Observational Loop QG-your opinion.

[marcus]

Observational Loop QG--your opinion.

Please read Bee Hossenfelder's recent post on this topic--you might also wish to glance at the papers/talks she discusses, if you haven't already done so.
http://backreaction.blogspot.com/2009/06/constraining-modified-dispersion.html

To what extent is the development of LQG (now primarily the spinfoam approach) being guided by astrophysics observations?

To what extent is it being tested by astro and collider experiments? Is LQG falsifiable at current collider energies?

============

Guidance is a looser less restrictive idea than falsifiability, and less technically stringent.
It is pretty clear that Loop research is influenced by the connection with DSR (deformed special rel) and it's significant which type of DSR you make a bridge to. Recent observations of delayed high energy photons tend to favor the type of DSR in which very high energy particles see more bumps in the geometry and have a longer harder road to travel, so can be very very slightly delayed. Or more generally, for whatever reason, in those versions the high energy photons appear to propagate slower thru the quantum geometry.

On the other hand, recent observations make less interesting the version(s) of DSR where higher energy photons have an advantage and arrive sooner. This gives Loop researchers some valuable clues or guidance in relation to DSR, but of course nothing here is confirmed yet. A lot more gammaray bursts need to be observed in order to establish that there is or is not a dispersion effect.

Strict falsifiability by experiment is a more demanding and technical matter. Of course presentday LQG is empirically testable/falsifiable because it assumes there are no extra space dimensions accessible at any energy. I doubt that extra space dimensions will ever be detected (it seems like a silly made-up idea for which there is no evidence) but if they were then today's LQG theories would be instantly out the window. It's quite a robust test.

To clarify the point by way of contrast, string extra dimensions are not being empirically tested because if one finds no evidence for them at a given level of energy one can imagine that you just have to look at some higher energy and they will appear. There is no falsifiability involved.

But subjectively it seems a bit unsatisfactory because of the unliklihood of LQG being falsified this way. One would like a test that LQG has more chance of failing.

================

So what do you think? I tend to agree with Bee Hossenfelder that what we are seen now is simple guidance. IOW that researchers in the field now have the potential to be guided by the gammaray observation data that is now coming in.

Here's an earlier PF thread about the main paper Bee talks about:
https://www.physicsforums.com/showthread.php?p=2248613#post2248613

 

[ccdantas]

I doubt that extra space dimensions will ever be detected (it seems like a silly made-up idea for which there is no evidence) but if they were then today's LQG theories would be instantly out the window. It's quite a robust test.

If somehow "extra-dimensions" are detected (something I doubt as well, but this is just a personal feeling), I wonder whether it is true that LQG would be instantly ruled out? I wonder whether it is possible to have a higher-dimensional LQG theory. I mean, start from a higher-dimensional Hamiltonian formulation of GR, do all the LQG quantization exercise, and see what comes up. I think such theoretical games have a lot of room to be played, the problem is how to rule them out according to observations, etc.

 

[turbo]

Bee's observations are well-reasoned. If high-energy gamma rays are delayed with respect to the bulk of the EM arriving from a GRB, we have to consider that Fotini Markopoulou-Kalamara was right, and that EM with shorter wavelengths should experience more interaction with the space through which they propagate. Of course, we will need a lot more GRB observations to see if this frequency-dependent slowing is a general feature of the bursts, and not a misinterpretation of some frequency-dependent evolution of EM at the source. Once we have enough observations (assuming more delays are observed), it will be interesting to see if the delays are proportional to the resdshift of the sources. That would be a very interesting development.

 

[ccdantas]

Of course, we will need a lot more GRB observations to see if this frequency-dependent slowing is a general feature of the bursts, and not a misinterpretation of some frequency-dependent evolution of EM at the source.

Yes, it is a major requirement to disentangle possible effects at the source, which could indeed happen, given that our knowledge of GRBs is still incomplete.

 

[nrqed]

. I doubt that extra space dimensions will ever be detected (it seems like a silly made-up idea for which there is no evidence) https://www.physicsforums.com/showthread.php?p=2248613#post2248613

Wow, you have insulted the intelligence of a large fraction of the theoretical particle physics community. It's no surprise that there are no string theorists spending time on this board to help discuss other things than LQG.

