Is Physicist Lubos Motl's Criticism of Loop Quantum Gravity Valid?

[ohwilleke]

A recent paper by Maldecena and Susskind entitled "Cool horizons for entangled black holes" (http://arxiv.org/abs/1306.0533) argues that every entangled complex EPR pair of quantum particles must be topologically connected by an Einstein-Rosen bridge (aka a pair of wormholes within the light cone of some point in space-time).

Lubos Motl, a conservative professional string theory physicist with no respect for decency or civility who blogs most about physics and his anti-global warming existence stances as well as a bit of politics and local Czech color (more credibly about the physics than other matters, but with credibility tainted by his incivility and belief in counterfactual climate theories) argues in a recent post that the Cool horizons paper's conclusions (which he finds to be credible) imply that loop quantum gravity and kindred pursuits are false in a post at http://motls.blogspot.com/2013/06/finding-and-abandoning-incorrect.html#more from which I quote:

General relativity is capable of producing spacetimes of nontrivial topology and the topology is a classical property of a spacetime. Therefore, it's been implicitly assumed that the discrete data defining the spacetime or space topology are observables that are represented by linear operators in the quantum theory.

Maldacena and Susskind have pretty much completely convinced me that this can't be the case, however. A spin-up electron here and a spin-down electron there, |↑↓⟩ , that propagate on a flat spacetime seem to be eigenstates of the "topology" operator with the "trivial topology" eigenvalue. The same seems to hold for |↓↑⟩ . If the "space topology" operator were linear, it would also obey

(space topology ^ )⋅|↑↓⟩=(trivial topology)⋅(|↑↓⟩−|↓↑⟩).

However, the correct eigenvalue is different, Maldacena and Susskind argue:

(space topology ^ )⋅|↑↓⟩=(topology with an ER bridge)⋅(|↑↓⟩−|↓↑⟩). . . .

After all, this singlet state is the simplest quantum state that lives on the background with a single Einstein-Rosen bridge. So there's no linear operator that would count the Einstein-Rosen bridges. It's not a good observable. You can't use the number of ER bridges as a quantity that defines coarse-grained histories, e.g. in the Consistent Histories approach to quantum mechanics. In other words, the state |↑↓⟩ isn't orthogonal to the singlet state above: they're not mutually excluding although the former seems to be living on a bridge-free spacetime while the latter is living in a spacetime with a bridge. . . .

You should notice that all the childish proposals to construct a theory of quantum gravity such as loop quantum gravity, causal dynamical triangulations, and so on are incompatible with the Maldacena+Susskind insight (not that it is the first lethal disease that has killed these stupidities, far from that). Why? Because they construct a spacetime (dreaming about its being large and nearly smooth) that clearly has some combinatorial properties and some topological invariants may be clearly extracted from their preferred basis of spin networks and other silly animals they use. So they have linear operators of "space topology".

It's easy to see why these approaches to quantum gravity have this property – which we now believe to be a pathological one. They have it because they belong among the naive models of quantum mechanics that start with a preferred basis and observables for which all the basis vectors are eigenvectors. But such theories are extremely unnatural as quantum theories because quantum theories don't come with any preferred bases or preferred operators. Viable quantum theories always offer us many different operators that don't commute with each other – which also implies that bases constructed from the eigenvectors of one set or another set are different bases.

Does he have a point or not? Why?

I suspect that he is wrong by virtue of attacking a straw man version of LQG and in particular failing to appreciate the background independence and meaning of locality in LQG, but I am really over my depth in evaluating his criticism and would appreciate the insights of others into his arguments.

 

[Demystifier]

The paper by Maldacena and Susskind contains a lot of hand-waving arguments and no serious quantitative analysis, so I can't take their paper seriously. Arguments for not taking Motl seriously are even more obvious.

 

[DimReg]

I doubt that Motl has any idea what the current status of LQG is. Approximately 10 years ago, there was some excitement because LQG seemed to predict a minimum length, which seems incompatible with special relativity. Motl remembers this, and as late as 2011 you can find record of him claiming that "LQG breaks the local Lorentz symmetry" (at this link: http://physics.stackexchange.com/qu...gravity-connect-in-any-way-with-string-theory). As early as 2004 for however, you can find writing in the LQG literature stating the opposite (such as Rovelli's book). From this I infer that basic and important results of LQG take at least 10 years to reach Motl. Certainly, if he actually tried to keep up with the field, he would be very interested in the status of local lorentz invariance.

If fact, attempting to make sense of his criticism, I think he's still trying to imply that the problem is linked to local lorentz invariance. At least, that's how I interpret his claim that there are "preferred" spin networks.

 

[audioloop]

I doubt that Motl has any idea what the current status of LQG is. Approximately 10 years ago, there was some excitement because LQG seemed to predict a minimum length, which seems incompatible with special relativity. Motl remembers this, and as late as 2011 you can find record of him claiming that "LQG breaks the local Lorentz symmetry" (at this link: http://physics.stackexchange.com/qu...gravity-connect-in-any-way-with-string-theory). As early as 2004 for however, you can find writing in the LQG literature stating the opposite (such as Rovelli's book). From this I infer that basic and important results of LQG take at least 10 years to reach Motl. Certainly, if he actually tried to keep up with the field, he would be very interested in the status of local lorentz invariance.

