Brownian Motion and the Appraisal of Theories

Abstract

Brownian movement (discovered by the botanist Brown in 1827) refers to the irregular motion of small suspended particles in various fluids which keeps them from steadily sinking due to gravitation. Attempts to explain this phenomenon are linked with the atomic debates from the late 19th century to the early 20th. In particular, the testing of the Einstein—Smoluchowski theory of Brownian motion by Perrin1 is considered to have provided the long sought-after evidence in favor of the molecular-kinetic theory of gases against classical thermodynamics. While this episode has often served as a case study for philosophers,2 such case studies, as remarked generally by Laudan et al. (1986, p. 159) “are not ‘tests’ of the theory in question at all, but applications of the theory to a particular case”. The aim of my discussion will be to employ this episode to carry out genuine tests, focusing on the following set of interrelated theses on theory appraisal: (T2.3, T2.4, T2.5, T2.6, T2.8 and T2.10). As many of these pertain to evaluating guiding assumptions, I shall suggest briefly how my results bear on a thesis on the appraisal of guiding assumptions (GA1.1), and a thesis on scientific revolutions (GA4.1). The atomic debate as a whole is too broad to be taken up here.