Thursday, October 11, 2012

PRL on "Testing Planck-Scale Gravity with Accelerators"

With astonishment I saw the other day that Phys. Rev. Lett. published a paper I had come across on the arxiv earlier this year:
I had ignored this paper for a simple reason. The author proposes a test for effects that are already excluded, by many orders of magnitude, by other measurements. The "Planck-Scale Gravity" that he writes about is nothing but 5th order Lorentz-invariance violating operators. These are known to be extremely tightly constrained by astrophysical measurements. And the existing bounds are much stronger than the constraints that can be reached, in the best case, by the tests proposed in the paper. We already know there's nothing to be found there.

The author himself mentions the current astrophysical constraints, in the PRL version at least, not in the arxiv version - peer review isn't entirely useless. But he omits to draw the obvious conclusion: The test he proposes will not test anything new. He vaguely writes that
"The limits, however, are based on assumptions about the origin, spatial or temporal distribution of the initial photons, and their possible interactions during the travel. Another critical assumption is a uniformly distributed birefringence over cosmological distances... In contrast to the astrophysical methods, an accelerator Compton experiment is sensitive to the local properties of space at the laser-electron interaction point and along the scattered photon direction."
He leaves the reader to wonder then what model he wants to test. One in which the vacuum birefringence just so happens to be 15 orders of magnitude larger at the collision point than anywhere else in space where particles from astrophysical sources might have passed through? Sorry, but that's a poor way of claiming to test a "new" regime. At the very least, I would like to hear a reason why we should expect an effect so much larger. Space-time here on Earth as well as in interstellar space is, for what quantum gravitational effects are concerned, essentially flat. Why should the results be so dramatically different?

I usually finish with a sentence saying that it's always good to test a new parameter regime, no matter how implausible the effect. In this case, I can't even say that, because it's just not testing a new parameter regime. The only good thing about the paper is that it drives home the point that we can test Planck scale effects. In fact, we have already done so, and Lorentz-invariance violation is the oldest example of this.

Here's one of the publication criteria that PRL lists on the journal website:
"Importance.
Important results are those that substantially advance a field, open a significant new area of research or solve–or take a crucial step toward solving – a critical outstanding problem, and thus facilitate notable progress in an existing field."
[x] Fails by a large amount.

Thanks to Elangel for the pointer.

39 comments:

Nidnus Rep said...

Hi Bee,

Why did you write this post? The only problem is that the article is published in PRL which should notice things like this. Except that, the arXiv (and journals) are filled with papers that miss references and / or prior result that somehow make their results trivial. Since your blog is fairly well known this will certainly reach the author and take away the good feeling he had from a PRL publication. Just let it Bee.

(I'm not the author btw :)

Cheers, P

Bee said...

I wrote this post because the author raises the impression that we might measure Planck scale effects with future lepton colliders. I don't want people to mistakenly believe this: we already know we won't find anything new there. If it's on the arxiv, who cares. If it's in PRL, some people might actually believe it. The very last thing this field needs is more claims for effects that aren't there. Best,

B.

Theophanes Raptis said...

btw, "I heard the news today", that more are coming out of the meta-Lorentz lore!

"Einstein's special relativity beyond the speed of light"

http://rspa.royalsocietypublishing.org/content/early/2012/09/25/rspa.2012.0340

Bee said...

Hmm. Haven't read that paper. I probably should. Just from reading the abstract let me say that it's no problem to have velocities faster than the speed of light. The big question is what do you do with causality if you allow that. Nothing in the abstract about that, which is a bad sign.

Theophanes Raptis said...

In any case, I feel that I have to emphasize, at least on my side, that this "never fail" philosophy is not the way to go for science. It is failure rather than anything else that teaches us. Human science, in many respects is a trial-and-error process as too many presumptions are involved. Even things like "General Covariance" are postulated rather than proved, so one should be carefull even with astrophysical data.

Bee said...

