By Don Lincoln, Fermilab
In the atomic saga, a person who contributed significantly toward our understanding of atoms wasn’t a physicist but, surprisingly, a botanist—Robert Brown. In fact, Brown himself never figured out that he’d made the advance that he did and became the cornerstone of Albert Einstein’s mathematical theory establishing the existence of atoms.
Robert Brown started his career as a medical student, but the discipline didn’t fascinate him the way plants did. Eventually, he dropped out to enlist as a surgeon’s assistant in a British regiment. He was assigned to Ireland and, since there were no invasions of England or Ireland at the time, he had a lot of time on his hands, which he devoted to studying plants.
And yet, botany wasn’t the skill that made Brown an important figure in the search for evidence of atoms. Instead it was his use of a new instrument that eventually led to this scientific advance. Brown was an early adopter of the microscope to study plants. In 1827, while leading a luminary in the European botany community, Brown began studying grains of pollen from the plant Clarkia pulchella, using his microscope.
Looking at these grain pollens suspended in water, Brown saw minute particles ejected from the grain pollens. When he stared at them through the eyepiece of his microscope, they moved and just sort of jittered around, in what we now call a random walk. The particles were staggering all over the place.
Now, this motion was indeed a curiosity. He investigated further and found that the random motion could also be seen when particles made entirely of inorganic matter and of similar size were suspended in water. The motion wasn’t a property of life, but rather a property of tiny particles in water.
Brown never did figure out what was going on, but he made the observation and the phenomenon is now called Brownian motion. However, the question remains-how does Brown’s Brownian motion lead us to knowing that atoms are real?
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Einstein and the Molecular Kinetic Theory of Heat
It’s said that all roads lead to Rome, and in science, it sure seems that an awful lot of roads lead to Albert Einstein. After all, he discovered his theories of relativity and he contributed to the discussion of whether light is a particle or a wave. He published both of those ideas in 1905. While both of those theories changed our understanding of physics, that wasn’t all he did that year.
The paper that Einstein wrote that helped establish the existence of atoms is his paper called On the Motion of Small Particles Suspended in a Stationary Liquid, as Required by the Molecular Kinetic Theory of Heat. It was submitted on May 11 and was published on July 18 in the prestigious journal Annalen der Physik. When translated from German, his paper begins in the following way. It says,
“It will be shown in this paper that, according to the molecular kinetic theory of heat, bodies of microscopically visible size suspended in liquids must, as a result of thermal molecular motions, perform motions of such magnitude that these motions can be easily detected by a microscope.”
The Brownian Molecular Motion Explained
Einstein proposed that his paper will explain what he called ‘Brownian molecular motion’. The basic gist was that he modelled water molecules as little marbles and worked out the motion of countless numbers of them, all bouncing around into one another.
Understandably, if the water molecules were bouncing into one another, they are bouncing into the little specks of dust or bits of pollen etc. It’s not at all that different from as if we were standing in a field somewhere and a huge crowd of people were constantly throwing soccer balls at us. We’d get knocked around and someone watching us, who couldn’t see the soccer balls, would just see us moving in essentially a Brownian kind of way.
The Einstein story doesn’t stop there. He uses his theory to not only explain why these little specs of dust move around in water, but, he uses it to make a crude determination of the size of water molecules.
A Determination of Avogadro’s Number
In the fifth and final section of the paper, Einstein worked out that if the dust particles were about a thousandth of a millimeter in size, that the particles could be expected to move about six millionths of a meter in a minute.
And, reversing the approach, one could use the measurement of the motion of the object and come up with a determination of Avogadro’s number, which can be used to determine the size of molecules. This, using the statistical motion of gas molecules to determine the size of the molecules was Einstein’s PhD thesis, which he had defended only a few months prior, so it’s understandable how he would bring this all together.
In any event, it was only a couple years later that very precise measurements of the motion of dust motes in water were shown to be in good agreement with Einstein’s predictions. Thus, in conclusion, essentially his mathematics supported and further built on Brown’s astute observation, proving that Brownian motion was caused by the dust being hit by water molecules.
Common Questions about Einstein and the Brownian Molecular Motion
Botany wasn’t the skill that made Robert Brown an important figure in the search for evidence of atoms. Instead it was his use of a new instrument that eventually led to this scientific advance. Brown was an early adopter of the microscope to study plants.
Albert Einstein proposed that his paper will explain what he called ‘Brownian molecular motion’. The basic gist was that he modelled water molecules as little marbles and worked out the motion of countless numbers of them, all bouncing around into one another.
Using the statistical motion of gas molecules to determine the size of the molecules was Albert Einstein’s PhD thesis.