Jason wrote:

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.

I think Feynman showed that a reversible computer has general computing power.

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.

That's exactly our situation.  But nobodies worried because the difference 
between the maximum entropy of the universe and it's current entropy is some 80 
orders of magnitude - besides which it's expansion is accelerating.

Brent Meeeker

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