When I met Quantum Physics

Of course I have had conversion experiences of a decidedly non-religious (conventionally speaking) sense as well. The one of these that sticks out most in my mind as a conversion was when I became a believer in the truth of quantum physics.

In Quantum I, on the 7th floor of RLM at the University of Texas, we were discussing the historical origins of quantum theory. We had been through Planck's explanation of the Black Body Radiation phenomenon, in which one particularly surprising logical result of his solution was the quantization of light. Planck's quantum conclusion was less than convincing to me, as it was to physicists of his day. The experiment that would later convince most physicists of the truth of Planck's conclusions, as well as me nearly a hundred years after the fact, was that of the Photoelectric Effect.

This phenomenon had been puzzling physicists for years. Essentially, when light towards the blue/violet end of the spectrum hits certain metals it causes the metal to eject electrons that all have approximately the same energy. If light is, strictly speaking, wavelike, then one would expect as the intensity, or brightness, of the light is increased then the energy of the electrons would increase similarly. Think about that for a moment; let that sink in.

It turns out that what actually happens in the Photoelectric effect is that as the intensity of light is increased, the energies of the electrons that are expelled from the metal remains constant. What we get instead of increased energies is simply an increase in the number of electrons expelled. I hope that you allowed ample time for the previous paragraph to sink in so that you realize how crazy this is. Einstein was able to resolve the problems one runs into in this experiment by treating light as if it were a particle, which worked wonderfully. What is more strange is that a constant that showed up in Planck's treatment of Black Body Radiation reappeared in Einstein's equations explaining the Photoelectric Effect. Einstein thus corroborated Planck's quantum conclusion, and essentially kicked off the quantum revolution in physics, as well as in me personally.

Sidebar: Einstein won the Nobel Prize in Physics in 1921 for his explanation of the Photoelectric Effect. Of course that work of his came out the same year (1905) as his more famous Special Theory of Relativity, and his similarly important explanation of Brownian Motion. Planck justifiably received the Nobel Prize for his work on Black Body Radiation in 1918