Fermi (GLAST) almost kills all Lorentz violating theories.

[MTd2]

http://www.symmetrymagazine.org/breaking/2009/02/19/most-extreme-gamma-ray-blast-also-probes-quantum-gravity/Fermi%20Observations%20of%20High-Energy%20Gamma-Ray%20Emission%20from%20GRB
http://www.nasa.gov/mission_pages/GLAST/news/high_grb.html
http://sciencemag.org/cgi/content/abstract/1169101v1The measurement sets the best bounds on Lorentz violation we have so far. In the parametrization popular among the Lorentz-violating self-described "quantum gravity theorists", the typical scale where the Lorentz violation starts to be big must be above 1/10 of the Planck mass. Everything below this gigantic mass scale has been proven to respect the Lorentz symmetry, if you wish.

People who have been promoting a complete denial of special relativity - in the form of various loop quantum gravities, doubly or deformed special relativities, dynamical triangulations, and all this stuff have been idiots, just like I have always emphasized."http://motls.blogspot.com/2009/02/fermi-glast-almost-kills-all-lorentz.html"

 

[atyy]

Isn't CDT Lorentz invariant? There is a discretization of space for the computation, but the action for the computation is not the continuum action, so CDT is more like a computational form of asymptotic safety, which shares the effective field theory framework with string theory, just differing in the guess as to what the "true" degrees of freedom are.

 

[MTd2]

Yes, that's what I was thinking. CDT was invented to help with strings computation...

 

[jostpuur]

http://motls.blogspot.com/2009/02/fermi-glast-almost-kills-all-lorentz.html"

Some confusion probably rose from the fact, that the link in the final period, indicating that your post contained a copy paste quotation from Lubos' blog, is quite difficult to detect

 

[marcus]

As it happens, the main form of LQG that has been researched in the past couple of years is Lorentz invariant.

 

[marcus]

Isn't CDT Lorentz invariant? ...

Not exactly, it fixes a particular time-slicing. I believe requiring Lorentz invariance would tend to favor fully covariant LQG over CDT.

Don't misunderstand me: I like the CDT work a lot.
And also I don't think Lorentz violation at Planck scale has been ruled out.
My only point is that besides the fact that the comments quoted in the original post do not make much sense, CDT (or string thinking for that matter) is not especially favored by the Fermi results.

 

[confinement]

Can anyone explain this:

Lubos from the link:

Any Lorentz violation will lead to minute differences in the speed of light for different "colors", using the words of Leslie Winkle. ;-) What are the differences in the speed? Well, by dimensional analysis, they must scale like

Delta (v/c) = Delta(E) / M

Why? Because (v/c) is dimensionless, the speed differences must be proportional to energy differences (if Delta(E) is small) or frequency differences, if you wish, and the denominator of mass "M" (to the first) had to be added for dimensional reasons. If you wanted to put "E" in the denominator, you would get a huge "Delta (v/c)" of order one which would be totally excluded.

 

[BenTheMan]

I thought this was one of the ways to "test LQG"---is that true, even in some limited sense?

 

[marcus]

Can anyone explain this:
...

There's a certain amount of unreliable and misleading material on the web which is better just to ignore, but I can explain the standard dispersion calculation which for several years has motivated people to look for Lorentz violation "suppressed" by the Planck energy in GRB and flares which have traveled distances on the order of a billion lightyears.

A good source on this is a 2007 article by several string theorists (John Ellis, Nick Mavromatos...) together with a team of astrophysicists. I'll get the link later.

I thought this was one of the ways to "test LQG"---is that true, even in some limited sense?

Unfortunately not. The mainstream LQG development (e.g. Rovelli and co-workers) has never predicted Lorentz violation. Socalled DSR was investigated a lot up to around 2006 but people have lost interest. After 2006 you don't hear much about that from LQG people. Before that considerable effort was made trying to prove that some form of LQG implied DSR, but it failed! (It worked in 3D, results by Freidel and Livine, so there was hope it would extend to 4D, but it didn't.)

