Friday, April 03, 2009

Quantum Gravity in GRB 080916C ?

The Fermi Collaboration has an interesting article in recent issue of Science

    Fermi Observations of High-Energy Gamma-Ray Emission from GRB 080916C
    Science 27 March 2009, Vol. 323. no. 5922, pp. 1688 - 1693

    Abstract: Gamma-ray bursts (GRBs) are highly energetic explosions signaling the death of massive stars in distant galaxies. The Gamma-ray Burst Monitor and Large Area Telescope onboard the Fermi Observatory together record GRBs over a broad energy range spanning about 7 decades of gammaray energy. In September 2008, Fermi observed the exceptionally luminous GRB 080916C, with the largest apparent energy release yet measured. The high-energy gamma rays are observed to start later and persist longer than the lower energy photons. A simple spectral form fits the entire GRB spectrum, providing strong constraints on emission models. The known distance of the burst enables placing lower limits on the bulk Lorentz factor of the outflow and on the quantum gravity mass.

The article presents details about the gamma ray burst 080916C observed on Sep 16 2008 with the recently launched Fermi satellite (formerly GLAST). Follow-up x-ray and optical observations measured a redshift of z approx 4.35. They have estimated an enormous energy release of ~4.9 times the solar mass, suggesting that the outflow was directed and occurred only into a narrow jet. The burst covered many orders of magnitude in energy, with the highest detected photon being at approx 13 GeV.

The interesting thing is that the higher energetic photons seem to be arriving later, with the MeV band peaking some seconds after the keV band, and the 13 GeV photon arriving approx 16.5 seconds after the onset of the burst. The article discusses various astrophysical reasons for this delay, such as spatially distinct regions of origin, a delay of high energetic photons through not well understood opacity properties of the source, or additional time needed to accelerate protons or ions sufficiently.

An alternative explanation they investigate is that the delay is due to an energy dependent speed of light caused by Planck-scale corrections to the dispersion relation, which could be a signature for quantum gravitational effects. With the assumption that the delay is entirely explained by astrophysical effects, they obtain a lower limit on the scale of quantum gravity, that is MQG > 1.3 x 1018GeV. Note that this is about one order of magnitude smaller than the Planck scale.

I would have found it more useful to think about it the other way round: if the delay is not caused by astrophysical effects but entirely by quantum gravitational effects, the scale must be below this limit to reproduce the observed delay. Since the scale is one order of magnitude below what one would have expected and thus the effect quite strong, I would then be concerned should this effect be due to a modified dispersion of photons it should already have been observed previously, for example in MAGIC's observation of a gamma ray flare in 2007.

In addition to this one has to keep in mind that one photon doesn't make a particularly great statistic. This gamma ray burst seems to be quite unusual in its luminosity anyway and an astrophysical origin of the delay is a more conservative explanation. To make a case for quantum gravity, much more data would be needed, in particular one would need to establish a dependence of the delay on the distance. If it is a quantum gravitational effect, the delay should increase with increasing distance. If the delay is of astrophysical origin, the delay should be independent of the distance.

Giovanni Amelino-Camelia gave a seminar at PI last week (I could not attend since I was in Stockholm as you know), in the first half he talks about the above discussed observation of GRB 080916C, see PIRSA: 09030039. (He then talks about the "mystery noise" in the GEO600 gravitational wave interferometer, previously discussed here.)

31 comments:

Anonymous said...

http://pirsa.org/09030039/ : Link broken

Bee said...

It's not the link, it's the server. Try again later.

Uncle al said...

Perhaps emission energy chirps with process time evolution. Is a GRB an explosively driven helical flux-compression generator?

Triggering a fat Z-pinch (not redshift z) requires time and space. 16.5 second frequency dispersion as mass shock propagation not lightspeed delay retains a compact source.

http://en.wikipedia.org/wiki/Explosively_pumped_flux_compression_generator

Giotis said...

This rings a bell. Around 10^18 is the string scale where the string dimension becomes important.

Bee said...

Just that there is no reason for the string scale to be relevant here and that doesn't change the fact that the scale should be the same for all observations.

Plato said...

Hi Bee,

I follow this information with utmost attention.

While you easily dismiss Giotis's statement, it's one of those cases where one might put on the rose coloured glasses and look at the universe "as if," the photon represents all the gravitational inclinations. Sort of like gearing the mind to see Lagrangian associations in this same way and represented.

