The search for quantum gravity signals

[wolram]

http://xyz.lanl.gov/pdf/gr-qc/0501053
The search for quantum gravity signals
jan 2005 an overview of ongoing searches for quantum gravity effects.

 

[marcus]

http://xyz.lanl.gov/pdf/gr-qc/0501053
The search for quantum gravity signals
jan 2005 an overview of ongoing searches for quantum gravity effects.

thanks for spotting and flagging this one, wolram.
Finding QG effects is potentially the most urgent and important effort underway and it is good to have an up-to-date overview.
I just added your link to the QG reference thread
https://www.physicsforums.com/showthread.php?p=444168#post444168

 

[wolram]

This link maybe a good overview ,but what are your thoughts? there is
still some leeway for originality, I mean testability has to be the focus
for theory

 

[marcus]

...is
still some leeway for originality,...

Absolutely right! There is still plenty of room for totally new approaches to appear:
ones we (non-professional amateur watchers) have no way to anticipate.

You are asking me simply for my opinion, which carries no weight besides that of the onlooker. YES the problem is totally open (in my opinion).
Neither LQG nor string nor noncommutativegeometry nor dynamicaltriangulations is credible as a monopoly---they are all immature attempts or they have serious problems or both.

And I don't think anybody is close to real rigorous testability either.

Smolin has gone out on a limb and unequivocally predicted some variation in the speed of light with photon energy----to be detected by GLAST.

But Ashtekar and Rovelli have not followed him out on the limb. They have not signed off on the prediction. I do not understand this division of opinion.

If no variation of gammaray photon speed is detected by GLAST this will be a serious embarrassment for Smolin. (but not for Ashtekar and Rovelli)

I guess you could say that LQG as Smolin understands it predicts this effect that GLAST is sensitive enough to detect if it is real and therefore LQG-as-Smolin-understands-it will be shot down or refuted or in serious trouble if the effect is not found.

Smolin's latest word on this came out just this month

http://arxiv.org/hep-th/0501091
Falsifiable predictions from semiclassical quantum gravity
Lee Smolin
9 pages

"Predictions are derived for the upcoming AUGER and GLAST experiments from a semiclassical approximation to quantum gravity. It is argued that to first order in the Planck length the effect of quantum gravity is to make the low energy effective spacetime metric energy dependent. The diffeomorphism invariance of the semiclassical theory forbids the appearance of a preferred frame of reference, consequently the local symmetry of this energy-dependent effective metric is a non-linear realization of the Lorentz transformations, which renders the Planck energy observer independent. This gives a form of deformed or doubly special relativity (DSR), previously explored with Magueijo, called the rainbow metric. The argument is general, and applies in all dimensions with and without supersymmetry, and is, at least to leading order, universal for all matter couplings. The argument is illustrated in detail in a specific example in loop quantum gravity.
A consequence of DSR realized with an energy dependent effective metric is a helicity independent energy dependence in the speed of light to first order in the Planck length. However, thresholds for Tev photons and GZK protons are unchanged from special relativistic predictions. These predictions of quantum gravity are falsifiable by the upcoming AUGER and GLAST experiments."

I think Smolin is a risk-taker and that he has laid it on the line here.
he says that the Planck energy is
observer-independent just like the speed of light (if LQG is right) and
so just like the speed of light looks the same to all observers (which is bad enough, we all find this a bit weird)
now, in addition, there is this other physical quantity (an energy this time, not a speed) which looks the same to all observers. And the only way this can happen is by a slight bending of the rules of special relativity which he says will be detectable by GLAST.

 

[wolram]

Thank you for your view Marcus, i guess we will all have to wait,
i hope delays are avoided.

 

[marcus]

Thank you for your view Marcus, i guess we will all have to wait,
i hope delays are avoided.

I understand your attitude----that we have to wait.
I agree that one has to hold off from believing anything.

On the other hand in another sense the action is just beginning.
Predictions AFTER the fact do not count in science. NOW is the time for any theory with pretensions to respectability to make its predictions.

Smolin's paper "Falsifiable Predictions..." is only the opening shot of what could be a round of predictions and even a small barroom brawl among academics. I have high hopes that Smolin's paper will make other theorists embarrassed that they don't have anything to offer and have been sitting on their hands

Especially since at the same time GLAST is planned to fly the LHC machine is also supposed to start working and sofar string theorizers have declined to predict anything about the particle physics that will be seen at LHC levels of energy.

So I hope for more than just waiting until 2007. I hope that there will be noisy arguments, perhaps that Leonard Susskind will make another scene, and even that Smolin will go on record with MORE predictions maybe involving other experiments besides AUGER and GLAST.

WHOAH we have not yet got anything from Loop Cosmologists! martin bojowald and that fellow parampreet singh should publish some firm predictions too in black and white. So there is more to be played out

 

[wolram]

http://xxx.itep.ru/pdf/gr-qc/0409077
Testing quantum gravity by neutrino flavor oscillations.
This paper proposes that significant tests for QG could provide
answers in 10 yrs.

