Brain Teaser: Brownian Motion


When Einstein wrote his famous article onBrownian motion in 1905, there were two main schools of thought about heat:thermodynamics (that only used macroscopic, observable quantities such astemperature or pressure) and the kinetic molecular theory of heat (this theoryexplained the macroscopic quantities of thermodynamics as the statisticalconsequence of the motion of molecules). The two theories seemed to make thesame predictions, and because before 1905 there was no strong empiricalevidence for the existence of molecules, the difference between the theorieswas mainly philosophical.


Einstein's article was immediately seen asproviding the first empirical grounds for accepting the existence of molecules.But how could the existence of molecules be inferred from the phenomenon of Brownianmotion? We have not discussed this in detail, but try to make an educated guessin the forum discussion below. If you want, you can also have a look atEinstein's paper on Brownian motion, available here.