Relativity: The Theory vs The Principle Edward G. Lake Independent Researcher October 16, 2017 detect@outlook.com Abstract: While it appears to be a common practice today to refer to the “Theory of Relativity” as if it were the same as the “Principle of Relativity,” according to Albert Einstein, his “Theory of Relativity” is that the “Principle of Relativity” is an illusion. Key words: Relativity; Theory; Principle; Postulate. I. INTRODUCTION When Albert Einstein wrote his 1905 paper titled paper On the Electrodynamics of Moving Bodies,[1] which introduced his Theory of Special Relativity to the world, he did not give his theory a name. The paper was just a description of his theory and how it worked. And since the paper was also about the long-existing Principle of Relativity that had been a basis for physics since the time of Galileo Galilei, Einstein’s theory was eventually described by others as Einstein’s “Theory of Relativity.” In a book Einstein co-wrote many years later,[2] Galileo’s Principle of Relativity is given as follows: if the laws of mechanics are valid in one C.S. [coordinate system], then they are valid in any other [coordinate system] moving uniformly relative to the first. Physicist Richard Feynman, who appears to have been in general agreement with Einstein’s theories, stated in one of his published lectures on “the principle of relativity,”[3]: The principle of relativity was first stated by Newton, in one of his corollaries to the laws of motion: “The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line.” This means, for example, that if a space ship is drifting along at a uniform speed, all experiments performed in the space ship and all the phenomena in the space ship will appear the same as if the ship were not moving, provided, of course, that one does not look outside. That is the meaning of the principle of relativity. This is a simple enough idea, and the only question is whether it is true