Why things fall

Abstract

In this paper we discuss Internal Relativity, a recent program to address the problem of quantum gravity. In our approach we change the relationship between spacetime and matter. Currently we view matter as propagating on spacetime. Einstein’s equations encode how spacetime curves due to the presence of matter and how spacetime, in turn, tells matter how to propagate. In internal realtivity matter and spacetime cease to exist as distinct entities, rather, they arise simultaneously from an underlying quantum system. It is through the emergent matter degrees of freedom that geometry is inferred. We have termed our program Internal Relativity to stress the importance of looking at the system from the point of view of an internal observer. We argue that special relativity is then a natural consequence of this viewpoint. The most important new aspect of Internal Relativity involves how gravity appears. It is not just a new quantum theory of gravity but a new theory of gravity. We also argue that the presence of a massive object implies curvature. In particular we show that Newtonian gravity arises in the appropriate limit. Our argument implies that there is no propagation without gravitation.