Rovelli/Perez: Physical Effects of the Immirzi Parameter

[marcus]

there's a new 3 page paper by Carlo Rovelli and Alejandro Perez
http://arxiv.org/abs/gr-qc/0505081
Physical effects of the Immirzi parameter

"The Immirzi parameter is a constant appearing in the version of the general relativity action utilized as a starting point for the loop quantization of gravity.The parameter is commonly believed not to show up in the equations of motion, because it appears in front of a term in the action that vanishes on shell. We show that in the presence of fermions, instead, the Immirzi term of the action does not vanish on shell, and the Immirzi parameter appears in the equations of motion. It is the coupling constant of a parity violating four-fermion interaction. Therefore the nontriviality of the Immirzi parameter leads to effects that are observables in principle, even independently from nonperturbative quantum gravity."

this paper was noted in the Loop-and-allied link library thread where new papers usually just get posted without much discussion
https://www.physicsforums.com/showthread.php?p=570481#post570481
and some interest arose, so maybe we should have a thread devoted to this paper

Spin_Network commented that the paper was helpful and ohwilleke had a request for explanation.

https://www.physicsforums.com/showthread.php?p=571326#post571326

BTW this paper cites the recent work of Freidel and Starodubtsev "Quantum Gravity in Terms of Topological Observables" (hep-th/0501191) and also a paper by Stephon Alexander (hep-th/0503146) which IIRC Kea started a thread about here at PF. Both of those papers caused some stir. they both showed the Immirzi parameter playing a significant (and for me unexpected role).

Personally I cannot give the "connect-the-dots" clarification that ohwilleke requested, but am hopeful some person or persons here can. You never know who will volunteer.

 

[marcus]

a key quote, from right at the end, which would benefit from some extra discussion:
"The value of the Immirzi parameter is therefore observable in principle also independently from its effect on the nonperturbative quantum theory. The analogy with the \Theta angle of QCD is, in this regard, misleading (see also [9]). The Immirzi parameter is precisely the ratio between the strengths of the parity preserving and the parity violating four fermion interactions."


Here is ohwilleke's question, post #338 from the "link basket" thread

Can anyone connect the dots and explain what sort of context one could observe the Immirzi parameter effects in?

Can anyone think up a context where the ratio between the strengths of two different interactions would come out, so that IF ROVELLI IS RIGHT the Immirzi parameter could at least in principle be observed?

 

[R0man]

Thank you for bringing this interesting paper to our attention. The Immirzi parameter has been a topic of much discussion in the field of loop quantum gravity, and this paper sheds new light on its physical effects.

The authors show that in the presence of fermions, the Immirzi term of the action does not vanish on shell, as it does in the absence of fermions. This leads to the Immirzi parameter appearing in the equations of motion, and it is the coupling constant of a parity violating four-fermion interaction.

This result is significant because it shows that the nontriviality of the Immirzi parameter can lead to observable effects, even without considering nonperturbative quantum gravity. This is a promising development, as it opens up the possibility of testing the Immirzi parameter experimentally.

The paper also cites the work of Freidel and Starodubtsev, as well as Stephon Alexander's paper, which have also shown the importance of the Immirzi parameter in different contexts. This further highlights the significance of this parameter in understanding loop quantum gravity and its physical implications.

Although the paper does not provide a comprehensive explanation of the connections between these different works, it does offer a new perspective on the role of the Immirzi parameter in the equations of motion. It will be interesting to see how this result is further developed and tested in future research.