Abstract

An outstanding problem of modern physics is the reconciliation of the two most successful broad theories of physics: general relativity, which describes the gravitational force exerted by massive objects in terms of the bending and rippling of spacetime, and quantum mechanics, which describes the behavior of matter on the minute scales of subatomic particles such as electrons. While both theories are remarkably successful independently, they appear to give contradictory predictions of phenomena that involve gravity at the small scales at which quantum mechanics is relevant. Such phenomena include the Big Bang, when the universe was small enough to be described by quantum mechanics and sufficiently dense to also require a gravitational description; also the process of the final stages of the death of a large star, when a black hole is formed and the star’s matter is concentrated in an extremely small volume. A proper description of these phenomena requires a unified theory of quantum gravity.