Three Roads To Quantum Gravity

180 THREE ROADS TO QUANTUM GRAVITY
to change the rules, not once but over and over again. There
are not particles, there are strings. There are not three
dimensions of space but nine. There are extra symmetries.
String theory is unique. Actually, it is not quite unique ± it
comes in an enormous number of versions. And in fact there
are not just strings, but membranes of many different
dimensions. And there are not nine dimensions, but ten.
And so on. String theory has been nothing but a series of
surprises, one after the other. We put in no principles ± all we
put in is the desire for a theory of gravitons that makes sense.
And we get out a long list of unexpected facts, a whole new
world to be explored.
For more than ten years, from about 1984 to 1996, these two
theories of quantum gravity were developed by two different
groups of people completely independently. Each group was
successful in solving the problems it set for itself. Although
we listened to each other's talks, and maintained friendships
formed before the split, it must be said that almost everyone
thought that their group was on the right path and the others
were misguided. To each group it was obvious why the other
could not succeed. The loop guys (and gals) said to the string
guys, `Your theory is not background independent, it cannot
be a real quantum theory of space and time. Only we know
how to make a successful background independent theory.'
The string guys said to the loop guys, `Your theory does not
give a consistent description of the interactions between
gravitons and other particles. Only our theory describes a
consistent uni®cation of gravity with the other interactions.' I
am ashamed to admit that few in either community rose to the
challenge. During this whole period, for example, there was
not a single person who worked on both theories. Many
seemed to make the understandable mistake of confusing the
solution of part of the problem of quantum gravity with the
solution of the whole problem.
Many misunderstandings have resulted. I have had the
experience more than once of sitting next to someone from
one camp listening to a talk by someone from the other. The
person next to me would get very agitated: "That young
person is so arrogant, they claim they have solved every-