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.