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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