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

[EMAIL PROTECTED]

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.


>



--~--~---------~--~----~------------~-------~--~----~
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
-~----------~----~----~----~------~----~------~--~---

Reply via email to