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.
Jason: '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 physics would involve entirely reversible computations? '
MP: This concept of 'reversible' is very useful, but to how great an
extent is it just a convenient fiction? My understanding is that you
can't fire *a particular* photon at a particular atom and guarantee that
your favourite electron will rise to the predicted level. I mean it
either will or it won't. Conversely as I understand it [AIUI] the
subsidence of an electron to a lower orbital is only predictable in a
statistical sense. Once again is it not that the real world entities
must be dealt with in a statistical manner, either as bulk substances,
predictable due to the averaging of activities of the individual quantum
particles, or as individual quantum items manifesting radical
indeterminacy? Either way AIUI, the computational model will manipulate
symbols denoting the real world physics and there is no guarantee that
any such computing system could overcome the limits imposed by entropy
and quantum indeterminacy.
Regards
Mark Peaty CDES
mpeaty.domain.name.hidden
http://www.arach.net.au/~mpeaty/
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.
>
> 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. This also hits on a topic Wei Dai brought up
> earlier about how it seems impossible to delete any information in this
> universe.
>
>
> >
>
>
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Received on Sun Jan 14 2007 - 05:01:15 PST