On the quantum scale, the dynamics are reversible, to a very high
level of accuracy. Irreversibility appears at macroscopic scales.

In answer to the original question, I would conjecture that an
evolutionary process is the only process capable of generating
complexity. Since we need a certain amount of complexity to be
conscious, it follows that the simplest universes are ensembles of
possibilities, on which anthropic selection acts to generate the
needed complexity. Ensemble universes can only evolve by reversible
processes - otherwise possibilities are irretrievably lost over time.

On Sun, Nov 23, 2003 at 10:55:10PM -0500, Stephen Paul King wrote:
> Dear David,
> 
>     Please explain the claim : "We observe that our universe uses a reversible 
> computation". I do not see how this follows from the observation that, on every 
> observable scale, there is a non-invertible (thermodynamic) arrow of time. I do not 
> see how this is possible if your claim holds. We can add to this the strong evidence 
> that our universe is open and very close to being "flat".
> 
> 
> Kindest regards,
> 
> Stephen
> 
>   ----- Original Message ----- 
>   From: David Barrett-Lennard 
>   To: [EMAIL PROTECTED] 
>   Sent: Sunday, November 23, 2003 9:14 PM
>   Subject: Move versus assign
> 
> 
>   We observe that our universe uses a reversible computation,  yet our brains only 
> appear to use irreversible computation.  It seems important to ask why.   Is it 
> possible for SAS's to live in a universe that is directly associated with an 
> irreversible computation?  If so then why are we special?
> 
>    
> 
>   Computer science seems to be centered around the concept of "assignment".  For 
> example, computer memory undergoes state changes in the form of assignments to 
> memory locations.  A Turing machine uses assignment operations each time a 1 or 0 is 
> written on the tape.  Assignment involves lost information because it simply 
> overwrites the previous value with a new value.  It is fundamentally irreversible.
> 
>    
> 
>   I have been wondering whether we can get a better understanding of reversible 
> computation by distinguishing between movement of information and assignment of 
> information.  The analogy of the Turing machine would be that we need to cut up the 
> tape with scissors - we are only allowed to move bits of tape around, rather than 
> reassign values on the tape.   This leads quickly to the view of particles that move 
> around,  rather than the idea of a particle that is stored in space (= memory) that 
> moves as the result of "assignments to space".
> 
>    
> 
>   So rather than think of a small piece of space having an attribute of what 
> particle is in it, we should think of a particle as having an attribute of where it 
> is in space.  The latter view makes space seem rather incidental - rather than 
> thinking of particles as being embedded in space.   I wonder to what extent 
> physicists distinguish between these two views.  
> 
>    
> 
>   I guess the distinction evaporates in string theory, where there is nothing but 
> (higher dimensional) space-time.   There is nothing to assign to because the 
> information is present in the topology of space itself.  Movement of information is 
> more like a ripple on a pond.
> 
>    
> 
>   The Turing machine seems to lack a direct relevance to our universe.   However, 
> can't a Turing machine emulate a reversible computation?
> 
>    
> 
>   - David
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-- 


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A/Prof Russell Standish                  Director
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