In fact, as you may remember, in 2007 when results from a imaging air cherenkov telescope (IACT) suggested a few minutes delay of high energy photons in a gammaray flare, it was some string theorists who rushed into print about it----claiming a variant of string theory predicted it .
The LQG community said nothing. Remarkably. None of the LQG folks said "I told you so."

 

[Haelfix]

I'd say its more problematic for things like Doubly special relativity and the host of modified dispersion relations, Lorentz breaking theories on the market that were always plagued with conceptual problems. Absent a workaround, it doesn't kill them, but it makes them much less relevant for Quantum Gravity.

 

[marcus]

Yes, it doesn't concern the main LQG research program. That was just innuendo of questionable sincerity.

Here, I found the article by the Cherenkov telescope team and the string theorists Ellis, Mavromatos, and Nanopoulos:
http://arxiv.org/abs/0708.2889
Probing quantum gravity using photons from a flare of the active galactic nucleus Markarian 501 observed by the MAGIC telescope
J. Albert et al. (for the MAGIC Collaboration), John Ellis, N.E. Mavromatos, D.V. Nanopoulos, A.S. Sakharov, E.K.G. Sarkisyan
12 pages, 3 figures, Phys. Lett. B
(Submitted on 21 Aug 2007)
"We analyze the timing of photons observed by the MAGIC telescope during a flare of the active galactic nucleus Mkn 501 for a possible correlation with energy, as suggested by some models of quantum gravity (QG), which predict a vacuum refractive index$$\simeq 1 + (E/M_{QGn})^n, n = 1,2$$. Parametrizing the delay between gamma-rays of different energies as$$\Delta t =\pm\tau_l E$$ or$$\Delta t =\pm\tau_q E^2$$, we find $$\tau_l=(0.030\pm0.012) s/GeV$$ at the 2.5-sigma level, and $$\tau_q=(3.71\pm2.57)x10^{-6} s/GeV^2$$, respectively. We use these results to establish lower limits $$M_{QG1} > 0.21x10^{18} GeV$$ and $$M_{QG2} > 0.26x10^{11} GeV$$ at the 95% C.L. Monte Carlo studies confirm the MAGIC sensitivity to propagation effects at these levels. Thermal plasma effects in the source are negligible, but we cannot exclude the importance of some other source effect."

Somebody asked about the math. Basically the hypothesis is that the higher energy photons are slowed by a tiny percentage proportional to
E/Eplanck. The fractional decrease in speed is proportional to DeltaE/Eplanck by a proportionality constant of order one.

The Cherenkov telescope was able to observe TeV photons from a brief flare at Markarian501, it could see their tracks as they entered the atmosphere and estimate the energy of individual photons. So Delta E was on the order of a TeV. You can divide a TeV by the Planck energy and see how large the expected fractional slowdown would be.

Now this tiny fractional slowdown, acting over a distance like a billion lightyears, translates into a delay on the order of minutes. So it is observable.
I don't have time to give more explanation now but if it isn't clear, ask. I'll get back to it.

 

[BenTheMan]

In fact, as you may remember, in 2007 when results from a imaging air cherenkov telescope (IACT) suggested a few minutes delay of high energy photons in a gammaray flare, it was some string theorists who rushed into print about it----claiming a variant of string theory predicted it

Sure, somewhere in the multiverse you can find Lorentz violations, probably.

I was under the impression that Lorentz violations was one of the only ways to test the current LQG theories, but apparently that's wrong? That's fine---are there other ways to test it?

 

[MTd2]

That's fine---are there other ways to test it?

Yes, by seeing if there is a kind of background that would radomly disturbe otherwise straight lines.

 

[BenTheMan]

How is that an experimental test of LQG?

 

[atyy]

Not exactly, it fixes a particular time-slicing. I believe requiring Lorentz invariance would tend to favor fully covariant LQG over CDT.

Don't misunderstand me: I like the CDT work a lot.
And also I don't think Lorentz violation at Planck scale has been ruled out.
My only point is that besides the fact that the comments quoted in the original post do not make much sense, CDT (or string thinking for that matter) is not especially favored by the Fermi results.

