Sunday, August 26, 2007

MAGIC's observation of Gamma Ray bursts

A couple of days ago, the MAGIC collaboration posted a paper on the arXiv:

Probing Quantum Gravity using Photons from a Mkn 501 Flare Observed by MAGIC

which I didn't mean to comment on, but it seems it has caused quite an astonishing amount of discussion. The SciAm blog sees hints of a breakdown of General Relativity, Lubos Motl expresses his skepticism, Slashdot proclaims it is a test of String Theory, which then causes Buffy Woit to fight against the living dead predictions, thereby unfortunately claiming to be in agreement with Lubos, who returns the favor with a second post denying any agreement with "the critique of science", fading out in a characteristic rant about 'aggressive imbeciles'. Even the more reasonable article by Chris Lee on Ars Technica which cautions that quantum gravity has probably not "made a sudden leap forward" proclaims that "this is the first real data against which such a theory can be tested", the caution of which however leads the SciAm bloggers to nag that those "who have been loudly complaining about the lack of [observations that probe string theory] have gone silent."

This is the summary, status Sunday evening. In case I loose one or the other reader here, let me give you my opinion up front: there is no experimental evidence for quantum gravity, and none of the present theories can be tested with currently available data. If you're looking for sensations, you're on the wrong blog. I suggest you try one of those mentioned above instead.

As a prerequisite, the issue is an observation of gamma ray flares from the active galaxy Markarian 501. The data was taken in summer 2005, the most characteristic event being on July 9th. The peak energy of the flare was around a TeV, it lasts about 2 minutes. The emitted spectrum spreads over some orders of magnitudes. I'm not an astrophysicist and details elude me, so please don't ask me in which constellation Mkn501 can be found, or what the status is about understanding AGNs.

The point is that if one knew the emitted signal, brightness and spectrum, measuring what arrives on earth would allow one to conclude what happened to the signal on the way. If the signal propagates through vacuum, General Relativity (GR) tells you what to expect. There is a redshift, brightness drops, but these effects are well known and computable within GR. If what happened to a photon during travel would depend in some unusual way on the energy of the emitted photon, it would distort the shape of the signal. This is called dispersion. If a family has to get through a crowd of many people, the small children will be the first to arrive, and then have to wait for their parents who have a harder time pushing their way through. Such, what originally started as a localized group arrives with a time delay between various parts. A similar effect happens to an emitted signal of photons if there is a dispersion.

Problem is, for the case at hand the emitted signal is not so very well known. However, Likelihood analysis allows to examine how high the chances a signal is distorted in a certain way for different frequency regions, and for the observed flare the MAGIC collaboration concludes that its shape "is unlikely, and consequently that the time shift of 4 ± 1 min between the highest and the lowest energies is more probable." [astro-ph/70702008, p. 13]. The time of this delay would be about the duration of the signal itself.

In their new paper the MAGICians write one can circumvent knowing the true shape of the time profile at the source by basically taking the result and assuming a parameter dependent blurring has taken place that distorts the signal differently in different energy regimes. If one reconstructs the original signal with that assumption, one can examine it for certain features, and ask for the parameter that optimizes that feature. If I understand it correctly, they have performed analyses with respect to two such features. The one is to "maximize the total energy in the most active part of the flare" and the other is "to optimize the sharpness of the transformed signal".

In both cases it is found that the signal can be optimized if one reconstructs it with a certain assumed dispersion relation that depends on a parameter. The best fit for the parameter is not for exactly zero which would correspond to no dispersion, aka standard propagation. It is interesting though that both analysis with respect to different features seem to fit together. There comes the chi-square fit, et voila, one has a signal at 95% CL. That is to say the detected signal would be more probable if there had been a dispersion. It's not striking how much one can improve it, but one can.

But anyhow, now the clue is to call the fitting parameter MQG and claim it is an effect of quantum gravity (QG) , since "QG effects [...] cause the fabric of space-time to fluctuate on the Planck time and distance scales. It has also been widely suggested that this ‘foaming’ of space-time might be reflected in modifications of the propagation of energetic particles, namely dispersive effects due to a nontrivial refractive index induced by the QG fluctuations in the space-time foam and that one is "probing the Planck mass range for the first time". Depending on your preferences you might then argue this is either a prediction of string theory, or contradicting predictions of string theory. Or a prediction of LQG, or not a prediction of LQG.


