In 1905 Albert Einstein published his seminal paper "On the Electrodynamics of Moving Bodies." In it he put forth his "Principle of Relativity," which soon gave the work the name by which it has been known since. All inertial (i.e., unaccelerated) observers, he said, regardless of their state of relative motion, must find the identical mathematical form for all laws of physics. (This is an example of what is today known as an "invariance principle"; in this case the laws are invariant--unchanging in form--when the space and time coordinates used by one observer are transformed into those used by another.) The laws known at that time were those of Isaac Newton's mechanics and James Clerk Maxwell's electrodynamics. Newton's laws of motion obeyed Einstein's principle, but Maxwell's equations did not.

The problem, Einstein realized, lay with the set of transformation equations relating position and time measurements made by observers...