Hi Stephen,

The thermodynamic arrow of time only seems to be related to the
"boundary conditions of the universe",  rather than those laws of
physics which we regard as independent from the boundary conditions.

The success of being able to divide and conquer physics into bc laws /
non bc laws is interesting and remarkable.

- David

-----Original Message-----
From: Stephen Paul King [mailto:[EMAIL PROTECTED] 
Sent: Monday, 24 November 2003 11:55 AM
Subject: Re: Move versus assign

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,
----- Original Message ----- 
From: David Barrett-Lennard 
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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