What is even worse than the advertisement in the paper is the echo it has caused, which is more attention that I think is appropriate. What about all those hard working people who don't wrap their results into easily digestible catchy phrases? Won't they wonder what all their effort is good for if they see one gets much larger attention with things like this?

Yes, there are motivations, and indications that within certain scenarios one can have such a dispersion. I myself find this very exciting, but as the authors state themselves "The calculation of such effects is beyond the scope of current theoretical methods". Also, as Lubos has pointed out, the same collaboration had a preprint already in February about the gamma ray emission of the same source Mkn501, in which they attributed observed spectral features to different effects, and remarked "A somewhat more speculative issue that blazar emission permits to explore concerns non-conventional physics. Energy-dependent arrival times are predicted by several models of Quantum Gravity, [...]". Even given that the signal is confirmed by sufficient other data, and proves to be statistically significant, the most likely explanation I'd think would still be our lack of understanding of AGNs.

The problem I have with that paper is not the analysis itself (which I wouldn't have found remarkable enough to mention here) but that it is clearly sold as a made up sensation. The whole introduction doesn't belong there. If they want to analyze their data with respect to dispersion, then they should do that, and not take several further leaps of faith towards quantum gravity. If at all, then this belongs into the discussion, but neither in the abstract, nor in the introduction, nor the conclusion of a scientific publication - the first paper e.g. is much better balanced. All that they have found is with unconvincing confidence a signal can be 'optimized' if a dispersion relation had a parameter that happens to be somewhere by the Planck scale. That itself has nothing to do with quantizing gravity, neither with a "Break down of General Relativity" as the SciAm blog put it.

However, even if there was a dispersion, it wouldn't be clear at all that this has anything to do with quantum gravity. If the signal undergoes a dispersion, then the signal undergoes dispersion. Full stop. Everything else is speculation, and should clearly be called such. The question what an effective description has to do with the potentially underlying fundamental theory I call the 'Inverse Problem' - for more details, please see my previous post on Phenomenological Quantum Gravity.

To sum up: 1) There is the question whether the signal is significant 2) Even if it was, it would most likely be something that we haven't understood about astrophysical processes 3) Even if that was outruled there would be the question whether it had something to do with quantum gravity.

Bottomline: If one adds a parameter one can fit curves better. If the original fit was good, the parameter will be small. (Another example for this sort of analysis you find here.)

Now you can blame those who are conservative and would attribute a potential signal to old-fashioned physics to be boring and pessimistic, but the only thing you get from hyping non-results is more people wasting time with alleged 'effects' and made-up 'predictions'. Yes, I do think scientific publications should be conservative and cautious, especially if they claim to have evidence for new physics. Reading papers and follow-up articles with exaggerated claims about testing quantum gravity annoys me considerably. It's like strawberry yogurt where the fineprint says 'no fruit'.

37 comments:

CapitalistImperialistPig said...

Now we know how physics must have been for the Greeks. All these theories, and no good ways to test them. If an idea in physics lives too long with out test, "Magical" thinking may be inevitable.

Bee said...

Patience... just wait two-thousand years or so for the indivisible to be divided ;-)

Anonymous said...

I disagree that this cannot be a testable QG effect. These types of observations may be repeated for other cases and systematically studied. If the effect is always there and varies with the distance to the source, then I think this would be strong evidence that the effect is indeed real.

Bee said...

Hi Anonymous:
I don't know who you disagree with, but I did not say 'this cannot be a testable QG effect'. Leaving aside the question whether it is an effect at all, what I said was the slight deviation from expectation in the data is unlikely to be a QG effect. And even if it was, establishing that there is a modified dispersion would not be sufficient to show it is a consequence of quantized gravity. Best,

B.

Arun said...

Dear Bee, thank du/sie for trying to put out a blog flare/burst. One of those could scorch the earth, you know, and make it uninhabitable :)

CIP, great observation!

