Re: Tegmark is too "physics-centric"

From: Kory Heath <kory.heath.domain.name.hidden>
Date: Sun, 18 Jan 2004 07:15:45 -0500

At 1/17/04, Hal Finney wrote:
>But let me ask if you agree that considering Conway's 2D
>Life world with simply-specified initial conditions as in your example,
>that conscious life would be extraordinarily rare?

I certainly agree that it would be "extraordinarily rare", in the sense
that the size of the lattice would need to be very big, and the number of
clock-ticks required would need to be very large. But "big" and "large" are
such relative terms! Clearly, our own universe is very, very big. The
question is, how can we sensibly determine whether life is more likely in
our universe or in Conway's Life universe?

I don't believe we have anywhere near enough data to answer this question,
but I don't think it's unanswerable in principle. Fredkin actually believes
that our universe is a 3+1D cellular automata, and if anyone ever found
such a description of our physics (or some other fundamentally
computational description), then we could directly compare it with Conway's
Life, determining for each one how big the lattice needs to be, and how
many clock-ticks are required, for life to appear with (say) 90%
probability. (Of course, this determination might be difficult even when we
know the rules of the CAs. But we can try.)

One thing that you'd have to take into account is the complexity of the
rules you're comparing, including the number of states allowed per cell.
Not only are the rules to Conway's Life extremely simple, but the cells are
binary. All things being equal, I would expect that an increase in the
complexity of the rules and the number of cell-states allowed would
decrease the necessary lattice-size and/or number of clock-ticks required
for SASs to grow out of a pseudo-random initial state. I mention this to
point out a problem with our intuitions about our universe vs. Conway's
Life: the description of our universe is almost certainly more complex than
the description of Conway's Life with a simple initial state. If Fredkin
actually succeeds in finding a 3+1D CA which describes our universe, it
will almost certainly require more than 2 cell-states, and its rules will
certainly be more complex than those of the Life universe. We have to take
this difference into account when trying to compare the two universes, but
we have nowhere near enough data to quantify the difference currently. We
really don't know what size of space in the Life universe is equivalent to
(say) a solar system in this universe.

In a way, this is all beside the point, since I have no problem believing
that one CA can evolve SASs much more easily than some other CA whose rules
and initial state are exactly as complex. (In fact, this must be true,
since for any CA that supports life at all, there's an equally complex one
that isn't even computation universal.) I have no problem believing that
the Life universe is, in some objective sense, not very conducive to SASs.
Perhaps it's less conducive to SASs than our own universe, although I'm not
convinced. What I have a problem believing is that CAs as a class are
somehow less conducive to observers than quantum-physical models as a
class. In fact, I think it's substantially more likely that there are
relatively simple CA models (and other computational models) that are much
more conducive to SASs than either Conway's Life universe or our own.
Models in which, for instance, neural-net structures arise much more
naturally from the basic physics of the system than they do in our
universe, or the Life universe.

>In many ways, our universe seems tailor made for creating observers.

I understand this perspective, but for what it's worth, I'm profoundly out
of sympathy with it. In my view, computation universality is the real key -
life and consciousness are going to pop up in any universe that's
computation universal, as long as the universe is big enough and/or it
lasts long enough. (And there's always enough time and space in the
Mathiverse!) When I think about the insane, teetering, jerry-rigged
contraptions that we call life in this universe - when I think about the
tortured complexity that matter has to twist itself into just to give us
single-celled replicators - and when I think about the insane reaches of
space we see around us (even if we end up finding life in practically every
solar system, there's a crazy amount of space even between planets, not to
mention stars) - I find it easy to believe that our universe is just one of
those countless universes out there in Mathspace which isn't especially
conducive to life at all, but is simply computation universal, so life pops
up eventually.

Because of the above conclusions, the problem of measure is a serious one
for me. I don't have a clue why I would be more likely to find myself in a
universe like this one instead of some CA universe. Regarding your
suggestion that we might judge universes not only by the complexity of
their rules and initial states, but also by the complexity of the simplest
program which "finds" SASs within them: as I understand the proposal,
universes (or observer-moments within the universes) which have simple
rules and initial states and yet generate SASs easily would have greater
measure, because the program needed to "find" these very common SASs would
also be simple. This is an intriguing idea, but it doesn't help me, because
I don't (yet) see why simplicity or complexity of any kind should affect
measure. I can imagine a CA world which is sparsely populated with SASs,
and one that's densely populated with them, but the number of SASs (or the
number of observer moments) in both worlds seems exactly the same -
(countably?) infinite. So why would I be more likely to find myself in one
of those universes rather than the other?

-- Kory
Received on Sun Jan 18 2004 - 07:18:45 PST