Arcadian Functor

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Marni D. Sheppeard

Sunday, August 16, 2009

Fermi on GRB 090510

We are (amusingly) told, once again, that string theory has been proven correct. Translation: observable naive Lorentz violation via frequency dependent photon speeds has been beautifully ruled out by GRB 090510.

But the new report, from the GMB and LAT collaborations, details other interesting features of the short gamma ray burst, such as:
We find no lags below 1 MeV (in agreement with the thus far known short GRB lags in that energy range), and above 30 MeV; however, we find that the bulk of the photons above 30 MeV arrive 258±34 ms later than those below 1 MeV.
which is in agreement with previous GRB results indicating some energy dependent time delay. The conservative explanation would be that the unknown details of the emission processes accounts for the apparent delay, but such event dependent delays could be ruled out once sufficiently many GRB spectra are obtained to test the correlation between spectra, redshift and delay times.

12 Comments:

Blogger L. Riofrio said...

As with "dark energy," acolytes of The String will seize upon any result to prove it exists. At the IAU meeting I told one man that supernova redshifts are the only evidence of cosmic acceleration. He was sure there were several tests, but couldn't think of what they were. Blame it on the media?

August 16, 2009 5:07 AM  
Anonymous Anonymous said...

If his memory was a little better he probably would have regurgitated some nonsense about (1) WMAP data and (2) structure formation theory, neither of which have anything to do with an observation of 'acceleration', as any child could see.

Remote Kea

August 16, 2009 6:03 AM  
Blogger Lumo said...

It is not in agreement with any delays. All previously conceivable hypotheses involving delay have been ruled out by the new Fermi result.

The May 2009 burst was shorter and the photon(s) had higher energies, which allowed to measure the coefficient of the Lorentz violation with much better accuracy, and it's zero, 100 times more accurate zero than needed to prove that the Lorentz invariance holds at the Planck scale.

All delays in MAGIC and other previous experiments appear because of the very creation of the burst. For example, the lower-energy rays are created earlier and the higher-energy rays are created later, much like in an accelerator - because the burst is a kind of a natural accelerator.

But the hypothesis that the delay arises on the journey would predict about 100 times bigger delay than the upper bound of the delay recently seen by Fermi.

Lorentz violation of all kinds is dead because if it doesn't appear by order-1 terms anywhere up to the Planck scale, it can't appear anywhere.

August 16, 2009 6:03 AM  
Anonymous Anonymous said...

Lubos, I was quoting the paper about the delay. I never mentioned (a) any stupid Lorentz violating theories or (b) any particular dependence of the delay on 'the path' taken (at least in the classical sense).

Please try to improve your reading skills.

Remote Kea

August 16, 2009 6:07 AM  
Anonymous Genius who is too modest to leave a name said...

"... the lower-energy rays are created earlier and the higher-energy rays are created later, much like in an accelerator - because the burst is a kind of a natural accelerator." - Lubos

Nobody has ever invented an accelerator for gamma rays. You can't accelerate them like charged particles! Gamma rays created by collisions in accelerators will be emitted when collisions occur. Since high energy charged particles travel faster than low energy charged particles, the higher energy ones will collide first, emitting higher energy gamma rays on the average. This is the opposite of the time sequence reported, where soft gamma rays are detected first and hard gamma rays are detected later.

A more successful explanation would be filtering by the mass of the exploding star as the fusion burns inward. Say the explosion starts with fusion occurring in the outer layers of the star witch are the least dense, light elements, which have the largest cross sections for fusion reactions. Because that's near the outside of the star, gamma rays escape with little shielding, so you can measure a lot of soft gamma rays which decrease the mean energy you detect. The deeper layers of the star with denser, heavier elements, then start to undergo fusion due to the heat generated by the fusion of the outer layers, but the gamma rays then emitted from deep layers have to penetrate through the matter produced by fusion of the outer layers before we can observe them. So those gamma rays suffer energy-dependent shielding. Softer gamma rays are absorbed easily, but the harder gamma rays are more prnetrating. Hence, as fusion burns in toward the core of the star, the mean energy of the gamma rays escaping increases because fewer soft gamma rays escape from the star, and the spectrum is predominantly the high energy, more penetrating gamma rays!

