'Quantum grains' smaller than 'Planck length'

[baguyette]

How is it possible that 'quantum grains' are many orders of magnitude smaller than 'strings' or the 'Planck length'?

 

[Naty1]

Never heard of a "quantum grain"...unless it's part of a new theory, I don't think anything can yet be described at smaller than Planck length nor at Planck energy...

 

[DevilsAvocado]

Welcome to PF baguyette!

The Planck length is closely related to the speed of light, Planck's constant, and the gravitational constant, thus getting anything smaller than this is going to cause some 'trouble'... I think...

 

[baguyette]

According to the July 1st article (see link below), space is made up of 'quantum grains' which have been shown to be many magnitudes of order smaller than the Planck length. I quote from the article;

"Some theories suggest that the quantum nature of space should manifest itself at the 'Planck scale': the minuscule 10-35 of a metre, where a millimetre is 10-3 m. However, Integral's observations are about 10 000 times more accurate than any previous and show that any quantum graininess must be at a level of 10-48 m or smaller. This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories,"

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

 

[Bill_K]

Yes, I saw the news stories too and wondered what on Earth they were talking about. The real paper is at http://arxiv.org/abs/1106.1068. It was an observation of the Integral satellite, studying polarized light from gamma ray bursts. The effect they looked for (and failed to find) is "birefringence of the vacuum". They excluded it to one part in a hundred trillion.

 

[phinds]

I saw this as well and was going to post a question here about it since it didn't seem to make sense.

Now, I find that it's incredible all right, but not the results stated, rather the fact that the ScienceDaily story seems to have been made up entirely. It bears no relation to the article that it references other than that it references the same experiment but makes statements that are nowhere found in the original paper.

I've never seen the ScienceDaily before (but I'm new to all this). Is it normally reliable?

 

[baguyette]

The ScienceDaily story was taken, verbatim, from the European Space Agency (ESA) website (http://www.esa.int/esaCP/SEM5B34TBPG_index_0.html). One of the authors of the original paper, Dr. Laurent, is quoted in the ESA article, but I could not tell if he made the statement that "quantum grains must be at a level of 10-48m or smaller". Not sure who made that determination or if it is even an accurate presumption from the facts.

 

[phinds]

The ScienceDaily story was taken, verbatim, from the European Space Agency (ESA) website (http://www.esa.int/esaCP/SEM5B34TBPG_index_0.html). One of the authors of the original paper, Dr. Laurent, is quoted in the ESA article, but I could not tell if he made the statement that "quantum grains must be at a level of 10-48m or smaller". Not sure who made that determination or if it is even an accurate presumption from the facts.

Seems like they should not have referenced the article they did since it has nothing about that discussion of guantum grains and as Bill_K pointed out is on a different topic entirely.

Be interesting to see if there's any followup on this since the quantum grains size statement is really radical.

 

[marcus]

How is it possible that 'quantum grains' are many orders of magnitude smaller than 'strings' or the 'Planck length'?

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.

 

[vela]

I might just be talking out of my butt here, but I'm guessing Laurent is referring to hypothetical violations of Lorentz invariance at high-energy scales.

http://en.wikipedia.org/wiki/Lorentz_covariance#Lorentz_violation

In the paper, Laurent et al. wrote, "On general grounds, Lorentz violating operators of dimension N = n + 2 modify the standard dispersion relations E² = p² + m² by terms of the order of f_npⁿ/M^n-2_Pl where M_Pl is the reduced Planck scale ... used as a reference scale since LIV is expected to arise in the quantum regime of gravity." They derive a correction to the polarization of order
$$\frac{\xi k}{M_\mathrm{Pl}}$$
and extract an upper limit of $\xi \lt 1.1\times 10^{-14}$ from their data. LIV will manifest when
$$\frac{\xi k}{M_\mathrm{Pl}} \backsim 1$$
which corresponds to a length scale of 1/k~10^-48 m. If LIV and quantum gravity arise at the same length scale, the results suggest quantum gravity doesn't kick in when most physicists expect it to. Laurent is interpreting this to mean spacetime remains smooth down to at least that length scale. (The paper does mention, though, that some symmetry, like supersymmetry, could imply $\xi = 0$.)

