, this means that reality is not well described by a simple one component collisionless dark matter particle model.

A. Lorentz Invariance

The breaking of Lorentz invariance at the Planck scale may simplify the construction of a quantum theory of gravity. This observation sparked a large theoretical enthusiasm for Lorentz-breaking theories some time ago, and rightly so. But that bubble of enthusiasm has been deflated by empirical observations. A large campaign of astrophysical observations has failed to reveal the Planck-scale breaking of the Lorentz invariance in situations where it would have been expected if this track for understanding quantum gravity had been the good one.

A methodological consideration is important at this point. Popperian falsifiability is an important demarcation criterium for scientific theories (that is, if a theory is not falsifiable, we better not call it “science”); however, Popperian falsification is rarely the way theories gain or lose credibility in science.

The way scientific theories gain or loose credibility in real science is rather through a Bayesian gradual increase or decrease of the positive or negative confirmation from empirical data. That is, when a theory predicts a novel phenomenon and we this to be right, our confidence in the theory grows; when it predicts a novel phenomenon and we do not find it, our confidence in the theory decreases. Failed predictions rarely definitely kill a theory, because theoreticians are very good at patching up and adjusting. But failed predictions do make the success of a research program far less probable: we loose confidence in it.

Hence, this has been the effect of not finding Lorentz violations in astrophysics: tentative quantum gravity theories that break Lorentz invariance might perhaps still be viable in principle, but in practice, far fewer people bet on them.

B. Supersymmetry

What I wrote above is particularly relevant to the spectacular non-discovery of supersymmetry at the LHC.

While in the Popperian sense, the non-appearance of supersymmetric particles at the TeV scale does not rule out all the theories based on supersymmetry, including string theory, in practice, the strong disappointment of not finding what was expected counts heavily as a strong dis-confirmation, in the Bayesian sense, of all those theories.

People have written that the non-discovery of supersymmetry is a crisis for theoretical physics. This is nonsense, of course. It is only a crisis for those who bet on supersymmetry and string theory. For all the alternative theoretical quantum gravity programs that were never convinced by the arguments for low-energy supersymmetry, the non-discovery of supersymmetry is not a crisis: it is a victory.

Precisely for the same reason that the discovery of supersymmetry would have been a confirmation of the ideas supporting the string supersymmetry research direction, the non-discovery of supersymmetry at the LHC is a strong empirical indication against the search for quantum gravity in the direction of supersymmetric theories and strings.

Nature talks, and we better listen.

C. Cosmological Constant

A case similar to the one above but even stronger concerns the sign of the cosmological constant. The cosmological constant is a fundamental constant of nature, part of the Einstein equations (since 1917), whose value had not been measured until recently. An entire research community has long worked, and is still working, under general hypotheses that lead to the expectation for the sign of the cosmological constant to be negative. Even today, the vast majority of the theoretical work in that community assume it to be so.

Except that the sign of the cosmological constant is not negative. It is positive, as observation has convincingly shown.

Once again, this counts as a strong dis-confirmation of the hypotheses on which a large community has worked in the past, and is still working on today.

So far, we lack any direct evidence of a quantum gravitational phenomenon; however, the non-detection of Lorentz violations around the Planck scale, the nondiscovery of super symmetric particles at the LHC, and the measurement of a positive cosmological constant are strong indications from Nature that disfavor the tentative quantum gravity theories that naturally imply these phenomena.

[Submitted on 15 Nov 2021]

Considerations on Quantum Gravity Phenomenology​

Carlo Rovelli

I describe two phenomenological windows on quantum gravity that seem promising to me. I argue that we already have important empirical inputs that should orient research in quantum gravity.

Comments:	3 Pages
Subjects:	General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Journal reference:	Universe 2021, 7(11), 439
Cite as:	arXiv:2111.07828 [gr-qc]

 

[MathematicalPhysicist]

*abandon LQG...

 

[Fra]

I suspect the take on the strategy and guidance here, depends if one tries to see the unification of gravity and QM, as topic concerning trying to understand the "quantisation procedure" for gravity separate from the general unification of forces or if they are deeply entangled.

