From: jbaez@riesz.mit.edu (John C. Baez)
Subject: Re: What We Don't Know (YET!) (Was: Black hole + anti-Black hole = ?)
Keywords: FAQ, ignorance
Organization: MIT Department of Mathematics, Cambridge, MA
Date: Thu, 10 Sep 92 01:48:16 GMT

Okay, starting with ones that have recently come up, here are
some things we don't know.  Here's how I would start off the FAQ; of
course everyone can suggest improvements but Scott is the boss.

While for the most part a FAQ covers the answers to frequently asked
questions whose answers are known, in physics there are also plenty of
simple and interesting questions whose answers are not known.  A real
"Theory of Everything" would answer all these questions.  Before you
set about answering these questions on your own, it's worth noting
that while nobody knows what the answers are, people have said a lot
of very intelligent things about many of these questions.  So do plenty
of research and ask around before you try to cook up a theory that'll 
answer one of these and win you the Nobel prize!  You can expect to
really know physics inside and out before you make any progress on these.

1.  What happened at, or before the big bang, assuming there really
was a big bang?  Of course, this question might not make sense, but it might.
Does the history of universe go back in time forever, or only a finite
amount?  

2.  Will the future of the universe go on forever or not?  Will there
be a "big crunch" in the future?

3.  Why is there an arrow of time; that is, why is the future so much
different from the past?

4.  Is the universe infinite in spatial extent or finite?

5.  Is spacetime really four-dimensional?   If so, why - or is that
just a silly question?  Or is spacetime not really a manifold at all if
examined on a short enough distance scale?

6.  Why is there more matter than antimatter, at least around here? 
Is there really more matter than antimatter throughout the universe?

7.  Why are the laws of physics not symmetrical between left and
right, future and past, and between matter and antimatter?  Why are
these asymmetries so small?

8.  Why are the strengths of the fundamental forces (electromagnetism,
weak and strong forces, and gravity) what they are?  For example, why
is the fine structure constant, which measures the strength of
electromagnetism, about 1/137.036?  Where did this dimensionless
constant of nature come from?

9.  Why do there seem to be 3 generations of leptons and quarks?  Why
are there mass ratios what they are?  For example, the muon is a
particle almost exactly like the electron except about 207 times
heavier.  Why does it exist and why precisely that much heavier?

10.  Do black holes really exist?  (It sure seems like it.)  Do they
really radiate energy and evaporate the way Hawking predicts?  If so,
what happens when, after a finite amount of time, they radiate
completely away?  What's left?  Do black holes really violate all
conservation laws except conservation of energy, momentum, angular
momentum and electric charge?

And some more theoretical questions... these are close to being
mathematics questions, but they bear heavily on physics.

1.  Is there a consistent and acceptable theory of classical point
charges coupled to electromagnetism by Maxwell's equations?

2.  Is there a consistent and acceptable relativistic quantum field
theory describing interacting (not free) fields in four spacetime
dimensions?  For example, is the standard model mathematically
consistent?  How about quantum electrodynamics?

3.  Is it possible to calculate masses of hadrons (e.g., protons,
neutrons, and mesons) precisely from the standard model?  Does it give
the right answer?
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