LQG is explained in his book Not Even Wrong where he points out that loops are a perfectly logical and self-consistent duality of the curvature of spacetime: ‘In loop quantum gravity, the basic idea is to use the standard methods of quantum theory, but to change the choice of fundamental variables that one is working with. It is well known among mathematicians that an alternative to thinking about geometry in terms of curvature fields at each point in a space is to instead think about the holonomy [whole rule] around loops in the space.’

LQG has the benefits of unifying Standard Model (Yang-Mills) quantum field theory with the verified non-landscape end of general relativity (curvature) without making a host of uncheckable extradimensional speculations. It is more economical with hype than string theory because the physical basis may be found in the Yang-Mills picture of exchange radiation. Fermions (non-integer spin particles) in the standard model don’t have intrinsic masses (masses vary with velocity for example), but their masses are due to their association with massive bosons having integer spin. Exchange of gauge boson radiations between these massive bosons gives the loops of LQG. If string theorists had any rationality they would take such facts as at least a serious alternative to string!

Dr Woit’s focus isn’t a complaint about the failure of string to accomplish checkable physics but is a complaint about the continuing hype and the underhanded attacking by string theorists at alternatives in general despite the hypocrisy that this involves! He makes it clear that he does not see string theory as wrong, only that so far it has only produced hype, hype, hype, plus extra loud hype when someone complains about alternatives being unheard.

Of course, he might see things from quite a different perspective if he was censored from posting papers to arXiv.org and so on. As it is, he can delete my embittered comments about string damaging physics as being mere noise.

“I think Peter Woit should start criticizing LQG too, only to be fair to “not even wrong” theories by his standard.”

I guess he won’t. A while ago some of his readers had the brilliant
idea of setting up a FAQ on his site. Great, I thought, let’s start
with proposing some questions, and came up with:
“What is the most overhyped theory of quantum gravity?”

He immediately deleted my post. I guess because he knew the answer ….

The review itself and some comments also express the craftmens view on this book (there is no argue for string theory _or_ loop).

For my opinion Lees objectives were different ones:

I think he took string theory as a current example for doing science the wrong way.

I agree with him totally that the current physics community is a kind of church and I think too that no relevant theory was born after the 1930ies. Thats a result of the dominating craftsmen in the physics community.

Frank.

You’ll never believe what the correct theory is! Unfortunately, I am unable to publish it.

‘It is shown that when a Burgers screw dislocation [in a crystal] moves with velocity v it suffers a longitudinal contraction by the factor (1 - v^2 /c^2)^1/2, where c is the velocity of transverse sound. The total energy of the moving dislocation is given by the formula E = E(o)/(1 - v^2 / c^2)^1/2, where E(o) is the potential energy of the dislocation at rest.’

Specifying that the distance/time ratio = c (constant velocity of light), then tells you that the time dilation factor is identical to the distance contraction factor.

here are just some funny stuff I couldn’t fail to notice about one post from LQG theorist Dr. Smolin:
“Dear Sean,
Thanks very much for an intelligent, perceptive review. If I may, then just a few words about the points where we disagree, because differences in judgment about these are at the heart of the issue”

:D:D It seems to me that the “a few words” are a lota words, more than a whole window on the Safari browser takes.

consequently ” the points where we disagree” is just about everything.

As dumb as I am, I have a side in the pick. Also hopefully you smart and serious guys out there don’t get confused between the functions of pop science and real science.

If asked the question which of the two (or one) theories are apparently more relevant to the real world, I wouldn’t hesitate a second to say it’s definitely string theory. and you guys who don’t know field theory well enough and criticize string theory for being irrelevant should really think twice before saying that again.

but there’s no proof LQG is ruled out. I can certainly agree to the statement that there’s a chance it’s a good theory for something, even maybe gravity of some worlds.

from the communist camp, we were told that philosophy that matters are extracted from things that happen obviously that’s not how the world outside that camp (which one existed) think. so I really agree a significant number of string theorists should dirty their hands now, as the LHC is at least in principle quite related to what many string theorists are doing, while (sadly) it cannot be said about the LQG at the present stage.

I think Peter Woit should start criticizing LQG too, only to be fair to “not even wrong” theories by his standard.

Unless we know all particle physics interactions all the way from accessible energy up to the Planck scale, we can never hope to extract any quantitative predictions about quantum gravitational effects.” I gave you an explicit example, in the work of Freidel and Livine, in a solvable (but perturbatively non-renormalizable) model, which contradicts that assertion.

You wish to quibble with the word “any” ? OK …

I am well-familiar with the work of Freidel and Livine, and I draw exactly the opposite conclusion from it.

In 2+1 dimensions, there is no local dynamics in the gravitational field. Ergo, the rationale for Georgi’s complaint: that there is no way to disentangle the effects of quantum gravitational dynamics from the effects of other (a-priori unknown to the low-energy observer) fields.

In 2+1 dimensions, there’s simply nothing to disentangle. And, indeed, Freidel and Livine show that the gravitational degrees of freedom can be integrated out once and for all (in closed form!), yielding a noncommutative effective field theory for the matter fields.

In 3+1 dimensions, there is local dynamics in the gravitational field, and so Georgi’s objection comes into play.

