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.

Comments

Not specific to String Theory

[E 99]

While this is great research, even if it can be demonstrated that the higher energy particles traveled faster, this is not a prediction specific to String Theories, but as the arstechnica.com article points out, this is common to most quantum gravity theories. Still, it would be an awesome thing to prove.

Re:Not specific to String Theory

[hedwards]

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.

Re:Not specific to String Theory

[Anonymous Coward]

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.

The neat thing about String Theory is that it is a coherent mathematical framework and a group of related models that unify the theories of several of the fundamental forces as they are currently understood. Essentially, it can't make predictions, because the if it does, in practice, the theories it unifies will have already made them. As of now, it is best understood as an "interpretation" in the sense that the Copenhagen or many-worlds interpretations are interpretations of quantum physics.

Note that individual string theories are falsifiable anyway. For example, there is a class of theories called the Super Symmetric String Theories that relies on the assumption of supersymmetry. If this assumption proves to be empirically false, the super symmetric theories will be abandoned.

If you're familiar with the methods of mathematical logic, an analogy can be useful. Some physicists have taken certain physical laws as axioms for something like a first-order logic. And many models (in the sense of model theory) have been created. Now the task is to figure out which models are representative of the physical world. This is kind of backwards from the scientific method, where a specific model (nature) is examined and attempts at an axiomatization of its working is attempted. But it is a logically sound technique, and as falsifiable as the axioms are.

Re:Not specific to String Theory

[Anonymous Coward]

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

Re:Not specific to String Theory

[bcrowell]

...even if it can be demonstrated that the higher energy particles traveled faster, this is not a prediction specific to String Theories, but as the arstechnica.com article points out, this is common to most quantum gravity theories.
Yeah, it's even possible to make a pretty reasonable model-independent argument that a variable speed of light must come out of any theory of quantum gravity. Lee Smolin makes a pretty simple model-independent argument that spacetime must be discrete in any theory of quantum gravity. The idea is that the Bekenstein bound says there's a maximum amount of information that can be contained in any region of spacetime (e.g., a black hole has a certain entropy, which is proportional to the surface area of its event horizon). However, if spacetime was continuous, then you could store an infinite amount of energy in any volume of space. (Here is a longer explanation.) Note that none of this requires any specific model such as string theory or loop quantum gravity. If spacetime is discrete, then there's a scale at which its discreteness occurs, and that corresponds to a certain minimum wavelength that a light wave can have. The propagation of light therefore has to be drastically modified as you approach that scale.

Relativity's Dead

[einsteindotcubed]

There is no need to confirm a breakdown of relativity. We already know that it is, at the least, incomplete, if not incorrect. Albert Einstein himself saw this, and was on his own quest for a "theory of everything" in his later years. String theory should become fully "testable" with the startup of the LHC (Large Hadron Collider, part of CERN) in May of 2008. Hopefully we may find proof for the God particle, also known as the Higgs boson. In any case, tremendous amounts of data will be reaped from this machine, and we may very well prove or at least expand upon string theory. (We could also completely disprove it, but I'm trying to be optimistic.)

Correction,experiment will test the standard model

[physicsphairy]

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!

Pah!

[Reed Solomon]

Nothing can stop Hulk from Smashing string theory to bits. Hulk will destroy puny humans who betrayed him. Wait, that's a Skrull. Is nobody a human anymore?

No no no

[Henry V .009]

There is a need to confirm a breakdown of relativity. It's an incredibly well-supported theory that predicts things on cosmic scales down to the Hydrogen atom.

The Higgs boson is predicted by the Standard Model, not String theory. String theory will be no more testable with LHC than it ever was. It's not even wrong.

Important caveat

[jlkelley]

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.

Re:String "theory"

[JetJaguar]

Actually, there are a great many phenomena that string theory explains, the subject of this story, for example is potentially one of them, there's also some things about black holes (like Hawking radiation) which string theory predicts, but other theories also predict Hawking radiation.... plus there's a whole host of things that it predicts that occur at very high energies. But that's essentially the problem with string theory. The kind of things string theory predicts that would confirm it require energies that we are simply incapable of achieving, and the more mundane predictions made by string theory also happen to match predictions by competing non-string theories, making it pretty much impossible for string theory to distinguish itself using modern technologies.

That being said, I think string theory is beautiful, however, it could very well turn out to be the most beautiful theory of physics ever constructed as well as the biggest dead end.

Re:Occam's Razor

[DynaSoar]

> OTOH: 100 or so years after the Principa was published a (French?) woman of noble
> birth corrected Newton's kinetic energy equation by emprical means (ie: dropped
> steel balls into clay and mesured the craters).

You're thinking of Emilie du Chatelet, paramour of Voltaire. I don't know how noble, but her family lived in a 30 room apartment overlooking Tuileries gardens in Paris. Certainly rich by birth, and married to a rich French military officer who conveniently left on a polar expedition.

And you're not quite correct about what she did; it was much better than that. The dropped ball and clay experiment was done by Willem 'sGravesande in the Netherlands, but he didn't have the theoretical background to understand what he had -- the craters got deeper with the square of the height (== energy). Liebniz had previously specified that energy should increase with the square of velocity, but that was somewhere between intuition, anti-Newtonian leanings (Newton got credit for calculus rather than he; Newton was pushing for mass times velocity, no square) and fortuitous guesswork. He didn't have the practical sense to develop a means to test it (or perhaps thought that beneath him). What du Chatelet did was put the two together and show the precise relationship between energy, mass and velocity that was supported by the data: E = mv^2.

Smiling Uncle Albert had it half written for him. What he plugged in was c for the Latin celeritas (rapidity), which he showed to have a limit of the speed of light, and that the E and m then equated completely and were thus interchangeable through it. Had she had the verification of Roemer's measurement of the speed of light to work with (said verification was just a few years old and not widely accepted yet) and had more time to work on it (she died from an infection after giving birth) she might have made progress towards that herself.

If she had done so, Poincare probably would have grasped the significance of his "theory of relativity" (Uncle A. never used that term until well after it became popularized, but Poincare used it explicitly in his own) and formulated the famous equation himself. He was, after all, right on the verge of it, and refused to talk about Ol' Al forever more because he failed to get all the way there first. It riled him no end, until the end of his days. Had he been younger and the age earlier, he might have challenged the young Bavarian Jew to a duel. A duel such as Francois-Marie Arouet threatened against a certain French nobleman, which resulted in his expulsion from France to England, where he learned of Newton and his work, which he brought back to France, along with his nom de plume, Voltaire. Or the duel (fencing match, actually) in which Jacques de Brun, the head of the King's bodyguards, was bested by a 16 year old girl named Emilie de Breteuil, as such was her family's name when they lived above Paris's Tuileries gardens.

If this was Connections, and I were James Burke, I'd be making a lot more money than what I'm getting for having written this. I am, however, every bit as pretty as Burke on camera, which is to say not at all.