Three Roads To Quantum Gravity

110 THREE ROADS TO QUANTUM GRAVITY
This discreteness of the magnetic ®eld lines in superconductors
is a curious phenomenon. It is unlike the discreteness
of the electric charge, or of matter, in that it has to do
with a ®eld that carries a force. Furthermore, it seems that we
can turn it on and off, depending on the material the magnetic
®eld is passing through.
The electric ®eld has ®eld lines as well, although there is no
equivalent of the iron ®lings experiment that allows us to see
them. But in all circumstances we know about they are
continuous: no material has been found which functions like
an electric superconductor to break electric ®eld lines into
discrete units. But we can still imagine something like an
electric superconductor, in which the ®eld lines of the
electric ®eld would be quantized. This idea has been very
successful in explaining a result from another seemingly
unrelated subject: experiments show that protons and neutrons
are each composed of three smaller entities called
quarks.
We have good evidence that there are quarks inside protons
and neutrons, just as there are electrons, protons and
neutrons inside the atom. There is one difference, however,
which is that the quarks seem to be trapped inside the
protons. No one has ever seen a quark moving freely, that
was not trapped inside a proton, neutron or other particle. It is
easy to free electrons from atoms ± one needs only to supply a
little energy, and the electrons jump out of the atom and move
freely. But no one has found a way to free a quark from a
proton or neutron. We say that the quarks are con®ned. What
we then need to understand is whether there is a force that
can act as the electric ®eld does in holding electrons around
the nucleus, but that does so in such a way that the quarks can
never come out.
From many different experiments we know that the force
that holds the quarks together inside a proton is quite similar
to the electric force. For one thing, we know that force is
transmitted by a ®eld that forms lines like electric and
magnetic ®eld lines. These lines connect charges which are
carried by the quarks, just as electric ®eld lines connect
positive and negative electric charges. However, the force