Albert Einstein's Special Theory of Relativity, or Special Relativity for short, came into being in 1905 in a paper with the unassuming title of “On the electrodynamics of moving bodies.” As the title suggests, Special Relativity is a theory of “moving bodies,” that is: motion. In particular, it is a theory of how motion is perceived differently by different observers. Since motion is the process in which an object's location in space changes with time, any theory of motion is also a theory of space and time. Therefore, Special Relativity can be said to be a theory of how space and time are perceived differently by different observers. The “electrodynamics” part of the paper title refers to the fact that the theory has something to do with light, which is an electromagnetic wave. As we will learn in this book, the speed of light in vacuum, which we will call c, plays a very special role in the theory of relativity.

Einstein (1879-1955) was not the first to construct a successful theory of motion. Building upon pioneering work by Galileo Galilei (1564-1642), Sir Isaac Newton (1642-1727) had constructed theories of motion and gravity which were spelled out in his famous book Philosophiae Naturalis Principia Mathematica, which is so famous that when people say the Principia, it is understood that they are referring to Newton's book.