According to Einstein’s general theory of relativity, first published in 1915, the phenomenon we experience as gravitation can be interpreted purely geometrically. Objects moving in the absence of external forces travel along “geodesics”, curves generalizing straight lines. However, in the presence of matter, the geometry of space-time is not Euclidean. Instead, nearby geodesics move closer together, similarly to longitude lines on the surface of the earth as one walks toward the north or south pole. Bodies moving along converging geodesics are, perforce, attracted to each other.
Among the predictions of general relativity is the bending of light by gravitation. In May 1919, Arthur Eddington led an expedition to the Isle of Principe in the Gulf of Guinea, West Africa. A companion expedition was sent to Sobral, in northern Brazil. A rare and timely astronomical coincidence was due to occur: a total solar eclipse as the sun passed through the Hyades, “an exceptional field of bright stars” in Eddington’s words. The expeditions’ goal was to photograph the deflection of starlight by the sun’s gravitational field. Newton’s law of gravitation also predicted deflection, but by an amount only half as great as Einstein’s prediction. The plates from Sobral showed, within bounds of experimental uncertainty, a greater deflection than predicted by Newtonian gravitation. Both sets of plates were entirely consistent with Einstein’s prediction. This initial experimental confirmation of general relativity catapulted Einstein to international fame.
Written in 1920, Eddington’s “Space, Time and Gravitation” is one of the first popular accounts of general relativity. The book begins with an imaginary conversation between a classical physicist, a pure mathematician, and a relativistic physicist who challenges the classical physicist (and therefore the reader) to reconsider the static, Galilean concepts of space and time. Chapter by chapter, using little more than the Pythagorean theorem, Eddington builds an ever-stronger case for the relativistic thesis: Space and time are not independent absolutes, but together comprise a single physical entity in which disparate physical ideas become unified.
The decades bracketing 1920 saw a dramatic shift in cosmology. In 1917, Einstein used general relativity to model the entire universe, introducing a “cosmological constant” in his field equations in order to obtain a static solution. In 1922, Alexander Friedmann found expanding solutions to Einstein’s original field equations (without a cosmological constant). By 1929, Edwin Hubble had discovered that distant galaxies were moving away from the earth at speeds proportional to their distance, an experimental suggestion that Friedmann’s model was closer to reality than Einstein’s. Eddington’s book predates these foundational discoveries, and even contains cosmological speculations that seem naive in retrospect. Despite this, the book remains a lively, accessible, and literate introduction to the rich geometric background underlying Einstein’s general theory of relativity.
This review was contributed by DP-volunteer adhere.