Moshe said...

Bee, clearly you are out of the loop, strawberry yogurts are always made "with" real fruit (among many other things).

paul valletta said...

Hi Bee, this really is another interesting posting! I had speculated on the very results seen in the papers some years ago, and amazingly, it comes up trumps.

There is contained within LQG, a very interesting notion for planck scale phenonema, a number of old papers( I will retrieve those specifically needed ), but for now I will mention this for comparision:In the UK there are a number of I.S.P's, there is competition to provide the best service, British Telecom and Virgin for instance are but two.

B.T runs all the local exchanges, where they allow communications to other providers, namely their competitors (Virgin). In the exchange buildings, it is their (BT) engineers who dictate and distribute the path where data is sent.

It has been known that BT have been anti-competetive for some years, they send "rivals" data around the exhange loops a few extra times, which then leaves the exhange, and goes onto the next exchange, so on...?

The web savy public then judge the ISP's based on the speed their system operate, for instance Virgin users have been complaining their service is much "slower" than the users of BT, as BT subscribers get their data without it being "looped" a few million times via the exchange,hope you get the basic principle, if somewhat crude, sorry?

Ok the jist of the Markovian paper could be based on the fact of "energy-looping" at specific quantum scales, operate a similar principle as my above notion, if this is so, then there are certain values of "energy-time" dispersions?

I have to dig out a Smolin paper that I recall, made discrete and specific handwaving, but for now thanks again for "untying our shoelaces" and thus allowing us the freedom to keep our feet on the ground, without us having to trip over top many times! , best to you both, paul.

Lumo said...

You're not distinguishing expectations and scientific knowledge.

I only say that these experiments won't test quantum gravity because local Lorentz invariance holds. So I predict that any dispersion-in-vacuum explanation will go away.

But if you understood your own (nonsensical) papers about this issue, you would know that you have implicitly stated that quantum gravity may be tested in MAGIC-like experiments and it will be tested.

If you don't know why this is what follows from your printed articles, then you really have no idea what you're talking about.

Bee said...

Hi Arun:

Well, the sad thing is that blogwise putting out flares is exactly what one does not want. Who is the smart player in this list? Slashdot who did upset the most people and got the most attention. I think about writing a post titled: Testable String Theory Prediction Measured! Seriously. How about string theory predicts all kinds of matter couple to gravity? wow! A testable prediction!

Reg dusie: how about Your Highness ;-)

Hi Paul,

Thanks. That is an interesting comparison. Is Virgin related to that weird guy who funded Virgin records and now does in interstellar travel or something? (sorry forgot the name)

Hi Moshe,

Which brings me directly to the question whether there is fruit in fruit loops.

Hi Lubos,

Expectations should be built on scientific knowledge, no? I don't know what you think you interpreted into my papers. If anything I might have said such experiments would test an energy dependent speed of light. However, as I have argued repeatedly, an energy dependent speed of light I find to be incompatible with Lorentz invariance. I.e. just 'deforming' does not work, one actually has to break observer independence.

This then means the explanation of a mdr would indeed imply Lorentz symmetry be broken. A scenario which I find improbably because there are all kinds of other constraints on such a setup (that were not mentioned in the MAGIC paper), esp. on the case with n=1. Best,

B.

Plato said...

As a lay person I can't help but think of Lagrangiam relations between the sun and earth and think, why not in our universe?

The color of garvity? You know what I mean. Glast is doing it's job.

In particle physics, the crypton is a hypothetical superheavy particle, thought to exist in a hidden sector of string theory. It has been proposed as a candidate particle to explain the dark matter content of the universe. Cryptons arising in the hidden sector of a superstring-derived flipped SU(5) GUT model have been shown to be metastable with a lifetime exceeding the age of the universe. Their slow decays may provide a source for the Ultra-high-energy cosmic rays (UHECR).

Plato said...

I guess this saids it all?

Eric Mayes said...

Hi Plato,
Thanks for quoting the abstracts and/or Wikipedia entry for some of my papers. I do enjoy the attention. However, I'm not sure what is the present relevance, especially considering that superheavy particle decay has now been ruled out by Auger as a possible source of the UHECRs. If I understand correctly from some of Bee's post, Lorentz violation has also been ruled out as a cause of the UHECRs.

