Jason wrote:
>
> It's been known since the 1970s that arbitrarily efficient computers
> could be constructed that could perform an infinite number of
> computations with a finite amount of energy, but only if the
> computations done on that computer are logically reversible.
> Performing a non-reversible computation results in an increase in
> entropy for the system and thus would not be sustainable. (see
> http://en.wikipedia.org/wiki/Reversible_computing)
>
> The physical interactions that occur in this universe are also
> reversible. e.g. An electron can accept a photon and move to a higher
> energy state or an electron can emit a photon and move to a lower
> energy state. Does reversible physics imply that a computational model
> of said physcis would involve entirely reversible computations? I
> believe that if past states of the universe could be calculated from
> future ones, then those computations would have to be reversible.
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.
>
> Assuming the above is true, it would have consequences for any
> civilization in a universe like this one (with finite energy); it would
> mean that said civilizations could only simulate universes using purely
> reversible computations without exhausting the finite amount of useful
> energy in their universe.
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.
Brent Meeker
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Received on Sun Jan 14 2007 - 03:11:04 PST