Quantum Gravity

wolram

Quantum Gravity: Evidence against

Lets take a moment to consider the evidence used by others to try and disprove this theory

Such as ..


http://cosmos.as.arizona.edu/~thomps...gazzoni03a.pdf

http://www.spacedaily.com/news/cosmology-03i.html

http://arxiv.org./abs/astro-ph/0303043

http://arxiv.org/list/gr-qc/0311

http://arxiv.org/PS_cache/gr-qc/pdf/0311/0311021.pdf

http://www.cerncourier.com/main/article/42/7/18.

http://arxiv.org/PS_cache/astro-ph/pdf/0301/0301145.pdf

marcus

The very first link on your list seems like a really solid and clearly-written paper. It is just 4 pages, so I printed it out.
Anyone who wants please start by focusing on that or suggest a better
one to start with. Let's go one at a time and see what they say.
My guess is that they are trying at this point to constrain rather than disprove, but its worth taking a closer look at some of these observational efforts.

marcus

Another reputable IMHO paper that someone interested in this
testing of quantum gravity parameters might want to print out is
dated August 2003 (a revision of an earlier entry so numbered
http://arxiv.org/astro-ph/0212190)

This was published in Nature journal and is by Jacobson, Liberati, and Mattingly and is called

"A strong astrophysical constraint on the violation of special relativity by quantum gravity"

The initial title, before revision, was
"Lorentz violation and Crab synchrotron emission: a new constraint for beyond the Planck scale."

that might be one to discuss as well

ranyart

One of the follow up that is quoted, this is a paper that must be considered:http://citebase.eprints.org/cgi-bin/...2Dph%2F9712103

selfAdjoint

Let's get back to that first one. So somebody suggested that quantum gravity effects might be seen in long distance light. And the astronomers looked and couldn't find those effects within the errors of their instruments. Right?

And this is supposed to be an argument against quantum gravity? It's another argument from a null result like the cranks' claim that the standard model was weakened by CERN's failure to find the Higgs.

ranyart

SelfAdjoint, can you be more specific as to the first paper?..are you refering to the first of the links in Wolrams first posting?..or to the Amelino-Camelia, G Ellis..et-al..Nature paper?

wolram

And this is supposed to be an argument against quantum gravity? It's another argument from a null result like the cranks' claim that the standard model was weakened by CERN's failure to find the Higgs.
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no sir, these are hard working mainstream scientists,
the null results you speak of are narrowing the
parameters for quantum gravity, my plebarian search
for what gravity is, has revealed more and more what
gravity is not, or can not be shown to be true
observationaly.

wolram

i would welcome clarification or rebutal of these
papers, my math is very poor but i have a keen
instinct, i came to PF to learn so any insights
you are willing to share will be helpfull

marcus

selfAdjoint, Ranyart already asked could you please go thru this a bit slower, tutorial-style.
I personally could use help understanding these observational tests of QG, so I am echoing what he said and asking for more discussion too.

You, and also Nereid, Ambitw etc could help us all understand how these tests work just by talking about it some more
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As I see it this is not the time to ask for guesses about the ultimate outcome---will astrophysical testing ultimately help refine LQG or will it invalidate LQG or will it confirm some other theory.
I dont want to engage in premature speculation! All I want to do now is just understand this testing in a little better concrete detail. A few nuts and bolts. Can we discuss this in terms that make sense to everybody here (you me Ranyart Wolram, the whole lot of us) or is this an inherently obscure highly technical subject.
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I have a very preliminary provisonal take on it. Maybe I will add a post following this up.
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BTW you said "lets get back to the first paper" meaning the first on Wolram's list
Ragazzoni, Turatio, Gaessler (RTG)
"The lack of observational evidence for the quantum structure of spacetime at planck scales."

That seems like a good idea. It is a strong clearly written paper (or looks so to me)

RTG are explicit about the limitations of their approach. On their first page they refer to the paper I gave a link to namely Jacobson, Liberati, Mattingly (JLM) and make a distinction between that paper and theirs.
RTG say they assume that planck-scale effects RANDOMLY alter the phase of light independently of the wavelength while JLM assume that the modification of the light depends on wavelength. Because of the different assumptions it might also be a good idea to look at the Jacobsen Liberati and Mattingly paper.
It is not easy to know which, if either, paper is putting observational limits on effects that a known version of quantum gravity might actually predict.

marcus

These are two alternative links to the Ragazzoni, Turatto, Gaessler article (RTG).

