[LEE
SMOLIN:] As a theoretical physicist, my main concern is space,
time and cosmology. The metaphor about information and computation
is interesting. There are some people in physics who have begun
to talk as if we all know that what's really behind physics is
computation and information, who find it very natural to say things
like anything that's happening in the world is a computation,
and all of physics can be understood in terms of information.
There's another set of physicists who have no idea what those
people are talking about. And there's a third set — and I'm
among them — who begin by saying we have no idea what you're
talking about, but we have reasons why it would be nice if it
was useful to talk about physics in terms of information.
I can mention two ways in which the metaphor of information
and computation may be infiltrating into our thinking about
fundamental physics, although we're a long way from really understanding
these things. The first is that the mathematical metaphor and
the conceptual metaphor of a system of relationships which evolves
in time is something which is found in physics. It is also something
that we clearly see when we talk to computer scientists and
biologists and people who work on evolutionary theory, that
they tend to model their systems in terms of networks where
there are nodes and there are relationships between the nodes,
and those things evolve in time, and they can be asking questions
about the time evolution, what happens after a long time, what
are the statistical properties of subsystems.
That kind of idea came into physics a long time ago with relativity
theory and general relativity. The idea that all the properties
of interest are really about relationships between things and
not a relationship between some thing and some absolute fixed
background that defines what anything means is an important
idea and an old idea in physics. In classical general relativity,
one sees the realization of the idea that all the properties
that we observe are about relationships. Those of us who are
interested in quantum gravity are thinking a lot about how to
bring that picture, in which the world is an evolving network
of relationships, into quantum physics.
And there are several different aspects of that. There are very
interesting ideas around but they're in the stage of interesting
ideas, interesting models, interesting attempts — it is
science in progress.
That's the first thing. To the extent to which our physics will
turn out to look like a network of relationships which are evolving
in time, physics will look like some system that computational
people or biologists using the computational metaphor may be
studying. Part of that is the questions of whether nature is
really discrete — that underlying the continuous notion
of space and time there's really some discrete structure, that's
also something that from different points of view — when
we work on quantum gravity we find evidence that space and time
are really discrete and are really made up on processes which
may have some discrete character. But again, this is something
in progress.
One piece of evidence that nature is discrete is something called
the holographic principle. This leads some of us physicists
to use the word information even when we don't really know what
we're talking about but it is interesting and worth exposing.
It comes from an idea called the Bekenstein Bound, a conjecture
of Jacob Bekenstein that there is more and more theoretical
evidence for. The Bekenstein Bound says that if I have a surface
and I'm making observations on that surface —that surface
could be my retina, or it could be some screen in front of me
— I observe the world through the screen, at any one moment
there's a limitation to the amount of information that could
be observed on that screen.
First of all that amount of information is finite, and it's
four bits of information per Planck area of the screen, where
a Planck area is 10 to the minus 66 centimeters squared. And
there are various arguments that if that bound were to be exceeded,
in a world where there is relativity and black holes, then we
would violate the Second Law of Thermodynamics. Since none of
us wants to violate the Second Law of Thermodynamics, I think
it's an important clue, and it says something important about
the underlying discreteness of nature. It also suggests that
information, although we don't know what information is, may
have some fundamental place in physics.
The holographic principle, of which there are several versions
by different people — the idea was invented by Dutch theoretical
physicist Gerard 't Hooft — is that the laws of physics
should be rewritten, or could be rewritten including dynamics,
how things evolve in time, so we're no longer talking about
things happening out there in the world in space, we're talking
about representing systems that we observe in terms of the information
as it evolves on the screen. The metaphor is that there's a
screen through which we're observing the world. There are various
claims that this idea is realized at least partly in several
different versions of string theory or quantum gravity This
is an idea there's a lot of interest in, but we really don't
know whether it can be realized completely or not.
One extreme form of it, which I like, is that perhaps the way
to read the Bekenstein Bound is not that there are two different
things, geometry and flow of information and a law that relates
them, but somehow we could try to envision the world as one
of these evolving networks. What happens is processes where
"information", whatever information is, flows from event to
event, and geometry is defined by saying that the measure of
the information capacity of some channel by which information
is flowing, from the past to the future, would be the area of
a surface, so that somehow geometry that is space would turn
out to be some derived quantity, like temperature or density,
and just the same way that temperature is a measure of the average
energy of some particles, the area of some surface would turn
out to be an approximate measure of the capacity of some channel
in the world would fundamentally be information flow. It's an
idea that some of us like to play with, but we have not yet
constructed physics on those grounds, and it's not at all clear
that it will work. This is a transition to a computational metaphor
in physics — it's something which is in progress, and may
or may not happen.