If one takes the latter view, dualities between very different theories (which need not be specifically AdS/CFT, which is just a major know explict case, it doesn't represent the general case even in conceptual sense) seems still like a highly relevant area, that is much more than just about the sign of the cosmological constant. Thinking in this direction does not necessarily mean advocating ST.

/Fredrik

 

[MathematicalPhysicist]

As far as I can see you cannot refute Superstring theory, LQG or Susy or Supergravity.
You can always say we need a bigger particle collider, larger energies.
If the models or theories don't have in them a built-in prediction of how much energies are needed, then they cannot be refuted.

 

[martinbn]

As far as I can see you cannot refute Superstring theory, LQG or Susy or Supergravity.
You can always say we need a bigger particle collider, larger energies.
If the models or theories don't have in them a built-in prediction of how much energies are needed, then they cannot be refuted.

That is in the article. His point is that even though you cannot disprove you can decrease/increase your confidence in a research approach. And that experiments and observations already show which programs are less promising.

 

[MathematicalPhysicist]

That is in the article. His point is that even though you cannot disprove you can decrease/increase your confidence in a research approach. And that experiments and observations already show which programs are less promising.

As I see it there is no difference between LQG, Superstring, etc.
Research in these will cease when the funding will dry out.

 

[martinbn]

As I see it there is no difference between LQG, Superstring, etc.
Research in these will cease when the funding will dry out.

Rovelli has a different view.

How can the funding dry out?! If you are hired at a university/institute you have your salary and you can do research. What other funding do you need?

 

[MathematicalPhysicist]

Rovelli has a different view.

How can the funding dry out?! If you are hired at a university/institute you have your salary and you can do research. What other funding do you need?

Your salary is composed of grants and teaching.
The committees that decide where the money goes might decide not to invest on these avenues.

 

[weirdoguy]

Your salary is composed of grants and teaching.

That is not true, there are scientists that don't teach and don't have grants but just normal salary for their scientific work. You tend to have a lot of strong but not really founded opinions...

 

[MathematicalPhysicist]

That is not true, there are scientists that don't teach and don't have grants but just normal salary for their scientific work. You tend to have a lot of strong but not really founded opinions...

So they can work on whatever they want to work on, without any need for writing proposals for their research?
I am skeptical...

 

[Demystifier]

So they can work on whatever they want to work on, without any need for writing proposals for their research?

Yes. For example, me.

 

[Demystifier]

As I see it there is no difference between LQG, Superstring, etc.
Research in these will cease when the funding will dry out.

By that criterion, do you see a difference between LQG and history of Roman Empire?

 

[weirdoguy]

I am skeptical...

What's funny is that you are skeptical on something that is a standard practice. Grants are relatively rare and really hard to get. Do you really think that each and every arXiv preprint is connected with some grant?

 

[MathematicalPhysicist]

Yes. For example, me.

Who pays for your work?

 

[MathematicalPhysicist]

By that criterion, do you see a difference between LQG and history of Roman Empire?

I guess both don't get funded.

 

[MathematicalPhysicist]

What's funny is that you are skeptical on something that is a standard practice. Grants are relatively rare and really hard to get. Do you really think that each and every arXiv preprint is connected with some grant?

Grant or some scholarship.

I know that for postdoc and phd you need to apply for some scholarships to fund your research.
So do you say that most professor's research isn't funded?
So what makes up their salary?

 

[Fra]

Having noting todo with the research politics but:

Rovelli wrote:

"As always in science, a priori everything is possible,but there is a profound difference between an implausible wild speculation and the predictions of a plausible, coherent framework. This is a distinction a bit too much disregarded in today’s fundamental physics, in my opinion."

This is true, but a problem is still that what IS a "plausible, coherent framework", is subjective so it seems still like am empty statement in the context. What appears to be a wild speculation from one perspective, may be the good inference from another perspective, because one has chose different fundamental starting points. There is no conflict in this, as there is no unique way to extrapolate known facts to guesses of the unknown. Only the future will tell.