 

[marcus]

If somehow "extra-dimensions" are detected (something I doubt as well, but this is just a personal feeling),...

Just a personal feeling with me too--as I said, it's how it seems and that means seems to me . But I suspect a substantial number of string and former string theorists would tend to agree with us! The reason I suspect that is a trend for string-theorizers to get out of versions requiring extra spatial dimensions and to get into various approaches to 4D quantum gravity including Horava's new one--as well as allied research (e.g. Steve Giddings in black holes) not involving extra-D.

This is a fascinating trend that we are watching happen right now. Maybe someone would like to comment. Large numbers getting into 4D, one way or another, and also into approaches focused primarily on the geometry (like LQG focuses) and only secondarily on the matter. (In LQG matter when present is mostly in some generic form, like a supporting-role player.) It's a question of priorities, a strategy of first getting background independent geometry right and then adding matter later. A growing number of researchers seem to be feeling that this could be the right way forward.

Earlier confidence in both susy and extra-D may have eroded somewhat. So two good reasons that more string folks could be intested in LQG these days are:
1. no extra dimensions
2. primary focus on 4D geometry, with matter on hold.

And one sees the interest because many of them are getting into research that has exactly these features.

 

[Civilized]

I scanned the recent Smolin paper that is the subject of this thread, but I was alarmed to see that the second equation and much of the discussion is straightfowardly about predicting violations of Lorentz symmetry. According to this article any violation of Lorentz symmetry could be used to construct a perpetual motion machine:

http://arxiv.org/abs/hep-th/0702124

We show that any Lorentz violating theory with two or more propagation speeds is in conflict with the generalized second law of black hole thermodynamics. We do this by identifying a classical energy-extraction method, analogous to the Penrose process, which would decrease the black hole entropy. Although the usual definitions of black hole entropy are ambiguous in this context, we require only very mild assumptions about its dependence on the mass. This extends the result found by Dubovsky and Sibiryakov, which uses the Hawking effect and applies only if the fields with different propagation speeds interact just through gravity. We also point out instabilities that could interfere with their black hole perpetuum mobile, but argue that these can be neglected if the black hole mass is sufficiently large.

I didn't see any parts of the Smolin paper that address this issue. Any comments?

 

[MTd2]

Yes, I have a comment. I believe that any consistent Lorentz violating theory must not predict black holes, but its mimickers. That is, everything is the same until you get to the horizon, when you reach there, quantum regime predominates and geometry breaks down.

 

[atyy]

I scanned the recent Smolin paper that is the subject of this thread, but I was alarmed to see that the second equation and much of the discussion is straightfowardly about predicting violations of Lorentz symmetry. According to this article any violation of Lorentz symmetry could be used to construct a perpetual motion machine:

http://arxiv.org/abs/hep-th/0702124

I wonder how this plays out in dumb holes.

 

[MTd2]

On second thought, maybe we should take the "naive" term of "naive Lorentz symmetry break", not as a break of the Lorentz symmetry, but as a quantity that measure the average deviation from the classical value of special relativity due to interactions of the particle with the space time foam. Think that a macroscopic effect of the space time foam is producing a extremely thin haze whose average refractive index increased with the frequency.

 

[atyy]

http://arxiv.org/abs/gr-qc/9712016
http://arxiv.org/abs/gr-qc/9901047

 

[MTd2]

Yes, I know him!

See this one, for exemple, he lists several types of mimickers:

http://arxiv.org/abs/0902.0346

 

[marcus]

MTd2 and atyy, the best recent review of modified Lorentz symmetry like DSR and also of outright Lorentz symmetry breaking is the paper by Liberati:
http://arXiv.org/abs/0906.0681

Even though Ted Jacobson has co-authored ten or more papers with Liberati, that particular paper of Jacobson http://arxiv.org/abs/hep-th/0702124 did not get mentioned in the review. Perhaps it is relevant only to the case where there is a preferred frame, and is therefore not of so much current interest (not relevant to the Smolin-Amelino paper for example) I really can't guess why it was not cited or mentioned.

