Rovelli/Perez: Physical Effects of the Immirzi Parameter

In summary, Carlo Rovelli and Alejandro Perez have published a new paper (arXiv:gr-qc/0505081) which discusses the physical effects of the Immirzi parameter in loop quantum gravity. They show that the parameter, which is usually believed to not show up in the equations of motion, can appear in the presence of fermions and is the coupling constant of a parity violating four-fermion interaction. This means that the nontriviality of the Immirzi parameter could potentially lead to observable effects, even independently from nonperturbative quantum gravity. The paper has sparked some interest and discussion in the physics community, as it cites previous work by Freidel and Starodubtsev and Steph
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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.
 
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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 [tex]\Theta [/tex] 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

ohwilleke said:
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?
 
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  • #3


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.
 

1. What is the Immirzi parameter and how does it relate to Rovelli/Perez's work?

The Immirzi parameter is a dimensionless constant in loop quantum gravity that is used to calculate the quantum mechanical effects of space-time geometry. Rovelli and Perez's work focuses on the physical consequences of this parameter and how it affects our understanding of black holes and other phenomena in the universe.

2. How does Rovelli/Perez's work contribute to our understanding of black holes?

Rovelli and Perez's work has shown that the Immirzi parameter plays a crucial role in determining the entropy (or disorder) of black holes. They have also proposed a new way of calculating the entropy of black holes that takes into account this parameter, which could potentially lead to a better understanding of the nature of black holes.

3. What are some of the key findings of Rovelli/Perez's research on the Immirzi parameter?

Rovelli and Perez's research has revealed that the Immirzi parameter has significant effects on the microscopic structure of space-time and the behavior of quantum particles. They have also found that this parameter can have a significant impact on the thermodynamic properties of black holes and other objects in the universe.

4. How does the Immirzi parameter affect the concept of "quantum gravity"?

The Immirzi parameter is a crucial factor in loop quantum gravity, which is a theory that aims to reconcile the principles of quantum mechanics and general relativity. Rovelli and Perez's work on this parameter has shed light on the nature of quantum gravity and how it relates to our understanding of space and time.

5. What are some potential applications of Rovelli/Perez's research on the Immirzi parameter?

Rovelli and Perez's work has the potential to impact various fields of physics, including cosmology, quantum information, and quantum gravity. Their findings could also have practical applications in areas such as quantum computing and space technology. Additionally, their research could lead to a better understanding of the fundamental laws of the universe and how they govern the behavior of matter and energy.

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