Tuesday, 14 February 2006
Einstein Yadda Yadda Yadda

Tuesday, February 14th was my turn to speak in the Center's lunchtime colloquium. Because of the hooplah surrounding the 100th anniversary  of Einstein's big year of 1905, I've done a lot of work on Einstein's  big year. Everyone knows that this was the year of special relativity  and E = mc². And people have a vague idea of other work. Something to do with atoms maybe. My goal was to make this "other work" come to life, to show that something very extraordinary also happened in this other work. Could I do this, I wondered as the usual crowd settled into their seats, sipping their coffee and munching on donuts and  bagels. Here goes, I thought.

The end point was Einstein's proposal of the light quantum. The idea was very simple. Sometimes light behaves as if its energy was localized at points in space. No big deal? Well it is a big deal if you are proposing it at a time when the greatest success of physics was Maxwell's theory of electrodynamics in which it is shown that light is a wave spread out in space. Given that background, 
Einstein's light quantum is just a bolt from the blue.

How could Einstein come up with it? The point of my talk was that Einstein's path to the result lay elsewhere. In the years leading up to 1905, Einstein had proved himself to be a master of statistical physics. When you try to analyze the physics of the many atoms that make up a gas or a liquid, there are so many of them that precise treatment of each atom is impossible. Instead you have to treat them 
by the methods of statistics. That work had been the focus of Einstein's earliest papers of 1901 and 1902; and the work continued through to another pair of his papers of 1905, his doctoral dissertation and his "Brownian motion" paper.

You can read about the details of this work on my website. (Go to "Goodies.") What matters in it is that Einstein had become a master at using the observed properties of gases and liquids to infer to their atomic microstructure. That atomic microstructure leaves a definite fingerprint on the observed properties of thermal systems, if only one knows how to read it. The ideal gas law, taught to every 
school child, is actually a fingerprint of many, localized independent atoms.

This skill at reading the signs, developed over the years, was the decisive tool Einstein needed to get to the light quantum. In poring over the measured properties of high frequency heat radiation, Einstein found the signature of atoms and his inference from that signature to the granular structure of radiation forms the core section of the his paper.

Once we see the path that Einstein followed, his proposing of the light quantum was not a bolt from the blue, but an adventurous extension of ideas that permeated his other work of 1905 and before.  Nonetheless Einstein's use of these statistical techniques was bold and inspired, so much so that his colleagues in physics needed 20 years before they would accept Einstein's conclusion.

John D. Norton

John D. Norton
14 February 2006
::: Atoms Entropy Quanta: Einstein's Statistical Physics of 1905
::: Lunchtime Colloquium