I mentioned already the Jacobson, Liberati, Mattingly (JLM) article
http://arxiv.org/astro-ph/0212190

In case anyone is interested, just today a new planck-scale phenomenology article was posted by Amelino-Camelia

http://arxiv.org./abs/astro-ph/0312014

It is 9 pages, dated 8 December 2003, and called
"Planck-scale structure of spacetime and some implications for astrophysics and cosmology"

It is the text of a talk he gave at some conference this September.
Whatever else you can say, the guy is prolific. This November's issue of "Physics World" magazine was devoted to Quantum Gravity and had three invited articles:
Amelino-Camelia on QG Phenomenology
Leonard Susskind on string
Carlo Rovelli on loop

Gives some idea of where A-C stands in relation to the field of planck-scale phenomenology. So he just posted a new paper and I'm going to take a look (but maybe it wont say anything new, just what he has already said several times in other papers)

marcus

well, some A-C papers I have found more helpful than this one. I cant recommend this one. But it is recent and here is an exerpt. He seems to give a lot of attention to TIME OF ARRIVAL dispersion that might be observed (or not observed) in short bursts.

Because of a modification of the energy-momentum relation (Smolin gave a similar discussion) there could be a dispersion effect---very high energy photons traveling at a slightly different speed.

So in short bursts of gamma rays one could look for the higher energy photons arriving slightly before or after the rest.

Here is an exerpt from A-C's article. Notation here is a rough substitute for his, because the article's symbol font didnt copy. His equation (1) is a modification of the basic energy-momentum relation or roughly-speaking of the basic ee-equals-emceesquare---same type of modification Smolin gave in his paper we discussed earlier.



------from A-C article-------
"...
One of the most studied scenarios is based on a modified dispersion relation of the type (1) [he gives the same kind of equation Smolin gave], using the associated small wavelength dependence of the speed of photons based on the relation v = d omega/dk, k = 1/wavelength.

The wavelength dependence of the speed of photons that is induced by
(1) is of order L_p/E, and is therefore completely negligible in nearly all physical contexts. It is however quite significant [1, 3] for the analysis of short-duration gamma-ray bursts that reach us
from cosmological distances.

For a gamma-ray burst a typical estimate of the time travelled before
reaching our Earth detectors is T = approx. 10^17 s. Microbursts within a burst can have very short duration, as short as 10^-4 s. Some of the photons in these bursts have energies in the 100MeV range and higher (and correspondingly small wavelengths). For two photons with energy difference of order 100MeV an Lp/E speed difference over a time of travel of 10^17 s leads to a relative time-of-arrival delay of order dt = approx 10^-3 s.

Such a quantum-gravity-induced time-of-arrival delay could be revealed [1, 3] upon comparison of the structure of the gamma-ray-burst signal in different energy channels, and these types of studies are planned for the next generation of gamma-ray telescopes, such as GLAST [51].

With advanced planned neutrino observatories, such as ANTARES [52], NEMO [53] and EUSO [54], it should be possible to observe neutrinos with energies between 10^14 and 10^19 eV , and according to current models [55] gamma-ray bursters should also emit a substantial amount of high-energy neutrinos. This might provide [56, 57] another opportunity for time-of-arrival analyses.
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Maybe time of arrival of light (and other) in short bursts is something to think about. Testing for some definite, not random as in RTG, dependence on energy.

marcus

this is easier than I thought it would be at first

the key thing is to put the photon energies into natural units!

a photon in one of these gammaray bursts could have an energy
of 100MeV, says A-C
but that is roughly the same as E-20 planck units!

then as a first approximation there is a fractional difference in speed by the fraction E-20

And the distance he is talking about is E17 lightseconds.

If two photons start out together to travel E17 lightseconds and one of them is slower by a fraction E-20, then how much later does it arrive?

Just multiply E17 by E-20 and get E-3
It arrives E-3 second later, one millisecond.

Might you see such a delay in the event of a gammaray burst? He says YES you might because sometimes the bursts have little short microbursts in them which are so brief that a delay of part of the spectrum by as little as one millisecond might be detectable.

wolram

Might you see such a delay in the event of a gammaray burst? He says YES you might because sometimes the bursts have little short microbursts in them which are so brief that a delay of part of the spectrum by as little as one millisecond might be detectable.
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seems you have been working hard MARCUS,
it also seems that not many are interested
in this subject, but whatever im going to
give it my full attention, thanks.





and this gives the blurring effects that are being searched
for in optical observations, so if one can compare like with
like at at different distances a pattern will emerge either
they are the same,"no quantum effect", or they will differ,
quantum effect",i guess a very large sample of observational
data is stored somewhere so it would be a matter of sifting
this information to find an answer.

marcus

it really is a lot simpler than I thought at first,
at least the gammaray delay part of it

if something travels a tiny bit slower, like a thousandth slower
or slower by a fractional difference E-3

and if it comes from the sun, which is 500 light seconds
then how much is its arrival delayed?

half a second. You just multiply 500 seconds by a thousandth, or E-3.
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In the same way if it travels E17 lightseconds and is slower by a fraction E-20 then it arrives one millisecond in retard. Again just multiply E17 seconds by the small fraction E-20 and get E-3

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Amelino-camelia is saying look the coefficients in physical law mostly turn out to be "order one" like 1/2, or 1/pi, or 8pi.
Now we dont know the exact coefficients in the LQG energy-momentum relations--we dont know what alpha and beta go into the formula.
But lets suppose the coefficients are "order one" and just take them to be one and lets see what size effects we get.

