In general, the principle of relativity is the requirement that the laws of physics be the same for all observers. Somewhat more particularly, the measurements an observer makes in his frame of reference of an important event are related in a particular way to those made by another observer. Several principles of "relativity" have been developed, starting with the Galilean version.
Galileo and others considered only inertial reference frames (see the discussion of such frames in frame of reference) in uniform translation relative to each other. The measurement of time was assumed to be the same for all observers, but the measurements of distance made by an observer differ from those by another depending on the relative velocity between the observers' frames of reference. The measurements made by one can be transformed to those by another one. Such a transformation is called the Galilean transformation.
Einstein saw, however, that if the vacuum speed of light was really the ultimate velocity in our physical world, a different kind of transformation was called for. If information about an important event cannot reach us faster than light (or any form of electromagnetic phenomenon) can, the Galilean assumption of universal time has to be discarded. The transformation of an observer's measurements to another observer's measurements, now changed, is called the Lorentz transformation. For details, seetheory of relativity and special relativity.
General relativity extended the principle of relativity to accelerated frames of reference in a gravity field.
According to relativity, this must be re-expressed as "The magnitude of the relative velocity between your car and the pavement must be less than 70 mph".
Special relativity was constructed to satisfy Maxwell's equations, which replaced the inverse square law electrostatic force by a set of equations describing the electromagnetic field.
Relativity also leads to interesting objects such as fl holes, but these are not very relevant to cosmology.
This divergence between Newtonian and relativistic physics was one of the most remarkable discoveries of this century.
When relativity is discussed at all, it often entails only a rather abstract introduction to Lorentz transforms, relegated to the back of a chapter (or the back of a book).
Sears and Brehme, Introduction to the Theory of Relativity (Addison Wesley, 1968).
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