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.
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