On reversibility, there is the observation (I think acredittable to Tom 
that most/all irreversible systems have a reversible subsystem and the 
dynamics arrive in that
subsystem after some (finite) time. Thus any system that we observe a 
after it has started will, with high likelihood, be reversible. In some 
sense the
irreversibility dissipates and leaves a reversible core.


On Apr 10, 2006, at 3:22 AM, Jesse Mazer wrote:

> Saibal Mitra wrote:
>> How would an observer know he is living in a universe in which 
>> information
>> is lost? Information loss means that time evolution can map two 
>> different
>> initial states to the same final state. The observer in the final 
>> state
>> thus
>> cannot know that information really has been lost.
> If he is able to figure out the fundamental laws of physics of his 
> universe,
> then he could see whether or not they have this property of it being
> possible to deduce past states from present ones (I think the name for 
> this
> property might be 'reversible', although I can't remember the 
> difference
> between 'reversible' and 'invertible' laws). For example, the rules of
> Conway's "Game of Life" cellular automaton are not reversible, but if 
> it
> were possible for such a world to support intelligent beings I don't 
> see why
> it wouldn't be in principle possible for them to deduce the underlying
> rules.
> As for the question of why we live in a universe that apparently has 
> this
> property, I don't think there's an anthropic explanation for it, I'd 
> see it
> as part of the larger question of why we live in a universe whose
> fundamental laws seem to be so elegant and posess so many symmetries, 
> one of
> which is time-symmetry (or to be more accurate, CPT-symmetry, which 
> means
> the laws of physics are unchanged if you switch particles with 
> antiparticles
> and flip the 'parity' along with reversing which direction of time is
> labeled 'the future' and which is labeled 'the past'). Some TOEs that 
> have
> been bandied about here say that we should expect to live in a universe
> whose laws are very compressible, so maybe this would be one possible 
> way of
> answering the question.
> Jesse
> >
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