If fact, attempting to make sense of his criticism, I think he's still trying to imply that the problem is linked to local lorentz invariance. At least, that's how I interpret his claim that there are "preferred" spin networks.

well then we have to give ten years more to LQG to see iblackf
same thing for string theory...

 

[Demystifier]

Some new papers on the ER=EPR conjecture:
http://arxiv.org/abs/1307.1132
http://arxiv.org/abs/1307.1604

 

[marcus]

Ohwilleke, Demy, Paulibus, excuse my changing the topic---I'm interested in Ohwilleke's reflections on a several other subjects and since he hasn't posted since middle of last year I want to encourage him to rejoin us at BtSM. In particular I'm interested in his views (as insightful outsider) on the future of particle physics. What are the issues around proposals for a future collider more powerful than LHC?

I saw a cogent listing of experimental HEP priorities by Ohwilleke elsewhere which I'd like to quote, something like it could function well as initial post of a discussion thread here at PF:

===quote Ohwilleke 28 February 2014 01:48===
A 100 TeV collider would be nice. So would a pony. I would prioritize about half a dozen other investments before that, however:

1. $100-$500 million for improved computing power to do lattice QCD calculations both to make theoretical predictions that reduce the MOE (especially for discriminating between backgrounds and signals), and increase the power of existing experimental data and new searches with the same equipment, and would allow more precise extraction of Standard Model constants from existing data. We are pretty much guaranteed to be able get seven or eight loop QCD beta functions (the current research effort is devoted to five loops and each successive term gets much harder than the one before it to calculate) and with more accurate calculations, greatly increased theoretical precision, for example, simply by investing the money to get the computational power to do the job.

2. $1-2 billion for deep space satellites. The only thing we can be absolutely sure exists in terms of BSM physics is something to explain dark matter phenomena, either at least one new particle or at least one new force or both. The best way to narrow the dark matter parameter space is with precision astronomy observations that the atmosphere obscures, not with a 100 TeV collider. We can be vastly more efficient in our search for dark matter particles at some future experiment yet to be designed if we use astronomy observations to more tightly narrow this parameter space first. It is the difference between looking for a face in a crowd based on a photograph v. a police sketch artist's effort from an eye witness's blurry recollection. We can also, for example, much more accurately triangulate star distances with a pair of distant deep space satellites which would calibrate all other astronomy observations. And, better observations of neutron stars, pulsars, cosmic rays, etc. provides a different way of doing super high energy HEP with nature paying the electric bills for it. Also, realistically, the only application of 100 TeV+ scale phenomena is cosmology anyway. Very early universe observations from deep space also narrow the parameter space of 100 TeV scale physics and let us know what we should be looking for.

3. $2-3 billion+ for neutrino physics experiments (e.g. astronomy, reactor, neutrino beam, double beta decay). A little more investment here to pin down the last few Standard Model parameters and determine things like the Dirac v. Majorana basis of the neutrino mass has an immense impact on the parameter space of BSM physics.

4. Continued funding of B factories. There are dozens of meson resonances that we don't really understand well. This is an area of particle physics where predictions are frequently not matching up with experimental results. You don't need a 100 TeV collider to investigate, e.g., 0.5 to 2.5 GeV scalar and axial vector mesons.

5. $100-500 million or so to push the envelope on exclusions for proton decay, magnetic and electric dipole moments, non-collider based axion field searches, entanglement experiments, etc.

6. $100-$200 million on non-SUSY/non-stringy theoretical work. We have all of our eggs in one or two baskets. The LHC has cracked many of the other contenders. Like anything, there is declining marginal benefit to funding yet another SUSY theorist.

If there is money left over, then by all means, let's go buy a 100 TeV collider.
==endquote==

I'm hoping something along these lines could germinate discussion here at BtSM.

 

[MTd2]

Well, EPR=EP is what I thought when I read about maximum acceleration. That means, the throat of a black hole will have a minimum curvature, it seems.

 

[Jimster41]

I don't understand the symbology of his summarization of Susskind et al. But I can imagine just in a purely abstract sense that "causality" might be defined in two distinct frames, one of which is a-causal with respect to the other. I could imagine there might be strict mechanisms (causal sets) that obey coherent rules that define behavior in their respective domains. And if they never met, then fine, but if they did by some requirement interact, couldn't such a situation potentially allow for strange things like "Spook-like Action at a distance" in one.

 

[Ilja]

6. $100-$200 million on non-SUSY/non-stringy theoretical work. We have all of our eggs in one or two baskets. The LHC has cracked many of the other contenders. Like anything, there is declining marginal benefit to funding yet another SUSY theorist.

As if this would help. Predictably these money would end in a third basket, probably LQG.

The problem of not only physics is that physicists are the most dependent people on Earth, given that they have to care all the time about the known end of the actual grant/job, and, therefore, to care about a new job. Thus, their predictable survival strategy is to follow the actual fashion. But there are not many fashions.

What we would need would be really independent scientists - they would not need a big income (the promise to receive one once they become successful would be enough), it would be sufficient to pay them not that much more than social security payments, but it should be safe, even in the case of complete failure of their personal research program. In this case, one would not have to worry about having two or three baskets in fundamental physics, one would have almost as much as people working there.