Re: Is the universe computable

From: Hal Finney <>
Date: Tue, 27 Jan 2004 10:33:33 -0800

Kory Heath writes:
> Forget about our own (potentially non-computable) universe for a second.
> Surely you agree that we can imagine some large-but-finite 3+1D CA (it
> doesn't have to be anything like our own universe) in which the state of
> each bit is dependent on the states of neighboring bits one tick in the
> "future" as well as one tick in the "past". Surely you agree that we could
> search through all the possible 4D cube bit-strings, discarding those that
> don't follow our rule. (This would take a Vast amount of computation, but
> that's irrelevant to the particular questions I'm interested in.) Some of
> the 4D cubes that we're left with will (assuming we've chosen a good rule
> for our CA) contain patterns that look all the world like SASs, moving
> through their world, reacting to their environment, having a sense of
> passing time, etc.

That is indeed a fascinating thought experiment, and I agree with
everything up to the last part. Are you sure that a CA whose state
depends on the future as well as the past can have self aware subsystems?
This seems different enough from our own physics that I'm not sure that we
can assume that it will work like that. I'm not saying it can't happen,
but I'm curious to see evidence that it can.

Our own universe's microphysics appears to be basically reversible, and
I remember that Wolfram's book had some CAs, I think universal ones,
which could be expressed in reversible terms. A reversible CA is one
where the present state can be deduced either from the future or the

But I think you're talking about something stronger and stranger, where
you'd need to know both the future and the past in order to compute
the present. This puts your questions about "when" the consciousness
exists in a much sharper light. (I do have answers to those questions
which I have somewhat explained in recent postings.)

One way to approach an answer to the question is to ask, is there such
a CA in which a universal computer can be constructed? That would be
evidence for at least a major prerequisite for conscious observations.
Do you have any examples like this?

Hal Finney
Received on Tue Jan 27 2004 - 15:57:49 PST

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