 

[marcus]

I might just be talking out of my butt here, but I'm guessing Laurent is referring to hypothetical violations of Lorentz invariance at high-energy scales.
...

That is right, vela. In my post I assumed that was what we were talking about.

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...
===

 

[unusualname]

I might just be talking out of my butt here, but I'm guessing Laurent is referring to hypothetical violations of Lorentz invariance at high-energy scales.

http://en.wikipedia.org/wiki/Lorentz_covariance#Lorentz_violation

In the paper, Laurent et al. wrote, "On general grounds, Lorentz violating operators of dimension N = n + 2 modify the standard dispersion relations E² = p² + m² by terms of the order of f_npⁿ/M^n-2_Pl where M_Pl is the reduced Planck scale ... used as a reference scale since LIV is expected to arise in the quantum regime of gravity." They derive a correction to the polarization of order
$$\frac{\xi k}{M_\mathrm{Pl}}$$
and extract an upper limit of $\xi \lt 1.1\times 10^{-14}$ from their data. LIV will manifest when
$$\frac{\xi k}{M_\mathrm{Pl}} \backsim 1$$
which corresponds to a length scale of 1/k~10^-48 m. If LIV and quantum gravity arise at the same length scale, the results suggest quantum gravity doesn't kick in when most physicists expect it to. Laurent is interpreting this to mean spacetime remains smooth down to at least that length scale. (The paper does mention, though, that some symmetry, like supersymmetry, could imply $\xi = 0$.)

Thanks for that, it wasn't obvious to me but your simple explanation looks right

 

[yoda jedi]

How is it possible that 'quantum grains' are many orders of magnitude smaller than 'strings' or the 'Planck length'?

, space is made up of 'quantum grains' which have been shown to be many magnitudes of order smaller than the Planck length. I quote from the article;

not seen yet... (perhaps never...)
the original article do not states that fact.
IF exist a quantum foam HAS to be smaller that Planck sclae, IF not, then, maybe space is continuous.from a previous experiment on high energy photons:
http://arxiv.org/abs/0908.1832

"Even more importantly, this photon sets limits on a possible linear energy dependence of the propagation speed of photons (Lorentz-invariance violation) requiring for the first time a quantum-gravity mass scale significantly above the Planck mass"

that is:
there is not small scale linearity altering the speed of light, According to relativity, the speed of light obeys Lorentz Invariance: it's the same for all observers and all energies of light. but some quantum gravity models suggest that Lorentz Invariance may break down near the Planck length, 1.62 x 10-³³cm, causing high-energy photons to travel at different speeds than their low-energy peers.
quantum theory implies that, at scales trillions of times smaller than an electron, spacetime should be discontinuous. Think of it as frothy or foamy.
In that case, we would expect that shorter wavelength light (at higher energies) would be slowed compared to light at longer wavelength.

"A value this close to the Planck length means that quantum gravity models in which there's a linear relationship between photon energy and speed are "highly implausible." That leaves other quantum gravity options open, including those in which the the relationship is non-linear"

string and loop model predicts a linear relationship....

 

[tom.stoer]

... but some quantum gravity models suggest that Lorentz Invariance may break down near the Planck length, 1.62 x 10-³³cm, causing high-energy photons to travel at different speeds than their low-energy peers.

which models?

 

[yoda jedi]

which models?

"a value this close to the Planck length means that quantum gravity models in which there's a linear relationship between photon energy and speed are "highly implausible." that leaves other quantum gravity options open, including those in which the the relationship is non-linear"

string and loop model predicts a linear relationship.

......
.

 

[tom.stoer]

Sorry, I don't understand; the paper says in the abstract "even more importantly, this photon sets limits on a possible linear energy dependence of the propagation speed of photons (Lorentz-invariance violation)"; the paper does not name one single specific model.

