Raffiniert ist der herrgott,
Aber boshaft ist er nicht.
[God is cunning, but not malicious.]
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,
After ten years of reflection such a principle (the principle of special relativity) resulted from a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam as an electromagnetic field constant in time, periodic in space. However, there seems to exist no such thing, neither on the basis of experience, nor according to Maxwell’s equations.