Yes, the time slicing means Lorentz invariance may not be there. But it doesn't mean it isn't there, since GR (or at least most physically meaningful bits of it) can be formulated in ADM or 3+1 form. Benedetti et al seem to think of CDT as computational AS in http://arxiv.org/abs/0901.2984 (see what they say about reference 17). If that's the case (is it?), then presumably CDT would have Lorentz invariance from AS?

 

[marcus]

Yes, by seeing if there is a kind of background that would radomly disturbe otherwise straight lines.

How is that an experimental test of LQG?

The topic of this thread is observational constraints on Lorentz violation.
The thread is not about ways to test LQG. (or string theories, for that matter.)
If anyone wants they can start a special thread about ideas for testing LQG.

There are quite a few papers on this, just in the past couple of years.

 

[BenTheMan]

Does this mean you can't summarize, in a single post, an answer to my question?

 

[xylbs]

Since Lorentz symmetry is violated in noncommutative spacetime,
does this result mean the noncommutative spacetime theories also be ruled out?

 

[marcus]

Since Lorentz symmetry is violated in noncommutative spacetime,
does this result mean the noncommutative spacetime theories also be ruled out?

Lorentz violation has not been ruled out.

 

[marcus]

I couldn't get one of the links in MTd2's top post to work. I think it is this one:
http://www.symmetrymagazine.org/bre...-gamma-ray-blast-also-probes-quantum-gravity/

So far I have not seen the actual article by Abdo et al, that all this fuss is about. It was published in a recent issue of Science
Here's the link to the abstract, given in the top post of this thread:
http://sciencemag.org/cgi/content/abstract/1169101v1

It sounds, from the summary and paraphrase I've seen, as if they came within a factor of 10 of ruling out something Smolin wrote about as a possibility, but did not establish as a LQG prediction, namely an order one proportion between the fractional change in speed of light and this fraction
E/M_Planck

Judging from the paraphrase, they didn't actually rule that out, they ruled out violations 10 times more extreme than what was conjectured, namely what you get if you replace the Planck energy by 1/10 of the Planck energy.

Then the fractional change in speed of photon is related to the photon energy E by order one proportion to
E/(0.1 M_Planck)
which is the same as 10 E/M_Planck
so the fractional change in photon speed is much larger.

But as far as I remember, nobody (not even the string theorists Ellis, Mavromatos,...) was talking about that.

So if I can believe, from the top post paraphrase, that this is all Abdo et al reported, then we have an exaggerated fuss. They have showed that they can get within a factor of ten---so the technology is promising and hopefully they can eventually succeed in testing Lorentz violation at the interesting order-one level.
And in any case nobody ever succeeded in deriving a prediction of Lorentz violation from the main version of LQG, although they tried a lot around 2005 and 2006. I don't know any version of 4D LQG that anybody managed to make predict Lorentz violation, but maybe someone can dig something up if you go back a few years.
However that may be, Ellis and Mavromatos say they have a version of non-critical string that predicts just the same thing that was being talked about in LQG circles. And this type of violation has not been ruled out by Abdo et al. If we can believe the lead post here, they only came within a factor of 10.

Maybe someone who has seen the full article in Science magazine will correct me on this, or say more precisely what the result was.

 

[Coin]

As it happens, the main form of LQG that has been researched in the past couple of years is Lorentz invariant.

Which variant of LQG do you refer to specifically here?

---

Also, a question which may or may not be sensible: Does this evidence against violation of Lorentz invariance in any way count as a mark against theories which have Lorentz invariance, but violate Poincare invariance?

EDIT: Yargh, my final paragraph contained a typo.

 

[marcus]

Which variant of LQG do you refer to specifically here?
...

There is really only one form of LQG that is getting the bulk of attention. It is designated EPRL (gamma<1) and also FK(gamma < 1). These two turned out to be equivalent as long as the Immirzi gamma is < 1. Two different vertex amplitude formulas which coincide in the important case.

Essentially every new paper I see about the full theory (not the LQ cosmo offspring, but LQG)
as well as every new video-stream seminar talk at PIRSA is about this one version of LQG.
From my perspective it is getting all the attention since sometime in 2007.

Rovelli and his group call it Covariant LQG.
EPRL is Engle Pereia Rovelli Livine, FK is Freidel Krasnov.
There was a revolution and convergence starting in 2006.

