Quantum grains (String theory & LQG in trouble?)

In summary, the article discusses a recent paper which suggests that Lorentz invariance may not be a necessary condition for the existence of quantum gravity. This paper suggests that the "grains" that underlie space may be much smaller than previously predicted, and that this theory may be a viable candidate for quantum gravity.
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  • #2
Fyzix said:

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.
 
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  • #3
marcus said:
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.

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
 
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  • #4
I think it's very missleading:
Science Daily said:
It has shown that any underlying quantum 'graininess' of space must be at much smaller scales than previously predicted
suggests that the "grains" themselves must be much smaller - which is not necessarily the case. Instead the effects of these grains need to be much smaller. So if there is a theory which is compatible with Planck-space grains but w/o any violation or deformation of Lorentz invariance at all (like LQG) then this theory remains to be a perfectly valid candidate theory for quantum gravity.

In that sense science daily does not make a good job.

Has anybody written a comment on their web page?
 

Related to Quantum grains (String theory & LQG in trouble?)

1. What is string theory and how does it relate to quantum grains?

String theory is a theoretical framework in physics that attempts to reconcile the theories of general relativity and quantum mechanics. It proposes that particles are not point-like objects, but instead are made up of tiny vibrating strings. Quantum grains, also known as Planck-scale structures, are the smallest possible units of space and time, and are predicted by string theory.

2. What is the current status of string theory and loop quantum gravity (LQG)?

Both string theory and LQG are still actively being researched and developed. However, they both face challenges and open questions, and it is currently unclear which theory (if either) will ultimately be able to fully explain the nature of the universe at the quantum level.

3. How do quantum grains play a role in the search for a theory of everything?

Quantum grains are a key concept in both string theory and LQG, and they are believed to be the fundamental building blocks of the universe. By understanding the properties and behavior of quantum grains, scientists hope to gain a deeper understanding of the fundamental laws of nature and potentially unify the theories of general relativity and quantum mechanics.

4. Are there any experimental tests being conducted to support or disprove string theory and LQG?

At this time, there are no direct experimental tests that can definitively prove or disprove string theory or LQG. However, scientists are exploring ways to indirectly test these theories through observations of the universe and particle accelerators.

5. What are some potential implications of string theory and LQG if they are proven to be correct?

If either string theory or LQG is proven to be correct, it would have vast implications for our understanding of the universe. It could potentially lead to a unified theory of physics that could explain phenomena such as gravity, the behavior of subatomic particles, and the origin and evolution of the universe itself.

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