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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...

This section contains 943 words(approx. 4 pages at 300 words per page) |