# Rovelli/Perez: Physical Effects of the Immirzi Parameter

1. May 18, 2005

### 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
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.

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.

Last edited: May 18, 2005
2. May 18, 2005

### 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."