The concepts of space, time, and matter are of central importance in any theory of the gravitational field. Here I discuss the role that these concepts might play in quantum theories of gravity. To be concrete, I will focus on the most conservative approach, which is quantum geometrodynamics. It turns out that spacetime is absent at the most fundamental level and emerges only in an appropriate limit. It is expected that the dynamics of matter can only be understood from a fundamental quantum theory of all interactions.
From classical to quantum gravityIn his famous habilitation colloquium on June 10, 1854, Bernhard Riemann concludedThe question of the validity of the hypotheses of geometry in the infinitely small is bound up with the question of the ground of the metric relations of space.. . . Either therefore the reality which underlies space must form a discrete manifoldness, or we must seek the ground of its metric relations outside it, in binding forces which act upon it. . . . This leads us into the domain of another science, of physic, into which the object of this work does not allow us to go to-day. (Riemann (1868); translated by William Kingdon Clifford 1873)1