It is misleading to say there are a whole lot of LQGs. There HAVE been in the past. But in 2007 the old vertex amplitude formula of BarrettCrane was thrown out and several new ones tried and the two I mentioned turned out to coincide and to give results on hypersurfaces consistent with those of the old pre-2000 canonical LQG. So it amounts to a rederivation of the whole theory.

Rovelli gives a good summary of this in his talk to the string theorists at Strings 2008. Ask if you don't have the link and want to watch the talk.
=================

Also, a question which may or may not be sensible: Does this evidence against Lorentz invariance in any way count as a mark against theories which have Lorentz invariance, but violate Poincare invariance?

The evidence is not against Lor. invar.----it is partial and inconclusive evidence against violation of Lor. invar.
But mutatis mutandis I don't see it as a mark against those theories, whatever they are.

 

[BenTheMan]

Then the fractional change in speed of photon is related to the photon energy E by order one proportion to
E/(0.1 M_Planck)
which is the same as 10 E/M_Planck
so the fractional change in photon speed is much larger.

But as far as I remember, nobody (not even the string theorists Ellis, Mavromatos,...) was talking about that.

I will paraphrase Lubos here, but if you take $$\delta E \sim 100 GeV$$ (which is probably as large as it could possibly be), then you're looking at a limit of $$\Delta v / c \sim 10^{-17}$$, which is the number you want to compare to.

Likely, the $$\Delta E$$ is much smaller than 100 GeV.

 

[Jon_Trevathan]

There have been some relatively new gamma-ray observations that are relevant to this discussion.

The article appeared in Nature (http://www.nature.com/nature/journal/v462/n7271/full/nature08574.html) and a summary of the findings appeared in ScienceNews
(http://www.sciencenews.org/view/generic/id/48891/title/Gamma-ray_observations_shrink_known_grain_size_of_spacetime_)

I have copied the Abstract to the Nature Article below:
"A cornerstone of Einstein's special relativity is Lorentz invariance—the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l_Planck approximately 1.62 times 10^-33 cm or E_Planck = M_Planckc² approximately 1.22 times 10¹⁹ GeV), at which quantum effects are expected to strongly affect the nature of space–time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy... Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves. Here we report the detection of emission up to approx 31 GeV from the distant and short GRB 090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E_Planck on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l_Planck/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light."

In deference to Marcus, I will not comment on the implications these results may have on LQG

 

[Jon_Trevathan]

According to the Wikipedia article on Loop Quantum Gravity, “The equations of LQG are not embedded in, or presuppose, space and time, except for its invariant topology. Instead, they are expected to give rise to space and time at distances which are large compared to the Planck length.” In other words, space-time is an emergent property of LQG, but in most of the LQG models, this emergence occurs at dimensions in excess of the Planck length. It is this prediction that may have failed in the experiment I cited above. (See: http://www.sciencenews.org/view/generic/id/48891/title/Gamma-ray_observations_shrink_known_grain_size_of_spacetime_).

Specifically, the experiment provides some evidence that space-time is less “grainy” than I believe most LQG theories had predicted and require.

I would hope that Marcus or others will critique this understanding.

 

[marcus]

There is no need to "critique" your understanding, Jon. You are just mistaken.

LQG researchers tried unsuccessfully around 2005-2006 to derive a prediction of energy-dependent speed of light, from the theory in 4D.

The authoritative review paper on LQG, as of May 2008, is Rovelli's article in Living Reviews of Relativity. As I recall, it examines the issue and points out that LQG does not predict energy-dependence of the speed that photons travel.
http://relativity.livingreviews.org/Articles/lrr-2008-5/

The impression that it does has been given in one or more hostile blogs, but that is misinformation. BTW be careful about relying on Wikipedia.

We've discussed this in several other threads.
Thanks for your interest, Jon, and the best of luck with your quantum gravity self-education!

 

[tom.stoer]

The old LQG calculations from which violation of Lorentz invariance (or better: deformation in the sense of DSR) seemed to follow were mostly based on crude approximations like weave states which are not solutions to the full LQG theory. So the Lorentz violation was an artefact of "wrong" or unphysical spin network states.

