ABSTRACT
The nature of time was the crux. It took Einstein’s insight to bring about the realization that a fundamental analysis of this kind was called for. But equally impressive was Einstein’s conviction that all observable physical phenomena must depend only on relative motions. A prescription of kind, a code of translation as it were, will generally consist of a mathematical system of transformation laws. It is in the nature of transformation laws to change most quantities but to leave some quantities unchanged. These [latter] are called the invariants of the transformation and serve to define its character. The relationship between measurements of a given motion as observed in different reference frames is expressed by a set of transformation equations. Clearly, if space is isotropic, all displacements transverse to the unique direction defined by the relative motion are equivalent, and it is not difficult to conclude that the appropriate transformations are of simple equality.
