The Nature of Mass
SINCE THE END of the nineteenth century physicists and philosophers have been cherishing the hope that all of the problems related to mass could be resolved if a theory could be constructed that reveals what they called “the nature of mass,” that is, a theory that explains the origin, existence, and phenomenological properties of mass. Of course, such an expectation was hardly compatible with the positivistic or operationalistic view that the concept of mass “involves as much as and nothing more than the set of operation by which it is determined”1 and that any talk about “the nature of mass” would be scientifically meaningless or metaphysical rigmarole. Nevertheless, it is a historical fact that even among positivistically inclined physicists there have been proponents of a theory of mass, as we shall see in what follows.
A theory of mass that goes beyond the quantitative determination of this concept does, indeed, come up against the serious problem of how to avoid the error of a logical circularity: If as noted above, it is the concept of mass that is required for the transition from kinematics to dynamics, it must contain a dynamical ingredient. A theory of mass can therefore not operate solely with kinematical conceptions. Rather, it must itself be a dynamical theory and as such somehow involve a notion of force that is defined in mechanics as the product of mass and acceleration, thus leading to a logical circle.
The quest for a theory of the nature of mass arises from a profound epistemological motivation. It is no exaggeration to say that all experiments and certainly all measurements in physics are in the last analysis essentially kinematic, for they are ultimately based on observations of the position of a particle or of a pointer on a scale as a function of time. In particular, all operational definitions of mass are kinematic in character. Mach, for example, defined the mass-ratio mA/mB of two bodies A and B as the (negative inverse) ratio of two accelerations, i.e., in terms of purely kinematically measurable quantities. Hence, the term “mass,” thus defined, has no absolute meaning since it always implies a relation to an object chosen to serve as the unit of mass. This is one of the reasons____________________