Lorentz violating severely restricted: Mqg/Mplank > 1200


by MTd2
Tags: 1200, lorentz, mqg or mplank, restricted, severely, violating
MTd2
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#91
Aug19-09, 11:56 AM
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Quote Quote by lumidek View Post
That's what the QCD fathers finally got their Nobel prize for. Before that point, one couldn't say anything sensible, deep, or useful about QCD, which is why no one should have studied it.
There would be no QCD fathers if they thought like that before studying it.
heinz
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#92
Aug19-09, 12:11 PM
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Quote Quote by lumidek View Post
In practice, I would bet 999:1 that these Planck-suppressed terms will never be measured. The only way how they could be measured would be to isolate an effect that doesn't exist without these terms at all, but appears as their consequence. I don't think that any such a phenomenon may exist, even in principle, because the higher-derivative terms mix with the lower-derivative terms if one changes the RG scale, so one can't even objectively say what the coefficients of these terms are - they depend on the RG scale. The only exceptions could be higher-derivative terms that violate a conservation law that is "accidentally" satisfied by the leading terms.

I agree with Woodard that quantum gravity has to agree with GR in most limits - in fact, I independently wrote it above. But I completely disagree that it is disappointing in any way.
Lubos,

thank you again for the clear answer. The reason I added "disappointing" is that the situation you describe is now the following: general relativity (with Einstein-Hilbert action) and string theory differ only by terms which cannot be measured in experiments (your 999:1 bet).

It is fun to see, when you google for "string theory" and "deviations from general relativity", that many pages come up. So thank you for stating so clearly that in fact, these deviations are probably not of any "measurable" importance.

Obviously, this distinguishes string theory from all other theories that predict deviations from general relativity, but it does not distinguish string theory from general relativity itself. We thus can confirm string theory only in the particle physics domain, not in the gravitation domain. I think that is a powerful conclusion. Thanks for saying this so clearly!

heinz
Finbar
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#93
Aug19-09, 12:57 PM
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Quote Quote by lumidek View Post
1. QCD is not only asymptotically "safe": it is asymptotically free (which means that the coupling goes to zero in the UV, instead of a finite constant, as in asymptotically safe theories). And it is a great example of yours showing what I meant.

Yes QCD is asymptotically safe, asymptotic freedom is a special case of asymptotic safety where the fixed point is Gaussian, as I said. But note that newtons constant would be asymptotically free in asymptotically safe gravity as it has a negative mass dimension.

Quote Quote by lumidek View Post
QCD only became a sensible theory worth studying when the people understood why it was asymptotically free - the negative beta-function. That's what the QCD fathers finally got their Nobel prize for. Before that point, one couldn't say anything sensible, deep, or useful about QCD, which is why no one should have studied it.
But by that logic the people who found the negative beta-function shouldn't of been studying it. People study the RG flow of gravity to see whether the beta-function's of gravitational constants have fixed points. Evidence has been found for the fixed so also it makes sense to look at the physical implications.



I was thinking of fields on black hole spacetimes in the entropy discussion. Sorry. Can you give me a reference with the calculation that entropy goes as T^d in a full QFT? I was looking at Susskind's book (BH, information and the ST revolution) where he notes this relationship S~V T^3 for a free scalar field. He then goes on to show that this implies the entropy diverges near the horizon. This though is at least a semi-classical calculation. Obviously a full theory of quantum gravity should solve this problem. It also doesn't seem unreasonable that within asymptotic safety this problem could be solved; the coupling of gravity to matter fields is asymptotically free so effects near the horizon should be reduced.
ccdantas
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#94
Aug19-09, 01:25 PM
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Quote Quote by lumidek View Post
a discussion of this paper by Thiemann involves some technicalities which are not terribly interesting. (...)
Thanks for your clarifications (although I was expecting a more detailed elaboration; but never mind). This is certainly something I should work myself.

Quote Quote by lumidek View Post
Do you really think that people like me should be wasting time with obscure LQG papers a week after the event that has falsified LQG and all similar research programs?
Evidently, every one is free to do whatever one desires, I'm not here to try to convince anyone otherwise.

Also, there is nothing particularly wrong in establishing one's own standard against which a given theory is found not to deserve further investiment of one's energy and time, so I have nothing to criticize you on thinking that LQG is a waste of your time. However, I do not agree to conclude that it has been "falsified and period". I think there is still a long way to ascertain the situation. This is quite normal in science. I am generally as skeptical as science requires, and I think it is healthy to keep that way. Evidently there is a limit to that and the limit is not often as clear as desirable. The situation in quantum gravity is exactly like that.