If you are able to identify the pathways that satellites travel "with least resistance," then you might consider that such pathways may also present the opportunity for energy inclination that the photon as it is travelling from it's destination, over a expanse of space?

We want the information "from it's source" to tell us something about the event. Calorimetric calculation present this for examination but have we discerned the event in this calculation and the travel "this information" passed through? Higher energy photons would have followed the path of least resistance?

Best,

Bee said...

I haven't 'dismissed' Giotis' comment I have merely pointed out a) there is no derivation of such a modification from string theory that would motivate why the appearing scale should have anything to do with the string scale. In fact the LQG folks have advertised modified dispersion relations as a signature for LQG exactly because of this b) waving hands and mumbling string theory doesn't make the evidence stronger. Best,

B.

Plato said...

Bee,

Strings and LQG are saying the same thing? It's procedural method that Lee's adopted?

It was Lee's position which drew my interest toward Fermi(formerly GLast)and subsequent information on the calorimeters. Dammaso Dorigo gives a nice explanation on the LHC ones.

Best,:)

Bee said...

I have no clue what method Lee has supposedly adopted, but to my best knowledge there is so far no derivation of DSR from string theory. Not that there is one from LQG either, at least not in 3+1 dim. So I guess you could indeed say they both say the same thing, namely no thing.

Anonymous said...

Dear Bee,
You said, 'there is no derivation of such a modification from string theory that would motivate why the appearing scale should have anything to do with the string scale'. This is simply not true as such models were proposed over ten years ago. Have a look at

http://arxiv.org/abs/0901.4052

Bee said...

Proposing a 'stringy model' isn't exactly the same as a derivation, is it?

Anonymous said...

If you will bother to read to literature, then perhaps you would have a better idea of how these string theory models are constructed.

Your 'quantum gravity' models aren't exactly a derivation either, are they? Especially given that you have no real theory since your prejudices forbids you from considering string theory.

Bee said...

You are barking up the wrong tree. I'm neither a supporter of LQG nor string theory or whatever approach to QG you like, I totally don't care either way. I only care about the phenomenology, and indeed - as I said above - none of these models is a derivation. I have a priori no problem with that, a model is a model and if it makes predictions one can learn something from it. I only have a problem if people pretend otherwise, people being LQG people or string theorists likewise, as you claimed above it is 'not true' that there is no derivation. Besides this, I read several of Mavromatos' papers at some point, he also was here for a conference I organized a while ago on the phenomenology of qg (you can find it somewhere in the archives I think). If you look at my publ list you'll find that I do have considered models motivated by string theory as well.

Phil Warnell said...

Hi Bee,

Interesting piece although as you know the first article that you point to I can’t read since it’s published in a journal and not anywhere in arxiv that I can find. The PIRSA lecture I was able to access. I’m not certain what to make of it other then to agree that much more data is needed. Just as a niave question is to ask when the energy per quanta of a photon rises is there no point at which it would come under the influence of the Higg’s field or does the slowing of high energy photons take it outside being explained by the standard model entirely say like perhaps with Bohm's diffusion effect?

Oh yes, welcome back and I hope the jet lags not too severe.

Best,

Phil

Plato said...

Bee:So I guess you could indeed say they both say the same thing, namely no thing.

High Energy Photons are indeed interesting.

The calorimeter design for GLAST produces flashes of light that are used to determine how much energy is in each gamma-ray. A calorimeter ("calorie-meter") is a device that measures the energy (heat: calor) of a particle when it is totally absorbed. CsI(Tl) bars, arranged in a segmented manner, give both longitudinal and transverse information about the energy deposition pattern. Once a gamma ray penetrates through the anticoincidence shield, the silicon-strip tracker and lead converter planes, it then passes into the cesium-iodide calorimeters. This causes a scintillation reaction in the cesium-iodide, and the resultant light flash is photoelectrically converted to a voltage. This voltage is then digitized, recorded and relayed to earth by the spacecraft's onboard computer and telemetry antenna. Cesium-iodide blocks are arranged in two perpendicular directions, to provide additional positional information about the shower.

Oh yes, putting the models aside sure, the procedure of "cause and effect" is an interesting method of measure when we consider the extension "through space and on earth" with which we have "designed methods" to push perspective ever deeper into understanding of the cosmological structures of the standard model in display.

