Evidence Against Quantum Gravity

In summary: A strong astrophysical constraint on the violation of special relativity by quantum gravity"So they are not claiming that quantum gravity has already been proven, but that it is a strong astrophysical constraint that it does not violate SR.The second paper you listed is also a good one."A new constraint for beyond the Planck scale."This is a paper that discusses how some theories of quantum gravity might produce effects that we would not be able to see with our current technology.The third paper is a revision of a paper from 2003."Lorentz violation and Crab synchrotron emission: a new constraint for beyond the Planck scale."This is a
  • #1
wolram
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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/~thompson/pubdb/docs/ragazzoni03a.pdf [Broken]

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 [Broken]

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

http://arxiv.org/PS_cache/astro-ph/pdf/0301/0301145.pdf [Broken]
 
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  • #2
Originally posted by 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/~thompson/pubdb/docs/ragazzoni03a.pdf [Broken]

...



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.
 
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  • #3
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 [Broken])

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
 
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  • #4
One of the follow up that is quoted, this is a paper that must be considered:http://citebase.eprints.org/cgi-bin/citations?id=oai%3AarXiv%2Eorg%3Aastro%2Dph%2F9712103 [Broken]
 
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  • #5
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.
 
  • #6
Originally posted by 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.

SelfAdjoint, can you be more specific as to the first paper?..are you referring to the first of the links in Wolrams first posting?..or to the Amelino-Camelia, G Ellis..et-al..Nature paper?
 
  • #7
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.
-------------------------------------------------------------------

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.
 
  • #8
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
 
  • #9
Originally posted by 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?

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
----------
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 don't 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.
----------
I have a very preliminary provisonal take on it. Maybe I will add a post following this up.
-----------

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.
 
  • #10
Originally posted by 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/~thompson/pubdb/docs/ragazzoni03a.pdf [Broken]
...

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

...

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 [Broken]

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 won't say anything new, just what he has already said several times in other papers)
 
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  • #11
well, some A-C papers I have found more helpful than this one. I can't 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 traveled 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.
---------------------------

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.
 
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  • #12
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.
 
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  • #13
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.
--------------------------------------------------------------------
seems you have been working hard MARCUS,
it also seems that not many are interested
in this subject, but whatever I am 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.
 
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  • #14
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.
--------------

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

-----------------

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 don't know the exact coefficients in the LQG energy-momentum relations--we don't know what alpha and beta go into the formula.
But let's suppose the coefficients are "order one" and just take them to be one and let's 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
 
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  • #15
Originally posted by 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.
--------------------------------------------------------------------


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.

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

has some interesting ideas 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-ph/pdf/0312/0312151.pdf
 
  • #16
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 [Broken]

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)
 
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  • #17
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)
 
  • #18
Originally posted by 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)

I am jumping ahead here, but I have my Mother coming to stay with me for over the xmas hols, I have to take all my papers into another room as my computer and work is in the main room!

You have rightly touched upon the detection of GWs, and I do recall a paper I have by A-C, and I also recall a Rovelli paper that I had written some questions into the margins, which relate to the probable limits of a detection system.

Its quite obvious the detection system has to be located in an enviromental isolation(I recall a system set-up whereby a detector must be in "levitation", and the Meisner Effect), but a recent proposal by these: http://arxiv.org/PS_cache/cond-mat/pdf/0307/0307491.pdf, [Broken] do tackle the problem of what I am conveying.

I do also recall that I know the detector has to be a sufficient distance away from the whole apparatus that contains it, and this 'detector' has to be a certain size itself in order to detect certain signals from far away locations, its 'like' the immirzi paramiter? but built up into cocentric layers of BEC's, the releasing Bose-Nova CAN BE CORRELATED TO DETECT FAR AWAY NOVA'S.
 
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  • #19
And he says some project named "GLAST" might be good enough to detect such an effect in long-distance gammaray bursts.
-------------------------------------------------------------------

GAMMA RAY LARGE AREA SPACE TELESCOPE, due for deployment
in 2004, i think.
 
  • #20
this theory seems to depend on the Planck scale being
homogenous and invariant throughout the universe, how
can local phenomena be ruled out?
------------------------------------------------------------------
i guess a predictive match of many samples would be
strong evidence for or against the quantum.
------------------------------------------------------------------
from what i can make out this theory is saying that
the high energy,"high frequency" photon will be retarded
in its travel, but a low energy photon,"low frequency",
will not, so could it be said that any EM signal will
travel slower through space if it has a high frequency?
 
