James Clerk Maxwell (1831–1879) achieved a synthesis of magnetism, electricity and electromagnetic radiation analogous to what Newton had achieved for motion. He introduced ideas that could not be explained by Newtonian physics, and in that sense, was a steppingstone to Einstein.
Maxwell combined the known facts concerning electricity and magnetism into a cohesive theory using a new abstract mathematical representation called field equations. From his equations, Maxwell predicted that a form of energy dubbed electromagnetic radiation could propagate at an enormous speed, represented as “c”. Realizing that the magnitude of “c” was almost exactly equal to the experimentally calculated speed of light, Maxwell proposed that light was a form of electromagnetic radiation. (Richard Feynman went on the public record stating that Maxwell’s discovery was the single most important event in the nineteenth century.[1])
Initially, scientists believed that electromagnetic waves required a substance for their propagation, in much the same way as waves in the sea or sound waves in the air travel through a medium. They dubbed this substance the “ether”. The ether was invisible and remarkably resistant to examination. Very precise experiments looking for evidence of the ether drew negative results. This unexpected finding suggested that something strange was going on. Did the ether exist at all? Could electromagnetic radiation move through empty space? In fact, the mathematical concept of fields (from which we derive terms like “gravitational field”) removed the need for “ether”; light moves through empty space.
[1] Feynman, Leighton, & Sands, 2010, p. 1~11 (Vol 2).