X-Message-Number: 14961
Date: Sun, 19 Nov 2000 18:19:05 -0700
From: Mike Perry <>
Subject: Machines and Brains

A lot has been said lately on whether a brain could be considered a "Turing
machine," or could be realized as such a device, but I see a need for
clarification.The point is made, and well made, that in practical terms we
may never see such an implementation. Certainly this is true if taken
literally, but come to think of it, what thing in our world today *is* a
Turing machine (ever see one?), and by extension, do we expect to see one in
the future, except as some curiosity? By this I am referring to the literal
notion of a finite-state device with an infinitely long (or finite but
indefinitely extendible), 1-dimensional tape it runs back and forth on,
marking and/or erasing symbols of a fixed, finite alphabet. (It might be fun
to try to implement this, just to show how it could be done. Maybe you could
use something like a model train set. Have the machine itself be an
electrically powered boxcar that runs back and forth on a track divided into
squares each of which is equipped with a liquid crystal display to show
different symbols. The car would sit on a square and have some way of
electronically detecting what symbol was showing, and also making changes to
both the symbol and its own state, then would move right or left one square
or halt. Probably straightforward to do, and I think it would make a good
instructional tool for computer science classes, but I've never heard of it
being done.)

But the point is made that Turing machines and real computers can be
considered equivalent devices, inasmuch as computations in either form can
be done in equivalent form in the other device. In either case the behavior
of the system is "quantized"--over finite time and distances you have a
finite number of discrete jumps or sudden changes, such that whatever is
happening in between the jumps is unimportant, so long as the jumps occur.
So in this more general sense we could ask if the brain too is such a
device, i.e. a "Turing machine." At the quantum level it seems we must
concur with that judgment; the Bekenstein bound puts a limit of around 4 x
10^53 (discrete) state changes per second in an object the size of a human
brain. In other ways the possible changes conform to a finite-state device,
with allowance for probabilistic effects. But the number of changes is so
large that in practical terms many possible ways of realizing them are
effectively ruled out, or maybe rendered as curiosities that, assuming
immortality is possible, we still won't encounter except very, very
marginally and rarely. Thus the Turing machine "running" the human seems
most unlikely, if again you mean the device with its 1-dimensional tape. But
the question remains of what other non-meat devices might make suitable
substitutes for your brain and mine, if well configured. For this we'll have
to wait and see. My hunch is we will find such devices, as our
miniaturization approaches the nano scale, and also, that consciousness is
really not so mysterious, being mainly a way to limit the attention of a
sensing, acting system or entity, so it can make important decisions more
efficiently.

Mike Perry

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