Brownian motion

The second paper that Einstein published in 1905, his miracle year, was on Brownian motion. Einstein was tackling the problem of the atom. That atomic theory, at the time, was remarkable at predicting “the properties of gases and chemical reactions”. However, there was no real direct proof of their existence. It was Einstein’s reasoning that “one might be able to prove the existence of atoms by noticing their effect on small particles in a liquid.” This is where the term “Brownian motion” comes in. Brownian motion is the random movement of small particles suspended in a liquid. Robert Brown, a botanist, discovered this phenomenon in 1828. Brown observed tiny pollen grains under a microscope exhibiting some strange motion. There were ideas of what was going on, however, nobody had a reasonable theory of them. Einstein reasoned that these atoms were to small to be observed, however, their size and behavior could be estimated. This was done by observing and calculating their impact on larger objects. The idea was that the atomic theory should be able to calculate the physical dimensions of atoms by analyzing this Brownian motion. This was the random motion of trillions upon trillions of molecules of water. This motion was causing the further motion of the dust particles. By analyzing this dust, the size and weight of atoms could be computed. This gave experimental evidence for the existence of atoms. 

Einstein showed that by peering into a microscope, a gram of hydrogen contained “3.03 x 10^23 atoms”. This is actually close to the correct value. The title of Einstein’s paper was “On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat”. The paper was published July 18 in 1905, Einstein’s miracle year. This was the first experimental evidence for the existence of atoms.