Lorentz violating severely restricted: Mqg/Mplank > 1200

[marcus]

In other words if no miracle happens at 3+1 dimensions, LQG is dead, right?

wrong

(You don't make any sense, Micha. They worked hard to reproduce the 2+1 result and couldn't. So one expects no bending.)

==============

What one expects to be the case does not qualify as a miracle if it happens.
For any chance reader: what happens in different dimensions is often very different. What we are dealing with here is evidence about a first order coefficient. Even a theory that predicted some Lorentz bending could have zero first order and nonzero second orded. It's inefficient to have to go thru this kind of detail. People should really read Smolin's post at Bee's blog.

I will give the links again:
Bee Hossenfelder's blog
http://backreaction.blogspot.com/2009/08/that-photon-from-grb090510.html
where she is discussing the May 2009 Gammaray Burst.
Smolin's comment:
http://backreaction.blogspot.com/2009/08/that-photon-from-grb090510.html#c8923384582399562257

 

[skippy1729]

A "denier of empirical evidence"! I can't remember the last time anyone has thrown that at me. I really hope that there are more observations so a realistic limit can be placed on LIV and good models of GRB. It would be too bad if this turns out to be another Monopole balloon observation or SETI WOW signal.

I was not suggesting the Banach-Tarski paradox has any physical interpretation. What I was suggesting is that to take the concept of an infinitely divisible space SERIOUSLY for a physical model you must accept that your model has inherent flaws. Fortunately the hypothesis "spacetime is an infinitely divisible medium" is a falsifiable theory (just display a discrete model which correctly models all physical phenomenon!); while the hypothesis "spacetime is discrete at SOME energy level" is not falsifiable (there will always be higher energies and smaller distances, whether or not they will ever be accessible).

Rejecting the Axiom of Choice is fine but its use has become commonplace in topology and functional analysis texts (with and without specific mention). An example of its use in physics is the theorem of Geroch, Choqut–Bruhat that there exists a unique maximal Cauchy development from a given set of initial data: "Global aspects of the Cauchy problem in general relativity" http://www.springerlink.com/content/r137914u14277831/ (Zorn's lemma, which amounts to the same thing).

Skippy

 

[yossell]

Fortunately the hypothesis "spacetime is an infinitely divisible medium" is a falsifiable theory (just display a discrete model which correctly models all physical phenomenon!);

Not taking sides here, but finding a discrete model correctly modelling all physical phenomena would not *falsify* "spacetime is an infinitely divisible medium". Finding a ¬P model of a phenomenon is not falsifying P.

yossell

 

[Micha]

What one expects to be the case does not qualify as a miracle if it happens.
For any chance reader: what happens in different dimensions is often very different. What we are dealing with here is evidence about a first order coefficient. Even a theory that predicted some Lorentz bending could have zero first order and nonzero second orded. It's inefficient to have to go thru this kind of detail. People should really read Smolin's post at Bee's blog.

From Smolin's comment:

"In fact, in 2+1 dimensions the argument from quantum group theory is correct and the low energy symmetry is kappa-Poincare (hep-th/0512113, hep-th/0502106). This suggests its not crazy that to hypothesize that the same is true in 3+1 but this is not a proof, it is a suggestion of a line of argument."

If Smolin expects things to be "very different" in 3+1 dimensions, he is hiding it quite well.

Arguing that first order terms could be zero without having a good argument for it (is there any?) to me sounds like quite a desperate move to save a theory.

 

[fleem]

Special relativity was derived solely from observations of the average, large-scale behavior of many objects. So are the concepts of continuums, manifolds, and even non-integral values. Yet the vast majority of scientists insist those 100% classical "fundamental" theories must unequivocally be the foundational theories for describing the behavior of a single one of those objects (a single particle-particle interaction). That's like insisting a theory that describes a swarm of bees be unequivocally used as the foundation of describing a single bee. In fact, I'll go so far as to say any theory describing single particle-particle interactions which uses those large-scale ideas, is suspect.

 

[atyy]

What he explicitly says here is that there is NOT a prediction of photon delay derived from Loop at present. And you have referenced an authoritative recent Loop review article, so that basically should settle the issue.

http://relativity.livingreviews.org/Articles/lrr-2008-5/
"A quantum-gravity interpretation of the MAGIC observation does not appear to be likely at present (see for instance [67]), but the measurement shows that quantum-gravity effects are within the reach of current technology."

Does this really unequivocally say that photon delay is not a prediction of LQG? The reference [67] is http://arxiv.org/abs/0804.0619 which seems to me to say that the delay is probably not due to violation of Lorentz invariance. This would suggest that Rovelli meant a violation of Lorentz invariance would indeed be a quantum gravity effect. But I do agree there is nothing definite to pin down by his specific choice of words - for example, he says "quantum gravity", rather than "loop quantum gravity" - in an article about "loop quantum gravity".

 

[marcus]

From Smolin's comment:

"In fact, in 2+1 dimensions the argument from quantum group theory is correct and the low energy symmetry is kappa-Poincare (hep-th/0512113, hep-th/0502106). This suggests its not crazy that to hypothesize that the same is true in 3+1 but this is not a proof, it is a suggestion of a line of argument."

