| Subject: Relational physics and quantum gravity
Date: Wed, 14 Apr 2004 15:39:37 +0300 From: Dimi Chakalov <dimi@chakalov.net> To: Olaf Dreyer <odreyer@perimeterinstitute.ca> CC: sorkin@physics.syr.edu, tthiemann@perimeterinstitute.ca, fotini@perimeterinstitute.ca, dgottesman@perimeterinstitute.ca, rmyers@perimeterinstitute.ca, dibarto@usb.ve, rgambini@fisica.edu.uy, pullin@rouge.phys.lsu.edu, mfrank@math.uchicago.edu, Curt.Cutler@aei.mpg.de Dear Dr. Dreyer, In your latest paper "Relational Physics and Quantum Space", gr-qc/0404054 [Ref. 1], you argued that "the dynamics of the theory is required to make progress on important issues like the question of the semi-classical limit of the theory", referring to loop quantum gravity and causal set theory. It seems to me that 'the dynamics of the theory' cannot be revealed in principle, since in GR we have some purely relational "coordinate time": there is no unique notion of time in GR. "Any observable is invariant under change of t , therefore everything is constant in this t just by gauge invariance", says Carlo Rovelli, http://members.aon.at/chakalov/Rovelli.html#note Hence the intrinsic dynamics is hidden in GR from the outset. Subsequently, we cannot, even in principle, isolate something called "the gravitational field stress-energy" in GR, http://members.aon.at/chakalov/Frank.html David Hilbert stressed this feature of Einstein's GR in November 1917, and Karel Kuchar identified two basic problems of quantum gravity in 1992: "classical geometrodynamics does not seem to possess a natural time variable, while standard quantum theory relies quite heavily on a preferred time", http://members.aon.at/chakalov/Pullin.html#7 Everything boils down to the nature of time, correct? See my proposal at http://members.aon.at/chakalov/Nicolai.html I will appreciate your professional feedback, as well as the opinion of your colleagues. It's about time, really. Regards, D. Chakalov [Ref. 1] Olaf Dreyer (April 14, 2004),
Relational Physics and Quantum Space, gr-qc/0404054 v1,
"To define a notion of distance in a relational way it
was necessary to have access to the dynamics of the theory. A purely kinematic
approach is not sufficient. It is here where some of the candidate theories
of quantum gravity, like Loop Quantum Gravity [3] or Causal Set Theory
[4], face their greatest problems. The arguments presented here suggest
that the dynamics of the theory is required to make progress on important
issues like the question of the semi-classical limit of the theory.
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