 

[marcus]

wolram, I am reserving judgement about Joy Christian's ideas
Here is another URL for the same paper
http://arxiv.org/abs/gr-qc/0409077
Now the indications are that the paper has passed peer review and will be published. But I am still skeptical. I am not sure why.
I see that the paper is in its FOURTH REVISION. that suggests that she got it back with some reviewers' comments and had to make some changes before she got it past them.
To my mind that is good, not bad, it is how the peer review system is supposed to work. But I still think one should be cautious.
Maybe I will have another look.

Testing Quantum Gravity via Cosmogenic Neutrino Oscillations
Authors: Joy Christian (Oxford)
Comments: 8 pages, RevTeX4; Essentially the published version
Journal-ref: Phys.Rev. D71 (2005) 024012
DOI: 10.1103/PhysRevD.71.024012

"Implications of some proposed theories of quantum gravity for neutrino flavor oscillations are explored within the context of modified dispersion relations of special relativity. In particular, approximate expressions for Planck-scale-induced deviations from the standard oscillation length are obtained as functions of neutrino mass, energy, and propagation distance. Grounding on these expressions, it is pointed out that, in general, even those deviations that are suppressed by the second power of the Planck energy may be observable for ultra-high-energy neutrinos, provided they originate at cosmological distances. In fact, for neutrinos in the highest energy range of EeV to ZeV, deviations that are suppressed by as much as the seventh power of the Planck energy may become observable. Accordingly, realistic possibilities of experimentally verifying these deviations by means of the next generation neutrino detectors--such as IceCube and ANITA--are investigated."
-------
Wolram, on the one hand Physical Review Series D is a very good journal
(if she really got it accepted) but on the other hand
saying "the seventh power of the Planck energy may become observable."
sounds like stark raving lunacy. If that is true then...I don't know what.
Maybe she is real good friends with Roger Penrose and he helped get her paper a sympathetic ear. Kea might very well know the inside story.

If she would only just say the square or the cube of Planck energy, I could endure it. But the seventh power sounds like the seventh heaven and is just too much to take. Sorry about the highly personal reaction

 

[wolram]

Marcus, i will forgive you anything, it is only testability i am in search
off, but it is like getting blood out of a stone, i have hopes that new
research will herald new answers, but i guess it will only herald new
questions.

 

[marcus]

Marcus, i will forgive you anything, it is only testability i am in search
off, but it is like getting blood out of a stone, i have hopes that new
research will herald new answers, but i guess it will only herald new
questions.

Feynmann said "science is organized doubt"
yes you are right squeezing even a drop of the most eagerly desired truth
out of science is like squeezing blood out of stone. very much like that.

But don't you love to hear them squabbling?

I think it is wonderful.

Listen. LQG together with a bunch of likeminded QG generically predict some slight energy dependence of speed gammaray travel.
If this is not observed in 2007 or 2008 by Glast then LQG is shot down!
Isnt that enough for you wolram?

Wolram I hope your health is good. It must be. You are only 50 and used to ride Triumph bikes (or nortons, I forget). Why can't you wait till 2007?
What does it take to make you happy?

there is going to be a great deal of shennanigans between now and 2007. It will be a great show.

Cumrun Vafa the commander in chief of theoretical physics at harvard, has put in a friendly mention of LQG in his string-talk at Toronto around 15 january.
The audience couldn't believe their ears and asked him to repeat.
Many young string researchers are probably experiencing a hollow feeling which they cannot explain.

there will be more changes in the picture between now and 2007 than you or I can keep track of, I promise. So be happy.

Science theories never reveal the truth anyway. they exist to make predictions and to eventually be shot down. The Dealer never shows you his cards.

 

[Kea]

There will be more changes in the picture between now and 2007 than you or I can keep track of, I promise. So be happy.

Keeeahhhhh!

 

[marcus]

Keeeahhhhh!

Busy busy busy...

 

[wolram]

Sapientissimus-a-um

 

[marcus]

Good! wolram says he might possibly be willing to wait till 2007 to see how it plays out.
wolram, and all of us, remember that even then there will be no fully satisfying answer about what is true, only the possibility that one or more of the struggling (and not fully developed) theories may be knocked down and dragged out.

and maybe only knocked down...it is very hard to anticipate. I think that if GLAST does not see some energydependence in speeds of gammarays then LQG is in very serious possibly fatal trouble. I do not see how the theory could be changed so as to recover, but perhaps it could be. And not only LQG would be in trouble, but all attempts at a quantum picture of space and time in which the Planck scale figures as a key element (or in which one expects the Planck length or Planck energy to be the same for all observers). I don't pretend to understand this fully but am, in part, relying on the judgement of Smolin, whom I respect. To the extent that I do understand the issues, it seems to me right that he should stand up at this point and risk a prediction that
could refute quantum gravity as he sees it.

 

[wolram]

Marcus you are the basilicus -a -um when explaining these matters, many thanks.