You should realize that your standard is not necessarily in agreement with other people's, not because other people are stupid and you are a genius, but just because in the present case, it is clear that there are still subtleties in the LQG formalism (I believe you would agree with this?). These are open for debate, and people are interested in investigating them further. This is not a big issue (although you do often make a big issue on this). You are free to put an end to your own curiosity about LQG, by your own standards. But this does not mean that you are 100% correct. What is needed is clean cut predictions and clean cut observations/experiments. LQG is not at that point yet; Fermi data lead to some interesting (possible) constraints, that need to be established with more data. More understanding of the source, a clear bound on the emission time of the highest energy photon.

A similar situation concerns your preferred approach, string theory, which is often claimed to be under construction. One needs clean cut predicitons of the theory in order to falsify it.

Thanks.
Christine
Micha
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#95
Aug19-09, 01:46 PM
P: 145
Hi Lubos,
as Christine and others pointed out, we are talking about one photon. I don't think that it is appropriate to say, that some theory has been falsified by measuring one photon. Never. Of course this does not change anything in all the arguments exchanged in this very interesting thread. You can always add the small if clause "if the result is confirmed then..." and then we can put a probability to that. But by completely dropping it, I think you make your position attackable at a point, where it is not necessary. Because whether this is confirmed or not we can just wait and see. No point to put energy into this if you are a theorist.
Eelco
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#96
Aug19-09, 02:23 PM
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Lubos:

While I am myself unsure of the merits of LQG, you make some fundemantally flawed claims concerning discrete spacetime. Small scale physics that breaks some laws deduced from macroscopic observations are perfectly capable of reproducing these macroscopic properties in the large scale limit. As an elementary example, just think of wave propagation over a mass-spring chain, which is dispersive but has the continuum wave propagation for wavelengths much larger than the grid spacing.

My impression of you and your likes, is that you are terrified of the possibility that all these fancy theories you have invested your life in, will be falsified by some future experiment. To claim victory over one photon merely shows desperation. But even if these dipersion predictions are falsified, at least thats more than string theory can lay claim to.

Since you seem to know so well, do you dare make any predictions that might be tested with the LHC? What if supersymmetric particles are not found, will that mean anything for your position on string theory?
MTd2
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#97
Aug19-09, 02:35 PM
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Quote Quote by Eelco View Post
will be falsified by some future experiment.
Not really. Just string theory models he likes best. The one by Mavromatos shows an average distribution for light speed, not of fundamental nature, and could fit possibly delayed fotons.
humanino
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#98
Aug19-09, 02:49 PM
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You know MTd2, Lubos admitted that he is on a political fight against LQG. I am not sure what is the point to continue any scientific argumentation for or against here, we merely have lobbying activity and it is against PF rules. So I am not sure how much will be necessary and whether it is worth pushing in this direction, for instance Lubos made several references to "God" which should be enough for moderation of a "regular" member.
Micha
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#99
Aug19-09, 03:32 PM
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Quote Quote by humanino View Post
for instance Lubos made several references to "God" which should be enough for moderation of a "regular" member.
That statement is completely ridiculous and you know it. The reference to God was clearly not made in a scientific sense.
Sauron
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#100
Aug19-09, 03:38 PM
P: 102
I would like to present the relevance from an slighly different viewpoint.

There are two asic options:
a) LQG predicts dispersion of speed of light. Them it has benn falsified.

b) It doesn't predict that dispersion. Them it doesn't predict anything measurable AFAIK. That's contrary to one of it's declared main purposes, to sacrify "ambition" of beeing an unified theory for the predictivity power.

Anyway this result is very bad for LQG.

Let's go with some of the subleties. Some people claim that LQG in fact doesn't predict that dispersions. Lubos, on the contrary, gives a general argument about the lack of imaginary values for areas in LQG (something shared by all the approach to LQG,canonical, spin foams, CDT's if I am not wrong) impliying, whatever LQG people agrees or not that dispersions. Well, I would like to see if Lubos has some reference for an actual paper where that argument is elaborated in detaill.