Yes, one should be careful with astrophysical data. But saying one has to be careful is not a particularly convincing argument to throw out a whole bunch of constraints that tell us there is no effect to be found where the author of this paper suggests to measure. One expects an argument why the constraints may not hold. Or an example for a model in which there might be an effect in the accelerator but not in interstellar space. But nothing like that to be found in the paper.

Theophanes Raptis said...

Anyway, I 'm no specialist on this so my main interest was on the paper's symmetrized Maxwell eqs (2) - (3). Can I ask under what constraints you would consider them valid? For I know a nice "massage" trick that could be applied on them.

Uncle Al said...

Photon vacuum symmetries poorly model fermionic matter (SUSY; quantum gravitation, dark matter). Heterodox models are not predictive (arxiv:1210.1281). Symmetry breakings and parameterizations are curve fits. Gharibyan's success would contradict prior observation.

Parity violations are trace vacuum chiral anisotropy toward fermions that is differentially measurable with dense, self-similar, enantiomorphic (pdf; Section 2) atom packings. Physics disfavors emergent properties. Dogma succumbs to observation. Look. "When you have eliminated the impossible, whatever remains, however improbable, must be the truth." Either way.

Bee said...

The relevant reference is Myers and Pospelov.

Plato Hagel said...

I am confused.

The electroweak length scaleis shorter, roughly \ell_{w} \sim 10^{-18} meters and is set by the rest mass of the weak vector bosons which is roughly 100 GeV. This length scale would be the distance where a Yukawa force is mediated by the weak vector bosons. The magnitude of weak length scale was initially inferred by the Fermi constant measured by neutron and muon decay.

So without consideration of the energy values phenomenology you may need does this not require methods for determination and use of Muon detection scenarios as measures in matter densities?

Best,

Plato Hagel said...

G-2 Experiment

Think of Gran Sasso as the backdrop for the detection, and the travel through the earth as a measure of?

Best,

Theophanes Raptis said...

Thanks a lot for the link! I am a bit confused though, because Gharibyan mentions another paper by Reinaldo J Gleiser, Carlos N Kozameh and Florencia Parisi
"On low energy quantum gravity induced effects on
the propagation of light."
http://arxiv.org/pdf/gr-qc/0304048v1.pdf
This on the other hand, nowhere mentions Pospelov and Myers work. Perhaps they were done at the same time independently. In any case, it is the second that justifies the symmetrized eqs used by Gharibyan.

Zephir said...

The domain of quantum scale is as distant from human observer scale, like the relativity (in natural logarithmic scale). Therefore the evidence of quantum mechanical effects to relativity manifest most pronouncedly just at the human observer scale with various forces violating the inverse square law: the dipole, Van derWaals and Casimir forces. In another words, the theorists are seeking the forrest between the woods and we are sufficiently stupid in supporting them in it.

Robert L. Oldershaw said...

Discrete Scale Relativity definitively predicts that Subquantum Scale systems with radii on the order of 10^-31 cm [individual particles], 10^-30 cm to 10^-29 cm [high energy clusters of particles], and 10^-26 cm [Subquantum Scale atom analogues] should eventually be observed in ultra-fine scale fluctuation experiments. These predicted values are 100 to 10,000,000 times larger than the putative conventional Planck length. Observational hints of such Subquantum Scale gravitational signatures in the 10^-31 cm to 10^-26 cm range have been seen in experiments at HERA (2.57 +/- 0.71 x 10-26 cm) and at SLC (3.50 +/- 0.04 x 10-28 cm), as reported by V. Gharibyan, arXiv, 2012 [arxiv.org/abs/1207.7297 ].

Plato Hagel said...

These scenarios can be tested with signals from gamma ray bursts, highly energetic flashes of light originating in faraway Galaxies. Their spectrum covers a large range of energies. If one records photons of different energies with an exact timing, one can compare their arrival time.

Constraining Modified Dispersion Relations with Gamma Ray Bursts


A method of choice or what's best right now in terms of phenomenology and interpretation of quantum gravity effects and Fermi?

Hmmm.....