It is also nice to see the effect of "faster then light" as a phenomena, traversing through a medium, that ultimately produces the Cerenkov effect. SNO or Ice Cube.

Iron wedges of the CMS forward calorimeter-Source from Quantum Diaries Survivor.

The future

If new detectors will ever be built to explore a yet higher energy regime than the one about to be probed by LHC, calorimeters will be as necessary as they are today. The following characteristics will be desirable in a design of new generation:

* self-triggering (the ability of independent portions of the system to identify and measure a signal, interpreting it and sending an accept signal to the data aquisition system)
* stand-alone tracking (the ability of the calorimeter system to independently determine the direction of crossing particles)
* an integrated time-of-flight measurement (the capability to separate different particle signals based on the delay between their arrival time and the interaction time)
* high resolution and granularity (attainable with silicon technology)

The needs of these fancy features, however, rests on the specific hunt that we will decide to embark on. Which, in turn, critically depends on the discoveries that the Large Hadron Collider will produce!
Calorimeters for High-Energy Physics-part 2, by Tommaso Dorigo (Bold added by me for emphasis)

When the photon escapes the gravity field, it will have a different frequency?

A "bulk perspective" changes things?

Yes, it is as Phil saids,"I’m not certain what to make of it other then to agree that much more data is needed."

Best,

Plato said...

A black hole is an object so massive that even light cannot escape from it. This requires the idea of a gravitational mass for a photon, which then allows the calculation of an escape energy for an object of that mass. When the escape energy is equal to the photon energy, the implication is that the object is a "black hole."

While these rose coloured glasses are put on, are we not defining for ourselves "differences," as we see this gravity in relation too, a location in the cosmos?

In theories contemplating a low quantum gravity scale, black holes could in principle be created in high energy collisions, but if a chance of detecting their creation exists, it is not by gravitational effects, which remain billions of billions of billions of billions (and then some) of times smaller than those caused by strong interactions.Dorigo

A "fifth dimensional perspective" is an integration of "gravity and electromagnetic energy," beyond spacetime.

You see? How can you fault me with this assumption?:)

Bee said...

Hi Phil,

The photon doesn't couple to the Higgs. This has nothing to do with its energy. Yes, I know the Science paper isn't open access, but there's nothing I can do about it. Best,

B.

Phil Warnell said...

Hi Bee,


Like I said just a silly question although its always interesting this threshold between matter and energy, with the coupling to the Higg’s being under the standard model what decides that in some respect. Also, I hope you didn’t take my comment about the journal article as a compliant that is at least not to you. With saying that an idea comes to mind, like perhaps entities such as Perimeter or University research centers who pay for all the journal access for their people could sell annual public memberships by where these members could access online what the students and facility can. Now we know that the journals would of course want some share of it, which if it remained reasonable would bring more money to the research institutes, journals and facilitate public outreach all at the same time. If this fee was just a few hundred dollars or less, I for one would be more then happy to pay it as I would suspect others would.


Best,

Phil

Bee said...

I am afraid you are seriously underestimating the cost for journal access. I think is not legally possible to extend institutional subscription to public memberships.

Phil Warnell said...

Hi Bee,


Perhaps, yet there must be some calculation, like what those cost per head are when you calculate that with the total facility and student body considered. I would be surprised if that amount to much greater then a few hundred a year. If it is then the journals are certainly taking from the education and research budgets far more then their entitled, as by way of having earned and/or being worth in the scheme of things.


Best,

Phil

Bee said...

Well, I can't give you exact numbers but I am pretty certain the cost is substantially higher than a few hundred dollar per year. You have to take into account that you're not done with one journal. In the field of physics alone there are hundreds of subscription journals. My guess would be a few hundred-thousand dollars per year to cover what's necessary (but please keep in mind, it's only a guess).

Phil Warnell said...

Hi Bee,


Yes, yet for a university if one takes $200,000.00 and divides by say 4000 students that would be $50.00 dollars a head. As you, I claim no great knowledge, yet perhaps as Stefan being evolved in the industry he would have some idea. Also, I think there are publisher packages (more then one journal) that such institutions purchase.


Best,

Phil

Bee said...