August 16, 2009 7:37 AM  
Blogger Lumo said...

No, you were not, Kea. At least not in the text I objected to. This paragraph:

"which is in agreement with previous GRB results indicating some energy dependent time delay. The conservative explanation would be that the unknown details of the emission processes accounts for the apparent delay, but such event dependent delays could be ruled out once sufficiently many GRB spectra are obtained to test the correlation between spectra, redshift and delay times."

is never written in the paper. It is your paragraph and it is just wrong because it says that the question whether the delay is created during the bursts, or on the journey, is open. It is not open: it's the very point of the new paper that it's not open and that the delay on the journey has to be 100-times smaller than expected from order-one Planck-scale Lorentz violations.

The event-dependent delay can't ever be ruled out, as you incorrectly claim, because it has been shown that all the multi-minute delays in such events are event-dependent, i.e. created during the very explosion.

This is the very point of the paper and you seem to misunderstand it even now. The statement that the delay was not created on the journey, and therefore had to be created during the burst, is not a "conservative explanation" but a proposition proved by this paper.

August 16, 2009 7:42 AM  
Blogger CarlBrannen said...

The basic problem with Lubos's logic is not in how GRB 090510 places limits on the speed of light, but instead in concluding that this eliminates all possible Lorentz violating theories. No, it eliminates Lorentz violating theories that conclude that the speed of light depends on its energy.

By careful measurements of the speed of sound you can't say anything useful about the speed of light. Similarly, careful measurements of the speed of light say nothing about the speed of gravity.

August 16, 2009 10:14 AM  
Blogger Lumo said...

Dear Carl, every theory that violates Lorentz symmetry implies that the speed of light is not universal.

A simple historical way to see it is that special relativity, including Lorentz symmetry, was derived just from two postulates - the equivalence of all inertial frames and the constancy of the speed of light (which is what we discuss here).

In more modern language, a constant, energy-independent speed of light in a Lorentz-violating theory would require an infinite amount of fine-tuning. It will surely never happen generically. One would need fine-tuning even to erase the first-order corrections (with the n=1 power law that is now excluded).

But in more specific theories, it is not just about vague arguments about fine-tuning. One can actually calculate the first-order corrections and they're nonzero and of order one.

To summarize, there's no loophole in my logic.

August 16, 2009 7:05 PM  
Blogger Kea said...

... a constant, energy independent speed of light in a Lorentz violating theory would require an infinite amount of fine tuning. It will surely never happen generically.

Generically? Surely you don't expect any decent theory of quantum gravity to be merely generic. What you really mean is that in string theory there is no Lorentz violation of any kind.

But some of your (nowadays not so stringy) colleagues would say that Lorentz invariance is a feature of an emergent classical world. At the classical level, all photons travel at a speed c, locally. All the photons we look at here are travelling at speed c. But we didn't observe all those GRB photons on their way here. And like in any simple 2 slit experiment, maybe they took many different paths. Maybe the path weighting for the higher energy ones gives a greater probability that these photons are slowed down more by gravity.

As you point out, the GRB events so far observed have different event characteristics. It would not be a prediction of string theory that there is a correlation between spectral delays and, say, redshift. This is therefore an opportunity for string theory to be proved incorrect.

August 16, 2009 9:57 PM  
Blogger CarlBrannen said...

"In more modern language, a constant, energy-independent speed of light in a Lorentz-violating theory would require an infinite amount of fine-tuning."

Any theory that requires a preferred reference frame will automatically have Lorentz violations built into its foundations, but that does not logically imply that any specific particle of the theory, for example photons, must propagate with Lorentz violations.

August 16, 2009 11:14 PM  
Blogger CarlBrannen said...

Kea: An interesting paper by Julian Schwinger. This, along with a previous paper by Weyl on unitary transformations, is the stuff on which Svetlichny built the interpretation of path integrals as products of MUBs.

August 16, 2009 11:31 PM  
Blogger Kea said...

Ah, yes. I have that one somewhere ... Schwinger and Weyl are underappreciated by modern physicists.

August 17, 2009 2:02 AM  

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