Plato said...

Yes of course the source was not given,


Bee:assuming a parameter dependent blurring has taken place that distorts the signal differently in different energy regimes. If one reconstructs the original signal with that assumption, one can examine it for certain features, and ask for the parameter that optimizes that feature.

Lensing would have had to be explained some how, and why not think of gravity as a dispersion in this case?

Plato said...

If one was to assume that such a place existed that satellites could have found "stationary orbits" then why not the position indicated in the sun earth relation L1 or L2. Routes in space?

In space such a equilibrium could be struck? Could be the "tipping point."

Layman thinking.

Plato said...

If the earth's gravitational self-energy does not precisely obey the Equivalence Principle, the orbits of the earth and moon around the sun would be slightly displaced from one another (think of this as a modification of Kepler's third law), which would show up as a signal in our lunar range data. Various string-motivated theories, quintessence, and other alternatives to General Relativity almost all predict a violation of the Equivalence Principle at some subtle level. Given the recent hints that there may be some new and mysterious modification to the laws of large-scale gravitational attraction (as indicated by supernovae and cosmic background anisotropies), it is important that we probe every available aspect of the basic nature of gravity.

Hmmmmm.....:)

Lumo said...

Dear Sabine, expectations are built-in in papers but one must still distinguish what is an expectation and what is a fact. The real point I don't understand is why you write papers on mutated Lorentz invariances and similar stuff if you're actually so skeptical - much like I am - that these effects exist.

When we say that the origin will be delayed because of astrophysical reasons, we are expressing a theoretical expectation. Such a theoretical expectation cannot be used as criticism against an experiment or its promotion as long as some sufficiently trustworthy theorist has a different expectation.

If my expectation is wrong, I want to know it. This is the first experiment where the sensitivity to these Planckian Lorentz-violation effects may be demonstrated. So let them work and let's give them the space to see what happens and how can one interpret it.

I don't share the attempts to sink them from the beginning. It seems very Woitian to me. There are not too many experiments around that would be much more relevant for new physics than this one.

The experimenters must be given freedom to measure and visibility for their results and the theorists must be given room to offer explanations and falsify wrong explanations.

Bee said...

Hi Lubos,

I don't want to sink any ship. I just want a balanced analysis and presentation of facts and potential explanations. Yes of course theorists should offer explanations, but then they should also take into account all other constraints that there are on a used setup, and give sufficient space to other explanations. Claiming that this signal is an indication for quantum gravity (or a breakdown of GR) is, at this stage, an exaggeration. I don't welcome this, because I am afraid that many people might jump on it, produce a lot of more or less irrelevant papers with more and more elaborated fits and models, which causes a lot of excitement and attention for the community but eventually amounts to nothing in the end - much like with the claimed GZK 'signatures'.

Reg my papers, I have explained repeatedly that in the models I consider there is no energy dependent speed of light, and thus no modified dispersion relation for free particles that would lead to an effect like the one discussed above.

Best,

B.

Who said...

**Reading papers and follow-up articles with exaggerated claims about testing quantum gravity annoys me considerably. It's like strawberry yogurt where the fineprint says 'no fruit'.**

there's fruit. It just might not be strawberries.
The IACT is a new kind of telescope. Imaging air cherenkov telescope. Theirs is a 17 meter optical dish.
They can detect the arrival of a single TeV photon and tell its source direction and energy by seeing the cherenkov from the air shower of electrons and positrons.
It is like a particle detector where you see trails.

You can say huckster to them, but it is important when a new kind of telescope comes into use. Their first bit of news is "this is the kind of thing we can do by seeing TeV photons."

You can say hype and criticize their style of presentation. And maybe their style stinks. But nobody seems to have noticed that they got a NEGATIVE result that could give tentative guidance to QG researchers.

Of course it was only one and a half flares and it should be replicated and so forth, but even with a sample of one good flare they got evidence that the speed does NOT INCREASE with energy.

