Relativity theory, a general term encompassing special and general relativity theories, sets forth a specific set of laws relating motion to mass, space, time, and gravity. Relativity theory allows calculations of the differences in mass, space, and time as measured in different reference frames.

At the start of the twentieth century the classical laws of physics contained in Sir Isaac Newton's (1642–1727) 1687 work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) adequately described the phenomena of everyday existence. In accord with these laws, more than century of experimental and mathematical work in electricity and magnetism resulted in Scottish physicist James Clerk Maxwell's (1831–1879) four equations describing light as an electromagnetic wave. Prior to Maxwell's equations it was thought that all waves required a medium or ether for propagation. Such an ether would also serve as an absolute reference frame against which absolute motion, space and time could...