  • #21

from what i can make out this theory is saying that
the high energy,"high frequency" photon will be retarded
in its travel, but a low energy photon,"low frequency",
will not, so could it be said that any EM signal will
travel slower through space if it has a high frequency?

edit: I may have misunderstood the effect as opposite from what it is really supposed to be. high energy might be associated with speeding up. still struggling with these papers

I believe that's right. And there is a rough estimate of the fractional size of the effect (the size of [edit: the change in speed] as a fraction of the normal speed photons go). The key to getting this estimate is to measure the photon's frequency in Planck frequency units.

There are natural units of mass and energy called Planck mass and Planck energy. (Im just reviewing common knowledge at PF). And associated with any energy there is a frequency----planck's h constant is the ratio connecting them----the frequency a photon would need to have if it carried that amount of energy.
So there is a natural unit of frequency----planck frequency---associated to the Planck energy unit.

When Amelino-Camelia talks about a "100 MeV" photon then roughly speaking he means one whose frequency is this small fraction of the natural unit
E-20, or 10-20

the estimate is that such a photon [has its speed changed] by that same amount. It basically goes at speed c, but slowed [or speeded] by a tiny fraction of c, namely by E-20 c, or 10-20 c.

In American English, a billionth is E-9, a trillionth is E-12,
a quintillionth is E-18

E-20 is "a quintillionth of a percent".

I realize that in UK a quintillionth may mean a different small number, but anyway I will use the language I am used to and put the conjecture about the 100 MeV photon this way:

Because the photon's frequency is a quintillionth of a percent of the natural unit of frequency, it is slowed down [edit: or speeded up] by a tiny amount of speed which is estimated to be a quintillionth of a percent of the natural unit of speed.
 
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  • #22
http://www.radio.gov.uk/topics/research/rcru/project52/finalreport/time.htm [Broken]
-------------------------------------------------------------------
i may be getting confused but, is this photon delay
phenomena connected with the electrical properties
of the vacuum, "permittivity, permeability" ?
 
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  • #23
Originally posted by wolram
http://www.radio.gov.uk/topics/research/rcru/project52/finalreport/time.htm [Broken]
-------------------------------------------------------------------
i may be getting confused but, is this photon delay
phenomena connected with the electrical properties
of the vacuum, "permittivity, permeability" ?

to a first approximation we are both roughly equally puzzled
maybe selfAdjoint or one of the others will step in and clarify
but I shall try to reply (on basis of incomplete understanding)

this [edit: speed change] effect is not AFAIK connected with those things
it is a conjecture which if evidence were found would bring about
new physics

the [speed change] comes from a distortion of the basic (ee equals emceesquare) formula.

E2 = p2 + m2

gets a tiny bit distorted by an energy-dependent term that creeps in from somewhere. Giovanni A-C prefers to write the new formula essentially this way:

E2(1 + const (E/Eplanck)n) = p2 + m2

I tell you flat-out, I do not understand why this happens. The best QGphenom. paper I know is GA-C http://arxiv.org/physics/0311037 [Broken] and this formula appears on page 4 (top of second column)
and he gives a footnote referring to a Smolin article
gr-qc/0207085 which may have more explanation.

This is where the [speed change] comes from. Eventually I am determined to understand how. For now, let's just repeat the conjecture slowly and understand the statement of it.
I will make a separate post about this.
 
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  • #24
just a thought, the higher the frequency of a wave
the more transitions from "electric to magnetic", it
has go through in a distance traveled, so this may
couple to the Planck energy more often.
 
  • #25
The real "emceesquare" formula is E2 = p2 + m2. The other thing is a simplification that refers to special case where momentum p is zero (the thing is at rest). If you take the real formula and put p =0 you get
E2 = m2.
Then if you put c back in (physicists save themselves trouble by imagining they are working in units with c =1 so they can drop c out of formulas) you get
E2 = m2c4
and that is the famous "emceesquare" formula but with both sides squared. All this is infuriating mumbledepeg about units. The gist of it is that the REAL 1905 formula is the "energy momentum relation":

E2 = p2 + m2

Somehow, I don't understand how, when this is worked out in LQG it can get deformed slightly by the Planckscale geometry of space. It can get deformed by multiplying the lefthand side by a number which is very close to one but not exactly one. GA-C claims he tries to give a nontechnical explanation of this in the 2002 Nature article but I looked at it and it didnt help, though it refers to the Smolin article.

the number that is almost but not quite one depends on expressing the energy of the particle as a fraction of the Planck energy
the fraction is (E/Eplanck) that is, the energy of the particle divided by the natural energy unit, Eplanck.
In the case of a 100MeV photon this fraction is 10-20.
In the case of a typical photon of sunlight it would be still smaller, like 10-28.

