- The principle of relativity rests on emptiness.
Edwin F. Taylor and John Archibald Wheeler
The Principle of Relativity derives from the common experience that we have no intrinsic sense of discerning constant motion from rest, in contrast to change of speed, i.e., acceleration. Imagine two observers in empty space; who is the one at rest, who is moving? There is no way to tell. We only observe motion with respect to something else, this something else being of our own choosing. In other words, 'motion' and 'rest' are a relative concepts.
Galileo Galilei (1564-1642) is usually considered to have been the first to explicitly state this concept in 1632 as a general principle of relativity. It should be mentioned, though, that Nicolas d’Oresme (c.1325-1382) and Nicolaus Copernicus (1473–1543), among others, wrote before about this idea.
Isaac Newton (1643-1727) added to this principle several other concepts, including laws of motion, inertial frames of reference, and an assertion of an absolute time. Corollary V in Newton’s Principia embodies his Principle of Relativity: "The motion of bodies included in a given space are the same among themselves, whether that space is at rest, or moves uniformly forwards in a straight line without any circular motion".
Newton’s Principle rests on the idea that inertial frames of reference are preferred. That just means this: if some law of physics is expressed in terms of the quantities of one inertial frame of reference, the resulting statement of the law will be exactly the same in any other inertial frame of reference. Thus, the Principle of Relativity can simply be stated as:
The laws of physics are the same in all inertial frames of reference.
The key feature of both classical mechanics and the theory of relativity is that no unique reference frame is determined. Any object that is not accelerating can be regarded as stationary ‘in its own inertial frame’. It defines a valid reference frame for the whole universe. There are many possible choices because given any particular reference frame, any other frame, defined to give everything a constant velocity relative to the first frame, is also a valid choice.
- In 1904 Henri Poincaré declared the relativity principle to be a 'general law of nature', on par with the law of energy conservation, and insisted on its unrestricted and universal validity.
- Albert Einstein elevated the principle of relativity to a postulate in his theory of relativity of 1905.