Three and a Half Principles: The Origins of Modern Relativity Theory

This article explores the origins of modern relativity theory. In his 1905 paper On the Electrodynamics of Moving Bodies, Albert Einstein directly addressed one of the largest issues of the time. Electrodynamics aims to describe the motion of charged particles (usually thought of as electrons), whose interaction through the electromagnetic field, as described by Maxwell’s equations, affects their respective motions. The problem was so complex because the electromagnetic field theory was not an action-at-a-distance theory. This article begins with an overview of the principle of relativity and of the constancy of the speed of light, followed by a discussion on the relativity of simultaneity, the mass–energy equivalence, and experimental tests of special relativity. It also examines the principle of equivalence, the concepts of spacetime curvature and general covariance, and Mach’s principle. Finally, it considers experimental predictions of general relativity.