If Smolin expects things to be "very different" in 3+1 dimensions, he is hiding it quite well.

Arguing that first order terms could be zero without having a good argument for it (is there any?) to me sounds like quite a desperate move to save a theory.

I'm glad you read the comment at Bee's blog and it is fine with me whatever your attitude/interpretation. Mathematics is indeed sometimes very different in spaces of different dimensionality. It may be not crazy to guess at some analogy, but often the analogy doesn't turn out. In this case Freidel Livine proved something in 2005 for 3D and everybody was hoping they could do something analogous. What Smolin's sentence means is that it wasn't foolish to try, and they tried hard, but they could not get a proof in 4D.

The possibility that Lorentz might be bent, at very high energy, either first order or second order has been around for a long time. I think papers by non-Loop folks back in the 1990s may have pre-dated Smolin's involvement. I don't know the history. These quantities M_QG1 and M_QG2 have been around for many years, and has always been pointed out that first order deviation would be easier to detect or rule out.

If you look at a non-Loop paper like Ellis Mavromatos Nanopoulos they have this notation, and they consider both first and second order, try to control both, and they cite papers of theirs about this from way back in the 1990s.

There is no question of "desperate move to save a theory". Nobody's QG theory is being tested. The first/second order thing is just how Nature is, when you have a symmetry you need to be aware of the possibility that it might be bent. Don't assume you know everything up to infinitely high energy. It has always been acknowledged that if Lorentz is bent it might be first order or it might be second etc. ---and that the latter case would be much harder to detect.

So the first agendum is to rule out first order bending. If observations can rule it out, that's great. If observations can eventually rule second order deviation out, that will be great too.
I don't know of any theory that says there should be either kind of deviation, but it is only reasonable to be on the look-out, now that we have an instrument like Fermi-LAT with adequate sensitivity.

http://relativity.livingreviews.org/Articles/lrr-2008-5/
"A quantum-gravity interpretation of the MAGIC observation does not appear to be likely at present (see for instance [67]), but the measurement shows that quantum-gravity effects are within the reach of current technology."

Does this really unequivocally say that photon delay is not a prediction of LQG? The reference [67] is http://arxiv.org/abs/0804.0619 which seems to me to say that the delay is probably not due to violation of Lorentz invariance. This would suggest that Rovelli meant a violation of Lorentz invariance would indeed be a quantum gravity effect. But I do agree there is nothing definite to pin down by his specific choice of words - for example, he says "quantum gravity", rather than "loop quantum gravity" - in an article about "loop quantum gravity".

That's right Thanks for reading carefully! Rovelli has never indicated that he thinks Lorentz bending is a LQG effect, derived from LQG. Indeed to the contrary, as in his 2002 paper where he takes the trouble to show LQG consistency with Lorentz.

What you quote means that QG-scale effects are now within reach of observational technology. This is good news for LQG research, as he is pointing out. It means the researchers will be getting guidance in the future from empirical data.

 

[atyy]

So the first agendum is to rule out first order bending. If observations can rule it out, that's great. If observations can eventually rule second order deviation out, that will be great too.
I don't know of any theory that says there should be either kind of deviation, but it is only reasonable to be on the look-out, now that we have an instrument like Fermi-LAT with adequate sensitivity.

I do - my favourite crackpot theories - Visser, Volovik, Wen - I still like them though

 

[marcus]

I do - my favourite crackpot theories - Visser, Volovik, Wen - I still like them though

QG with a condensed matter perspective! I should have thought of that! I don't know much about VV&W's work but I wouldn't cry crackpot just yet.
BTW the analog models people seem to have put up a strong showing at Vancouver.

 

[atyy]

QG with a condensed matter perspective! I should have thought of that! I don't know much about VV&W's work but I wouldn't cry crackpot just yet.
BTW the analog models people seem to have put up a strong showing at Vancouver.

I mean "crackpot" as a high compliment BTW, although AdS/CFT was derived from a different viewpoint, I would actually count it as instantiating the "emergent philosophy" of condensed matter - I think they've also got a session at Vancouver.

 

[Micha]

Sorry, Marcus.

there is no particular sentence in your reply which I can point to and say this is wrong. But all in all I have the impression I am arguing with a rubber band, always stretching so much, it is forced to in the light of new results.

The problem I have is I read Smolin's book (the one whose sales figures you analyzed here for months) and while I do not remember every single detail, I clearly remember the general line of thought. On the one hand there was string theory not being able to make any predictions and sticking exactly to Lorentz symmetry. On the other hand there were all these new approaches like DSR and LQG, where experimental results were around the corner, an energy dependent speed of light being the most important one.

Yes, I remember clearly, that Smolin also said in his book, that in 3+1 LQG there were so far no clear predictions about an energy dependent speed of light, but he would like to have them, before experimental results were coming in.

You can always put some ifs and whens, but according to you, the new experimental result makes not the slightest difference on how you view the different quantum gravity approaches. Not the slightest disappointment. No reference to Smolin's book. Nothing. Now Smolin suddenly is a figure on the sideline of LQG (which he might even be today). This makes me wonder what result ever would make an impression on you.

