Introduction

Einstein's recognition early last century that gravity can be interpreted as the curvature of space and time represented an enormous step forward in the way we think about fundamental physics. Besides its obvious impact for understanding gravity over astrophysical distances – complete with resolutions of earlier puzzles (like the detailed properties of Mercury's orbit) and novel predictions for new phenomena (like the bending of light and the slowing of clocks by gravitational fields) – its implications for other branches of physics have been equally profound.

These implications include many ideas we nowadays take for granted. One such is the universal association of fundamental degrees of freedom with fields (first identified for electromagnetism, but then cemented with its extension to gravity, together with the universal relativistic rejection of action at a distance).