Three Roads To Quantum Gravity

112 THREE ROADS TO QUANTUM GRAVITY
q
q
q
q
q
q
FIGURE 21
Quarks are held together by strings made of quantized ¯ux lines of a ®eld,
called the QCD ®eld, which are analogous to the quantized magnetic ¯ux
lines in a superconductor (Figure 20). As the quarks are pulled farther apart,
the ¯ux lines are stretched, and the force between the quarks is the same no
matter how far apart they are. The result is that the quarks cannot be pulled
apart.
perhaps empty space is very like a superconductor, except
that what ends up discrete is the lines of force holding the
colour charges of quarks together rather than the lines of
magnetic ¯ux. In this picture the lines of force between the
coloured charges on the quarks are analogous to the electric
rather than the magnetic ®eld. So this hypothesis can be put
very succinctly as follows: empty space is a colour-electric
superconductor. This has been one of the most seminal ideas
in elementary particle physics over the last few decades. It
explains why quarks are con®ned in protons and neutrons, as
well as many other facts about elementary particles. But what
is really interesting is that the idea, clear as it is, contains a
puzzle, for it can be looked it in two quite different ways.
One can take the colour-electric ®eld as the fundamental
entity, and then try to understand the picture of a string
stretched between the quarks as a consequence of space
having properties that make it something like an electric
version of a superconductor. This is the route taken by those
physicists who work on QCD. For them, the key problem is to
understand why empty space has properties that make it
behave in certain circumstances like a superconductor. This