Tuesday, September 17, 2013

Quantum Gravity in Gamma Ray Bursts: Still Nothing

Small wavelength photons (blue) travel faster
than their long wavelength companions (red).
[Image Source]
Gamma ray bursts emit highly energetic photons that, by the time they reach us, have a long journey behind them. That makes these photons excellent candidates to test new physics: Because both the energies and the distance are extreme they potentially give us access to so-far undiscovered effects.

However, in contrast to supernova of type Ia, gamma ray bursts are one of a type – they’re not so much standard candles but surprise fireworks. That makes these photons not quite so excellent candidates to test new physics.

A story that dates back now more than a decade and that has been hailed as a ‘test of quantum gravity’ is that certain quantum gravitational effects could lead to an energy-dependence of the speed of light. In this case, photons of high energy would travel either faster or slower than the low energetic photons (depending on the sign of a parameter). Such an effect is not allowed by the presently established theories, and looking for a signal of an energy-dependent speed of light therefore tests deviations from Einstein’s theory.

Theoretically, there are two different ways this could happen, either by a breaking of Lorentz-invariance or by a deformation of Lorentz-invariance, and these cases have to be carefully distinguished. Both cases lead to an energy-dependent speed of light, but if Lorentz-invariance is broken, meaning there is a preferred restframe, then this would lead also to other effects that we should have seen already. This means if we do see such an unexpected effect in the emissions of gamma ray bursts, we’d know it’s not a breaking of Lorentz-invariance but a deformation. This would be considerably more exciting, but is also much more speculative.

My position on this has been, and still is, that a deformation of Lorentz-invariance is not well motivated and theoretically highly problematic, thus I don’t think an energy-dependent speed of light is plausible. But in the end the question is what the data says.

Data however is a reserved companion who just politely asks to be analyzed, and given that no two gamma ray bursts are alike it’s not at all clear how to do the analysis. It seems to me experimentalists are still poking around and trying out new methods. Occasionally a constraint comes out of this. The most recent constraint came out in two papers by Vlasios Vasileiou and a whole list of other people in no particular alphabetic order (if somebody can fill me in on the authorship order in that part of the community, please enlighten me).

To make a long story short, they propose three new ways to arrive at new bounds, all with advantages and disadvantages, and arrive at a bound that constrains new quantum gravitational effects to be beyond 7.6 times the Planck scale, at 95% confidence level. This means the new bound is both weaker and at a lower confidence level than the bound by Nemiroff et al that we previously discussed, so it’s non-news really. And that doesn’t even take into account that the more ways you try to extract a signal from the data, the less likely it will eventually be a real effect.

In a footnote in the discussion the authors of the new paper criticize the Nemiroff et al result basically for the same reasons that I put forward in my earlier blogpost: The constraint hinges very strongly on a few pairs of photons. But the advantage of the Nemiroff analysis is that it’s a clear and clean method that can rapidly increase to higher statistical relevance with more observations, provided we see just a couple more of such pairs. It merely relies on the statement that it’s quantifiably unlikely that a few photons arrive almost simultaneously if they weren't emitted simultaneously and traveled together – at the same speed. Unfortunately, the significance of that result could also decrease in relevance, and that for reasons that have nothing to do with the energy-dependence of the speed of light, just with the physics at the source.

The new approach in the Vasileiou et al paper is valuable however for trying to take into account an intrinsic dispersion of the source. But I think the great weakness of this bound is the same as the previous bounds: low statistics with results that strongly depend on one or a few gamma ray bursts. I doubt we’ll ever get rid of the possibility that source effects play a role unless red-shift is taken into account and different distances are sampled over. That’s because an energy-dependent speed of light should yield a stronger effect the farther away the source, while a source-dependent effect does not get stronger.

Either way, for me it’s a win-win situation :o) There’s either quantum gravity in the gamma ray burst measurements or there isn’t. If there is, it’s a huge boost for the field I work in. If not, I was right all along saying that there is no effect. At the moment however the situation isn’t entirely settled, so stay tuned.


Uncle Al said...

Assume photon vacuum symmetry (QM) is identically matter vacuum symmetry (GR). Axiomatic systems cannot self-falsify. Rigorous derivation is then errorless and unphysical. Challenge spacetime geometry with the orthogonal geometries of enantiomorphic self-similar atomic mass distributions. Geometric Eötvös experiments demonstrate vacuum symmetry, photons vs. matter, trace diverges. GR has fermion nightmares - and well it should.

Zephir said...

In AWT the Lorentz symmetry breaking really happens inside of gamma ray bursts, because the more energetic photons are heavier and they're traveling slower. But the same effect leads the less massive photons into revolving of heavier photons around Kepler orbits - so that whole group of photon reaches the Earth in a same moment - despite each photon travels along different path with different speed.

Robert Nemiroff said...

The main "millisecond / GeV" result in the Nemiroff et al. paper is based on three separate groups of GeV+ photons, each separated by (about) a millisecond. The first group, which is the most important as it has the highest energy photon, has three photons in it, not two. Yes, it is possible that the distribution of these seven high energy GRB 090510 photons -- or the three in the main group -- is just lucky bunching from a longer time-scale parent distribution. Let's say, though, that the bunching of "the three" photons really occurs at the distant source. Seems favored, statistically. An interesting attribute is that the highest energy photon comes in the MIDDLE of the three. Wow! Then even the temporal width of this fleeting bunch is likely a source effect and not energy-dependent dispersion -- and therefore not quantum gravity dispersion. Then QG dispersion, if it exists, is confined to scales below 1/500 of the Planck length.