It's a three-way thing. Now, when GLAST flies, I expect to hear that either there is no dispersion or if they see some dispersion I expect it will be the speed decreasing with energy. The real surprise would be if GLAST goes in the opposite direction.

So I claim that MAGIC told me something.
1. there is a new kind of telescope

2. it is sensitive enough to pick up dispersion (if it exists)

3. if there is any dispersion at all it is probably in the direction of decreased speed, not increased.

Hope those more expert will correct me on any point where I've blundered.

Uncle Al said...

Postulating vacuum dichroism based upon a received signal envelope from an unknown orginal shape is madness. This class of ambiguation is used to rationalize superluminal signal propagation. Chirping a waveform is not overturning the foundations of physics.

Plato said...

Chirping a waveform is not overturning the foundations of physics

If seen in "another light":)as the joining of "gravity and electromagnetism" maybe?

One does not discount the "geometrical implication" of the absence of?

QGP may be one extend, while the other end is? "Strange qualities" of the viscosity allow for an state of inertia for "relativistic flows?"

QUASAR9 said...

but the only thing you get from hyping non-results is more people wasting time with alleged 'effects' and made-up 'predictions'. Yes, I do think scientific publications should be conservative and cautious, especially if they claim to have evidence for new physics.

Bee, I think they (and Sciam) admit to wanting to stimulate debate

Reading papers and follow-up articles with exaggerated claims about testing quantum gravity annoys me considerably. It's like strawberry yogurt where the fineprint says 'no fruit'.

So what do you think of Warping of spacetime around neutron stars

Bee said...

Hi Who:

Yes. I am not saying the result is completely uninteresting, and there is of course a non-vanishing probability that it will be confirmed. However, for a flare with a delay of the order of minutes I'd think the challenge is not so much detecting the delay, but to cope with the lack of knowledge about the original signal - i.e. confirming that it is indeed a delay, and not a feature of the signal to begin with. Uncle Al formulated that in his usual eloquent style ;-) I think the situation with gamma ray bursts that GLAST could observe is better. Though the duration itself is much shorter (of the order microseconds if I recall correctly), our knowledge about the processes that cause (long) GRBs is much more reliable and so the expectations we have on the spectrum. Sure, I am looking forward to the data, it's an exciting time for theorists as well as for experimentalists.

Hi Quasar:

They don't want to cause discussion for the sake of the topic, they want links, comments, and attention for their own advantage, otherwise they would have invested a minimum of effort into their writing besides trying to get as many reactions as possible, and even complaining that people were too reserved with their reaction. Articles like this show imho that they don't care about the real science behind it. Sorry, this is the fast-food equivalent of science - it's worthwhile every now and then but I don't want it to take overhand. I am usually a big fan and a frequent reader of SciAm (and have been since I can think), and so I can only hope they won't be falling into the same trap as other magazines, and sacrifice well-founded and researched information for the sake of fast and catchy headlines.

Best,

B.

QUASAR9 said...

lol Bee, 'fast' & 'catchy' headlines
I like your analogy of a packet of photons being like a family pushing its way thru a crowd. The individual members of the family are 'quanta'

and higher energy phonoss more affected by QG than lower energy?

Who said...

*Yes. I am not saying the result is completely uninteresting, and there is of course a non-vanishing probability that it will be confirmed.*

Thanks for the friendly and reasonable reply, Bee. People's tempers seem frayed by this and they seem more worked up than usual.

What would you say the result is, and what would you estimate the probability it will be confirmed?

For me the important result is that the first-order coefficient is not positive. I think there is a high probability it will be confirmed.

Before the report I'd guess my subjective prior was 60-30-10.
60 that there is no dispersion, 30 that the coefficient is positive, and 10 that it is negative. Or maybe 75-20-5, in another mood.

If you had told me someone you know was working on a dispersion model then I would have guessed it was on some model with a positive correlation between energy and speed.

Now I see the odds more as 60-10-30
(or in a different mood 75-5-20.)
And if you were to tell me of someone working on Lorentz-bending then I would hope it was on some version with NEGATIVE first or second order coefficient.