The other subletie I see is that some people claim that as an experimental result the conclusions are not absolutly settled because it is argued that it is necessary to obtain result for ensemmbles of photons because the dispersion is an average result. Aout this particlar point Lubbos says that the natural thing would be to do an satatistic about the number of collisions of the photon with the "atoms of space time" (inthe markopolulos justification of the phenomena). Form this viewpoint a single photon will have by far enought numer of collisions to do a good statistichal average as far as I see (I think that is the essence of Lubos argument).

About the question of predictions of string theory surelly Lubos can give more detaills (and correct me if I make some mistake in what I say about F-theory). I can point to the F-theory GUT scenaries. In the strings 2009 Vafa gave a brief account of results stating two clear predictions. One was the there were not WIMP's candidates foro dark matter. That implied that the aparent excess of positrons observed by ARTIC and PAMELA were false. Curiously FERMI/GLAST could also prove this.

For the LHC it is predicted some particular particle (I don't remembberthe name just now), with a very clear trace when leaving the detectors.

If that results are found that particular approach, the F-theory GUTS will be clearly favoured. If not that particular approach of string theory (a very good one which reproduces all the characteristics of the standard model) will have been falsified. Still it is possible that other phenomenological models based on string theory could be shown valids.
Finbar
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#101
Aug19-09, 05:26 PM
P: 343
Quote Quote by Sauron View Post
Let's go with some of the subleties. Some people claim that LQG in fact doesn't predict that dispersions. Lubos, on the contrary, gives a general argument about the lack of imaginary values for areas in LQG (something shared by all the approach to LQG,canonical, spin foams, CDT's if I am not wrong) impliying, whatever LQG people agrees or not that dispersions. Well, I would like to see if Lubos has some reference for an actual paper where that argument is elaborated in detaill.
Nice post. I just want to know more about this lack of imaginary area. Does this stem from the Hamilton approach of LQG where by they split space time d=3+1 such that areas can only be real i.e spatial? I see then that this could crop up in CDT as there they seem to give time a direction. As I see it the singling out time could well be the downfall of these theories if this makes them break Lorentz invariance physically. On the other hand it could be that this singling out of time is no more than a gauge fixing procedure for example if one gauge fixes a Lagrangian in the path integral approach this breaks Lorentz invariance in the Lagrangian but the theory still gives the correct gauge independent results.

Clearly a lack of imaginary areas seems like we area seriously restricting the number of metrics that we include in a path integral approach like CDT. Perhaps this restriction is to server on the other hand restriction is needed such that double counting doesn't occur.

In my opinion if your starting principles are general relativity and quantum mechanics and you end up with a theory that breaks local Lorentz invariance you haven't applied those principles. If this is so then you should really restate your guiding principles, change your approach, so you retain Lorentz invariance, or give up on the theory altogether. I must say that the first one seems least appealing therapeutically but experimentally it obviously leads to predictions.
marcus
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#102
Aug19-09, 06:27 PM
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Quote Quote by Sauron View Post
...
Anyway this result is very bad for LQG.
...
I don't understand your reasoning, Sauron. LQG researchers tried for some years to derive a prediction of dispersion, but could not make the 4D theory yield such a prediction.
This observation makes dispersion less likely. If it is born out by other similar observations then this will help guide their development of LQG and save them trouble.

It certainly does not falsify the approach since there was no prediction that actually derived from the theory. I see this kind of Fermi-LAT observation as stimulating for LQG and the other QG approaches.

The task of deriving predictions still remains, and various avenues are being explored. But that is a separate topic. All this observational result does is give more direction and focus to the effort. Or? Please explain if you see it differently.
atyy
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#103
Aug19-09, 06:36 PM
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Quote Quote by marcus View Post
I don't understand your reasoning, Sauron. LQG researchers tried for some years to derive a prediction of dispersion, but could not make the 4D theory yield such a prediction.
This observation makes dispersion less likely. If it is born out by other similar observations then this will help guide their development of LQG and save them trouble.

It certainly does not falsify the approach since there was no prediction that actually derived from the theory. I see this kind of Fermi-LAT observation as stimulating for LQG and the other QG approaches.