Bee said...

Hi Plato,

The muon decays into an electron and two neutrinos. This might help your confusion. Best,

B.

Phillip Helbig said...

"Discrete Scale Relativity definitively predicts that Subquantum Scale systems"

What will happen when they are not observed? DSR also predicted substructure of the electron at a level below the resolution of experiments at the time of the prediction, but will within that level now. It was a "definitive prediction". Since it was not observed, DSR is falsified. You cannot move the goalposts on a "definitive prediction".

Reference: http://adsabs.harvard.edu/abs/1987ApJ...322...34O

Plato Hagel said...

Hi Bee,

I guess the distinction here is a matter of choice as to respective researcher's opinion of what is fundamental in the research? Phenomenologically successful at interpretation?

If we are to say,"simply quantize the gravitational field while keeping it separate from the other forces." then we have understood the position of some researchers over others?

Would this be correct as a basis of preference as to understanding Quantum Gravity in your case?

Best,

Plato Hagel said...

Secondly, as failure recognized in Opera's result due to mechanical error, the hope is to recognize fundamental attributes of relativistic interpretations as faster then light limits, as speed of light?

Would you see extension here beyond Gran Sasso as not in concert with fundamental research of QG using LHC?

Best,

Robert L. Oldershaw said...


Mr. Helbig,

We have already discussed the electron substructure issue 4 times.

I have patiently explained why your hackneyed criticism is false and misleading to others.

What is your problem with basic scientific reasoning processes? Has some malfunction compromised your mental capacity?

Joel said...

Some time ago I saw this video:
http://www.youtube.com/watch?v=nByekIx7XXw&feature=related
suggesting to me that spacetime may well not be discrete.

I have tried to find a paper with a summary of the "Fermi Gamma-ray Space Telescope" results in respect to a discreteness of space(and time) and what constraints these impose but couldn't find one. Can anybody help me ?

Bee said...

Hi Joel,

Maybe this helps? Best,

B.

Plato Hagel said...

One can expect "holes" in the argument when it comes to lensing/dilation and how photon travel is seen in relation to Lagrangian?

Continuity of expression helps to paint a picture of the cosmos that is smooth and fluid in expression, and raises the question of constraints as the photon travels through space? Is it not affect in terms of its speed as a color may be assigned to an energy value?

So you may say discrete values yet gravity is understood as it is seen in relation to speed and slowing of speed as an image in Lagrangian?

I mean how do yo see the cosmos?

Backdrops here on earth are well understood as to what arrives here and is expressed in terms of the information that is received?

LIGO as well as Fermi have arrival times that should be very close in terms of what is measurable in correspondence from events in the cosmos right?

The beam of light in LIGO arm is susceptible of fluctuation as well as calorimeter measure coordinated in Fermi as a energy value?

Hmmmm......

Joel said...

Hi, Bee
thanks for the link, yes it's very helpful.
If I understand reference [14] correctly the constraints from Fermi in respect to birefringence are pretty weak, dispelling my skepticism.

Bee said...

Hi Joel,

I don't know reference [14] of which paper you refer to. As to vacuum birefringence, I believe this is presently the state of the art. Best,

B.

Theophanes Raptis said...

Hi Bee

I have news from the Lorentz lore again! A very nice quote from G. Sudharhan on Tachyons

"(…)Suppose that someone studying the distribution of population on the Hindustan Peninsula cockshuredly believes that there are no poeple north of the Himalayas, because nobody can pass throught the mountain ranges! That would be an absurd conclusion. The inhabitants of Central Asia have been born there; they are not obliged to be born in India and then cross the mountain ranges. The same can be said about superluminal particles(…)"

http://thespectrumofriemannium.wordpress.com/2012/10/13/log043-tachyons-and-sr-i/

Theophanes Raptis said...

...and more news!

"Meta" relativity: Against special relativity?

Jakub Rembielinski, Marta Wlodarczyk

http://arxiv.org/abs/1206.0841

Joel said...

