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Lorentz violating severely restricted: Mqg/Mplank > 1200 
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#181
Aug2209, 09:19 PM

Astronomy
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PF Gold
P: 23,235

(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/200...grb090510.html where she is discussing the May 2009 Gammaray Burst. Smolin's comment: http://backreaction.blogspot.com/200...84582399562257 


#182
Aug2309, 12:06 AM

P: 148

[QUOTE=lumidek;2317421
.... The more clear measurements of this kind one needs, the more appropriate it is to call him a denier of empirical evidence..... ....The opinion that the BanachTarski paradox has any consequences for physics or any natural science is downright ludicrous. It is pure maths, and very abstract and pathological maths. The most straightforward physical way to deal with the BanachTarski result is simply to deny the axiom of choice for infinite sets  one doesn't really need it in this form in any physically relevant maths.... [/QUOTE] 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 BanachTarski 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 


#183
Aug2309, 05:03 AM

P: 341

yossell 


#184
Aug2309, 05:41 AM

P: 145

"In fact, in 2+1 dimensions the argument from quantum group theory is correct and the low energy symmetry is kappaPoincare (hepth/0512113, hepth/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. 


#185
Aug2309, 07:16 AM

P: 461

Special relativity was derived solely from observations of the average, largescale behavior of many objects. So are the concepts of continuums, manifolds, and even nonintegral 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 particleparticle 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 particleparticle interactions which uses those largescale ideas, is suspect.



#186
Aug2309, 09:08 AM

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P: 8,660

"A quantumgravity interpretation of the MAGIC observation does not appear to be likely at present (see for instance [67]), but the measurement shows that quantumgravity 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". 


#187
Aug2309, 10:26 AM

Astronomy
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PF Gold
P: 23,235

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 nonLoop folks back in the 1990s may have predated 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 nonLoop 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 lookout, now that we have an instrument like FermiLAT with adequate sensitivity. What you quote means that QGscale 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. 


#188
Aug2309, 11:02 AM

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#189
Aug2309, 11:13 AM

Astronomy
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BTW the analog models people seem to have put up a strong showing at Vancouver. 


#190
Aug2309, 11:21 AM

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#191
Aug2309, 11:24 AM

P: 145

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. 


#192
Aug2309, 12:23 PM

Astronomy
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PF Gold
P: 23,235

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 popularwritten book. The popularwritten 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 uptodate. 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 Smolintype 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. 


#193
Aug2309, 02:02 PM

P: 145

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. 


#194
Aug2309, 02:11 PM

PF Gold
P: 1,961

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



#195
Aug2309, 02:13 PM

P: 145




#196
Aug2309, 05:36 PM

PF Gold
P: 1,961




#197
Aug2409, 01:55 PM

PF Gold
P: 1,961

What if lorentz violating effects were supressed like 1sech(Mpl/Mqg^2)? I guess that wouldn't rule out DSR, right?
Edit: perhaps, a 1erf^2 http://en.wikipedia.org/wiki/Error_function 


#198
Aug2609, 02:15 PM

P: 145

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. 


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