A fresh view on string orbifolds

Understand

In quantum field theory, an orbifold is a way to obtain a new theory from an old one by gauging a finite global symmetry.

• This definition of orbifold does not make sense for quantum gravity theories, that admit (conjecturally) no global symmetries.
• In string theory, the orbifold procedure involves the gauging of a global symmetry on the world-sheet theory describing the fundamental string.
• Alternatively, it is a way to obtain a new string background from an old one by quotienting some isometry.