In the case of a photon (because we like to work with light) the mass m is zero so the basic formula simplifies to

E2 = p2

or, if you put the c back in (which physicists so often leave out)

E2 = p2c2

Now there is that damned factor just slightly more than one which has to be put in so it can multiply the lefthand side.
Apparently no one knows the exact formula for it (and it is not experimentally verified anyway) but GA-C says it is something like

(1 + const (E/Eplanck)N) for some number N and some constant.

that is you take the particle's energy and express it as a fraction of Planck, which might be 10-20 and then you raise it to some power N, which might simply be one, and then you multiply it by some constant, which might be two.

In fact since the whole business is so strange and new let's just assume for the same of getting some preliminary estimates (this is could be what GA-C is thinking) that the const = 2 that N = 1 so then the factor is

(1 + 2(E/Eplanck))

I want to see how that effects the speed. this is very very tentative on my part. as soon as i get a chance i will look at the Smolin article and see if that helps

http://arxiv.org/gr-qc/0207085 [Broken]
 
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  • #26
well thank goodness that Smolin article
has a bit more of the technical detail
there may be hope of understanding this yet
(wolram your intuition about how frequency translates
into this [edit: speed change] effect MAY actually be on the right track!
but I can't respond because I have no intuition of my own about
this as yet and i haven't yet understood the technical explanation
given in this article)

who is Joe Magueijo and what kind of reputation does he have as a theoretical physicist?

this could matter because what I called the Smolin paper is actually by
Maguieijo and Smolin.
It is called "Generalized Lorentz invariance with an invariant energy scale"

Magueijo is at the Blackett Lab of the Imperial College in London.

I don't know how it happened that I didnt see this paper earlier. an oversight.
 
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  • #27
i don't know how to put this in a kinder way so------,
this theory seems to be a fudge factor to keep
observational evidence in step with particle
physics, but as you know i rely on intuition
rather than knowledge.
that said a deeper understanding may change
my mind.
 
  • #28
Originally posted by wolram
i don't know how to put this in a kinder way so------,
this theory seems to be a fudge factor to keep
observational evidence in step with particle
physics, but as you know i rely on intuition
rather than knowledge.
that said a deeper understanding may change
my mind.

I sympathize.
Anything with a changing speed of light is an abomination
on the face of it. But I am going to try to understand what
they are saying anyway. And, as you suggest, more understanding
might change one's mind. Or might not, I'll see.

(BTW I am not even sure that loop gravity necessarily predicts
a varying speed of light, it may be something that GA-C sees
that the theory could predict and so he is encouraging people to look for it. But there actually seems to be a difference of opinion between Rovelli and Smolin about it.)
 
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  • #29
I think I found the source of this.
Rovelli does not think that LQG needs any
modification of the energy-momentum relation and
any energy-dependence of the speed of light.
So he shows a way around it, for example in a paper
by Rovelli and Speziale
"Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald constraction"
http://arxiv.org/gr-qc/0205108 [Broken]

On the other hand Magueijo/Smolin although they CITE
Rovelli/Speziale (footnote on page 4) as giving an
alternative viewpoint, present an argument to persuade the
reader that LQG should have this non-linearity (they call it)
in the way energy and momentum transform and this energy-dependent
speed of light.

Observational data is not yet good enough to force the issue
either way. Eventually if some program like GLAST finds
some dispersion (like higherenergy photons arriving slightly
ahead of lower) then that will favor Magueijo/Smolin at the
expense of Rovelli's viewpoint. Or it may find no dispersion
which will lend support to what Rovelli says in
"Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald constraction"

As a spectator I guess I am a bit let down. I thought the prospect was of observations settling some large issue. But it now looks as if the GLAST can only help settle a kind of "minor internal difference" within LQG. And maybe will not even do THAT in a fully conclusive way.

[edit:] double-checked about GLAST "Gammaray Large Area Space Telescope"
http://glast.gsfc.nasa.gov/project/gsmo/GDS_Ops_ProjSched_071803.pdf
It looks like the launch is scheduled for 2006.

But it won't settle the question "Is Loop Gravity right?", only, at best, give some guidance as to which VERSION of loop gravity is preferred. I thought, for a while today, that some more conclusive results were in store, and also that the Glast thing was happening sooner.
 
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  • #30
well it seems there is no option other than to leave
this one on the back burner.
sorry about GLAST date, the article i read some time
ago said 2004.
until shown otherwise i prefer the constant speed of
light theory.
thanks MARCUS.
 