Then you say "string and loop model predicts a linear relationship". Where do you get your statement from? Where and how is this prediction of Lorentz invariance derived? Currently afaik the loop community says that LQG does not predict violation of Lorentz invariance, neither linear nor non-linear.

 

[yoda jedi]

for lqg:

http://arxiv.org/PS_cache/gr-qc/pdf/9809/9809038v1.pdf
http://arxiv.org/PS_cache/hep-ph/pdf/0402/0402271v1.pdf
http://arxiv.org/PS_cache/hep-th/pdf/0603/0603002v1.pdf


for string theory

http://arxiv.org/PS_cache/arxiv/pdf/0804/0804.3566v1.pdf
http://arxiv.org/PS_cache/gr-qc/pdf/9904/9904068v1.pdf



.

 

[unusualname]

for lqg:

http://arxiv.org/PS_cache/gr-qc/pdf/9809/9809038v1.pdf
http://arxiv.org/PS_cache/hep-ph/pdf/0402/0402271v1.pdf
http://arxiv.org/PS_cache/hep-th/pdf/0603/0603002v1.pdffor string theory

http://arxiv.org/PS_cache/arxiv/pdf/0804/0804.3566v1.pdf
http://arxiv.org/PS_cache/gr-qc/pdf/9904/9904068v1.pdf
.

Please post links to abstracts if possible

http://arxiv.org/abs/gr-qc/9809038v1
http://arxiv.org/abs/hep-ph/0402271v1
http://arxiv.org/abs/hep-th/0603002v1

http://arxiv.org/abs/0804.3566v1
http://arxiv.org/abs/gr-qc/9904068v1

 

[tom.stoer]

afaik the results for LQG do not hold in the covariant approach, therefore something must be wrong with this construction. I guess this is de to the fact that the correct regularized Hamiltonian H is still unknown (and therefore the effective ones are incorrect)and that the semiclassical states used are not in the kernel of H.

I don't know a paper where these conclusions have been withdrawn, but today nobody in the LQG community believes in violation of Lorentz invariance.

Perhaps marcus knows more avout the history.

 

[yoda jedi]

Please post links to abstracts if possible

http://arxiv.org/abs/gr-qc/9809038v1
http://arxiv.org/abs/hep-ph/0402271v1
http://arxiv.org/abs/hep-th/0603002v1

http://arxiv.org/abs/0804.3566v1
http://arxiv.org/abs/gr-qc/9904068v1

thanks usualname you did.

june 2011
"we show that non-perturbative treatments like those of loop quantum gravity may generate deviations of Lorentz invariance"

from rodolfo gambini one of the leaders in loop quantum gravity.
.

 

[tom.stoer]

june 2011
"we show that non-perturbative treatments like those of loop quantum gravity may generate deviations of Lorentz invariance"

suprising; again:is there a reference? a paper? or a talk?

 

[yoda jedi]

suprising; again:is there a reference? a paper? or a talk?

a paper,
<< comment removed by Mentor >>

.

 

[eiyaz]

So what is the final outcome on this debate? Does the physics community consider this a blow to ST and LQG as Yoda is saying?

Or do most agree with tom.stoer?

 

[marcus]

*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==

 

[eiyaz]

Oh I see, I understood that the magazine was incorrect, I just wasnt sure what the opnion was. Thanks for the insigit!

 

[yoda jedi]

.


http://arxiv.org/PS_cache/gr-qc/pdf/0502/0502093v1.pdf

"Much research has been done in the latter years on the subject of Lorentz violation induced by Quantum Gravity effects. On the theoretical side it has been shown that both Loop Quantum Gravity and String Theory predict that Lorentz violation can be induced at an energy near to the Planck scale. On the other hand, most of the experimental results in the latter years, have confirmed that the laws of physics are Lorentz invariant at low energy with very high accuracy"

"Quantum Gravity effects would modify the dispersion relations for particle propagation, such as photons. These modifications in turn would change the propagation velocity of photons, introducing delays for particles of different energies which could be detected if these particles would travel cosmological distances. Such modifications of the dispersion relations have been found in the two most popular approaches to Quantum Gravity: Loop Quantum Gravity and String theory. These theories predict corrections to the dispersion relations which depend on energy in the form (EℓP )n, with ℓP = 1.6×10−33 cm the Planck length scale"


.