The task of deriving predictions still remains, and various avenues are being explored. But that is a separate topic. All this observational result does is give more direction and focus to the effort. Or? Please explain if you see it differently.
What's your take on Henson's http://arxiv.org/abs/0901.4009 ? He claims spin foams violate Lorentz invariance, specifically photon dispersion tests.
Micha
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#104
Aug19-09, 06:42 PM
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Quote Quote by marcus View Post
I don't understand your reasoning, Sauron. LQG researchers tried for some years to derive a prediction of dispersion, but could not make the 4D theory yield such a prediction.
This observation makes dispersion less likely. If it is born out by other similar observations then this will help guide their development of LQG and save them trouble.

It certainly does not falsify the approach since there was no prediction that actually derived from the theory. I see this kind of Fermi-LAT observation as stimulating for LQG and the other QG approaches.

The task of deriving predictions still remains, and various avenues are being explored. But that is a separate topic. All this observational result does is give more direction and focus to the effort. Or? Please explain if you see it differently.
I must have dreamt when I read Smolin's book. Lubos Motl concludes, that LQG is dead. Where is your voice, Marcus?
marcus
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#105
Aug19-09, 06:50 PM
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Quote Quote by atyy View Post
What's your take on Henson's http://arxiv.org/abs/0901.4009 ? He claims spin foams violate Lorentz invariance, specifically photon dispersion tests.
I guess you know Joe Henson is not a Lqg researcher. He is a postdoc currently at Perimeter who has done almost all his work in Causal Sets. He indicates that Carlo Rovelli and Daniele Oriti (more experienced representatives of mainstream Lqg) had serious objections to the paper. The paper is iffy and handwaving. It says if such and such then maybe so and so. Ultimately doesn't derive hard prediction.

You asked my take. Well, in essence that paper seems to have been Joe's contribution to one of the parallel sessions at the Potsdam Loops 2005 conference,
http://loops05.aei.mpg.de/index_file...ct_henson.html
I am not sure why it didn't get published earlier. The preprint is January 2009. Before spending a lot of time on it, I would wait to see how it fares in peer review and the normal publication channels. If he thinks the idea is good maybe he will follow it up with something less tentative.
marcus
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#106
Aug19-09, 07:01 PM
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Quote Quote by Micha View Post
... Lubos Motl concludes, that LQG is dead. Where is your voice, Marcus?
Hi Micha. I have never found L especially reliable on the facts or successful at anticipating the future course of research. I really have no business getting involved in this thread. It is an exciting lively discussion and everybody is doing a great job. I think I will try to keep "my voice" out of it (unless it gets too tempting to resist) but thanks so much for asking! Maybe I will start a quieter thread trying to anticipate how QG research is likely to go, emphasis-wise, over the next 6 months or so. Then if I make predictions, and they turn out wrong, anybody who bothers to read can laugh at me.
turbo
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#107
Aug19-09, 08:34 PM
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It is interesting that a paper (cited in the OP) suggesting that there is no frequency-dependent photon dispersion (to some constraint) rests on the capture of one high-energy photon in one observation. That's not really good science, regardless of the number of names and sponsoring agencies on the paper. That signal could have been unrelated to the GRB in question.

Astronomy is an observational science, and if it going to be used to test and constrain cosmology (a really good idea, IMO) people have to take a breath and wait for trends in repeatable observations to lead the way.
lumidek
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#108
Aug19-09, 11:47 PM
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Quote Quote by atyy View Post
I agree that the major question about asymptotic safety is its existence. But suppose the UV fixed point for gravity exists, and the critical surface is finite dimensional - will that be enough to make predictions, or will there still be a problem coming in from electroweak theory not having a continuum limit (ie. can asymptotically safe gravity and electroweak theory be combined at Planck scale energies)?
Dear atyy, this is a whole research project of yours. Great questions. ;-)

But there are no clearcut answers known to these questions. It is not known whether the UV fixed surface is finite-dimensional without electroweak stuff added. So of course, it's also unknown whether it would remain finite-dimensional with the non-gravitational forces included. And it is unclear how the Landau poles would be treated. At any rate, it's clear that one cannot "neglect" the nongravitational forces at the Planck scale because they're actually *guaranteed* to be stronger and more important than gravity, so one would be neglecting the bulk of the forces, see

http://arxiv.org/abs/hep-th/0601001

Also, it is unknown how to actually extract predictions from asymptotically safe theories although it should be possible: but no systematic procedure is known. If it were known, it would have to be possible to prove the finiteness of the UV surface, too. And so on. So asymptotic safety's only realistic goal at this moment is to defend an idea, not predict or explain things beyond this idea, which I find too little, too late.


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