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Gamma Ray Anomaly Could Test String Theory
exploder writes "String theory is notorious for its lack of testable predictions. But if the MAGIC gamma-ray telescope team's interpretation is correct, then a delay in the arrival of higher-energy gamma rays could point to a breakdown of relativity theory. A type of 'quantum lensing effect' is postulated to cause the delay, which is approximately four minutes over a half-billion year journey." Ars's writeup is a little more fleshed-out than the Scientific American blog posting.
If the standard model fails, string theorists will laugh, jump and down, and point their fingers at their former naysayers.
If the string theory model fails, it will be replaced with a newer, better version of string theory, with bountiful opportunities for new books, conferences, papers, and maybe even some derivative specialities of study.
YOU CAN'T KILL WHAT LIVES ONLY THE MINDS OF MEN... BUWAHAHAHAHAAAAA!
When things get complex, multiply by the complex conjugate.
I think that it is definitely important to note, as you did, that this isn't just a matter for string theorists.
I really wish that string theory wouldn't be glorified the way that it is. I am not aware of a single hypothesis that has been successfully tested and validated under it. And as you mentioned, string theory does predict something like this, but so do other forms of physics.
This is definitely a significant finding, because gamma rays should be traveling at the speed of light, and only that speed through a vacuum. I read through things quickly, but it doesn't appear that any reasoning was advanced in the article for the delay. But as long as the rays left at the same time, this would be a problem for relativistic physics. Unless it turns out that there is some sort of mass in the medium, in which case the relativity is still fine.
IAAA [I am an astrophysicist], and I'd like to point out what I feel is an important caveat to this nevertheless very interesting work. From the paper itself:
"We cannot exclude the possibility that the delay we find [...] may be due to some energy-dependent effect at the source."
What they are saying is that there are still details we don't understand about AGN [active galactic nuclei] like Markarian 501. So, while this effect could be a first sign of quantum gravity (*not* string theory in particular, as others have pointed out), it could also simply be something going on in the intrinsic spectrum of the flares themselves. I'd personally consider the second explanation more likely at this stage.
As they also point out, one approach to sort out the ambiguity would be to observe other flary AGN at different redshifts (distances). One could then, for example, see if the delay gets shorter or longer as the distance changes, as one would expect with a quantum gravity effect due to propagation to Earth.
If my understanding is right, though, string theories usually predict an infinite spectrum of increasingly massive particles. What is equivalent to the standard model is the low-energy limit, where we can ignore all but a finite number of low-mass particles. Thus string theories do make predictions that are testable - namely, that we'll keep finding new particles.
Some physicists have taken certain physical laws as axioms for something like a first-order logic.
The great part about loop quantum gravity is that it takes tried and true principles of physics (specifically background independence) and works from there. Thus loop quantum gravity can be thought of as a somewhat "minimal" theory of quantum gravity: it takes Einstein's GR, which is well tested, based on solid principles, and has a wonderful economy in the sense that it is roughly the simplest nontrivial theory that fits those principles. This is a trait shared in many ways with LQG; this paper seems to be saying that under weak assumptions, the fundamental basis of LQG is in fact unique.
String theory, on the other hand, keeps seeming like it has the opposite properties. It throws out the principle of background independence, instead making ad hoc assumptions about the shape of spacetime. In addition to the string particles it hypothesizes, it must add an additional object (D-branes) to give a reasonable low energy limit. In addition, the specific assumptions about the shape of spacetime give wildly different low-energy limits, and the restriction that this must match up with our experience (which is a terrible principle to base physics on) doesn't even give us a unique theory, so the "theory" is even more resistant to falsifiability, as a slightly different configuration can be pulled out to explain why the previous test failed.
In short, LQG has going for it:
- mathematical elegance and economy
- strong, tested founding principles
Whereas string theory has:
- slightly less ugly than the standard model