Lorentz violating severely restricted: Mqg/Mplank > 1200

humanino

I have to take back what I said. I know of one instance among professionals.

Finbar

Oh so this is how science works. Some one writes down a theory and then we gather evidence for that supports that theory for an arbitary amount of time, say I dunno 104 years, then we conclude that the theory is correct and unquestionable.

ensabah6

Lubos,
what about condense-matter analogue approaches like Volvovik and Wen?

Perhaps the "atoms" of spacetime are discrete, but they give rise via collective emergent properties into a superfluid spacetime that appears continuous and lorentz invariant to particles (in 4D, SUSY optional)

lumidek

Dear ensabah6,

I don't think that you're quite understanding the observation. The observation implies that the Lorentz symmetry not only "appears" to be there but it "is" actually there, up to 100 times the Planck scale. If the Lorentz symmetry were only an artifact of emergent or collective or blah blah features of many degrees of freedom, it would be violated at the Planck scale, but it is demonstrably not violated.

All these condensed matter-like theories of spacetime were obviously falsified, too. Sorry I didn't include them to the list but I thought it was obvious that they were dead, too.

Cheers
LM

ccdantas

Lubos,

In the supporting material document to that paper (Fermi collab.), the authors mention on page 24:

"A specific model of particular interest that has been proposed is a space-time foam scenario inspired by string theory that predicts a small retardation of photon velocity to first order in Eph/MQG(...)"

and cite this paper:

SI39 - Ellis, J., Mavromatos, N. E., & Nanopoulos, D. V. “Derivation of a vacuum refractive index in a stringy space time foam model”, Phys. Lett. B 665, 412–417 (2008), and references therein.

Do you have any particular comments on that paper (Ellis et al 2008)?

Thanks.

Christine

lumidek

Dear Christine, the most important fact about the paper is that their predictions have been falsified as cleanly as the predictions of any other kind of fundamental Lorentz-violating theories on the market. What they call the "most conservative" scenario has been proved right and it is not relevant for anything they want to speculate about in the paper.

Not even the word "stringy" could have saved them.

I respect at least some of the co-authors of this paper but I have always found such models dumb. By the way, they may have called it "stringy" but the model has nothing to do with string theory. The closest feature of this model to "string theory" is that they cite a paper or two co-written by people who are otherwise "string theorists" (like Myers, coincidentally at the Perimeter Institute), but those papers don't build on string theory, and they usually don't even pretend so (unlike your particular paper): Myers et al. just write some effective field theories. And Ellis et al. here cite many "anti-stringy" people (Amelino-Camelia, Jacobson, Gambini, Pullin, Magueijo, Smolin etc.) and essentially call their work "stringy", even though it's demonstrably not stringy: they do this trick probably to order to increase the credibility of those authors who are the real background of the paper by Ellis et al.

String theory doesn't allow any kind of "foamy" violations of the Lorentz symmetry near the Planck scale. The latter is fundamentally incorporated into the theory, and it can only be broken by configurations (e.g. B-fields) of matter, and such breaking normally starts at low energies, while the violation is *smaller* at very high energies, much like in all other kinds of spontaneous symmetry breaking. Every well-known string theorist, and every grad student who is on her way to learn string theory from the textbooks, knows this much.

I don't really believe that e.g. Ellis doesn't know, but if he doesn't, he may be getting too old. But this question - stringy or not - is less important than the basic adjective about the paper: it is wrong. So while the superficial label could be perhaps compatible, because string theory predicts no lags here, none of the details is compatible with reality, so the paper's model is exactly on the same level of falsification as any model that deliberately wanted to start with a "non-stringy" vocabulary.

Best wishes
Lubos

MTd2

No, it wasn't. They predict a distribuition for a given photon energy over a range for values of delay. That one photon that arrived to early is just one lucky that wasn't significantely delayed by the quantum foam.

lumidek

I have already explained that this may only be an interpretation of a downright crackpot.

The probability that a multi-hour hour would be erased by "chance" is effectively zero, because it is the value of the probability distribution 10 sigma away from the central value etc. The photon would have to be created long time after (or before) the actual burst, and it's just negligibly unlikely.

At any rate, your new, increasingly awkward hypothesis will be easily yet gradually falsified by further bursts in the future. When Fermi sees another burst of the same kind with a 30+ GeV photon, when do you think it will probably arrive? Together with others, like in the May 2009 case, or two hours or two weeks later? This is a test of basic intelligence and if you answer b), you should seek medical help.

atyy

Volovik actually envisages the Lorentz breaking scale far-above the Planck scale (Eqn 4.1, http://arxiv.org/abs/0801.0724).

I don't really understand why the latest observation change the likelihood of the various theories much, because the other theories had much bigger problems even before this - Volovik had a massive graviton, Xu and Wen got quadratic and cubic graviton dispersions respectively, Wen cannot (yet?) get chiral interactions, and Horava has an extra scalar mode. So they were all already dead (are they deader now?) - but I've always found them well-motivated and really like their playful style.

