Concepts of Mass in Contemporary Physics and Philosophy

Book Preface

This book intends to provide a comprehensive and self-contained study of the concept of mass as defined, employed, and interpreted in contemporary theoretical and experimental physics and as critically examined in the modern philosophy of science. It studies in particular how far, if at all, present-day physics contributes to a more profound understanding of the nature of mass.

In order to make this book accessible not only to the professional physicist but also to the nonspecialist interested in the foundations of physics, unnecessary technicalities and complicated mathematical calculations have been avoided without, however, impairing the accuracy and logical rigor of the presentation.

Next to space and time, mass is the most fundamental notion in physics, especially once its so-called equivalence with energy had been established by Albert Einstein. Moreover, it has even been argued repeatedly that “space-time does not exist without mass-energy,” as a prominent astrophysicist has phrased it.1

Although for the sake of completeness and comprehension the text includes some historical and explanatory comments, it deals mainly with developments that occurred after 1960. In fact, the year 1960 marks the beginning of a new era of experimental and theoretical research on gravitation and general relativity, the two main bases of our modern conception of mass. In 1960 the first laboratory measurement of the gravitational redshift was performed by P. V. Pound and G. A. Rebka, and the first recording of a radar echo from a planet (Venus) was made. In 1960 the spinor approach to general relativity was developed by R. Penrose. In the same year V. W. Hughes and independently R.W.P. Drever confirmed the isotropy of inertial mass by what has been called the most precise null experiment ever performed; and R. H. Dicke, together with P. G. Roll and R. Krokov, planned the construction of their famous “Princeton experiment,” which was soon to confirm the equivalence of inertial and gravitational mass with an unprecedented degree of accuracy. All these events rekindled interest in studying the properties of mass and endowed the study with a vigor that has not abated since.

As this book deals primarily with developments that occurred during the relatively short interval of only four decades, its presentation is predominantly thematic and not chronological. The first chapter discusses the notion of inertial mass and in particular the still problematic issue of its noncircular definability. Chapter 2 deals with problems related to the concept of relativistic or velocity-dependent mass and to the notion of velocity-independent rest mass. Chapter 3 clarifies certain misconceptions concerning the derivations of the mass-energy relation, usually symbolized by the equation E D mc2, and comments on various interpretations of this relation. Chapter 4 analyzes the trichotomy of mass into the categories of inertial, active gravitational, and passive gravitational mass and studies the validity of the equivalence principle for test particles and for massive bodies. The final chapter, probably the most controversial one, discusses recently proposed global and local theories of the nature of mass.