Einstein contra Thermodynamics

From todays's point of view Einstein's starting point for his work on Brownian motion is rather surprising. Classical thermodynamics implies that there is no osmotic pressure in suspensions. Einstein did not intend to explain Brownian motion, the small irregular motion of particles first observed by the Scottish botanist R. Brown under the microscope, but to show that the statistical theory of heat required the motion of particles in suspensions, and therefore both diffusion and an osmotic pressure.

R. Brown (1773-1858)

Video of Brownian motion of nano particles in water (YouTube-Rutger Saly)

Contrary to thermodynamics which only works with macroscopic state variables, the statistical theory of heat tries to answer the question which microscopic changes are originated by the addition or removal of heat. Heat is related to an irregular state of motion of the microscopic building blocks of matter , such as atoms and molecules - addition/removal of heat therefore corresponds to an increase/decrease in motion.