CHAPTER I.
CLASSICAL RELATIVITY.
Before entering upon the subject proper of this volume, namely, the modern doctrine of Relativity and the history of its origin and development, it seems desirable to dwell a little on the more familiar ground of what might be called the classical relativity, and to consider two particular points which are of fundamental importance, not only for the appreciation of the whole subject to follow, but also for an adequate understanding of almost all physico-mathematical considerations. What I am alluding to are the following questions: 1° the choice of a framework of axes or, more generally, of a system of reference in space, and 2° the definition of physical time, or the selection of a clock or time-keeper, to be employed for the quantitative determination of a succession of physical events.
Both of these questions existed and were solved, at least implicitly, a long time before the invention of the modern Principle of Relativity, in fact centuries ago, in their essence as early as Copernicus founded his system.[1]
The question of a space-framework is obvious enough and widely known; it will require therefore only a few simple remarks.
The most superficial observation of everyday life would suffice to show that the form and the degree of simplicity of the statement of the laws of physical phenomena, more especially of the laws of motion of what are called material bodies, depend essentially on our selection of a system of reference in space. Certain frameworks of reference are peculiarly fitted for the representation of a particular
- ↑ A clear and beautiful statement of the fundamental importance of the Copernican idea is to be found in P. Painlevé's article 'Mécanique' in the collective volume De la Méthode dans les Sciences, edited by Émile Borel. (Paris, F. Alcan, 1910.)