So he looks at the energy-momentum relation (that modified ee-equals-emceesquare) and that determines a "dispersion" formula that says if something has higher energy it goes slower by a tiny fraction.

That fraction is (up to an "order one" coefficient) simply the size of the photon's energy expressed in natural units!

So if the photons energy is E-20 planck energy units (which is very powerful gamma rays) then it goes slower by a fraction E-20 of the normal speed.

[edit: I misunderstoon something GA-C and got the effect backwards. I now believe he means the high energy gamma burst travels faster and the lower energy visible light is delayed and arrives later, still confused about this

And over billions of years (like E17 seconds) that could build up and amount to something detectable!

And he says some project named "GLAST" might be good enough to detect such an effect in long-distance gammaray bursts.

These are strange events which some people think are caused by
the collapse of neutron stars to form black holes.

Gammaray photons are very powerful. E-20 planck is an extremely energetic photon. Sunlight is more like E-28 and E-27 planck.
the thermal Xrays at the core of the sun, where thermonuclear energy is released are IIRC something like E-25 planck units. So in a gammaray burst light is reaching the orbital observatory which is much more hot than light at center of sun. I guess it does not get down thru atmosphere to us. But the visible light that accompanys these gammaray bursts does reach us at earth surface.
However that light, being like E-27, would not have a detectable delay. Very strange business. Seems outrageous for the "heaviness" of a photon to slow it down even a tiny bit.

edit: I may have misunderstood and its the more energetic photons that go faster.

and then Amelino-Camelia is also talking about delay in very energetic neutrinos. so we have some interesting things to look forward to when those detectors are ready as well

ranyart

This paper by:Loukas Vlahos, George Voyatzis and Demetrios Papadopoulos

has some interesting idea's a Quote:In this article, we re-investigate the non-linear interaction of an electron with a GW inside a magnetic field, using the Hamiltonian formalism. Our study is applicable at the neighborhood of the central engine (collapsing massive magnetic star (Fryer et al. 2002; Dimmelmeir et al. 2002;
Baumgarte and Shapiro 2003)) or during the final stages of the merging of neutron star binaries (Ruffert and Janka 1998; Shibata and Uryu 2002). We find that a strong but low frequency (10KHz) GW can resonate with ambient electrons only in the neighborhood of magnetic neutral sheets and accelerates them to very high energies in milliseconds. Relativistic electrons travel along the magnetic field, escaping from the neutral sheet to the super strong magnetic field, and emitting synchrotron radiation. We propose that the passage of a GW through numerous localized neutral sheets will create spiky sources which collectively produce the highly variable in time.
The collective emission of thousands of short lived (less than a second) synchrotron pulses, created during the passage of the GW through a relatively large volume. The relativistic electrons
loosing most of their perpendicular to the magnetic field energy to synchrotron radiation retain the parallel energy and heat the ambient plasma emitting other types of longer lived bursts, e.g.
X-rays and optical flushes.
We have also shown that if the acceleration length follows a simple scaling law (N(§¤acc) ¡_ §¤−b
acc) and since, as shown, < ¥ã >¡_ td acc, the energy distribution also follows a power law scaling (N(< ¥ã >) ¡_< ¥ã >(−b+1−d)/d), which for reasonable values of b ¡_ 3/2 and d ¡_ 1/2 agrees remarkably
well with the energy distributions inferred from the observations.

full pape here:http://uk.arxiv.org/PS_cache/astro-p...12/0312151.pdf

marcus

Ranyart I am creeping along in low gear here and havent
gotten to thinking about Gravity Waves (GW as you abbreviate)
as yet.

I just printed out another Amelino-Camelia article that I think is better-written than that other one and covers some of the same things, particularly the gammaray burst test of quantum gravity theories.

It is called "Quantum Gravity Phenomenology" and I gather is essentially his invited article for the November issue of "Physics
World"
http://arxiv.org/physics/0311037

I'm still thinking about the tests people are discussing that just involve high energy light and that dispersion relation where the
planck-scale effect depends on the energy of the light

it is a miniscule effect, I'd be tempted to say zero for all practical purposes, but the fact that it depends in a definite way on the energy of the photon makes it potentially detectable.

I think that may keep me busy thinking about it for a while, without
even getting to the next state of testing the theories, which I gather is high-energy neutrinos from the same (gammaray burst) events.
Forgive my absorption in this one thing.
(in principle you are right that GW are another venue for testing QG)

marcus

BTW can you imagine one of these events that
produces a gammaray burst?

people call them "hypernovas"
incredible amounts of energy, orders of magnitude
more than supernovas

maybe you have found some articles that discuss possible
mechanisms

the observation of these things began fairly recently
with orbital observatories that would notice the
gammaray flash and then radio coordinates to an automated
groundbased telescope that homes in on the same spot to catch
the visible light coming along with the gammaray

it seems appropriate that these things should be among the first
types of event to afford checks for quantum gravity
(since they are so extreme and it is an extreme theory)