From: rickert@mp.cs.niu.edu (Neil Rickert)
Subject: Re: Robert Rosen & Physical form of Church's Thesis
Organization: Northern Illinois University
Date: Fri, 10 Apr 1992 14:24:24 GMT

In article <TogZiB1w164w@cybernet.cse.fau.edu> tomh.bbs@cybernet.cse.fau.edu writes:
>Robert Rosen has a new book out, called "Life Itself" which
>addresses many of the issues that have been discussed here.
>It is similar in spirit to Penrose's book, but Rosen argues
>the points much more effectively, I think.  I heartily
>recommend it to anyone who reads this group.

  Sounds interesting.

>Rosen claims that the physical form of Church's Thesis is false.
>This means there are physically realizable processes that are
>not effectively calculable.  He also claims that life and probably
>'intelligence' are such processes.

  Has anybody claimed that life is effectively computable?

>One candidate is a turbulent flow, which (mathematically) is
>aperiodic.  Clearly, any simulation on a digital computer is
>going to be periodic, by the finiteness of the number of states
>of the computer.  Therefore a digital computer cannot simulate an
>aperiodic process perfectly.

  It would not surprise me if turbulent flow were shown to be not effectively
computable.  But the argument based on periodicity is quite unconvincing.
Even a very small computer has enough states that it can change state every
nanosecond and not repeat itself for a time much longer than the expected
life time of the galaxy containing the turbulent flow.  If you prefer to
think about an idealized eternal turbulent flow, it is only fair to also
allow an idealized computer (with an infinite tape).

>Issues this raises are:
>1) Is a physical turbulent flow really aperiodic?  If not then brains
>   are computers and we can all go home.

  It is not apparent how you reach this conclusion about brains.

>2) Is a turbulent flow an effective form of computation?  That is,
>   does a turbulent flow comprise an algorithm for computing something?
>3) What happens if we use an analog computer instead of a digital one?
>   The analog computer still has the 'initial condition' problem.

  Unless you hypothesize an analog computer of infinite precision, I don't
see what the analog computer would provide.  You can simulate an analog
computer on a digital computer.

>Perhaps the most important issue:
>...
>Another way to say this is that two simulations of a brain (with even the
>tiniest difference in input) will diverge from each other, as well as

  This sounds like a way of saying that even two identical twins will not
always behave identically.  I don't see it as causing any problems as far
as AI is concerned.

>Here's the reference:
>
>Life Itself
>Robert Rosen
>Columbia University Press, NY, 1992
>Complexity in Ecological Systems Series

  Thanks.

>The book covers a lot of ground, syntax/semantics, Goedel, etc.
>He uses Category theory to formalize it all.  I may post a
>summary later, but I wanted to get these questions posted first.

  I suspect many readers of this newsgroup would appreciate the summary.

-- 
=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
  Neil W. Rickert, Computer Science               <rickert@cs.niu.edu>
  Northern Illinois Univ.
  DeKalb, IL 60115                                   +1-815-753-6940
Newsgroups: comp.ai.philosophy