Throughout this book we are concerned with fields – that is, functions of space and time, such as gravitational fields, electromagnetic fields, and velocity fields – and density distributions that describe masses and charges. Spacetime is the arena in which these fields perform their joint evolutions. It is therefore clear that we must first get to know the structure and geometry of spacetime. Unfortunately, because the velocity of light is so large, everyday experience leads us to acquire various misconceptions about the geometry of spacetime. This set of misconceptions goes under the name of Newtonian, or Galilean, spacetime. The true (or more true) geometry of spacetime was discovered through the development of Einstein’s theory of special relativity, starting in 1905. The keystone of this theory is the principle of relativity, according to which the laws of physics are the same in all inertial reference frames. Einstein was led to this principle by his investigation of Maxwell’s equations. As he wrote in his autobiographical notes,