Existence of Atoms Reaffirmed

A new experiment has reproduced a landmark 1908 study that demonstrated the physical existence of atoms, even to many of those (such as the chemist William Ostwald) who had doubted that matter consisted of microscopic particles rather than being continuous in nature.

The new experiment, conducted partly as an educational exercise for undergraduates at Harvard, reproduced (with modern equipment) the work in 1908 of Jean-Baptiste Perrin, a French physicist, who in turn was seeking to test a prediction of Albert Einstein.

Einstein's miraculous 1905 output included famous papers on special relativity (bearing on features of space-time and on the equivalence of matter and energy) and the photoelectric effect (explaining the quantum nature of light). The propositions of relativity and quantum theory proved to be extremely fruitful and are put to frequent experimental test.

A third paper from that year, one devoted to explaining Brownian motion, is perhaps less well known, but also of great importance. Brownian motion, first observed by Robert Brown in 1827, is the jostling of one set of tiny particles (in this case, pollen grains) by other, even smaller, particles (the surrounding water molecules).

Einstein interpreted the jostling as the incessant and fluctuating aggregate effect of all the presumed atoms or molecules on the grains; occasionally the net force on the grain would push it to the side. Einstein worked out a formula relating the size of the pollen grains and their median momentary excursion (part of what we would now call a "random walk") and the size of the surrounding and invisible buffeting particles (atoms and molecules).

Perrin performed his experiment using emulsions containing microscopic particles of gamboge (a type of pigment) or mastic (a clear plastic). Using a microscope he painstakingly watched, measured, and tabulated many displacements of individual gamboge particles. From this he confirmed Einstein's predictions about the statistical nature of the agitations, and from this one could calculate Avogadro's Number, the number of atoms or molecules in a single mole of that substance. And this in turn supported the atomistic view of matter.

The new Harvard version of this experiment is faithful to the 1908 work except that a CCD camera viewed the particle movements and analyzed the displacements by means of a computer program.

Newburgh, Peidle, and Rueckner, American Journal of Physics, June 2006
Contact Ronald Newburgh, rgnew@verizon.net