I am not absolutely sure about that; Marcus, do you remember?

 

[marcus]

Off the top of my head, as I recall it, before 2006 there was a lot of wishful thinking, mainly on the part of Smolin. Smolin and a few others WANTED to have a prediction which could be tested.

In 2005 and 2006, GLAST was getting near time to launch and Smolin felt it was urgent to derive some kind of testable prediction, so he wrote some inconclusive papers involving guesswork. I think he motivated some younger people, like Freidel and Kowalski-Glikman, to work on the general problem. Freidel got a result for 3d gravity, but it could not be extended to the interesting case of 4d. Kowalski-Glikman studied DSR and found that (contrary to most people's expectation) it did NOT imply a variation in photon speed. I'm not clear on how that conclusion was drawn--there seem to be several versions of DSR. It wasn't clear which version, if any, was related to 4d LQG.

Essentially a Perimeter group tried to prove that LQG predicted variation in the speed of light, back then, because they wanted to be able to TEST LQG when GLAST was launched. In science your theory has to make a prediction BEFORE the experiment. So there was a time-pressure which Smolin and a few others felt. But despite their good efforts, they were unable to derive such a prediction. So in 2008 GLAST was launched without their going on record with something definite.

This was not representative of the broader LQG community. The main community did not think LQG had any prediction like that and they didn't bother with it. As far as I can tell, at this time it is only outsiders who imagine that LQG should imply variation in speed of photons.

About the earlier wishful thinking, and the handwaving arguments, I regret to say I don't remember what the technical grounds were.

It's worth pointing out that Jon is talking about the issue of energy-dependent photon speed: a separate issue from DSR (deformed special rel), which you asked about.
I'm sorry to say I don't know what the deal is currently with DSR. I'll try to find out.

 

[atyy]

In science your theory has to make a prediction BEFORE the experiment.

But we'll make an exception for mercury's perihelion precession, right?

I'm sorry to say I don't know what the deal is currently with DSR. I'll try to find out.

I'm also confused about this, especially in the light of http://arxiv.org/abs/0903.3475, which is a 4D result.

 

[marcus]

I'm also confused about this, especially in the light of http://arxiv.org/abs/0903.3475, which is a 4D result.

Spot on! It does not seem to make a rigorous connection with LQG, and I don't see an energy-dependent speed of light, but it is probably the best indication of the current status of this question. We can learn by studying this paper!
http://arxiv.org/pdf/0903.3475
Page 2:
==quote==
Very interesting results have been obtained in the 3d context [16, 17, 18] where it has been shown that effective models with quantum group symmetries and a non-commutative spacetime structure (although different from the DSR one) arise very naturally when considering the coupling of point particles to a spin foam model for 3d quantum gravity, in the Riemannian setting, with the physics of these particles being that of non-commutative field theories on Lie algebra spaces. While no similarly solid links between spin foam models and non-commutative field theories have been discovered in the 4d context, several arguments have been put forward suggesting that these links should exist and that the relevant effective models in 4d should indeed be of the DSR type [19, 20].

For reasons that should become apparent in the following, group field theories are a natural framework for establishing such links,...
...
...What we do in this paper is to apply the same procedure to the more technically challenging case of four spacetime dimensions, and Lorentzian signature, and derive from a group field theory model related to 4-dimensional quantum gravity an effective non-commutative field theory of the DSR type and living on κ-Minkowski spacetime.

As said, not only this is the first example of a derivation of a DSR model for matter from a more fundamental quantum gravity model, and one further example of the link between non-commutative geometry and quantum gravity formulated in terms of spin foam/loop quantum gravity ideas, but it is of great interest from the point of view of quantum gravity phenomenology. It is also interesting, more generally, as another possible way of bridging the gap between quantum gravity at the Planck scale and effective physics at low energies and macroscopic distances...
==endquote==

As they indicate in the conclusions, they haven't yet connected with LQG. But they have connected with, or at least see a way to connect with some LQG "ideas".
==quote page 22==
Further investigations are needed to establish a better link between our initial GFT model, classical solutions and effective field theory on the one hand, and a spin foam formulation of the Freidel-Starodubstev classical gravity theory [33] and the particle observable insertions à la Kowalski-Glikman-Starodubtsev [20] on the other, which represent...
==endquote==

It's an exciting prospect!