 

[unusualname]

.


http://arxiv.org/PS_cache/gr-qc/pdf/0502/0502093v1.pdf

"Much research has been done in the latter years on the subject of Lorentz violation induced by Quantum Gravity effects. On the theoretical side it has been shown that both Loop Quantum Gravity and String Theory predict that Lorentz violation can be induced at an energy near to the Planck scale. On the other hand, most of the experimental results in the latter years, have confirmed that the laws of physics are Lorentz invariant at low energy with very high accuracy"

"Quantum Gravity effects would modify the dispersion relations for particle propagation, such as photons. These modifications in turn would change the propagation velocity of photons, introducing delays for particles of different energies which could be detected if these particles would travel cosmological distances. Such modifications of the dispersion relations have been found in the two most popular approaches to Quantum Gravity: Loop Quantum Gravity and String theory. These theories predict corrections to the dispersion relations which depend on energy in the form (EℓP )n, with ℓP = 1.6×10−33 cm the Planck length scale"


.

One good reason for posting a link to the abstract http://arxiv.org/abs/gr-qc/0502093v1

is that it enables checking for citations for the paper http://arxiv.org/cits/gr-qc/0502093

in this case there are 27, some from peer reviewed papers (even though the paper itself is unpublished)

 

[qsa]

.


http://arxiv.org/PS_cache/gr-qc/pdf/0502/0502093v1.pdf

"Much research has been done in the latter years on the subject of Lorentz violation induced by Quantum Gravity effects. On the theoretical side it has been shown that both Loop Quantum Gravity and String Theory predict that Lorentz violation can be induced at an energy near to the Planck scale. On the other hand, most of the experimental results in the latter years, have confirmed that the laws of physics are Lorentz invariant at low energy with very high accuracy"

"Quantum Gravity effects would modify the dispersion relations for particle propagation, such as photons. These modifications in turn would change the propagation velocity of photons, introducing delays for particles of different energies which could be detected if these particles would travel cosmological distances. Such modifications of the dispersion relations have been found in the two most popular approaches to Quantum Gravity: Loop Quantum Gravity and String theory. These theories predict corrections to the dispersion relations which depend on energy in the form (EℓP )n, with ℓP = 1.6×10−33 cm the Planck length scale"


.

from his conclusion


"It should be noted that there is not necessarily a conflict between discreteness and
the absence of a preferred frame. For instance in reference [125] a Lorentz invariant
macroscopic space is constructed by the use of a random causal set of points. On
the other hand, the authors of [126] argue that the existence of a minimum length
does not imply local Lorentz invariance violation, anymore than the discreteness of
angular momentum eigenvalues signal a violation of rotational invariance."

 

[tom.stoer]

The weave states discussed ~5 years ago are problematic as they are not physical states, i.e. they do not belong to the kernel of H, i.e. do not solve the constraint H|phys> = 0.

In http://arxiv.org/PS_cache/gr-qc/pdf/0502/0502093v1.pdf you are mentioning Vucetich explicitly says "There is currently no clear characterization in Loop Quantum Gravity of a semiclassical state. ... In the following, we shall assume that the weave state correctly describes the privileged reference frame S."

In http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.4188v3.pdf Sahlmann writes "We should however point out that since the semiclassical states used in this context
are not strict solutions of the constraints, the results obtained with them are only approximations ... of poorly controllable quality ... and should not be interpreted as firm predictions of the theory".

@Marcus: covariant LQG with local LI does not automatically imply global LI; the latter one is typically broken (like in GR - especially for flat Minkowski space).