 

[marcus]

...Yes, I remember clearly, that Smolin also said in his book, that in 3+1 LQG there were so far no clear predictions about an energy dependent speed of light, but he would like to have them, before experimental results were coming in.

... according to you, the new experimental result makes not the slightest difference on how you view the different quantum gravity approaches...

Micha, I'm glad you remember so clearly from that popular 2006 book! I read some of it when it came out. It had an important function, but it was not something to learn all about LQG from! In this case what you remember is right. As we know, a number of people tried hard including top people like Freidel and Kowalski-Glikman. The result just doesn't seem to extend to 4D! Maybe sometime we will understand the underlying reason. Or even it could happen that some new approach to QG will actually imply some kind of Lorentz bending, and it will check out observationally! We are at the beginning stages of observational QG. Theories don't stand still either.

I have an idea to propose to you. Since you show signs of being a thoughtful reader, how about you try Rovelli's 2008 review of LQG?
This is a scholarly review article, not a popular-written book. The popular-written books can not and do not properly characterize an intellectual discussion, obviously.

BTW my introduction to QG was basically the online 2003 draft of Rovelli's book. That book is still worth looking at, for its depth. But I mention the review article because shorter and more up-to-date. It is true that I tend to see LQG "thru Rovelli glasses" so to speak. I also follow AsymSafe QG of Reuter/Percacci/Weinberg and the Triangulations QG of Loll's group. These are all exciting QG approaches in a phase of rapid growth and change.

It is useless to harp on the past. Like what looked hopeful in 2003 to someone writing an informal memoir for a poplar online magazine
LQG has gone thru enormous change even since 2006. A new spinfoam model is now the basis of a new dynamics. (Before there was no satisfactory dynamics, and no good grasp of the semiclassical limit.) The cosmo subfield LQC has also been completely revolutionized since 2006. Basic equations of LQC changed, and fresh results. It is really time now for a new Smolin-type book that will bring the general audience readership up to date.

I guess I should point out that my personality or your personality or what we think of each others thought process is not the issue. We are talking about the real research world out there.

 

[Micha]

I guess I should point out that my personality or your personality or what we think of each others thought process is not the issue. We are talking about the real research world out there.

I agree with that.

But it is also true that real research is paid by the general society. So the public deserves to get an accurate picture about the status of various research approaches. It is a moral requirement and public perception surely also has an effect on funding agencies and hiring at universities and so on, although you will never be able to exactly quantify that. And the physics forum is a small piece of this public perception. So therefore it is not unimportant, what is written here. And as you are a key contributor to this forum, I think, that you have a responsibility as well.

Best regards Michael.

PS: I haven't anything against you personally. To the contrary. I am just curious about scientific truth.

 

[MTd2]

Unfortunantely, this forum is not supported by any public agency.

 

[Micha]

Unfortunantely, this forum is not supported by any public agency.

I don't think, this invalidates any of my points.

 

[MTd2]

I don't think, this invalidates any of my points.

Of course it does not invalidate your points. But many of us do not have much time left because of our jobs, family, etc. And many of the recent developments in most of new areas did have a proper conference on the subject, so there is not much consolidation even among experts.

 

[MTd2]

What if lorentz violating effects were supressed like 1-sech(Mpl/Mqg^2)? I guess that wouldn't rule out DSR, right?

Edit: perhaps, a 1-erf^2

http://en.wikipedia.org/wiki/Error_function

 

[Micha]

Hi MTd2,
as any smooth function, the Lorentz violating effects, in this case the deviation of the speed of high energy photons from the known speed of light as a function of energy, can be developed into a Taylor expansion. So whatever the exact form of the function is, it is zero in the low energy limit (the speed of light of low energy photons is what it is) and then what you first would notice are linear effects. These have been excluded by the measured result now (if confirmed). Quadratic or higher order terms can't be excluded so far. Maybe quadratic terms can be measured or excluded by the experiment in the future, but not yet. So you can save a Lorentz violating theory by having it only predict quadratic effects at maximum. This however smells like a trick, if you don't have a good reason, why the linear term should be zero.

 

[MTd2]

Hi Micha, look at this:

http://backreaction.blogspot.com/2009/08/that-photon-from-grb090510.html

At 12:06 PM, August 25, 2009, Daniel de França MTd2 said...
You mean, the linear dispertion was not ruled out?

At 12:25 PM, August 25, 2009, Lee Smolin said...
Dear Daniel de França MTd2,

This is a key question which was covered in many places, for example my paper with Amelino-Camelia or my post above. See those for details.

The linear correction is parity odd in the case of Lorentz symmetry breaking and parity even in the case of DSR. The former is ruled out by several orders of magnitude because it leads to rotations of planes of polarization, see the reference by Gleiser and Kozemeh I mentioned above. The latter, DSR, parity even case is not ruled out, although further observations by Fermi may be able to do that.

The distinction between broken and deformed lorentz invariance is a key point in this whole discussion.

Thanks,

Lee

Thanks,

Lee

 

[Micha]

Yes I read it.

When I read your discussion of the error function, I just thought, that I remind people of the Taylor expansion. It is a trivial point of course.