Gravitational Mass and the
Principle of Equivalence
SO FAR THE SUBJECT of our discussions has been almost exclusively the concept of inertial mass, which determines the inertial behavior of particles or bodies. Now we shall turn our attention to the concept of gravitational mass, which determines the gravitational behavior of matter. Since every body is a source of a gravitational field and is in turn affected by it, it has become common practice, as we noted in chapter 1, to assign to every body, apart from its inertial mass mi, an active gravitational mass ma, which specifies the body's role as the source of a gravitational field, and a passive gravitational mass mp, which specifies the body's susceptibility to being affected by this field. In many respects ma and mp can be conceived of as gravitational analogues to electrical charges and are therefore sometimes referred to as “gravitational charges.”
Since the history of the conceptual development that led to the classification of mass into mi, ma, and mp appears never to have been studied before, it seems appropriate to comment upon it briefly. This trichotomy, which is of rather recent origin, was preceded by the dichotomy of mass into inertial and gravitational mass or, symbolically, into mi and mg, where mg denotes either ma or mp. But even this dichotomy was rarely, if ever, explicitly emphasized prior to the twentieth century.
True, Newton, as we shall see very soon, did distinguish between what he called “quantity of matter” (“quantitas materiae,” “massa,” or “corpus”), which corresponds to mi, and “weight” (“pondus”), but he never regarded “weight” as the product of a gravitational mass and the acceleration that is denoted by g. Nevertheless, until about 1900 physicists and philosophers who dealt with the foundations of physics often confounded the notions of mass and weight, a historical fact that was noted with disapproval as early as 1908 by Émile Meyerson.1____________________