On 9/16/2012 6:11 PM, Jason Resch wrote:

On Sep 16, 2012, at 5:00 PM, "Stephen P. King" <stephe...@charter.net> wrote:

On 9/16/2012 3:06 PM, Jason Resch wrote:
Why does a physical system have to be non-invertible? My understanding is that current physical laws imply that systems are invertible.
Hi Jason,

   Say hello to the "problem of time".

What's the problem?


    Try this: http://arxiv.org/abs/1009.2157

 The Problem of Time in Quantum Gravity

Edward Anderson <http://arxiv.org/find/gr-qc/1/au:+Anderson_E/0/1/0/all/0/1>
(Submitted on 11 Sep 2010 (v1 <http://arxiv.org/abs/1009.2157v1>), last revised 20 Jul 2012 (this version, v3))

   The problem of time in quantum gravity occurs because `time' is
   taken to have a different meaning in each of general relativity and
   ordinary quantum theory. This incompatibility creates serious
   problems with trying to replace these two branches of physics with a
   single framework in regimes in which neither quantum theory nor
   general relativity can be neglected, such as in black holes or in
   the very early universe. Strategies for resolving the Problem of
   Time have evolved somewhat since Kuchar and Isham's well-known
   reviews from the early 90's. These come in the following divisions
   I) time before quantization, such as hidden time or matter time. II)
   Time after quantization, such as emergent semiclassical time. III)
   Timeless strategies of Type 1: naive Schrodinger interpretation,
   conditional probabilities interpretation and various forms of
   records theories, and Type 2 `Rovelli': in terms of evolving
   constants of the motion, complete observables and partial
   observables. IV) I argue for histories theories to be a separate
   class of strategy. Additionally, various combinations of these
   strategies have begun to appear in the literature; I discuss a
   number of such. Finally, I comment on loop quantum gravity,
   supergravity and string/M-theory from the problem of time perspective.




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