Brownian Motion

Brownian Motion and Microphysical Reality c. 1900

Charlotte Bigg

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The irregular motion of suspended particles in a solution as recorded by Jean Perrin

In the years around 1900 scientific research became increasingly concerned with sub-microscopic entities, including atoms, molecules, ions, bacteria, and all sorts of minute organisms. In different fields, scientists groped for ways of perceiving, studying, and measuring—indeed in most cases of ascertaining the very existence of—these entities. Vacuum tubes, oil drops, and cloud chambers for instance constitute some of the experimental and instrumental methods devised to probe the microphysical world. The opening up of new dimensions also stimulated the development of theoretical tools to account for the workings of the microcosmos and to relate it to the macroscopic realm: new optical theories helped analyze the visual appearance of phenomena at the limit of optical resolution; statistical methods were introduced to treat the behavior of large numbers of particles and account for their effect on a macroscopic scale.

The eruption of new microscales on scientific research agendas arguably contributed to a profound transformation in scientific practices and social organization in the early twentieth century. In this project, the case for this argument was made for the physical sciences on the basis of a study of Brownian motion research in the 1900s. Charlotte Bigg investigated the investigations carried out by a handful of physicists and chemists in these years, most notably Albert Einstein and Jean Perrin.

Brownian motion, the perpetual and irregular motion of particles suspended in a solution, had long been known but until then little noticed. In the 1900s it came to encapsulate the fundamental issues at stake in early twentieth-century physical sciences: the nature and structure of matter, the relationship between statistical mechanics, kinetic theory and thermodynamics, and more broadly the validity of hypotheses and mechanical models in science.

Specifically, Charlotte Bigg examined how Perrin and Einstein deployed theory and experiment to produce, for the first time, "visual’" evidence of the existence of atoms and of the statistical nature of the second law of thermodynamics, e.g,. how they developed methods to make sense of the behavior of floating submicroscopic particles and connected it with broader issues in the physical sciences. Close attention was paid to Einstein’s intricate interweaving of chemical and physical theories to account for the individual and collective behavior of particles, the significance of his application of Boltzmann’s statistical mechanics for this purpose, and its implications for assessing the commensurability of the macro- and microscopic dimensions and for the development of thermodynamics. Bigg also investigated how the Brownian motion of submicroscopic particles was experimentally turned into "visual" evidence of atoms, most notably through the use of the ultramicroscope, a new instrument enabling the visualization of particles below theoretical resolution (though not of atoms), and how Perrin in particular worked to make the molecular dimension intelligible by extending the domain of application of different theories into the molecular or macroscopic realms (e.g., extension of the kinetic theory of gases to suspended particles). In this respect, a comparison could be made with the simultaneous discovery of the syphilis bacillus using the ultramicroscope, and how microbiologists negotiated similar issues of scale shifting.

Through a close analysis of the relatively circumscribed field of Brownian motion research, Charlotte Bigg unwrapped the momentous scientific, disciplinary, and social stakes at play in this period of profound transformation of the physical sciences.

In physics, Brownian motion research played a significant part in the overhaul of physics in the early decades of the twentieth century, and the emergence of a "modern" physics opposed to the "classical" physics of previous decades. As Einstein first recognized, the perpetual motion of particles seemed to contradict the second law of thermodynamics, implying that this law could no longer be considered as absolutely true at the submicroscopic level. Brownian motion was thereby turned into a crucial test for contemporary physics. The fact that these conclusions were obtained strictly though mathematical reasoning must moreover be seen as an important moment in the ascendancy of theoretical physics as a distinct and fundamental field of research, and of Einstein as one of its archetypal representatives.

In chemistry Brownian motion was interpreted, most notably by Perrin, as conclusive evidence of the existence of atoms. The significance of Brownian motion for the resolution of the century-old debate among chemists about atomic reality was portentous, albeit perhaps on a more symbolic than pragmatic level; as few chemists had in previous years actually attempted to settle the question experimentally. Rather, Perrin’s work was interpreted by chemists as the vindication of an approach described as "physical," which involved a set of instrumental and theoretical methods adapted from physics; a predilection for working with mechanical models instead of energetic ones; a willingness to work with hypotheses against the empiricist bend of many contemporary chemists. Indeed, in institutional terms, Brownian motion work was instrumental for the emergence of a recognizable field of physical chemistry in France. Through popular books and exhibitions, Perrin, one of its most vocal representatives, helped turn Brownian motion experiments into showcase of physical chemistry but also of science’s power and insight.