As astronomical phenomena were discovered, such as quasars (1963), the 3-kelvin microwave background radiation (1965), pulsars (1967), and the first black hole candidates (1981), Special relativity is a theory of the structure of spacetime.It was introduced in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies" (for the contributions of many other physicists see History of special relativity).These are the Michelson–Morley experiment, the Kennedy–Thorndike experiment, and the Ives–Stilwell experiment.Einstein derived the Lorentz transformations from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence.Moreover, the theory has many surprising and counterintuitive consequences.
New mathematical techniques to apply to general relativity streamlined calculations and made its concepts more easily visualized.
In the discussion section of the same paper, Alfred Bucherer used for the first time the expression "theory of relativity" (German: It rapidly became a significant and necessary tool for theorists and experimentalists in the new fields of atomic physics, nuclear physics, and quantum mechanics.
By comparison, general relativity did not appear to be as useful, beyond making minor corrections to predictions of Newtonian gravitation theory.
Maxwell's equations—the foundation of classical electromagnetism—describe light as a wave that moves with a characteristic velocity.
The modern view is that light needs no medium of transmission, but Maxwell and his contemporaries were convinced that light waves were propagated in a medium, analogous to sound propagating in air, and ripples propagating on the surface of a pond.