In any case Liberati talks about DSR (and Planck suppressed dispersion generally) around page 8 of the review paper. It's a good place to get a critical expert phenomenologist look at it. Liberati points out some of the unresolved problems. He also cites DSR papers, which the 2007 Jacobson et al does not, since apparently it is talking about something different.

In DSR the speed c is the same for all observers. No preferred frame. I think most likely you both know that but mention it in case other readers do not and might wish to know.

Liberati's article was prepared for the the Annual Review of Nuclear and Particle Science. He would be the person that Annual Reviews would ask

 

[MTd2]

Indeed, the speed of light is constant, but the point is that light does not move at the speed of light, but it is delayed like it was passing through a refractive media. I guess this analogy was why the case of birefringence was considered

At first sight I seemed Smolin was going to case IV.a, but in the end of his article, I guess he was going to something different, that is, a large gaussian distribution of the delay for a given frequency.

 

[turbo]

(snip)...for typical GRB redshifts of ∼ 1, and for observations of multi-GeV photons, the expected time delays would be of the order of tens of seconds. This time scale is safely larger than the typical variability time scales one expects for the astrophysics of GRBs.

Is this widely accepted? Do we understand the evolution of GRBs well enough to assume that delays of tens of seconds in highly energetic photons is indicative of frequency-dependent dispersion, and not due to some mechanism at the source?

(Edit) I ask because there is a very large (and growing) body of work modeling the light-curves of GRBs and there seem to be lots of open questions.

 

[marcus]

...
there is a very large (and growing) body of work modeling the light-curves of GRBs and there seem to be lots of open questions.

I'm glad to hear that! this business we are discussing here is only one part of understanding GRB, the processes that cause them as revealed, for instance, by their different types of light-curves.

What we are discussing here is something that has not been reliably observed yet---a delay proportional to the travel time (for photons of a given energy).

We have nothing, no empirical grounds to say anything, until we have observed many GRB at various distances and can plausibly extract a travel-time effect.

I would say that the point of Liberati's review article (for Annual Reviews) which I would urge anyone to read before trying to discuss the Smolin-Amelino is essentially that now that the Fermi spacecraft is up and running we do expect to see lots of GRB, and we will be able to tell, before very long, whether there is a travel-time effect. Or not!

 

[turbo]

The largest systematic error affecting this method is the uncertainty about whether photons of different energy are produced simultaneously in the source.

And there is the rub. We have to make some assumptions about simultaneity of emission if we are to cite frequency-dependent time-of-flight effects.

Some interesting stuff going on, including time-referenced analysis of light-curves, here.
http://arxiv.org/abs/0904.4786

These three results provide potentially new constraints on the theory of prompt GRB emissions. For example, time lags between different energy photons are predicted by quantum gravity in the framework of string theory (e.g. Amelino-Camelia et al. 1998). However, in such a case optical photons should arrive before gamma ones. As we observe the opposite, one can rule out this hypothesis for the GRB 081126’s optical lag. Gamma-ray photons comptonization on cold electrons could explain the profile of the optical flare. However, this cannot explain the positive lag observed.

Within the internal shock framework, this temporal lag implies that optical photons were emitted after the gamma−ray ones. However, it is surprising that the flux increases so dramatically during this process. This is not well understood in the standard model for the inelastic internal shock and our results provide new tools for refining the standard model.

Hopefully, astrophysicists will make enough progress in understanding GRBs to make time-of-flight effects measurable, even if only to rule them out to some level of confidence. Perhaps by constraining observations to gamma-rays only and binning them by energy and arrival time? ...

 

[MTd2]

Hey people, have you ever thought that photons can decay to lower wavelenghts and gravitons?

 

[marcus]

Hey people, have you ever thought that photons can decay to lower wavelenghts and gravitons?

A charming thought! But isn't what you are imagining rather like the scattering process that we already know about with Xrays? In the Compton experiment the x-ray comes in and hits an electron and gives him some of her energy and then she goes away with a slightly longer wavelength.