Obviously, attempting to emulate Freidel and Livine in 3+1 dimensions is doomed to failure. The gravitational field has local propagating massless degrees of freedom. Integrating out gravity would yields an intractable, hopelesly nonlocal mess.

This is already clear at the semiclassical level, which is why I was puzzled by your previous comment:

If this turns out to be true also in 3+1, as indicated by semiclassical calculations, it implies predictions for GLAST.

I would love to believe you that LQG actually makes testable predictions for GLAST. But I don’t. And, if GLAST returns a negative result, I suspect that we will hear that neither do you.

Anyway, if you feel that the word “any” in the above statement of Georgi’s objection is too strong, I’d be happy amending it to something less categorical.

But the spirit of his objection still holds..

Suppose we could go back in time and educate ancient Greek Mathematicians about Classical Mechanics, Special and General Realitivity, and Classical Electrodynamics. Then sooner or later they would have found out that there are inconsistencies in elctrodynamics when you attempt to take into account the back reaction of emitted radiation in a fully consistent way.

But I doubt that they would have been able to invent quantum mechanics as the correct solution without doing experiments.

The key sentence in your assertion is “Unless we know all particle physics interactions all the way from accessible energy up to the Planck scale, we can never hope to extract any quantitative predictions about quantum gravitational effects.” I gave you an explicit example, in the work of Freidel and Livine, in a solvable (but perturbatively non-renormalizable) model, which contradicts that assertion because the global symmetry of the ground state is deformed in a way that leads to “quantitative predictions.”

If you examine the actual calculation you can see how this works. They define a scattering amplitude in terms of a spin foam model. This gives a sum over diagrams. These can be organized in terms of matter Feynman diagrams. For each matter Feynman diagrams one has to sum over the degrees of freedom of the gravitational field. These are topological degrees of freedom in the 3d spacetime mod the Feynman diagram. This sum can be done for each diagram, and the effect is a universal deformation of the Feynman diagrams corresponding to a quantum deformation of Poincare symmetry.

It seems to me this is a counterexample to your claim above,

Thanks,

Lee

This is true unless there is a universal mechanism that cuts off quantum gravitational fluctuations and the fluctuations of anything else…

No!

This is true, unless there is a universal mechanism that cuts off quantum fluctuations of everything else except gravity. Then, and only then, would we have a regime where quantum gravity effects were important, but where the effects of other interactions were negligible.

Georgi’s assertion is that there is no such mechanism.

Is the nature of his objection clear, now?

It seems to me we have answered Georgi’s objection over and over again. I’ll try it again. First, is this the complete statement of it (from http://golem.ph.utexas.edu/~distler/blog/archives/000639.html): “The point is that there’s no decoupling regime in which quantum “pure gravity” effects are important, while other particle interactions can be neglected. “Universality” in field theory — usually our friend — is, here, our enemy. Unless we know all particle physics interactions all the way from accessible energy up to the Planck scale, we can never hope to extract any quantitative predictions about quantum gravitational effects.”

This is true unless there is a universal mechanism that cuts off quantum gravitational fluctuations and the fluctuations of anything else, because as a consequence of this mechanism there are simply no degrees of freedom with wavelength smaller than the Planck length. In fact, there is a such a mechanism, and it is understood, as I said, both heuristically and rigorously. To understand it heuristically you have to think carefully about how imposing spatial diffeomorphism invariance limits what can come out of an operator product, regulated through a point splitting procedure.

This is clearly described in the literature now for more than 10 years, please read the literature at whatever of rigor you are happy with. Then come back and either indicate you agree or indicate that there is a technical error somewhere in the proofs of this.

This implies that the uv problem does not suffice to fix the matter couplings. This however does not imply that the theory can make no predictions. An example is 2+1 gravity with matter as solved by Freidel and Livine. A universal effect is a deformation of the Poincare symmetry governed by a single computable parameter. If this turns out to be true also in 3+1, as indicated by semiclassical calculations, it implies predictions for GLAST.

Please tell me why this does not answer Georgi’s objection.

Also, to Sean, “whether the theory recovers GR (or some close relative thereof) in the classical limit, is not a minor technicality. It’s basically the whole point…” Certainly, and therefore you should be celebrating with us the results of Rovelli et al that show that the graviton propagator emerges from a spin foam path integral with the correct low energy behavior, demonstrating that the theory has gravitons and also a Newtonian gravitational force. You should also be celebrating with us the Freidel-Livine results I just mentioned as they show that in this interacting, perturbatively non-renormalizable model (2+1 gravity coupled to matter fields) the low energy limit emerges and it is QFT in a background spacetime, but on a non-commutative manifold.

These are important developments, but they were not the first indications that LQG has a good low energy limit. There were also various results showing that there are semiclassical states which approximate classical metrics and that QFT on background manifolds emerges as an approximation when one studies excitations of those states.

And while I agree with your sentiment, it didn’t have to turn out that the quantization of GR does give a rigorously defined hilbert space and observables algebra, but it did. Shouldn’t this be a clue? The fact that we now have good evidence that the low energy limit has gravitons is then I would think compelling.

Thanks,

Lee