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marcus
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Jul5-06, 03:26 PM
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Fh/Distler conversation is interesting

From the comments at Prof Distler's blog
http://golem.ph.utexas.edu/~distler/....html#comments
Picking up from that last post by Urs

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RE: WELCOME

If quantization of 2d gravity gives different results than everything else (including the lattice = dynamical triangulation) and if even the harmonic oscillator comes out wrong, how can you just go on?

Both examples you mention live in CQL. According to Lee Smolin, most practitioners of LQG have abandoned that and moved from CQL to spin foams.

It seems to be me that hence the question is if there is a spin foam model for d-dimensional gravity, in particular for d>3 , that we could throw all these questions at.

Posted by: urs on July 4, 2006 10:54 AM
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RE: A NEW KIND OF SCIENCE?

“Do you really want to assert that LQG is a “new kind of Science”, which cannot be discussed using conventional scientific criteria?”

I want to assert that science (or more precisely fundamental physics) is not equal to standard QFT.

“You can’t “generalize” something that you can’t reproduce.”

See the work by Freidel (this is getting repetitive and tiresome) Spin Foams can reproduce ordinary flat spacetime QFT.

Also way to go to clip the context, the sentences preceding what you make bold are:

“It is an old speculation in physics that, once the gravitational field is successfully quantized, it should serve as the natural regulator of infrared and ultraviolet singularities that plague quantum field theories in a background metric. We demonstrate that this idea is implemented in a precise sense within the framework of four-dimensional canonical Lorentzian quantum gravity in the continuum.”

So we have a theory that has the Gauge group and algebra of the Standard Modell but we do not know and nowhere claim to know if and how this theory reproduces the flat spacetime limit. Though we suspect that it does we can’t prove that. The situation is the same as Lattice QFT.

Anyways, what are you suggesting? That THiemanns results are wrong? That he’s lying about what he is doing?

Posted by: fh on July 5, 2006 03:49 AM
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RE: A NEW KIND OF SCIENCE?

So we have a theory that has the Gauge group and algebra of the Standard Modell but we do not know and nowhere claim to know if and how this theory reproduces the flat spacetime limit. Though we suspect that it does we can’t prove that.

As I pointed out above, nothing of Thiemann’s analysis would have changed, had he attempted to couple an anomalous chiral gauge theory in place of the (nonanomalous) Standard Model.

That he would claim to have a consistent nonperturbative quantization of an anomalous chiral gauge theory coupled to quantum gravity ought to tell you something.

The situation is the same as Lattice QFT.

Not even close. First of all, the Lattice Gauge theorists know that they cannot currently study chiral gauge theories. Second, they have abundant numerical evidence that they can achieve the continuum limit.

Anyways, what are you suggesting? That THiemanns results are wrong?

Yes.

That he’s lying about what he is doing?

“Wild exaggeration” would be more accurate.

Posted by: Jacques Distler on July 5, 2006 10:47 AM
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RE: A NEW KIND OF SCIENCE?

You can tell that they are wrong from reading the abstract?

While demonstrating amply that you have not attempted to understand what he is claiming or trying to do?

While it is clear that there is a different use of language as well, resulting from the different perspectives, that you fail to acknowledge?

I’m sorry but this is a pure (and not the first in this thread) ad hominem. You have not pointed out ANYTHING about Thiemanns paper that is wrong.

You’re free to claim that the quantization of SM+GR THiemann is producing is unphysical for various reasons (including the fact that it apparently doesn’t know about anomalies), you can not claim thaat he doesn’t have one.

Posted by: fh on July 5, 2006 11:46 AM
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RE: A NEW KIND OF SCIENCE?

The fact that he claims to be able to quantize an anomalous gauge theory, merely by coupling it to quantum gravity, is, indeed sufficient grounds to be confident that he is wrong.

Yes, I could carefully work through his paper to find the precise point where he goes astray. But what would that gain?

True-believers, like yourself, would dismiss it as a mean-spirited attack. And people like Lee would say, "It doesn’t matter, because we’ve all moved on to study spin foams, anyway.”

The US Patent Office does not accept applications for Perpetuum Mobile, without a working model. Similarly, I am not interested in entertaining any purported quantizations of chiral gauge theories, without a detailed explanation of how anomalies are realized.

Posted by: Jacques Distler on July 5, 2006 12:08 PM

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RE: THE LQG LANDSCAPE

Dear MoveOn,

There are lots of finite theories that have running effective coupling constants: take any lattice QFT and choose the lattice spacing short enough to accomodate the present limits on uv breaking of Lorentz invariance.

Dear Jacques and others,

Yes, the results together amount to a demonstration of the existence of a new kind of QFT which has no background metric but is diffeomorphism invariant. Moreover, there are “semiclassical states” when gravity is coupled to matter fields, such that expansion around them reproduces at long wavelength a cutoff version of the matter QFT.

I understand your skepticism as I was also trained as a conventional QFT, but at some point you have to decide to take the chance that we are neither dishonest nor stupid and investigate whether these claims have merit. To do this there is no alternative but to study the books and papers. We do take great pains to be honest about open questions, but we also have to insist when questions are not open. These claims are not new (Thiemann’s paper in question is nine years old) and they have been thoroughly checked and examined by a community of smart, critically minded people. If your interest is, as I hope it is, in good faith-that is if this is science and not a debating club-you might consider taking the time to study the papers and understand exactly what the claims are and how they are demonstrated.

I would urge that this is only fair. I only expressed criticisms of string theory after having learned the basics, taught a graduate course in the subject from the standard textbooks, and published a dozen technical papers in the subject.

I promise you that if you do the same you will understand that everything we have claimed is true, as we have stated them. But I do not see how more repetition of those claims can help if you are implacably hostile to the possibility that they are right.

Thanks,

Lee

Posted by: Lee Smolin on July 5, 2006 10:43 AM

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