  • #31
Originally posted by marcus
well
who is Joe Magueijo and what kind of reputation does he have as a theoretical physicist?

this could matter because what I called the Smolin paper is actually by
Maguieijo and Smolin.
It is called "Generalized Lorentz invariance with an invariant energy scale"

Magueijo is at the Blackett Lab of the Imperial College in London.

I don't know how it happened that I didnt see this paper earlier. an oversight.

Marcus,:who is Joe Magueijo and what kind of reputation does he have as a theoretical physicist?

Surely you must be joking!

The paper here quoted:"Generalized Lorentz invariance with an invariant energy scale"
Came out some years ago, my copy is well worn, and there are some significant insights contained within.

I say this with some conviction that I was a porter at Swansea University at the Halls of Residence. In the mid-nineties Ed Witten was giving a 're-scheduled' lecture to the worlds formost Post Grads,

It is certain that Joao, benefeted from a discussion I gave to a number of Post-Grads in the 'snooker-hall' one evening. I gave a precise talk on how the speed of light had to be different at the early-universe, I am in no way saying that I had been the first to state this 'Varying Speed of Light', but I certainly left a good impression, and being that Ed Witten himself was presenting his talks to the Post-grads in such a way, most(PGs) were contemplating falling out and leaving the Superstring 'M' Theory arena.

back to the said paper, there is relevence to a diverging speed of light and energy, I will be locating the papers I have on Joao, and will make some speculations, it will relate to the breaking of the Second Law of Thermo dynamics (the result of which has played an important part in the evolution of our observed Universe), this as I recall was touched upon in a paper by Joao, but until I retrieve the paperwork I will say no more.
 
  • #32
MARCUS,
i had quick google for J MAGUEIJO, most of the sites
are advertising his book, it seems the media like him
for his contravertial views.
number 2 in my list may interest you.
-------------------------------------------------------------------


an interview with JOAO MAGUEIJO.

http://www.abc.net.au/rn/science/ss/stories/s212674.htm [Broken]
-----------------------------------------------------------
can a changing "a"explain the supernova results.
john d. barrow and joao magueijo

http://xxx.lanl.gov/PS_cache/astro-ph/pdf/9907/9907354.pdf
-----------------------------------------------------------
joao magueijo, lee smolin.
generalized lorentz invariance---------

http://www.lns.cornell.edu/spr/2002-07/msg0043162.html
 
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  • #33
Thanks ranyart, thanks wolram!

the link to the discussion on SPR, by Jeremie Vinet,
was especially helpful in my case. I had somehow just
filtered out a lot of the talk about variable speed of light
as too wild to consider.

this whole business (whether one thinks of light's speed varying in the course of time or being constant in time but varying with the energy of elementary particles) is a can of worms and would be safer
left on the back shelf, as was suggested

It seems to me at this point that Rovelli must have been at pains to distance his "vanilla" (plain unvarying speed of light) version of LQG from this more exotic approach: one that obviously intrigues Smolin.
This would have been one of the motivations for the May 2002 paper
by Rovelli and Speziale "Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald constraction".

As a spectator I don't need to favor one over the other and have no business doing so, but my sympathies are more on the vanilla side.

As I understand it, the main reason (perhaps fundamentally the only reason) this comes up is that at least from a naive perspective ANY theory that distinguishes the Planck scale as a place where new things start to happen is effectively setting up a quantity which (like the speed of light) should be the same to all observers. Rovelli attempts to deflect this, and indeed may succeed in showing what is wrong with it. I am going to take another look at what he says (and also at the naive argument that all observers should agree on what lengths, energies etc are Planck-scale because new behavior starts to happen around that scale.)

there is more discussion of these issues at the start of this earlier thread:

https://www.physicsforums.com/showthread.php?s=&threadid=8514
"how area in LQG transforms under Lorentz boosts"
 
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  • #34
Im kinda jumping into the discussion late here, but I got a couple questions I need to clear up.

1) I understand the GLAST experiment, except for it's implications to LQG. How does this experiment relate to LQG's well known spin networks? Does it imply that because the gamma particle has more frequency (less wavelength) that it takes a shorter path in the network than the particle with less frequency? I'm a little lost here.
Paden Roder
 
  • #35
hello Paden, anyone who can help out with explanation is invited to jump in
In what I've read the connection (which Smolin sees but Rovelli seems to be reserving judgement about) between Loop Gravity and some "DSR"-like theory with its energy-dependent speed is not a firm one.