 

[atyy]

Science 2 April 2010: Vol. 328. no. 5974, p. 27, DOI: 10.1126/science.328.5974.27
Thought Experiment Torpedoes Variable-Speed-of-Light Theories
Adrian Cho

"...

The speed variations must be at least 23 orders of magnitude smaller than experimental limits set last year, she says.

"It's incredibly hard to find an observable effect of quantum gravity, so I'm a little bit sorry about the result," says Sabine Hossenfelder of the Nordic Institute for Theoretical Physics in Stockholm. ...

The debate centers on a decade-old idea known as DSR—for "doubly special relativity" or "deformed special relativity." DSR attempts to reconcile Einstein's theory of special relativity—which says the speed of light is the same for all observers, even if they're moving relative to one another—with the possibility that the speed of light also depends on its wavelength. Such a dependence had been suggested by theories of "noncommutative geometry" and emerges from some theories of "loop quantum gravity"

...

Developers of DSR aren't ready to concede the point, however. ... Giovanni Amelino-Camelia .... Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, agrees that the effects of quantum spacetime may resolve the paradox and says he's studying the matter."

 

[Physics Monkey]

There seems to be a lot of confusion on this subject. I would suspect that whatever physics LQG predicts, it has to involve some modification of the notions of distance, time, speed, etc at the Planck scale. Perhaps an energy varying speed of light isn't the right way to think, although again I suspect the speed of light must become less meaningful in some sense at the Planck energy in any theory of quantum gravity. If the modification is more subtle then maybe the cosmic ray experiments can't get at it.

However, there appears to be a more mundane explanation. Namely, if the modification of the speed is second order in one over the Planck energy then the authors of the paper http://www.nature.com/nature/journal/v462/n7271/full/nature08574.html point out that they can't offer any meaningful constraints. I think such a modification is more natural anyway because the quantity $$|\vec{p}|^3 = (p_x^2 + p_y^2 + p_z^2 )^{3/2}$$ isn't nice at $$\vec{p} = 0$$. This is a common observation in condensed matter systems where terms in the low energy effective action like $$| \nabla \phi |^3$$ usually don't appear. I would consider a formula like $$E^2 = p^2 f(p^2 / M^2 )$$ with M of order the Planck mass much more natural, but this formula gives only a second order correction to the speed provided f(x) behaves like 1 + c x + ... generically. And whether or not you like my argument, its remains true that these second order theories are basically unconstrained. That second power of E/M really kills you.

Since the first order models seem highly constrained up to the Planck scale, it would be interesting to see what constraints on the mass scale they could put for second order models. In other words, a long standing hope has been that the energy scale of quantum gravity might be much lower than the Planck scale, so if we assume a lower energy scale but a second order model how low can the energy scale be?

 

[marcus]

Science 2 April 2010: Vol. 328. no. 5974, p. 27, DOI: 10.1126/science.328.5974.27
Thought Experiment Torpedoes Variable-Speed-of-Light Theories
Adrian Cho

"...The speed variations must be at least 23 orders of magnitude smaller than experimental limits set last year, she says.

"It's incredibly hard to find an observable effect of quantum gravity, so I'm a little bit sorry about the result," says Sabine Hossenfelder of the Nordic Institute for Theoretical Physics in Stockholm. ...

The debate centers on a decade-old idea known as DSR—for "doubly special relativity" or "deformed special relativity." DSR attempts to reconcile Einstein's theory of special relativity—which says the speed of light is the same for all observers, even if they're moving relative to one another—with the possibility that the speed of light also depends on its wavelength. ...

