Danny Mayes writes:
> Interested in thoughts on this excerpt from Martin Rees
> Which approximates my ideas on the nature of reality and the possible role
> of intelligence.
Well, no offense to Martin or you, but that's pretty ordinary stuff which
we have been discussing on this list since 1998. There is an entire
website devoted to the speculation that Rees describes, the possibility
that we live in a simulation: simulation-argument.com, by former list
member Nick Bostrom.
It's always kind of disconcerting to read popularizations of the kinds
of ideas we discuss here. Writers have to approach them so delicately,
taking such pains to marvel at the amazing and perhaps outlandish
imagination it takes to consider them seriously. I always read these
articles with a sinking feeling, knowing that they aren't going to
say anything new to me and frustrated that the concepts can't just be
delivered straight, the way we take them here.
I've been wanting to write something about the collision between physics
and the anthropic principle, and maybe this is a good time. As Rees
describes, recent developments in string theory, have introduced the idea
of a "landscape" of possible stable models. Each point on the landscape
would correspond to a possible set of particles and physical laws.
I haven't gotten a clear picture of whether this landscape is actually
infinite or merely very large; most writers seem to think the distinction
is unimportant, because the "large" is so "very". Rees is typical in
how he inconsistently says first one and then the other. (I imagine
that most of us would see the distinction as rather more important.)
Anyway, as Rees describes, this is forcing physicists to consider the
possibility that the fundamental physics of our universe is no more
meaningful than the distance from the earth to the sun: a mere accident
of nature, with the anthropic principle explaining why we are here to
see it and ask about it. I've seen this same earth-sun analogy used in
talks by other physicists.
But what Rees does not say is that many or most physicists are being
dragged down this path kicking and screaming. Physics is not going gently
into the anthropic night. Most physicists, I think, see this as the
death of physics. Rees himself admits that it in effect blurs the line
between physics and philosophy. It calls into question the entire program
of fundamental physics, to explain why the universe is the way it is.
The real problem is not just that it is a philosophical speculation,
it is that it does not lead to any testable physical predictions.
The string theory landscape, even if finite, is far too large for
systematic exploration. Our ideas, with an infinite number of possible
universes, are even worse. Physicists see acceptance of anthropic
explanations as the end of physics because there is no way to make
quantitative predictions when there are so many degrees of freedom.
Now, granted, predictions may still be possible in principle - given
that the string theory landscape is finite, you could in theory explore
every part of it and create probabilistic predictions contingent on what
has been observed so far. The problem is that there is no prospect of
actually being able to do this. Clearly it can't be done by brute force.
Maybe someone will come up with some kind of clever sampling approach,
or perhaps some structure can eventually be found in the landscape.
But at this point there is no reason to believe that such efforts will
amount to anything.
The result is that string theory in particular has reached a dead end.
This is what physicists have been banking on for twenty years to achieve
a grand unification, a theory of everything. And instead, they have
found within the past couple of years that it is more like a "theory of
anything", in the sense that seemingly any hypothetical universe might
be consistent with string theory. Such a theory is no theory at all,
to physicists.
The situation has gotten so bad that there is a growing backlash
against string theory. Proponents of rival theories, who have labored
in obscurity for decades, are coming out of the woodwork and demanding
their share of attention (and the substantial research funding of which
string theory has long received the lion's share). Two major books
are coming out this fall which proclaim the death of string theory.
One is "Not Even Wrong: The Failure of String Theory And the Search for
Unity in Physical Law" by Peter Woit (his blog of the same name is here,
http://www.math.columbia.edu/~woit/wordpress/ ). The other is by Lee
Smolin: "The Trouble With Physics: The Rise of String Theory, The Fall
of a Science, and What Comes Next". Amazon even pairs the books for
you so you can get a double dose of anti-string theory. These books
have been well reviewed and there seems to be a sense that indeed,
the general approach of physics needs to be substantially rethought.
So the bottom line is that Rees leaves us with a highly misleading
perspective. While he personally may be happy with anthropic ideas,
most physicists are not. Where I live in Santa Barbara, Nobelist David
Gross, head of the KITP at UCSB, is famous for his active hostility to
the concept. So opposed are physicists to adopting all-universe models
that they are ready to abandon twenty years of work and strike off in
a new direction rather than face the immensity of the anthropic universe.
Now, I'm sure that some physicists will continue to work on these ideas,
just as a minority has continued to work on rivals to string theory
all these years. The bottom line is that unless some way is found to
make specific, testable predictions (and not the kind of hand-waving we
sometimes get away with around here, explaining why bunnies can't fly),
the anthropic universe is not physics. It is philosophy, and physicists
want nothing to do with it.
Hal Finney
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Received on Wed Jul 26 2006 - 03:37:15 PDT