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 usefulenergy 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 --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-list@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/everything-list?hl=en -~----------~----~----~----~------~----~------~--~---