Interesting! The piece in Science was, I gather, based on this:
http://arxiv.org/abs/0912.0090
The Box-Problem in Deformed Special Relativity
20 pages, 3 figures
S. Hossenfelder
(Submitted on 1 Dec 2009)
"We examine the transformation of particle trajectories in models with deformations of Special Relativity that have an energy-dependent and observer-independent speed of light. These transformations necessarily imply that the notion of what constitutes the same space-time event becomes dependent on the observer's inertial frame. To preserve observer-independence, the such arising nonlocality should not be in conflict with our knowledge of particle interactions. This requirement allows us to derive strong bounds on deformations of Special Relativity and rule out a modification to first order in energy over the Planck mass."
Peter Woit included a mention of the Adrian Cho piece in Science.

 

[marcus]

To sum up, there has long been a hope (going back well before 2003, I know) that LQG could be connected to DSR. And it was also pointed out early on that DSR does not necessarily have energy dependent photon speed (dispersion). But the hope was that LQG would connect with some version of DSR that DID have dispersion.

So then there would be one possible way to test. (Other potential prospects for testing have now come on the scene, but dispersion used to be the only one people could think of.)

The first paper ( http://arxiv.org/abs/0903.3475 ) by Girelli Livine Oriti shows that they have not yet established a connection between 4d LQG and DSR but there is hope and some researchers are interested in it.

But then the problem remains, even if they got a connection to DSR would that predict a measurable energy-dependence in photon speed?

It seems unlikely, judging from the second paper, by Hossenfelder. If we accept her result, she has ruled out a first-order dependence on the energy.
The term in question is the ratio E/E_Planck where the numerator E is the photon energy and the denominator is the Planck energy.

Given the highest energy gamma rays which have been observed, and the distances traveled, it is only possible to constrain a FIRST ORDER coefficient of dependence. We fall many orders of magnitude short of being able to constrain a second order dependence.

Because in practice E/E_Planck is quite small, so when you square it, the ratio gets even smaller. It is only the very long travel time (on the order of a billion years) that offsets the small ratio and produces a measurable delay. Travel times long enough to offset the ratio squared, and produce a measurable delay, are simply not available.

So it is possible that people like Girelli Livine Oriti will, in fact, eventually prove a connection of 4d LQG to a type of DSR, and that DSR (if it predicted dispersion) might allow a second order dependence of photon speed, which would however not be able to be tested or constrained at least in the way we have seen tried---by measuring Gammaray Burst photon delays.

All still pretty speculative. I haven't followed Hossenfelder's thought experiments through carefully, just provisionally assume her arguments are right. Adrian Cho is a good science journalist, who follows QG. He's the one who broke the Renate Loll 4d CDT story in 2004 (when they first got CDT to work in 4d). The more notice Hossenfelder's paper gets, the more scrutiny by people like Amelino-Camelia, and if it survives intense scrutiny the more likelihood that it's right.

In any case it is certainly not true that the Fermi (GLAST) gammaray observations have "killed all Lorentz violating theories." That is a ridiculous idea. It has not even come within 22 orders of magnitude, one could say.

And it has also not been shown that LQG necessarily bends Lorentz. So if not LQG, what quantum gravity theories are we talking about, exactly?

 

[Nudiustertian]

Not "all"

What Fermi's observational data are actually implying is that in the series expansion of the photon velocity,

v_ph /c = 1 + a₁ (E/E_QG) + a₂ (E/E_QG)² + a₃ (E/E_QG)³ + ...,
​

where E_QG ~ 10¹⁹ GeV, the coefficient a₁ must be << 1 (in relativity all a_i must vanish), i.e., the data are imposing an upper bound on these coefficients.

While these data possibly rule out some LIV theories such as DSR, there may exist theories according to which the coefficients a_i are ought to be small. For example, in http://arxiv.org/abs/0906.4282 (in v4 see eqs 21-25) the coefficient a₁ is proportional to the constant of refraction of the physical (nontrivial) vacuum and thus is naturally small.

Moreover, according to that theory, the velocity dispersion relation is represented in series w.r.t. not E/E_QG but rather E/E₀ where E₀ is the characteristic vacuum energy. Only in the limit when no strong external fields exist the value of E₀ tends to the Planck-scale E_QG. In other words, from the point of view of that theory the current astrophysical observations are just imposing an upper bound on the averaged constant of refraction for the physical vacuum which was subjected to the cumulative combination of weak inter- and intragalactic electromagnetic fields, scattering processes, etcetera.