#9 – J.J. Thomson

The entire edifice of chemistry is a theme and variation on the study of the properties of atomic electrons. Tremendous sections of physics, from solid state to quantum optics to AMO and beyond hings almost entirely on electron behavior. Astrophysics, spectroscopy, and large chunks of high energy physics rely on the understanding of the electron. And those are just the leading edge of what the electron means to modern science. More than anyone else, we owe J.J. Thomson for what we know about the electron.

No one knew for sure that there was any such thing as an electron until Thomson discovered it in 1897. Before Thomson’s experiments, it was known that a scientist could produce something called cathode rays which were thought to be associated with a negative charge based on their behavior in a magnetic field. Not much else was known about them.

Thomson proved that in fact the rays themselves were charged and could be deflected by an electric field. He then was able to experimentally measure the charge to mass ratio of the rays, proving that in fact it was made out of particles. No matter how the rays were produced, the charge to mass ratio stayed constant. Taking the state of the art to its limit, he was able to make good estimates of the actual charge and mass of each individual electron. From that information, he found that in fact it was these same particles that composed the beta particles in radioactivity and the charged particles emitted by the photoelectric effect. He made progress in understanding that process, in which electrons were kicked out of metals which were exposed to light, and he observed the change in intensity of the emitted electrons as a function of the intensity and frequency of the incoming light. It remained for Einstein to explain this in terms of the photon concept of light a few years later in his great year of 1905. But it was Thomson who discovered that the electron was the link between these formerly separate processes.

All this seems obvious now, but that’s just because electrons and their applications are so ubiquitous. At the time it was a staggering leap forward. There were particles smaller than atoms, and they came from the atoms themselves. In other words, atoms were divisible, not fundamental unbreakable units of mass. Thompson later went on to demonstrate that in fact the atoms of an element were not even necessarily all the same – they came in different masses called isotopes. This anticipated the discovery that electrons were responsible for the chemical properties of elements while the later-discovered nucleus carried most of the mass of each atom. And furthermore his general experimental technique became what’s now known as mass spectrometry which appears in practical applications all over science and industry. He also first characterized and explained Thomson scattering.

Among his students were several more titans of physics. Ernest Rutherford, J. Robert Oppenheimer, Max Born, Charles Wilson, and William Bragg among others all studied under Thomson and went on to make their own dramatic discoveries (and have a shot at the top 10!). Thomson’s son George Thomson went on to win his own physics Nobel by discovering electron diffraction, which illuminated the wave-particle duality of matter for the first time.

Not half bad. For all that, he’s the 9th greatest physicist.

The list so far:
10. Wolfgang Pauli
9. J.J. Thomson