This year is the centenary of four scientific
papers by Albert Einstein.
It seems that the least known paper of the four is 'On the Motion
of Small Particles Suspended in a Stationary Liquid According to
the Molecular Kinetic Theory of Heat'. Einstein extended the work
of Ludwig Boltzmann, calculated the average trajectory of a
microscopic particle buffeted by random collisions with molecules
in a fluid or in a gas, and gave a estimate for the size of the
molecules.
The paper presented a mathematical formula for Brownian motion, the
random displacement of particles suspended in a fluid. The motion
could be explained by molecules with random energy moving in random
directions, randomly striking the particles. Einstein used the
framework of James Clerk Maxwell's and Ludwig Boltzmann's kinetic
theory of gases to estimate the size of molecules. One of the
assumptions, unproven and controversial in 1905, of the kinetic
theory is that gases are composed of a large collection of hard,
spherical objects in constant motion.
Introduction to the Kinetic-Molecular Theory of Gases. This is
an
ideal
gas, provided that the collisions are perfectly elastic.
Robert Brown was the observer who gave his name to the phenomenon.
Brown's investigations of fertilization in flowering plants
observed with
his microscope
lead hem to dissect pollen sacs and suspend the pollen in water.
The suspended grains were filled with
particles in
constant motion. Was this the motion of life? Brown argued
'no', since he observed the same motion in pollen grains preserved
for eleven months in alcohol. His further observations of minerals
in suspension supported his conclusion that the motion belonged to
the particles themselves, and not to any activity of life.
Brown was a member of the Linnean Society, and the week Charles
Darwin received Alfred Wallace's paper on the 'survival of the
fittest', Brown died. His death on June 10, 1858, caused the
postponement of the Society's regular meeting until July 1. At the
July 1 meeting, the Darwin-Wallace paper was read to members.
Until Einstein's explanation, many scientists doubted whether atoms
and molecules physically existed. Atoms and molecules were a very
useful assumption, but other scientists rejected the atomic theory
of matter. In 1905, the first law of thermodynamics could be
understood in terms of the motion of Newtonian atoms. But
mechanical behavior could not explain the second law, because the
second law is irreversible: a melted ice cube does not refreeze.
Einstein's estimate of the size of molecules provided critical
support for the atomic theory, and his analysis used statistical
methods to describe the behavior of the enormous number of atoms or
molecules in a gas. The kinetic theory of gases and the atomic
theory of matter (rather than Wilhelm Ostwald's energeticist view),
began to decisively win out. Ostwald later said that reading
Einstein's paper converted him to the atomic explanation.
(Einstein's father unsuccessfully wrote to Ostwald in 1901 seeking
a position for his son, when he couldn't find work.)
Tragically, Boltzmann, after decades of work on the kinetic theory
and the application of statistics to describe the behavior of atoms
and molecules, committed suicide in 1906.