Following up on the work of John Dalton, Robert Brown and, later, Albert Einstein, would finally help to convince the world of the existence of atoms.

Although the concept of atoms had been around since the time of Democritus in the 5th century B.C., by the time the 19th century had come around, the idea still hadn’t caught on. This began to change, however, with the impressively refined theories of John Dalton in the first decade of the 1800’s.

The farfetched concept of a universe made up of an uncountable number of tiny particles, all bound together by some unknown force to create all matter, however, was still rather difficult for most reputable scientists to believe. While Dalton’s theory was clever, and certainly very interesting from the perspective of chemistry, it had yet to be experimentally verified.

Experimental Evidence of Atoms

This next major step – experimental examination of Dalton’s atomic theory – came in 1827, by way of a Botanist, of all people – Mr. Robert Brown of Scotland.

In his observations, Brown had noticed that pollen grains placed into a clear liquid (probably water) and viewed through a microscope tend to behave somewhat strangely, moving around almost as if they were alive, jumping and skipping about in random motions with no visible impetus.

This effect is what today is known as Brownian Motion, and was soon discovered to be caused not by the pollen grains being alive (as was a common explanation of the time), but by the pollen grains being pushed around by the molecules within the water itself – in other words, they were being “jostled” about by the atoms. Although Brown himself did not provide such an explanation for this phenomenon, it is still named in his honor, and at least provides a framework for later work on proving the existence of atoms.

It is interesting to consider the fact that, even in the first years of the 20th century, there were still holdouts in the scientific community who had not yet come around to a belief in atoms as the building blocks of matter. Even though a tremendous amount of work had already been done on them – the discovery of electrons, an examination of atomic radiation – some apparently just weren’t ready to believe. Before too long, though, even these most ardent skeptics would have no choice but to accept the atomic theory.

The fact of Brownian motion was important in that it went great lengths to verify the existence of tiny particles even in a seemingly continuous fluid such as water. Just how significant wouldn't be known until nearly 80 years later, when a soon-to-be-famous physicist named Albert Einstein took the concept of Brownian motion to the next level.

Einstein’s Contribution

The second of Einstein’s four papers published during the “Miracle Year” of 1905 was entitled A New Measurement of Molecular Dimensions & On the Motion of Small Particles Suspended in a Stationary Liquid. While Einstein may have failed at creating a clever title for his piece, its content (as well as the content of his later doctoral thesis, which is really just an extension of this paper) is rather interesting.

By giving the subject of Brownian Motion a more intense study than had ever been undertaken, Einstein was able to calculate the number of water molecules per square inch (to a surprising degree of accuracy), as well as to provide statistical and mathematical formulas for the motion that was evidenced.

His theory was based on the assumption that as small particles (such as pollen grains) move about in a liquid, they are being pushed about by much, much smaller atoms in every direction. Normally, there are roughly the same number of atoms on each side of the pollen grain, all pushing and bumping against each other in random directions, so naturally, such movement should tend to cancel each other out most of the time. Seeing as it truly is a random process, however, it is not infrequent that the pollen grain is pushed a little bit more in one direction, so it moves that way, then later it is pushed in a different direction and moves another way.

So Einstein determined that while such movement is completely random and unpredictable, it also obeys certain laws of probability, which Einstein was able to determine using a mathematical formula which became known as “The Random Walk.”

An Alternate Use of the Random Walk

Interestingly enough, mathematicians have observed that Einstein’s formula, which describes random changes in motion, can actually be used to determine the number of extra steps will be needed for a drunk person attempting to walk from one place to another, staggering back and forth as they lose balance. Remember, though, that this is all from a statistical standpoint, meaning that while it is not exact, it will tend to hold true on average.

The Fate of Atoms

With the important foundation laid by Robert Brown in the 1820’s and the intensive follow up by Einstein in the early years of the 1900’s – combined with other work done by many other, wonderful scientists, atomic theory finally had the foothold it needed to be fully recognized as the fundamental explanation for every piece of matter in the universe.

It was not clear that any of these early atomic scientists understood the can of worms that was being opened here.

Further Reading:

Democritus and Ancient Atomic Theories

John Dalton and the First Modern Atomic Theory

The Scale of Atoms

J.J. Thompson and the Discovery of Electrons

Ernest Rutherford and the Discovery of Protons and the Atomic Nucleus

James Chadwick and the Discovery of the Neutron

References:

Moring, G. F. (2004). The Complete Idiot's Guide to Understanding Einstein. New York, NY: Alpha Books.

Gribbin, J. (2002). The Scientists: A History of the Science Told Through the Lives of its Greatest Inventors. New York: Random House.

Einstein, A. (1905). Investigations on the Theory of the Brownian Movement. Annalen der Physik .