Modular Spacetime and Metastring Theory

In this talk we review our recent work on metastring theory and its habitat, a new form of quantum spacetime, called modular spacetime. We emphasize that the geometry underlying modular spacetime, i.e. the background geometry of metastring theory is also the geometry underlying any quantum theory as formulated in terms of Aharonov's modular variables. Thus the metastring sheds light on the foundations of quantum theory, and it represents a new formulation of string theory and quantum gravity based on the principle of relative locality.