So this report has not changed my sense of how likely there is zero correlation. I still suspect there is most likely zero dispersion.

But the report has had a big effect on how I see the main issue, which is whether, if it goes, it will go positive or negative. This question is critical for researchers because the logic is different, I think, in the two cases. It is not a simple matter of doing the same math in either case and just chosing the sign. Correct me if I am mistaken but I think it is fundamental.

**I think the situation with gamma ray bursts that GLAST could observe is better.**

Maybe, but IACT can see higher energy photons than GLAST. If there is any energy-dependence there will be more advance or delay in the photons that MAGIC sees than in the photons GLAST sees. I think it is even possible that the imaging air cherenkov telescope IACT could be the better instrument in some respects.

Of course there are GRB every day and flares are rare, so GLAST can study more events.

**Sure, I am looking forward to the data, it's an exciting time for theorists as well as for experimentalists.**

I too. I am also looking forward to MAGIC results from flares at different distances. This offers a simple way to exclude effects at the source.

It is an exciting time, certainly, and for onlookers as well.

I don't begrudge MAGIC the use of the phrase "Probing Quantum Gravity..." in their title. Because they did in fact probe QG. They showed they had sufficient sensitivity to detect positive correlation (energy with speed) and they report they did not find positive correlation. That changes the picture considerably, in my view.

For some reason it seems to be a minority view right now :-)

thanks

Plato said...

George Musser(Sciam) follows the conversations quite a lot. You can see him at cosmic variance and such, stimulated by such conversations.

Nurturing the public media reach with regards to science, a scientist would know to correct?

I mean we can all learn by the responses given, and of course by our reading from the learned individuals, who present us information on this or any other blog.

Asking the question on survey takes some thinking, as I would tend toward uncovering the connections, whilst of course, may have wrong basis of thinking.

That is not my motivation and does not fuel my search. There seems to be a deeper mystery?:)

Intuitively it seems to be a thread through it all, and it falls into place eventually:)

Bee said...

Hi Who:

The result is a 2.5 sigma signal, I will think about it should it be confirmed. I don't like to assign probabilities, as they hardly reflect my opinion - I hope the post did a better job there, but since you asked. Probability for the signal to be confirmed with other signals (other sources): 5%. Probability that in case it was confirmed it is a quantum gravitational effect: 10 %. I am being optimistic today. Reg. the sign, yes it makes a difference esp. for other signatures that should have followed from the same scenario (anybody recalls the threshold modifications?) but as far as I know neither sign is a problem to come up with theoretically. Basically one can use the relation c(E) as an input and cook up a model that produces it. Yes, people seem to favour certain versions of DSR, but these are not the only possible ones, just the ones most used.

Best,

B.

Bee said...

PS: You say "They showed they had sufficient sensitivity to detect positive correlation (energy with speed) and they report they did not find positive correlation." Again, let me point out the problem here is not to have sufficient sensitivity to measure a delay but to find out whether this delay originated during the propagation from the source to us.

Who said...

you have already been very kind and patient in your response, thanks!

I shall understand if you don't have time for further dialog, but I shall pursue this using the Martin Reuter case

**as far as I know neither sign is a problem to come up with theoretically.**

Reuter model has G_k and Lambda_k that run with inverse scale k, leading to a metric g_k

the distance to a particular active galaxy differs according to what g_k. As a little wheel rolling along a bumpy road goes farther than a bigger wheel in traveling from town A to town B, so a photon with small scale (high k) might have a different distance to travel. In the case of the photon it might be a greater distance or it might be less, but Reuter would already have decided on one or the other. His theory is, in a sense, complete.

this is mere speculation, but I think that if Reuter wanted to explain energydependent dispersion he could not so easily flip the sign.

As for DSR, I guess I am just plain confused. You know more about that.

As for sensitivity, you quote and reply as follows:
**PS: You say "They showed they had sufficient sensitivity to detect positive correlation (energy with speed) and they report they did not find positive correlation." Again, let me point out the problem here is not to have sufficient sensitivity to measure a delay but to find out whether this delay originated during the propagation from the source to us.**

I am trying to see the MAGIC result as disfavoring a positive correlation (energy and speed). MAGIC is sensitive enough to show a negative correlation. If we want to suppose there is a positive one that is obscured by effects at the source then we have to suppose the source effect is strong enough to both cancel the positive and produce the appearance of a negative correlation. That's asking a lot.

thanks again!
Marcus aka Who

Eric Gisse said...

