The end of String Theory and Loop Quantum Gravity ?

[czes]

If spacetime is composed of tiny quantum "grains," the gamma-ray photons' polarization should change from random polarization (at the GRB source) to biased toward a certain polarization when received by the Integral spacecraft .
The Integral polarization results depend on spacetime being constructed from discrete quanta that behave in a way that fits with quantum theory. The holographic universe hypothesis goes one step further, constructing 3-dimensional spacetime from projections of a 2-dimensional "shell" -- perhaps gamma-ray photons behave differently in this fuzzy, projected, quantum world, and this could be why no polarization difference between gamma-ray photons are detected.
"This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories," said Laurent in the ESA press release.
http://news.discovery.com/space/we-might-not-live-in-a-hologram-after-all-110701.html

 

[tom.stoer]

I don't know what the current string theory an covariant loop quantum gravity approaches currently say regarding polarization - and whether we already a have conclusive results from these theories.

 

[francesca]

This ESA result was already discussed in this thread
https://www.physicsforums.com/showthread.php?t=511288"

and I can also add that what Philippe Laurent actually said was badly reported by the journalist.

http://www.sciencedaily.com/releases/2011/06/110630111540.htm

What do you guys think?

Irrelevant to presentday LQG. The results only concerns theories (I don't know which) that have been shown to be Lorentz violating. LQG has not been and is not in that class of theories.

I looked at the Philippe Laurent et al paper on arxiv yesterday. It does not mention LQG as far as I could see. What you quote is Science Daily---pop journalism. Can't rely on it.

The technical paper is
http://arxiv.org/abs/1106.1068

I just checked, and in the scholarly paper I can't find any citation to any of the standard LQG sources at all! Nor, of course, does it mention LQG. Philippe Laurent is quoted by the Science Daily reporter as saying something which, if he actually said it, just shows he does not know what he is talking about LQG-wise. But in any case the actual scholarly paper avoided that.

Exact Marcus, LQG is compatible with local Lorentz invariance.
Two explicit references about this are the old paper (2003)

``Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald contraction",
C Rovelli, S Speziale, Physical Review D67 064019; http://arxiv.org/abs/gr-qc/0205108"

and a more recent one, in the covariant theory:

``Lorentz covariance of loop quantum gravity",
C Rovelli, S Speziale, Physical Review D83 104029; http://arxiv.org/abs/1012.1739"

There are interesting proposals on the possibility of Lorentz violations (Pullin, Gambini, Smolin, Amelino Camelia...). These are still viable and compatible with the observations so far. But notice that these possibilities are not necessarily implied by LQG.

Cheers,
Francesca

 

[francesca]

see also:

*Sigh*
The original ESA press release and the Sci Daily piece contained garbage. This has been pointed out in several threads. People kept starting new threads about the stupid business.
There is no "debate". Yoda is misapplying a Gambini paper ( http://arxiv.org/abs/1106.1417 ) in a misleading fashion---a paper he refused to give the link to when asked.

I already addressed the issue, if you can call it an issue , in post #10 and #12 of this thread:

==quote #10==
The press release and the Sci Daily make a wildly inaccurate presentation of the actual result of the actual paper.

Baguette I think you suspected as much.

What Philippe Laurent was quoted as saying was just hype: The paper is not about grains but about Lorentz Invariant Violation.

http://arxiv.org/abs/1106.1068
Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
P. Laurent, D. Gotz, P. Binetruy, S. Covino, A. Fernandez-Soto
(Submitted on 6 Jun 2011)

LQG was mentioned in the hype, but LQG does not predict LIV! As far as we know it is Lorentz covariant so bounds on LIV do not constrain it. So the Laurent et al result is not about LQG.
And indeed, LQG was not mentioned in the paper, which is at a more professional level than the public hype.

The LQG theory does says something about the measurement of area and volume, and the quantum states. The area and vol operators have discrete spectra. That is at the level of measurement---geometric observables. It also does not represent space on which geometry lives as consisting of physical grains. Subtle distinction.

But in any case, no Lorentz violation.
===

Some people tried to show LIV a few years back but failed. Recent papers have shown the theory is Lorentz covariant.

But this is irrelevant. The Laurent et al paper only constrains Lorentz violation. LQG does not violate Lorentz. So the paper has no bearing on LQG---neither the standard version established since around 2007 nor any other versions that I know of.
==endquote==

NOTE THAT IN THE PAPER YODA CITES, GAMBINi ET AL DID NOT DERIVE LORENTZ VIOLATION FROM LQG. They derived it for some thing "like" LQG---having some resemblance. The actual LQG that people are working with has been shown to be Lorentz invariant. So the whole thing would not apply it.

