Three Roads To Quantum Gravity

108 THREE ROADS TO QUANTUM GRAVITY
superconductors has been a very fertile source of ideas about
how physical systems might behave. This is undoubtedly
because in these ®elds there is a close interaction between
theory and experiment which makes it possible to discover
new ways for physical systems to organize themselves.
Elementary particle physicists do not have access to such
direct probes of the systems they model, so it has happened
that on several occasions we have raided the physics of
materials for new ideas.
Superconductivity is a peculiar phase that certain metals can
be put into in which their electrical resistance falls to zero. A
metal can be turned into a superconductor by cooling it below
what is called its critical temperature. This critical temperature
is usually very low, just a few degrees above absolute zero. At this
temperature the metal undergoes a change of phase something
like freezing. Of course, it is already a solid, but something
profound happens to its internal structure which liberates the
electrons from its atoms, and the electrons can then travel
through it with no resistance. Since the early 1990s there has
been an intensive quest to ®nd materials that are superconducting
at room temperature. If such a material were to be found there
would be profound economic implications, as it might greatly
reduce the cost of supplying electricity. But the set of ideas I want
to discuss go back to the 1950s, when people ®rst understood
how simple superconductors work. A seminal step was the
invention of a theory by John Bardeen, Leon Cooper and John
Schrieffer, known as the BCS theory of superconductivity. Their
discovery was so important that it has in¯uenced not only many
later developments in the theory of materials, but also developments
in elementary particle physics and quantum gravity.
You may remember a simple experiment you did at school
with a magnet, a piece of paper and some iron ®lings. The
idea was to visualize the ®eld of the magnet by spreading the
®lings on a piece of paper placed over the magnet. You would
have seen a series of curved lines running from one pole of the
magnet to the other (Figure 19). As your teacher may have told
you, the apparent discreteness of the ®eld lines is an illusion.
In nature they are distributed continuously; they only appear
to be a discrete set of lines because of the ®nite size of the iron