Thanks for the analysis, Bee!

Kris Krogh said...

Great post Bee!

Can anyone bring me up to speed on the ordinary optical properties of the interstellar and intergalactic media at these wavelengths? Are they well-established?

Anonymous said...

If the observed delay were due to a effect such as scattering as the gammmas propagate through interstellar space, I think this would only effect the intensity of the gammas rays and not affect the time of arrival. Furthermore, one would expect the shorter wavelength photons to be affected less since they have a small scattering cross-section.

Bee said...

Hi Kris,

Anonymous above is right that interstellar medium wouldn't affect the travel time, but the intensity (Though the frequency dependence depends on the kind of scattering. Lubos had something to say about that.) Also, the medium would have to be very thin, otherwise it would have a lensing effect. However, I don't understand their fitting procedure well enough to know whether an unknown intensity distortion would affect something like "maximiz[ing] the total energy in the most active part of the flare". Best,

B.

Anonymous said...

This is what 30 years of String has done to physics. Wild speculations sold as facts form the foundation of career advancement. The authors would be fools not to play the game, so I find Bee's astonishment astonishing.

Kris Krogh said...

Anonymous and Bee,

Thanks!

Here's a description of the parsec-scale jet producing these Tev gamma-rays:

http://arxiv.org/abs/astro-ph/0210482

Maybe it's possible to imagine some time and wavelength-dependent effects originating there.

alessandro said...

Let us start from the experimental facts. The MAGIC collaboration has detected a short flare (about 2 minutes) from the galaxy Mkn 501; this flare has been seen from energies of about 100 GeV to energies 100 times higher. The data show a time delay of about 4 minutes between the lowest and the highest energy gamma rays; this delay is significant beyond the 95% C.L..

Now the interpretation. We cannot exclude the possibility that the delay we find is due to some energy-dependent effect at the source. However, if the delay is due to propagation effects, under some hypotheses clearly stated in the paper, the effect we see can be interpreted as an evidence for Lorentz violation at a scale close to the Planck scale. As said in the paper, we must now observe more flares, preferably of different AGNs at varying redshifts.

I believe that physics, especially experimental physics, is simple. And I must answer to a criticism that offended me a bit, when it is said in the blog that “The only thing that I find morally problematic about the recent paper is that those 100+ experimenters were forced or convinced to believe and promote a particular theoretical explanation that is prominently featured in their paper. I don't believe that these 100+ experimenters universally started to believe that this explanation is correct or likely, especially because the same team of 100+ people has published a previous paper with a completely different explanation half a year ago.” Well, starting from the end: (1) The previous paper by MAGIC had basically no interpretation. (2) The rules negotiated between the MAGIC Collaboration and the funding agencies impose that the experimenters sign a paper whenever new data are used in it – and in the new MAGIC paper new data are used, with respect to the old one. Thus, for a reason that the author of the blog did not probably know, the comment “That's why I think that the theorists should have written their interpretation of the MAGIC data separately from the experimenters. There has been no real collaboration here: there was just a pragmatic coalition in which the theorists were used to make the experimental results sexier than they are” is not appropriate. (3) The MAGIC paper just says that the subject is important, it never says that we detected a violation of the invariance of the speed of light. We believe that this explanation is not unlikely, but as experimenters we are very careful before making extraordinary claims. We’ll need to measure more sources. Nevertheless, we hope we pointed out the importance of the subject and the sensitivity of the Cherenkov instruments, that is now close to the Planck mass scale.

Thanks for the attention and for hospitality in the blog,

Alessandro De Angelis, professor.
Physics Coordinator of the MAGIC Collaboration

Bee said...

Hi Alessandro:

Thanks for your comment. However, your remarks 'it is said in the blog' and 'the author of the blog did not probably know' does not refer to my blog and this post. Best,

B.

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