As a lot of theoretical research on physics is not about producing explicit phenomenological predictions, but about playing around in a theoryspace, which is defined and constrained in different ways depending on what paradigm one is using. The hope is that one day something nice will come out. So as I see, focus is more on which "theory toolbox" is likely to be the most efficient and successful one, in eventually either making explicit phenomenolgical predictions (that are DOABLE) or making process by increaseing explanatory power (which I see as the prime mission) in the sense of for example, reudcing the number of free parameters, and providing a conceptual framework for navigating in theory space. There will never be an consensus agreement on this among scienticst, and there need not be. This is just the way it is, it is not a foundational problem per see.

Lets say we one does not like Strings, not LQG, then what is the alternative? Shall we just stop wasting time on anything that is not explicit phenomenology?

From a pragmatic perspective, and short time scale resource planning, it may seen like an option. But there is also a problem with such an approch, that "science" risks beeing just a big "statistics", with less and less explanatory power. I think what drives some of us, is not just about collecting statistics about the past, but to gain deeper insight in how things are causally related. This is for me what foundations of physics is about, not just "applying" the mature theory to experiments.

/Fredrik

 

[Demystifier]

Who pays for your work?

Government. (Which gets money from taxpayers.)

 

[Demystifier]

I know that for postdoc and phd you need to apply for some scholarships to fund your research.

It depends on the country (among other things). Where do you live? I suspect in US, because it's typical for US citizens to assume that the whole world works the same way as their country does. (Of course, not all US citizens think that way, but in my experience many do.)

 

[Demystifier]

So do you say that most professor's research isn't funded?
So what makes up their salary?

The primary job of a professor is teaching. But leaving that aside, getting a salary for research doesn't imply that you have to tell in advance what will you study and what are your expected results. In fact, in my opinion (with which not everybody will agree), it is contrary to the spirit, the idea and the purpose of fundamental research to make a specific research proposal before actually doing the research. If you know in advance what your results will be, then maybe you are doing straightforward stuff which will not produce a true progress. As Einstein said, if we knew what it was we were doing, it wouldn't be called research, would it?

 

[weirdoguy]

So what makes up their salary?

University just pays them for doing research and writing papers on that. That's the way it is in Poland. Grants are relatively rare. The same goes with PhD, one doesn't need to apply for any funding.

 

[MathematicalPhysicist]

It depends on the country (among other things). Where do you live? I suspect in US, because it's typical for US citizens to assume that the whole world works the same way as their country does. (Of course, not all US citizens think that way, but in my experience many do.)

No, I don't live in the USA.
I got this wrong impression from google search.
I did pursue two MScs one in maths and the other in physics but didn't complete them (the average grade in the courses for maths was 95 and for the physics was 75, I didn't complete my thesis writing component in the required alleged time).
I didn't need to apply for scholarships, but I did work as a grader (a work which I still have).

But for PhD I guess from what I had seen, you need to write research proposals, and to explain your progress in detailed reports that's at least how it looks in the University which I did my two Msc above.

 

[MathematicalPhysicist]

But I agree, if you know what your results will be then it cannot be called research.
But nowadays, in the capitalist's eyes do people still conduct such research as Albert Einstein is quoted?

 

[Fra]

But nowadays, in the capitalist's eyes do people still conduct such research as Albert Einstein is quoted?

To a certain extent, there would be a compromise. Either one have to accept some bias over the research direction, in order to get payed (for example if your goal is to become a professional researcher), or one has to accept to not get payed and do it on your free time (if the goal is to ansewr your own questions). But in the latter case the compromise is still that one has less time to spend. I suspect that those people that backed up financially and are free to do whatever they want are rare.

I made the latter choice long time ago. Had I aligned and followed advice of supervisors at the time I should have pursued string theory as that is where the "opportunities" were.

/Fredrik

 

[Demystifier]

But nowadays, in the capitalist's eyes do people still conduct such research as Albert Einstein is quoted?

Those who pay for the research must have a lot of money, and those who have a lot of money tend to see everything from the capitalist point of view. Fundamental research is just one of the victims of that. The result is that grant proposals for fundamental research look like business proposals. To get a grant, a scientist must write a lot of bull...t that makes little sense from a scientific point of view. It is supposed to make sense to bureaucrats who make decisions about grants, but I doubt that it makes sense to them either. The way how scientists actually conduct research has little to do with what they write in the grant proposals.

 

[mitchell porter]

I just went and looked at the five most recent papers on hep-th. All of them acknowledged support from some kind of grant, in countries as different as US, Japan, China, various EU, and Chile.

