Quantum gravity

Abstract

Quantum gravity (QG) is the problem of finding a theory that describes the quantum effects on gravity. These effects escape the currently accepted physical theories. In particular, QG is an investigation on the nature of space and time. The structure and the nature of physical space are expected to change radically at the Planck scale; and the conventional way of conceptualizing of time evolution is expected to cease to be viable at this scale. The numerous ideas and approaches towards quantum gravity are described in this chapter. A large number of other ideas and directions of investigation about QG have been proposed. The problem of QG raises basic methodological issues and involves conceptual and foundational questions. Some of these are similar to the foundational questions that physics addressed at the time of other major conceptual shifts —the birth of classical mechanics, field theory, relativity, or quantum mechanics. Old problems demand new answers, in the light of the twentieth century's novel insights.