Lorentz violating severely restricted: Mqg/Mplank > 1200

lumidek

Dear Christine,

a discussion of this paper by Thiemann involves some technicalities which are not terribly interesting. Thiemann is a champion of the spin foam models (unlike Ashtekar and others), and he criticizes Nicolai et al. for not giving much attention to spin foams.

Indeed, they don't give enough time to spin foams. But any consistent theory can be, at least in principle, formulated via the spacetime perspective, like in spin foams, or in the time-sliced perspective, like in the canonical quantization. The results must be automatically equivalent when done correctly.

So Thiemann doesn't address any of the main points by Nicolai et al. He's just redirecting the discussion elsewhere. For example, Nicolai show, in the operator approach, that the algebra doesn't close. Thiemann doesn't find any mistake in Nicolai et al. Instead, he presents a calculation in his formalism, leading him to the conclusion that the algebra closes - but he can only say up to zeroth order in hbar. That's really entertaining because it's like saying that the classical diffeomorphism groups etc. exist.

Similarly, Nicolai et al. explain that many constructions are ambiguous and must be allowed to lead to objects with many possible values. Thiemann doesn't show anything wrong about the multiparameter classes of solutions: he just presents a random one among them and declares it to be the right answer, without showing that his answer is any better than any other answer in any way.

I think it's obvious that Thiemann doesn't disprove any basic propositions by Nicolai et al. That's not because Thiemann is not bright enough in this case: it's because the Nicolai et al. arguments are correct so they cannot really be disproved in any way.

Note that despite the self-promoting character of the LQG community - that makes sure that all published papers about LQG are "positive" and hide all the inconsistencies under the rug - the Nicolai et al. paper still beats Thiemann by citations. It probably makes no sense to write new papers of the Nicolai et al. type (or to rewrite the arguments into the spin foam language) because the people who have already gotten the message would learn nothing new, and the people who haven't gotten it will probably never get it, either because they're blinded by preconceptions, or because they're just intellectually limited.

I don't plan to read Thiemann's paper or any other paper on LQG because this story is over and no sensible person should continue to waste her time with this stuff. And I surely don't plan to become Thiemann's personal historian because I don"t think that he deserves one. Do you understand the concept that in science, one eliminates theories that have been falsified and focus on the viable ones? 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?

Best wishes
Lubos

lumidek

No, I don't realize that I am the most prominent figure in the discipline you wrote.

Indeed, it is not true that I disagree just with scientific propositions. I disagree with their moral values (or, more precisely, the absence of values). I disagree with their nasty plans to politicize science, their focus on populism and brainwashing of ordinary people, and their desire to transform science into another left-wing mouthpiece. I protest against their double-faced approaches and hypocrisy, and about their readiness to deny evidence or facts whenever they find it convenient. Indeed, this is about much more than a disagreement about a technicality in science.

I think that these people are not good ones. Still, the amount of my attacks against them is negligible relatively to their attacks on good science and its fundamental principles.

For example, I didn't put "Rise of Smolins, fall of science" into the subtitle of my books I wrote so far, even though such a subtitle would be very accurate because the rise of this crap and loud sub-par would-be scientists is really threatening to cause a fall of science. I want to do everything I can to stop the influence of these people on this world which has been devastating for science and all other values I find important.

humanino

Dear LubosI sincerely appreciate that you clarified your position. Thank you very much.

I also apologize to those who think I polluted this thread, but I do believe that strong opinions expressed earlier in this thread must be read in the light of this political position.

lumidek

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.

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. The situation with asymptotic safety is analogous except that there's no understanding why gravity should be asymptotically safe. So no one should really study it because all insights depend on it, and predictions of any other questions are probably at least as difficult as the "proofs" of asymptotic safety. So the output from asymptotic safety is at most as large as the input. One wants to believe it's there, and given this assumption, it's there, but one can't learn anything else because any other question is as difficult as the proof that it's asymptotically safe, and this proof doesn't exist.

2. Yes, once again, the information getting out of the black hole violates the causal rules as classically determined from the black hole causal diagram. It is a violation of relativity in the same sense as quantum tunneling may violate the energy conservation law for a while, by allowing the particle to be in classically inaccessible regions.

Let me just say that this is what pretty much all experts think that actually happens in string theory = quantum gravity (regardless of the approach). There's nothing paradoxical about it. The metric tensor is a fluctuating quantum degree of freedom and the causality defined by it simply cannot be quite exact for objects whose lifetime is finite.

3. I didn't talk about any classical field theories. I talked about the full quantum field theories, the entropy density was not divergent, but finite, and it was determined fully by the conformal invariance i.e. dimensional analysis. In field theory (and this term always means "quantum field theory" in these HEP discussions!), it is C times Temperature^d where d is a spatial dimension. You seem to respond to something that has nothing to do with my argument, and is clearly irrelevant for this discussion because it's just some artifact of classical field theories. Again, we are talking about (asymptotically safe) quantum field theories, right?

atyy

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)?

MTd2

There would be no QCD fathers if they thought like that before studying it.

heinz

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

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.

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

Thanks for your clarifications (although I was expecting a more detailed elaboration; but never mind). This is certainly something I should work myself.

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

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

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

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

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

That statement is completely ridiculous and you know it. The reference to God was clearly not made in a scientific sense.

Sauron

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

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

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.