It isn't based on any mechanism that one can visualize. In loop gravity space is not "made" of spin networks---the spin networks serve to catalog the quantum exitations of space. they provide a orthogonal basis of a hilbertspace of the quantum states space can be in.

so it doesn't seem to help to visualize a gammaray photon traveling thru a spin network! I haven't read anybody thinking of it that way

The intuitive argument for DSR (doubly special relativity) is of a different kind. Main proponent seems to be G Amelino-Camelia. He says that since quantum effects are expected to become important at Planck scale the main milestone or boundary-marker of that scale (namely Planck length or Planck energy) should be the same for all observers.

If we are going to see remarkable weird new behavior at Planck scale then moving observers ought to be able to agree what that scale is! what wavelengths and particle energies are at that scale and what are not.

This is a dreadful realization because it means that instead of ONLY ONE thing, the speed of light, that has to be the same for all observers and has to be invariant under lorentz group, you now have TWO quantities (speed of light AND Planck length or energy for elementary particles) that have to be invariant.

then there is a kind of mathematical thing that comes in, if you want TWO quantities to be invariant (and yet retain the principle of relativity that all observers are equal, there being no one distinguished frame) then there has to be some "give" in the system.
there can be a "normal" speed of light that is the same for all observers and which photons have in the low energy limit but we are forced to consider the possibility that very high energy photons
get away from this

Fortunately Rovelli has found a way around this, and has avoided getting on the DSR track. Things seem a lot simpler if you just have one invariant quantity (c) that looks the same to all observers. All light can travel that speed including very high energy photons. Whew! What a relief!

He describes his viewpoint in the paper called "Reconcile..." I gave a link earlier.

But at least for now Smolin is going a different way and exploring the consequences of Giovanni AmelinoCamelia's idea---what if there were not just one but instead TWO invariant quantities----the speed limit c and the boundary marker of the Planckscale domain (plancklength or Planckenergy). This is what the GLAST can test because it can see if there is some variation of speed with elementary particle energy.

Part of your question is what physical MECHANISM for varying speed has been imagined, and I don't know of any----it is more a mathematical revenge that nature takes on you if you try to pin her down at two points instead of just one---with two invariant quantities instead of only a single one.
 
<H2>1. What is the current evidence against quantum gravity?</H2><p>Currently, there is no definitive evidence against quantum gravity. However, there are several theories and experiments that suggest discrepancies between general relativity and quantum mechanics, which are the two main pillars of quantum gravity.</p><H2>2. How does the theory of general relativity contradict quantum mechanics?</H2><p>The theory of general relativity describes gravity as a curvature of spacetime, while quantum mechanics suggests that gravity is caused by the exchange of particles called gravitons. These two theories have not yet been successfully reconciled, leading to the search for a theory of quantum gravity.</p><H2>3. What experiments have been conducted to test quantum gravity?</H2><p>Several experiments have been conducted, such as the search for gravitational waves and the study of the cosmic microwave background radiation. However, these experiments have not yet provided conclusive evidence for or against quantum gravity.</p><H2>4. Are there any alternative theories to quantum gravity?</H2><p>Yes, there are several alternative theories to quantum gravity, such as loop quantum gravity, string theory, and causal dynamical triangulation. These theories attempt to reconcile general relativity and quantum mechanics in different ways.</p><H2>5. What are the implications of finding evidence against quantum gravity?</H2><p>If evidence against quantum gravity is found, it would require a major shift in our understanding of the fundamental laws of the universe. It could also have implications for our understanding of black holes, the beginning of the universe, and the nature of time and space.</p>

1. What is the current evidence against quantum gravity?

Currently, there is no definitive evidence against quantum gravity. However, there are several theories and experiments that suggest discrepancies between general relativity and quantum mechanics, which are the two main pillars of quantum gravity.

2. How does the theory of general relativity contradict quantum mechanics?

The theory of general relativity describes gravity as a curvature of spacetime, while quantum mechanics suggests that gravity is caused by the exchange of particles called gravitons. These two theories have not yet been successfully reconciled, leading to the search for a theory of quantum gravity.

3. What experiments have been conducted to test quantum gravity?

Several experiments have been conducted, such as the search for gravitational waves and the study of the cosmic microwave background radiation. However, these experiments have not yet provided conclusive evidence for or against quantum gravity.

4. Are there any alternative theories to quantum gravity?

Yes, there are several alternative theories to quantum gravity, such as loop quantum gravity, string theory, and causal dynamical triangulation. These theories attempt to reconcile general relativity and quantum mechanics in different ways.

5. What are the implications of finding evidence against quantum gravity?

If evidence against quantum gravity is found, it would require a major shift in our understanding of the fundamental laws of the universe. It could also have implications for our understanding of black holes, the beginning of the universe, and the nature of time and space.

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