 

[maline]

Summary:: One of the leading researchers in Loop Quantum Gravity discusses existing observations that should guide future quantum gravity research in a three page paper.

stop working on the anti-deSitter/conformal field theory relationship in gravity and cosmology work

I don't see that he's saying that. AdS/CFT never made any claim that our universe should be AdS. Their claim is that the duality defines a quantum gravity theory, which happens to live in AdS. Hopefully, gravity in our world works more-or-less similarly to how it works in AdS, at least as far as the high-energy stuff is concerned; so finding a full quantum gravity for AdS would be a great and very relevant achievement.

I understood Rovelli's remark to be referring to the "swampland" problem, that string theories with Calabi-Yau compactifications seem to predict a negative cosmological constant.

 

[Demystifier]

Hopefully, gravity in our world works more-or-less similarly to how it works in AdS

Yes. AdS is somewhat like a harmonic oscillator; our world is not exactly a harmonic oscillator, but understanding of the harmonic oscillator helps a lot to understand the real world as well.

 

[martinbn]

Yes. AdS is somewhat like a harmonic oscillator; our world is not exactly a harmonic oscillator, but understanding of the harmonic oscillator helps a lot to understand the real world as well.

Is it like the harmonic oscillator or is it hoped to be?

 

[Demystifier]

Is it like the harmonic oscillator or is it hoped to be?

Good point, it's more like a hope.

 

[Nullstein]

So is Rovelli arguing against LQG now? As far as I'm concerned, it isn't locally Lorentz-invariant either, due to the inevitable singular excitations of geometry, at least at sufficiently small scales.

 

[martinbn]

So is Rovelli arguing against LQG now? As far as I'm concerned, it isn't locally Lorentz-invariant either, due to the inevitable singular excitations of geometry, at least at sufficiently small scales.

How so!?

 

[Nullstein]

How so!?

Local Lorentz-invariance means that spacetime is locally Minkowski, i.e. given any point in spacetime and any desired accuracy, there must be a small neighborhood around that point such that this neighborhood is isometric to a region of Minkowski spacetime, at least up to the specified accuracy. However, in all LQG models, geometry is defined by spin networks or spin foams. These are lattice-like structures embedded into the spacetime manifold. If you choose a point directly on a spin network, you will find its geometry to be excited. But if you pick any neighborhood of that point and any point within that neighborhood (other than the ones that lie on the lattice), the geometry around this other point will not be excited. There is a discontinuous jump. So there is no neighborhood around the points that lie on a spin network/foam that are isometric to a region of Minkowski spacetime. Minkowski spacetime is translation-invariant. You would have to be able to shift one point onto each other and still have the same geometry. But that's impossible due to the discontinuity.

 

[Fra]

I see a conceptual difference in violating a continuous symmetry due to that it is in fact violated(and spacetime still is a justified continuum), or just because the continuum model may not be physically justified. Then it seems that, at before point where the symmetry is "violated", the set which is subject to the symmetry is no longer well defined. I think is may be more of a problem for the continuum model itself.

/Fredrik

 

[martinbn]

Local Lorentz-invariance means that spacetime is locally Minkowski, i.e. given any point in spacetime and any desired accuracy, there must be a small neighborhood around that point such that this neighborhood is isometric to a region of Minkowski spacetime, at least up to the specified accuracy.

This of course is not true. The neighborhood will be homeomerphic not isometric to a region in Minkowski.

However, in all LQG models, geometry is defined by spin networks or spin foams. These are lattice-like structures embedded into the spacetime manifold. If you choose a point directly on a spin network, you will find its geometry to be excited. But if you pick any neighborhood of that point and any point within that neighborhood (other than the ones that lie on the lattice), the geometry around this other point will not be excited. There is a discontinuous jump. So there is no neighborhood around the points that lie on a spin network/foam that are isometric to a region of Minkowski spacetime. Minkowski spacetime is translation-invariant. You would have to be able to shift one point onto each other and still have the same geometry. But that's impossible due to the discontinuity.

I know very little about this, so I cannot really respond, but I think that your description is not right (of course I am probably wrong). Can you point to a source about this?