Similarly with LQG - I've not found it well-motivated, because, for example, Ashtekar kinda uses Asymptotic Safety to backup one of his points in his FAQ. But then why not just pursue Asymptotic Safety, which is a logical possibility and a well-defined programme?

MTd2

Don't try to convince Lubos. You will waste your time.

jal

It appears that the discussion has hit the "wall".
Discrete, continuous and at what scale.
(Planck scale or some other minimum length.)

If you have a perfect liquid, do you have confinement?

What experimental approach (CERN?) will shed light on which theories to pursue?
jal

ccdantas

Lubos,

Thanks for your response.

As far as I understand, the limits found in that paper are specially worrisome for the n=1 (linear) models. On what grounds do you claim that the whole programme of LQG has been falsified by the Fermi observations? See the question #6 by Ashtekar's FAQ paper arxiv:0705.2222: "Will Lorentz invariance be violated in the low energy limit of LQG dynamics?". Please, if possible, state your counter-arguments according to the exposition presented in that paper by Ashtekar.

I would like to invert the question the other way around. Would it be correct to affirm that *if* Lorentz violations were observed, string theory would be promptly falsified?

Thanks.
Christine

lumidek

Dear Christine,

the research program of LQG has been falsified because an observation showed that its basic prediction about the character of spacetime - Lorentz violation at the Planck scale - is incorrect. This procedure of "falsification" is the main part of the scientific method.

For the same reason, many other classes of theories have also been falsified, including causal dynamical triangulations, emergent condensed matter-like spacetimes, Horava-Lifshitz gravity, and many others.

To address your particular question and to see why LQG predicts that the Lorentz invariance can't exactly hold, see either my much more concise argument, or any of dozens of papers about this very question, e.g.

http://arxiv.org/abs/gr-qc/9809038
http://arxiv.org/abs/hep-th/0108061
http://arxiv.org/abs/gr-qc/0411101
http://arxiv.org/abs/gr-qc/0403053
http://arxiv.org/abs/hep-th/0603002
http://arxiv.org/abs/hep-th/0111176
http://arxiv.org/abs/hep-th/0208193
http://arxiv.org/abs/hep-th/0501116
http://arxiv.org/abs/gr-qc/0207030
http://arxiv.org/abs/gr-qc/0207031
http://arxiv.org/abs/gr-qc/0207085
http://arxiv.org/abs/hep-th/0501091
http://arxiv.org/abs/hep-th/0605052
http://arxiv.org/abs/gr-qc/0404113

Best wishes
Lubos

lumidek

Two sentences I didn't address. Yes, if the spacetime were found to deviate from Lorentz symmetry by order-one terms at the Planck scale, string theory - as understood by real string theorists and taught by Polchinski or GSW or Becker or other textbooks would be instantly falsified.

Second point. Ashtekar arguments that he would love to have Lorentz invariance in LQG are nothing else than a wishful thinking, and all his detailed statements - especially those in between the lines - are just plain wrong. It is not true that the split of dimensions to 3+1 is the only or main feature that makes LQG violate Lorentz symmetry. It is not enough to be able to define generators on a Hilbert space if one wants the dynamical laws to be symmetric - because the former condition is kinematic and knows nothing about the dynamics, while LI invariance is a dynamical question.

Also, it is not true that one can actually define proper generators on the spin network Hilbert space. Also, it is not true that discrete area spectrum may be compatible with the Lorentz symmetry. If there is any formula for the areas that is a manifestly a sum of real discrete numbers, the theory automatically violates the Lorentz invariance - for example because areas in Lorentz-invariant theories can be both real and imaginary (spacelike vs timelike).

So all his verbal proclamations seem to be wrong and there's no calculation. So what should I do with that? It's just rubbish. The other papers at least try to calculate something, and of course, they end up with the only possible answer they can: LQG much like any other theory with a naive mechanistic discrete picture of space at the Planck scale violates the Lorentz symmetry. I am sure that you know very well that Ashtekar's paragraph is pure babbling and there exists not a single paper that would make a single calculation supporting the wishful thinking in the paragraph.

Best
Lubos

atyy

Maybe not CDT - I believe CDT is more like computational asymptotic safety - CDT itself is not a complete theory - and asymptotic safety, although it may have other problems, surely respects Lorentz invariance?

ccdantas

Hmm, I'm not here in defense of anyone, but I do not find evidences for that claim in his paper, only that

"In full non-perturbative quantum gravity there is no background metric whence some
care is needed to speak of Lorentz invariance. The question can only refer either to asymp-
totic symmetries in the asymptotically flat context or effective low energy descriptions. I
would expect LQG will have the first type of Lorentz invariance generated by global charges
corresponding to asymptotic symmetries. But unfortunately so far global issues related to
asymptotic flatness have received very little attention." p. 13.

Would you please elaborate on that?

The paper is a summary for a general audience at the 11th Marcel Grossmann meeting, so there are no detailed calculations, as expected, but I suppose some can be found in the list of references that he provides (see also his footnote #8). I would have to check on that, though.

lumidek

There is no asymptotically safe theory of gravity, because of technical RG reasons and because of wrong scaling for the entropy at high energies that should be dominated by black holes. And even if there were one, CDT couldn't be its approximation.