Brian Greene is an American physicist, professor of physics and mathematics at Columbia University, and director of its Center for Theoretical Physics. His research focuses on string theory, where he is known for co-discovering mirror symmetry, which relates two different Calabi–Yau manifolds. He also described the flop transition, a mild form of topology change, and the conifold transition, a more severe transformation of space, demonstrating that topology can smoothly change in string theory.