Spacetime

In 1905 Albert Einstein published a paper entitled, "On the Electrodynamics of Moving Bodies." In that paper, he put forth his "principle of relativity" which soon gave the work the name by which it has been known since. Einstein wrote that regardless of their state of relative motion, all inertial (that is, unaccelerated) observers 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. Two well-known sets of laws at that time were those of Isaac Newton's mechanics and...