While Kapteyn was occupied in Groningen measuring the positions of stars on photographic plates a revolution in physics was occurring elsewhere in Europe – a revolution that, over two decades, fundamentally changed the perception of the physical world that rested on the well-tested foundations of Newton's theory of dynamics and gravity and Maxwell's theory of electromagnetism. To say that these developments have had a profound impact on astronomy and astrophysics is a drastic understatement.
There were, in effect, two parallel revolutions. The first, following Max Planck, who in 1900 proposed that electromagnetic radiation came in discrete units of energy, was the quantum revolution. This permanently altered human perception of determinism and the fundamental limits of experimental precision and led to the concept of particle-wave dualism. The second revolution, the theory of relativity, forever changed the view of space and time as absolute entities and shook the foundations of Newtonian mechanics. The new theory of relativistic gravity, General Relativity, predicted phenomena that were discovered decades later, such as gravitational lenses, and, of particular relevance to this discussion, the existence of a new class of objects that are the consequence of gravitational collapse – black holes. It this second revolution that is most relevant to the discussion here.
It all began rather quietly in 1905 in Zurich where an obscure clerk in the patent office, Albert Einstein (Figure 3.1), wrote a fundamental paper dealing with the way in which the laws of particle dynamics and electromagnetism should be transformed between different reference frames moving at a constant relative velocity with respect to one another.