Speed of Light, Historical Review to 1972

Abstract

Light is the principal means for finding out about our surroundings, for the most primitive requirements of finding our way, as well as for obtaining a profound understanding of our universe (especially the case if one includes the whole electromagnetic spectrum). Its nature has always posed some mystery, exemplified by the sometimes acrimonious debate as to whether it consisted of particles or waves, questions as to how it interacts with matter and its role in relativity. All that need concern us here however is that it is propagated as a sinusoidal wave whose speed of propagation (c) is very high; the latter is an important constant of physics, it is now being used to make accurate measurements of distance and is being considered as the basis for defining the primary standard of length. The measurement of c has preoccupied scientists for a long time, almost continuously for the last three hundred years. Even though its speed is so high that it appears to travel over earth distances instantaneously, the fact that it was obviously created at a source must have stimulated very early attempts to detect and measure a time of propagation. As will be seen in this review, there are indirect ways to observe the speed of light: two measurements resulted from astronomical observations and an important series of determinations were made by finding the ratio of electrostatic to electromagnetic units, which Maxwell’s electromagnetic theory of light showed to be equal to the speed of its propagation. Most measurements, however have been made by more direct approaches.