Hi Phil,

An institute is not a university, and the average university has considerably more departments than the one dept. of physics for which my above guess was. But yes, the easiest and probably least expensive way for you to get well covered journal access is to become affiliated in one way or the other with a public university. Isn't a public education system a great thing? Now you can complain that tuition fees are too high, but that's an entirely different discussion. Either way, you can of course just purchase single articles if you are interested. The above one is priced at $15 (I don't know if that includes the supplementary material). Best,

B.

Phil Warnell said...

Hi Bee,

Yes I’m aware of all those options yet what my point was is to widen general access, while at the same time increasing overall funding for the ivory towers and the journals. I’ve exchanged my views on this with Stefan in the past, coupling this with the tenuous position journals are in these days; with things like arxiv along with other internet sources.

I think that along with any print media which are experiencing difficultly with the shift this could be one of those ways they can transition. The way I look at it a large part of public outreach is in effect inclusion and finding ways to facilitate and nurture more of this. I commend you and some others like yourself with the creation and maintenance of blogs such as this, in giving your own time and effort in this regard. It’s just I wish others who being in even a better position stop just giving it mostly lip service and begin in helping shoulder the load.

Best,

Phil

Bee said...

Hi Phil,

You are preaching to the choir. I, as I believe most bloggers, am all in favor of open access. I am aware it raises significant challenges, but these can be addressed with a little effort. These challenges are not so different from those newspapers are facing, and I have said many times eg here the solution is to recognize that what is a public good should be financed like a public good. That means among other things, journals should apply for grants to finance their expenses like scientists do, but then their services should be free for everybody. Even more radically, I believe information should generally be free (where not in conflict with privacy rights). I vaguely recall that 15 years ago or so I tried to convince the youth organization of the Social Democrats where I was active then that we should have a right for information. It didn't pass. I am still convinced they will come back to it, maybe in another decade or so. Best,

B.

Phil Warnell said...

Hi Bee,

In being reminded of the huge energy density that’s contained within some of the photons detected emanating from such bursts, it’s had me to wonder about the theoretical proposals of the threshold for energy densities beyond which any entity would, if only for an instant form to be a black hole. That is to say what are the speed limitations of a primordial black hole as opposed to photons?

It would appear that a black hole would be required to respond to the Higg’s field presence which would mandate its speed to be less then the limit of c . Also, like as in the expectations of such black holes being produced with the LHC what are the upper energy limits of the photons that would emanate from such black holes upon their disintegration resultant of the Hawkings’ effect.

Like Hawking speculated primordial black holes might have been created at the beginnings of the universe, then couldn’t perhaps even smaller ones be created in the death throws of a massive star and this account for the observation of such delays? The key here would be that such delays would not be distance dependent, since such black holes would dissolve quite rapidly. Probably a lot of nothing, yet I thought I throw one more log on the fire or is this in effect what the mechanism for the quantum gravity explanation relates to?



Best,

Phil

stefan said...

Hi Phil,

searching for the title of the paper at google scholar sometimes leads to PDF copies of papers not on the arxive, but hosted at the websites of the scientists involved instead. It works in this case, here is the PDF of the science express version of the paper.

The supplementary material at Science is generally not restricted in access - in this case, it even gives much more details about the Planck Scale estimate than the short paragraph at the end of the actual paper.

Cheers, Stefan

Phil Warnell said...

Hi Stefan,

Thank you so much, as it goes to show that along with Bee access to information and with it the benefit it holds, along with the promise is a truly a worthy goal as considered by many; even more so in our rapidly changing times. You if anyone must find this to be even more of a relevant concern since your bread and butter are tied up in such matters in regards to change and strategies for the for transition.

Thanks again,

Phil

Phil Warnell said...

Hi Stefan,

Yes now that I’ve had a chance to look at it all this is certainly great stuff, in particular that time lapsed Quick Time movie of the burst’s reception by the detectors . I can recall the time not all that long ago when such bursts presented as being a total mystery and now it is well on its way to being unraveled to be understood. When I see something like this I’m reminded how it is only in what is considered the violent end of a star that elements that are common here on earth yet rare in the overall scheme of things are and can be created which in and of themselves also play a part in what we are and able to do.

Best,

Phil

Giotis said...

Presumably these guys don't know what free access to knowledge and information means. Socrates was one of the few philosophers that didn't want money for his teachings. He believed that access to knowledge is every man's right. 2500 years later in our modern times people still have to pay to execute this right.

Plato said...

Yes Goitis,

Socrates was a very smart man.