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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Received on Sun Jan 14 2007 - 15:35:23 PST