What I mean is, the photon does not really decay in the usual sense, it just gets its wavelength lengthened and its frequency lowered by some interaction.
Perhaps I'm not getting something, and need you to explain a bit more.

 

[MTd2]

Hmm, it would be like this:

photon comes and interacts with an inertial random fluctuation of space time, the photon loses a bit of energy and this random fluctuation is scattered, that is, it becomes a graviton. The higher the energy of a photon, the higher is the cross section of this interaction.

 

[marcus]

Anyway the issue is how do we see the situation. Is Loop QG entering an observational phase?
The most obvious world authority to consult is Stefano Liberati, who belongs to no "camp" and is the person Annual Reviews would naturally tap to write a QG phenomenology survey paper.

So I will try to dig out a quote from Liberati's latest review paper that can give some perspective. The big thing is that the Fermi spacecraft was launched this year and is operating. There are other gammaray and cosmicray instruments, but that is the main observational development.

Also I should mention that when there is a big international conference related to Gravitation and GR/cosmology whether it is the GRG triennial, the Marcel Grossmann triennial, or some other biggie, they often get Amelino-Camelia to chair the QG phenomenology parallel session. He's also one of the main experts. He's younger (Liberati is more senior) but he has recognized cred.

QG phenomenology/testing is a field. So we should get Liberati's perspective on it. I'll go dig up a quote or two. The Review paper appeared June 2009, we are lucky to have something this authoritative that is this recent.
http://arXiv.org/abs/0906.0681
Lorentz Violation: Motivation and new constraints
Stefano Liberati (SISSA, Trieste and INFN, Trieste), Luca Maccione (DESY, Hamburg)

(Submitted on 3 Jun 2009 (v1), last revised 26 Jun 2009 (this version, v3))
"We review the main theoretical motivations and observational constraints on Planck scale suppressed violations of Lorentz invariance. After introducing the problems related to the phenomenological study of quantum gravitational effects, we discuss the main theoretical frameworks within which possible departures from Lorentz invariance can be described. In particular, we focus on the framework of Effective Field Theory, describing several possible ways of including Lorentz violation therein and discussing their theoretical viability. We review the main low energy effects that are expected in this framework. We discuss the current observational constraints on such a framework, focusing on those achievable through high-energy astrophysics observations. In this context we present a summary of the most recent and strongest constraints on QED with Lorentz violating non-renormalizable operators. Finally, we discuss the present status of the field and its future perspectives."

Comments: prepared for Annual Review of Nuclear and Particle Science; v3: several references added

==excerpts==
Infact, models of gravitation beyond GR and models of QG have shown that there can be several of
what we term low energy “relic signatures” of these models, which would lead to deviation from the standard theory
predictions (standard model of particle interactions (SM) plusGR) in specific regimes. Some of these new phenomena,
which comprise what is often termed “QG phenomenology”, include:

•Quantum decoherence and state collapse[4]
• QG imprint on initial cosmological perturbations [5]
• Cosmological variation of couplings [6, 7]
• TeV Black Holes, related to extra-dimensions [8]
• Violation of discrete symmetries [9]
• Violation of space-time symmetries [10]

In this review we focus upon the phenomenology of violations of fundamental symmetries, given that a convenient
way to perform high-precision tests is to look for experimental deviations from symmetries that are believed to hold
exactly in nature and that could be broken by QG...
...
...

...We hope that this review has convinced even the most skeptical reader that it is now possible to strongly constrain
Planck-suppressed effects motivated by QG scenarios. The above discussion makes clear that this can be achieved
because even tiny violations of a fundamental symmetry such as Lorentz invariance can lead to detectable effects
at energies well below the Planck scale.
...
...
As we discussed at the beginning of this review, LV is not the only possible low energy QG signature. Nonetheless,
it is encouraging that it was possible to gather such strong constraints on this phenomenology in only a few years.
This should motivate researchers to further explore this possibility as well as to look even harder for new QG induced
phenomena that will be amenable to observational tests. This will not be an easy task, but the data so far obtained
prove that the Planck scale is not so untestable after all.
==endquote==