The laws of physics distill our observations
about what can and cannot happen in the
physical world. Powerful conservation laws
tell us that certain physical attributes are
immune to change. Energy, for example, can
take many different forms, but it can be
neither created nor destroyed.
In particle physics, conservation laws
result from symmetries in the forces that
govern how nature works. A symmetry of
the electromagnetic force tells us that
electric charge is exactly conserved, that
all electrons are created equal, and that
the photon—the particle of light—is
exactly massless. Such symmetries have
provided some of the most dramatic
insights in particle physics.
An understanding of all the symmetries of
nature would be equivalent to knowing how
the world works at its most basic level.
There are many reasons to believe that we
have not found all the symmetries of nature.
String theory, for example, requires a new
symmetry called supersymmetry. It introduces
new quantum dimensions to spacetime and gives
meaning to many of the particles we know. It
predicts that every elementary particle has a
superpartner, just as every particle is
accompanied by an antiparticle. Scientists
have not yet discovered the supersymmetric
partners; fi nding them is among the greatest
challenges for the current and upcoming
generations of particle accelerators.