From 1905 to 1915 Albert Einstein revolutionized the conception of space and time and gravity that had been central in physics since Isaac Newton. For a brief discussion of the history of the development of relativity see the entry "Einstein, Albert." This entry describes the content of the theories.

The special and general theories of relativity are, at heart, theories of spatiotemporal structure. They are not particularly about observers or reference frames or ways to synchronize clocks, although as fundamental physical theories they have implications about what observers will observe and what various physical procedures for coordinating clocks will accomplish. It is easy to fall under the impression that these theories are basically concerned about coordinate systems or reference frames because physical events are typically described by means of coordinates or reference frames, but that temptation ought to be avoided.

Perhaps the easiest way to understand special relativity...