From: jbaez@riesz.mit.edu (John C. Baez)
Subject: Re: Baez's latest post on "links".
Organization: MIT Department of Mathematics, Cambridge, MA
Date: Sun, 20 Dec 92 21:57:29 GMT

In article <BzKn2q.41q@well.sf.ca.us> sarfatti@well.sf.ca.us (Jack Sarfatti) writes:
>
>Very formidable and I don't pretend to understand it. I'm sure it's good
>math and good intellectual exercise for those who do it - but is it physics?
>It would help if John could give a short exposition of what kinds of physics
>this work could explain - even in principle - any contact with experiment
>potentially possible - even in future assuming a really advanced super-
>technology. 

I already posted one reply to this, but let's suppose one really doesn't
see what the point of unifying general relativity and quantum theory is.
It's true that most of the experiments one can *easily* imagine to test
quantum gravity *would* require a really advanced super-technology to
carry out.  (E.g., rigging up a particle collider to wham charged
Planck-mass mini-black-holes into each other, or trying to start a new
universe in your basement by messing with the Higgs potential.)  Being
an optimist in certain matters, I feel that if humanity doesn't croak
first it will eventually learn how to mess with the fabric of spacetime,
and that's what quantum gravity is about.  But it's not going to happen
soon -- unless, as is so often in the case with physics, something
unexpected happens!!  (Who'd have thought that messing around with rocks
that spoiled photographic plates would have led to the ability to
vaporize whole cities?)  

I'm inclined to think, though, that the first payoffs of quantum gravity
will be of 2 different sorts.  First, there are many riddles of particle
physics on which we are (more or less) utterly stuck: why 3 generations of
quarks and leptons, with precisely the masses they have, etc..  We can
make up models to fit the data, but not to "explain" it, that is, to
reduce the number of free parameters or ad hoc constructs.  Particle
physics is also well suited to describing all forces EXCEPT GRAVITY.  It
doesn't take a genius to see that gravity may be the key to some of
these riddles.  Especially since mass has a lot to do with gravity - and
since quantum gravity kicks in at small distance scales, where the
mathematical foundations of particle physics collapse under the strain
of ultraviolet divergences.  While it is still way too early to see how
(or whether) quantum gravity saves the day, it's an avenue worth
pursuing (among many).  

Secondly, and perhaps most dear to my heart, is the purely philosophical
challenge of understanding a world in which 1) spacetime is a dynamical
entity rather than a static arena on which events play and 2) even
complete knowledge of the state of affairs does not permit a certain
answer to every question about the state of affairs.  To my mind, these
are the 2 most interesting discoveries of the 20th century, and the task
of the 21st century should be to reconcile them, to see how they can
BOTH be true.  As Smolin points out in the review article I mentioned,
we should not think of this intellectual revolution as being over.  It
is
more like halfway through.  To quote Smolin:

With this example in mind, let me suggest that, rather than
think of ourselves as living in
a period following a great scientific revolution, we should think of
ourselves as living in the middle period of a revolution that is taking
a long time for the same reason the Copernican revolution took
so long.  We are, perhaps, in the position
of Galileo and Kepler:  we are sure that quantum mechanics and
relativity are more right than Newtonian physics, but all of
our expectations about what physics is and how it works are
at heart Newtonian.  This is as it must be, because quantum mechanics
and relativity have, neither separately or together, given us a world
view or a view of science that is coherent and complete enough to
replace the Newtonian views.  For exactly this reason we are
convinced of the need to replace the current situation of having two,
apparently incompatible, but allegedly fundamental, theories with
one synthesis.  This indeed, *is* the problem of quantum gravity. But
we are having a hard time doing this and I, at least, suspect that the
reason is that the final synthesis, when it comes, will be as far from
our expectations as to what a physical theory should be as
Newtonian mechanics was to the expectations of Copernicus, Kepler and
Galileo.

In this situation, what can we usefully do?  I think, first of all, that
we should lower a little our expectations.  We should stop trying
every five years to invent a candidate for the final theory of
everything.  Let me put forward the proposition that almost anything
that we
can now invent, educated as we are mostly in a classical framework, is
unlikely to be radical enough.  For what changes during a scientific
revolution is not only the answers to questions, but the questions
themselves.  Those brought up on Aristotle,
including Copernicus, Kepler and
Galileo, were stuck on trying to answer the problem: what is the
shape of the orbits of the planets.  It never occurred to them that this
question was to become much less important and that the new physics
would center around completely different questions: what are the
laws of motion and what are the forces.  Similarly, by trying
to invent "The" lagrangian and "The" symmetry we are, perhaps, acting
out
of our Newtonian instincts; we are trying to answer the important
questions of the old science.  Mathematics will do us little good if
we have not yet stumbled upon the right new questions; had
Copernicus known Fourier analysis he could have made a much
better epicycle theory (indeed, he could have used it,
there were more epicycles in his theory than in Ptolemy's) but he never
would have hit on the idea of a law of motion.

The main problem, then, is what to do while we are waiting to
stumble upon the right questions.