Brent Meeker wrote:
I don't think the calculation has to be reversible in order to be the 
calculation of a reversible phenomena.  We use irreversible computations all 
the time to calculate simple Newtonian processes which are certainly reversible.

I agree that a computation of a reversible phenomenon could make use of
non-revsible computations, but the bigger question is: are any
reversible physical processes that cannot be simulated using only
reversible computations?  I haven't been able to think of any such
examples off hand, but if you are aware of any I would be interesting
in seeing them.

They could simulate a smaller or simpler universe, which is what, for example, 
a computer game does.  The irreversible computation just dumps entropy into the 
universe which is far below it's maximum entropy (thanks to inflation).  This 
of course depends on a a coarse-grained view of entropy.  At the microscopic 
level, if we could keep track of all the quantum entanglement we'd presumably 
see that the fine-grained entropy doesn't increase.

Even if the simulated universe was much smaller and simpler than their
own, each time a non-reversible computation was performed it would
decrease the amount of useful energy available to that civilization,
and when that amount reached zero they would die.  Note that I am also
assuming that the minds of everyone in such a civilization are also
being computed on this maximally efficient computer which is designed
to run forever.  Therefore those inside it would be averse to running
any computations that would deplete useful energy, even if only a small

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