Hi Bee,
sorry I was a bit sloppy. [14] is the reference in the paper in regards: arxiv:1207.7297.

There is an interesting table in
http://arxiv.org/abs/1104.4438
So, yes, this seems to be the state of the art (in terms of vacuum birefringence) on the astrophysics side.

If one could really push the limit further with accelerators, that would be fantastic.

Plato Hagel said...

Interesting......2011 Lorentz and CPT Violation in Astrophysics and Cosmology

You know it becomes an accumulative thing in the repertoire of seeing(lagrangian) the cosmos in certain ways that it now reveals a "magnetic lensing" which I had not entertained before.

I mean if we had sought to identify chemical and mineralogical refractions as elemental construct faster, is the circumference of the event(spherical cow) then we want to assemble a complete unit before arriving at the source of measure here on earth.

In a sense, this to me seems more of the questing toward electromagnetism and gravity unification as a method of approach toward understanding QG?

It is not to far a shot then to consider the Dark energy/matter scenarios as fundamental extension of the collider perspective.

It may have tended to a matter of choice as to the method of research and phenomenological experimentation here? I know you are just asking the question as to the Collider's use while already in use versus "another approach."

Do you really need a collider as large as the cosmos?

Best,

Bee said...

Hi Joel,

I think we're talking past each other. Look, the current astrophysical constraint is \xi < 10^{-14}. The limit that you can reach with accellerators is, in the best case, of the order \xi < 1. This isn't a stronger limit. It's weaker by many orders of magnitude. This is why I am saying: there is nothing to be found there. We already know that. Best,

B.

Zephir said...

First, the domain of quantum gravity isn't some area of obscure physics somewhere around Planck scale. The quantum mechanics begins at the 10E-9 meter scale, the relativistic scale begins at the 10E9 meter scale. The quantum gravity struggles to interpolate between general relativity and quantum mechanics, so it applies just at the intermediate scale, which is traditionally the domain of so-called classical mechanics. If you seek for quantum gravity phenomena outside of this scale, you may still get some grants and money for it, but you aren't doing the research of quantum gravity.

Lets face the sad reality: the quantum gravitists have no idea about the actual scope of their subject, not to say about its phenomenology.

Zephir said...

We have essentially two ways, how to define the scope of quantum gravity along dimensional scale of the observable universe. The first one is "maximalistic" and it considers the quantum gravity as a sorta interpolation between "pure relativity" scope and "pure quantum mechanics" scope.

The second way is minimalistic and it considers the quantum gravity as a model, which deals with dimensional/energy density scale inside of the thin zones around "pure relativity" and around "pure quantum mechanics", which still cannot be described with classical physics.

Joel said...

Hi Bee,

we both agree on the astrophysical constraint. You may well be right with the accelerator constraint, but I haven't really looked at that yet.

Just found that:
http://phys.org/news/2012-10-planck-scale-gravity.html

Best,

Joel

Joel said...

Hi Zephir,

you may be right. In general quantum mechanics isn't a matter of scale, see for instance quantum phase transitions.

I wonder if quantum gravity couldn't also involve macroscopic quantum effects.


Bee said...

Hi Joel,

Yes, I had seen that. And I left a comment, but got a message saying it's held for approval. The only thing you need to do is look at the image in the phys.org post, which is from the paper above. The horizontal line at l_p corresponds to \xi = 1. That's as good as it will get. Best,

B.

Joel said...

Hi, Bee
"The horizontal line at l_p corresponds to \xi = 1. That's as good as it will get."
I see.
"This is why I am saying: there is nothing to be found there. We already know that."
Now I got that too, thanks a lot.

Best,

Joel

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

Bee: Look, the current astrophysical constraint is \xi < 10^{-14 The limit that you can reach with accellerators is, in the best case, of the order \xi < 1.

Hmmmmm.....finally, specifics that one can see:)


Best,

Plato Hagel said...

fine....why even exist on the internet:)

physorg.com/newman/gfx/news/hires/2012/planckscalegravitytest.jpg