QMD running G and Lambda challenges Loop Gravity

[marcus]

"QMD" running G and Lambda challenges Loop Gravity

As I see it, and I may be missing the point entirely, this is a moment of opportunity and change for LQG and related researches.

In brief, the story is that for several years Reuter has been saying that Newton's G and the cosmological Lambda RUN with increasing k. He and other people have been showing how this happens, but until now the QG consequences haven't been adequately studied.

The k parameter is an index of proximity and energy of interaction---its dimension is reciprocal length. As k --> infty, says Reuter, G and Lambda converge to their BARE values. (and the corresponding action coverges to what he calls the bare action.)
If LQG or any other non-string QG is to be considered a FUNDAMENTAL theory of the microscopic degrees of freedom where geometry and matter interact, then presumably it should be based on the bare Einstein-Hilbert action determined by the bare G and Lambda.
The bare G is less than everyday value (something Reuter calls "anti-screening") and the bare Lambda is much larger than the measured cosmological constant.

The bare E-H action is what Reuter et al have discovered to be the FIXED POINT towards which the renormalization flow converges. They seemingly do not have to put it in by hand---they specify what the theory is about and what its symmetries are and they claim they get bare E-H out, in other words it is not an assumption but a prediction of the theory.

If one takes seriously what Reuter says, then it seems as if LQG theorists should be using NOT the everyday (low k) versions of G and Lambda, which we can measure by macroscopic observation in our low-energy world, but rather the bare (high k) versions of G and Lambda.

Until around June 27 I had been thinking that Reuter's work was very interesting---and I was glad that the LQG community included him as a plenary speaker at Loops 05 and the Zakopane QG school---but I wasnt thinking much about the consequences of having constants run. Reuter's June 27 talk at Loops 07 triggered something.

One event that rang a bell, or sounded an alert, was that, just a month after Reuter's talk, Ted Jacobson posted his Renormalization and Black Hole paper. This takes seriously the idea that G runs and that the bare value is different from the everyday. And it seems to urge that people should take stock of the consequences of that. And, whether it turns out to be significant or not, in his recent paper Jacobson cites a Reuter paper. He doesn't make a big deal of it, but he includes that gesture.

Curiously enough this is not even new, in a sense. Reading LQG papers even from back in the 1990s, I recall having encountered references to the bare value of G---the awareness that G may vary with scale has always been there! But I don't think people back then had as clear a picture as they do now of HOW it runs. Reuter has plotted a trajectories for both G and Lambda. So the whole business seems to have a less abstract, more down-to-earth feel. Jacobson's paper exemplifies this---he shows how it can make a difference when one considers black hole entropy.

Jacobson's paper: http://arxiv.org/abs/0707.4026

Reuter's 27 June talk at Loops 07
https://www.physicsforums.com/showthread.php?t=178572

slides PDF:
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.pdf

audio MP3 :
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.mp3

 

[AndyW]

Thank you for bringing this interesting topic to our attention. I agree that the running of G and Lambda has important implications for Loop Quantum Gravity and other non-string Quantum Gravity theories. It is certainly a moment of opportunity and change, as you said.

The idea of a fixed point towards which the renormalization flow converges is intriguing and certainly deserves further investigation. If the bare E-H action is indeed a prediction of the theory, it would be a significant step towards a more fundamental understanding of gravity and its interaction with matter.

I also find it interesting that this idea is not entirely new and has been mentioned in LQG papers from the 1990s. However, with the recent work of Reuter and Jacobson, we now have a clearer picture of how G and Lambda actually run and what the consequences may be.

I believe it is important for the LQG community to consider these implications and potentially revise our understanding of G and Lambda in light of this new information. I look forward to seeing how this research develops and what new insights it may bring to our understanding of Quantum Gravity. Thank you again for bringing this to our attention.

 

[Entropia]

Thank you for sharing this information. It is certainly an interesting and potentially game-changing development in the field of loop quantum gravity. The idea that the fundamental constants of gravity, G and Lambda, may actually vary with scale is a significant departure from the traditional understanding of these constants as fixed values. This could have far-reaching implications for our understanding of the microscopic world and the interactions between geometry and matter.

It is encouraging to see that Reuter's work is being taken seriously and that it has sparked further research and discussion, as evidenced by Jacobson's recent paper. It will be important for the LQG community to carefully consider the consequences of this new understanding of G and Lambda, and to explore how it may impact our current theories and models.

I look forward to seeing how this development unfolds and how it may contribute to our understanding of quantum gravity. Thank you for bringing this to my attention.