==quote #12==
...
It turned out to be impossible to derive LIV at very high energy scales from LQG when this was a research goal around 2005-2007.
So either Laurent was misquoted (mentioning Lqg as one theory that might be effected) or he doesn't know the current state of the field.

Since you seem interested in the Lorentz covariance of quantum gravity theories you might like to look at this paper published in Physical Review D May 15, 2011, so quite recent!
===
http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity
Carlo Rovelli, Simone Speziale
(Submitted on 8 Dec 2010, last revised 18 Apr 2011)
The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covari...
==endquote==

 

[atyy]

LQG may be compatible with Lorentz-invariance - but is it compatible with the ESA result?

In particular, the classical limit so far seems to be discuss Regge gravity, which is grainy.

 

[francesca]

Hi Atyy!
One should be careful not to confuse two kinds of discreteness.
The discreteness introduced by the fact that the theory is approximated on a graph, as in Regge calculus, is different from the fundamental quantum discreteness of the theory, that comes from the discrete spectrum of area and volume. The first is like saying that you see a certain numbers of (discrete) modes for a field in a box, the second is related to the actual quantization of the field.
A discussion about this and further references can be found in the http://arxiv.org/abs/1102.3660" .
Now, look at the figure at page 21: to recover full GR, one has to remove both discreteness, that correspond to take two distinct limits: respectively these are the "continuous limit" and the "classical limit".

 

[atyy]

Hi Atyy!
One should be careful not to confuse two kinds of discreteness.
The discreteness introduced by the fact that the theory is approximated on a graph, as in Regge calculus, is different from the fundamental quantum discreteness of the theory, that comes from the discrete spectrum of area and volume. The first is like saying that you see a certain numbers of (discrete) modes for a field in a box, the second is related to the actual quantization of the field.
A discussion about this and further references can be found in the http://arxiv.org/abs/1102.3660" .
Now, look at the figure at page 21: to recover full GR, one has to remove both discreteness, that correspond to take two distinct limits: respectively these are the "continuous limit" and the "classical limit".

Yes, that's exactly the figure I had in mind. So far the continuous limit hasn't been shown, has it?

 

[tom.stoer]

I think there are different experiments, namely regarding dispersion relations and regarding polarizations, respectively. That's different. Does LQG say anything regarding polarizations?

 

[czes]

I didn't know it was discussed before. Thank you for comments.
The journalist wrote about a space as something material made of particles and antiparticles. There is something as the polarisation of the space but it doesn't mean there is a material stoff.
Due to Quantum Decoherence there aren't the particles, distance, space nor time on a quantum fundamental level.
http://www.decoherence.de/
There is a superposition of the quantum information which are creating the Wave Function. The Wave Function is not material it is a mathematical description. Therefore the space also might be something mathematical (relation between the information on a 2D screen) only. The Integral satellite project will not detect the particles of the space then if it is not real.
Do I understand it properly ?

 

[francesca]

Yes, that's exactly the figure I had in mind. So far the continuous limit hasn't been shown, has it?

I would not say that the continuos limit has not been proven. We know that the continuos limit of Regge calculus is GR. Therefore in LQG you can take first the classical limit, and you get Regge, and then you take the continuous limit, and you get GR. Every thing seems fine to me.

The discretization of Regge calculus is an approximation. Therefore you may ask which is the regime in which this approximation is valid. A good physicist would choose the appropriate discretization depending on the system that s/he wants to describe.

 

[francesca]

I think there are different experiments, namely regarding dispersion relations and regarding polarizations, respectively. That's different. Does LQG say anything regarding polarizations?

I'm not aware of any discussion about the polarization of light in LQG. The only polarization that I know it is interesting for LQG is the polarization in the CMB, because this could let us detect possible effects coming from the big bounce.

 

[atyy]

I would not say that the continuos limit has not been proven. We know that the continuos limit of Regge calculus is GR. Therefore in LQG you can take first the classical limit, and you get Regge, and then you take the continuous limit, and you get GR. Every thing seems fine to me.

The discretization of Regge calculus is an approximation. Therefore you may ask which is the regime in which this approximation is valid. A good physicist would choose the appropriate discretization depending on the system that s/he wants to describe.

But does the Regge-GR limit exist within LQG?

First the truncated to full QG limit must exist, then the classical and continuous limits must commute (for the scenario you bring up - but I think it's also fine if they don't commute, as long as one can take continuous then classical starting from truncated?)