We should pause a moment to take stock of what we have learned. In the Standard Model we have a framework, based upon the beautiful principles of quantum field theory and local gauge symmetry, that successfully explains all the particle physics experiments that have ever been performed. But the Standard Model is incomplete: it contains two different sectors that seem to demand unification, and it completely neglects gravity. The first problem is addressed by grand unification, which expands the gauge symmetry of the Standard Model. Grand unified theories themselves suffer from problems, but the problems become much less severe when we introduce another type of symmetry: supersymmetry. And if we make supersymmetry a local symmetry, then we find that gravity appears automatically. We are thus led to supergravity theories. Finally, if we introduce the notion of higher spatial dimensions — an old idea for physicists who have tried to describe gravity and electromagnetism in a unified framework — and combine it with supergravity, we are led to a theory that is almost a unified theory of all the fundamental particles and their interactions: 11-dimensional supergravity.
KeywordsString Theory Gauge Symmetry Unify Theory Open String Heterotic String
Unable to display preview. Download preview PDF.