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The Roman Lucretius's
scientific poem "On the Nature of Things" (c. 60 BC) has
a remarkable description of Brownian motion of dust particles. He uses this as a
proof of the existence of atoms:
"Observe what happens when sunbeams are admitted into a building and shed
light on its shadowy places. You will see a multitude of tiny particles mingling
in a multitude of ways... their dancing is an actual indication of underlying
movements of matter that are hidden from our sight... It originates with the
atoms which move of themselves [i.e. spontaneously]. Then those small compound
bodies that are least removed from the impetus of the atoms are set in motion by
the impact of their invisible blows and in turn cannon against slightly larger
bodies. So the movement mounts up from the atoms and gradually emerges to the
level of our senses, so that those bodies are in motion that we see in sunbeams,
moved by blows that remain invisible."
Although the mingling motion of dust particles is caused largely by air
currents, the glittering, tumbling motion of small dust particles is, indeed,
caused chiefly by true Brownian dynamics.
Jan Ingenhousz had
described the irregular motion of coal dust particles on the surface of alcohol in 1785. Nevertheless Brownian
motion is traditionally regarded as discovered by the botanist Robert
Brown in 1827. It is believed that Brown was studying pollen particles floating in water under the microscope.
He then observed minute particles within the vacuoles of the pollen grains
executing a jittery motion. By repeating the experiment with particles of dust,
he was able to rule out that the motion was due to pollen particles being
'alive', although the origin of the motion was yet to be explained.
The first person to describe the mathematics behind Brownian motion was Thorvald N.
Thiele in 1880 in a paper on the method of least squares. This was followed independently by
Louis Bachelier in
1900 in his PhD thesis "The theory of speculation", in which he presented a
stochastic analysis of the stock and option markets. However Einstein published in 1905 five important papers,
including two papers on the special relativity. Another is the paper on
photo-electric effect for which he received the Nobel Prize later. And still
another is the paper on the so-called Brownian Motion which encouraged Jean
Perrin to pursue his experimental work for confirming the kinetic theory and
showing the existence of atoms. Here is the beginning paragraph of this famous
paper.
In this paper it will be shown that, according to the molecular-kinetic
theory of heat, bodies of a microscopically visible size suspended in liquids
must, as a result of thermal molecular motions, perform motions of such
magnitudes that they can be easily observed with a microscope. It is possible
that the motions to be discussed here are identical with so-called Brownian
molecular motion; however, the data available to me on the latter are so
imprecise that I could not form a judgment on the question. (John Stachel, ed.,
Einstein's Miraculous Year: Five papers that changed the face of physics,
Princeton University Press, 1998, 85; Einstein's original papers are included in
the Collected Papers of Albert Einstein, vol. 2) What is crucial in this paper is that Einstein predicted "motions of such
magnitudes that they can be easily observed with a microscope". Many people,
before Einstein, studied and performed experiments on, the Brownian motion, but
they could not obtain any decisive results. The reader should read Stachel's
introduction for details. In order to supplement Mayo's description, I will
quote Stachel's nice summary on the significance of Einstein's paper.
In summary, the study of previous explanations of Brownian motion shows that
three elements of Einstein's approach are characteristic of his decisive
progress: (1) he based his analysis on the osmotic pressure rather than on the
equipartition theorem; (2) he identified the mean-square displacements of
suspended particles rather than their velocities as suitable observable
quantities; and (3) he simultaneously applied the molecular theory of heat and
the macroscopic theory of dissipation to the same phenomenon, rather than
resticting each of these conceptual tools to a single scale, molecular or
macroscopic. (op. cit., 77) At first the predictions of Einstein's formula were seemingly refuted by a
series of experiments by Svedberg in 1906 and 1907, which gave displacements of
the particles as 4 to 6 times the predicted value, and by Henri in 1908 who
found displacements 3 times greater than Einstein's formula predicted. But Einstein's
predictions were finally confirmed in a series of experiments carried out by
Chaidesaigues in 1908 and Perrin in 1909. The confirmation of Einstein's theory
constituted empirical progress for the kinetic theory of heat. In essence,
Einstein showed that the motion can be predicted directly from the kinetic model
of thermal equilibrium. The importance of the theory lay in the fact that it
confirmed the kinetic theory's account of the second law of thermodynamics as being an
essentially statistical law.
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