Three Roads To Quantum Gravity

158 THREE ROADS TO QUANTUM GRAVITY
gravitational force from varying in space and time. And
almost all consistent string theories predict symmetries
among their particles beyond those that are seen. The most
important of these are supersymmetries.
Supersymmetry is an important idea, so it is worth while
making a detour here to discuss it. To understand supersymmetry
one must know that elementary particles come into
two general types: bosons and fermions. Bosons, which
include photons and gravitons, are particles whose angular
momentum, when measured in units of Planck's constant, are
simple integers. Fermions, which include electrons, quarks
and neutrinos, have angular momenta that come in units of
one-half. Fermions also satisfy the Pauli exclusion principle,
which states that no two of them can be put in the same state.
Supersymmetry requires fermions and bosons to come in
pairs consisting of one of each, with the same mass. This is
de®nitely not observed in nature. If there were such things as
bosonic electrons and quarks, the world would be a very
different place, for the Pauli exclusion principle would have
no force, and no form of matter would be stable. If supersymmetry
is true of our world, then it has been spontaneously
broken, which is to say that the background ®elds must confer
a large mass on one member of each pair and not on the other.
The only reason to entertain the idea of such a strange
symmetry is that it seems to be required for most, if not all,
versions of string theory to give consistent answers.
The search for evidence of supersymmetry is a major
priority of experiments now under way at particle accelerators.
String theorists very much hope that evidence for
supersymmetry will be found. If supersymmetry is not found
experimentally, it would still be possible to concoct a string
theory that agrees with experiment, but this would be a less
happy outcome than if experimental support for supersymmetry
were forthcoming.
There is obviously something very wonderful about string
theory. Among its strong points are the natural way it leads to
a uni®cation of all particles and forces, and the fact that there
are many consistent string theories that include gravity.
String theory is also the perfect realization of the hypothesis