Uncle Al said...

I got yer orbit riiight heeere. It's a töricht not a torus.

Eric said...

Bee, have you considered that there is a much more prosaic example of highly energized photons acting differently? Perhaps neutrinos are just an example of single or multi photons that have acquired mass and slowed down slightly.

I always think that an encyclopedic description of nature where all observables are neatly categorized but not understood is the fertile ground from which physical relationships are later understood. Maybe neutrinos are what you are really thinking about.

Sabine Hossenfelder said...

Robert: Thanks for chiming in.

Eric: Neutrinos are fermions. Photons not.

Eric said...

"Eric: Neutrinos are fermions. Photons not."

Exactly. What do think the process of turning bosons into fermions consists of? It's displaced energy during acceleration.

If it turned out that gamma ray photons actually changed speed and/ or attained mass what do you think that change would consist of? It would be the equivalent of turning a boson in a fermion.

Sabine Hossenfelder said...

Eric: Look, make that verbal explanation mathematically exact, publish it, and then we can talk again.

Eric said...

Bee, I know this might be hard to accept but I've learned in this world things aren't really fair. I just put it out there on these comments because some smart person deserves to know about it minus all the math that wouldn't really add to the idea. If you don't like hearing really good ideas because they aren't published in the archive or because my name doesn't have a credential after it then I consider that your loss.

But I actually believe the real reason you said what you just said is that you would then never have to suffer the embarrassment in future of crediting me if you ever decided to use my logic.

Actually I never even thought of trying to get official credit for it. But it does strike me as small minded, and not fooling too many people either, to go out of your way to act unimpressed. It's not fooling me and probably not anyone else either.

Sabine Hossenfelder said...


Look, you've previously falsely accused me of allegednly 'stealing' your great ideas and I was hoping you learned something from that.

'Great ideas' come in dozens. Maybe the neutrino is a photon or the other way round or maybe a graviton is two electrons, or maybe spacetime is fractal or maybe there's a universe inside every quark or maybe Planck's constant is not actually constant, or maybe Feynman diagrams are knots and braids or maybe black holes are elementary particles, and so on and so forth. I am pretty damned sure that for every single one of these 'ideas' you'll find a hundred people who've had them, and most of them probably think they're very creative and understand a lot about physics. These are ideas, Eric, that are born out of the social context you swim in. They're hybrids of concepts that already exists, which is the easiest and most common way to be creative. It's also the cheapest way. It's not a bad way and often successful. But success doesn't come from having an idea. Success comes from making it work.

Rest assured, I have zero interest in stealing your idea, thank you. And if you don't want me, or anybody else to 'steal' them, then why are you dumping them (off topic) on my blog to begin with? Best,


Uncle Al said...

"minus all the math that wouldn't really add to the idea." Science is theory (math) and observation (becomes math). Alain Connes' non-commutative algebra; GR appears. Thermodynamics plus the Beckenstein bound, GR appears. Einstein-Cartan-Kibble-Sciama gravitation is GR’s superset. Test the increment: "Opposite shoes" violate the Equivalence Principle.

Eight years grinding CHI with mathematician Michel Petitjean for molecules then crystal lattices. y = mx + b models intensely complex CHI, with mathematician Penelope Smith. Three years with crystallographer John Osborn on space groups. Caltech chair parade mining crystal structure databases. It's a hobby.

A vast gulp of superlative theory contradicts observation. Science tolerates error but not folly, even for a hobby. Then, peer review.

Symmetry: Culture and Science 19(4) 233-247; 307-316 (2008)
Bull. Am. Phys. Soc. 49(2) 54 (2004)

Eric said...
This comment has been removed by the author.
Eric said...

Bee, you missed my point entirely. We all move on and learn from mistakes, including me. What I was saying, which you obviously didn't get, is that I like the idea of freely putting out good ideas on the net. I accept no "official" credit. Not even a little. To me that's what this stuff is for. That's why I've never promoted or characterized my ideas within some official theoretical framework. For instance Aether theory or Discreet blah blah.

What I expect in return for no expectation of credit is the simple engagement with a good idea instead of pathological movement away from it because of the form the idea comes in and the source of the idea.

It requires nothing more than the simple admission that it might be a good idea to think about things that way. And like I said, that admission would not in any way ever make me think you owed me any kind of credit whatsoever in later publication. I understand how this works. What I'm really talking about is just acting honestly within the usual social contract between people. Your going back to my response from before is outdated. I've movee on.

Sabine Hossenfelder said...


Okay, I'm sorry for the misunderstanding and apologize. Let me just say that I don't think it's a good idea, and I have no intention to spend time thinking about it. I would also appreciate if you could keep your comments on-topic. Topic of this post are the new constraints on spectral dispersion in gamma ray bursts, just as a reminder. Best,


johnduffieldblog said...

I feel this work is more lose-lose than win-win actually. I also feel that I should look closely at the papers, and if do I feel like saying something, maybe a public blog isn't the best place for it.

Phillip Helbig said...

Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats.
---Howard Aiken

Robert L. Oldershaw said...

Sometimes it's best to let nature do the persuading.

The Tunneller said...

Hie Volks

I have been working on quantization of spacetime using Dirac's techniques and "discovered" the graviton. This is a promising approach to quantum gravity. You can download my paper for free at www.scirp.org/journal/ijaa and discuss whether it is an approach worthy of further pursuit. The paper is called Nexus: A Quantum Theory of Spacetime Gravity and the Quantum Vacuum.