Mark Peaty wrote:
> Hello Jason,
> please excuse my ignorant interjections here but, as a
> non-mathematician, non-philosopher, I need to work things into a plain
> English version before I can feel that I understand them, and even then
> the edges of things get fuzzy with far more ease than they get straight
> and clear cut. Furthermore I am beginning to wonder if the apparently
> 'straight' and clear cut boundaries to concepts and so forth are not
> merely figments of my imagination. I don't think I go anywhere as far as
> John M. in this but then maybe that is just because I fear to let go of
> my sceptical reductionist walking stick. :-)
>
> Jason: 'perform an infinite number of
> computations with a finite amount of energy, but only if the
> computations done on that computer are logically reversible.'
>
> MP: Surely 'logically reversible' does not necessarily imply no entropy,
> just that for the purposes of the concerned observer, the computing
> system can return to a state that is sufficiently close to the original
> state so that the inputs can be discovered. More or less by definition,
> entropy increases and manifests as deterioration of the substrate and as
> the need to supply more energy to travel through the system than
> otherwise is calculated to be necessary to obtain the minimum changes
> needed to embody the changes of state in the calculating system.
I think there is a confusion creeping in here. I don't think "logically reversible" is misleading. It is only physical processes that can be termed reversible or irreversible. Logic lives in a timeless Platonia. Computers operated irreversibly, they dissipate heat when they they erase data. Feynman pointed out that this was not necessary and a computer that did not erase data could operate without dissipating heat (no increase in entropy).
Brent Meeker
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Received on Sun Jan 14 2007 - 15:33:42 PST