Now that we have the Compton Effect sort of humming in a preliminary way in 1920 as Arthur Compton leaves the Cavendish Lab and gamma-rays to go to St. Louis and get back to X-rays, we can do some reflecting on why the interactions of X-rays and electrons were so hard to picture in 1920. “Picturing” the interaction is, of course, part of the problem and part of the solution to that is images from cloud chambers developed by C. T. R. Wilson, but, somewhat paradoxically, what we now know about the various continua around the discontinuities of quantum events can help us understand why it was so hard to define the Compton Effect in the world of 1920.

Stuewer’s story of the Compton Effect (page
161) features a facsimile of Compton’s notes written on *Aquitania* stationary as he voyaged on that ship back across the Atlantic. In these notes, Compton outlines his research
plan under the heading “Problems to be tackled at St. Louis.” His plan was to study the scattering of X-rays
at various angles off of various metals.
The basic problem would be abandoning the electromagnetic field model
for interpreting his data for a quantum model when there was already a workable
“classical” (ie not quantum) model – Thomson Scattering – to account for the
scattering of radiation less energetic than X-rays and a lot of confusion about
how the photoelectric effect was to be interpreted without quantized packets of
energy.

Clearly, part of the problem in 1920 (and we will see this again with Cosmic Rays in the 1930s) was that there was a continuum of energies that had to be understood and what happened at different energies could be very different. So, to look at what Compton was faced with in terms of how the Compton Effect is now understood, Compton was in a good position to isolate single instances of a single type of quantum event but he had to disentangle that single type from many different continua – most notably from the range of different effects associated with the whole continuum of photon energies, from other types of scattering, from the electromagnetic field and from what we now think of as higher order effects in the quantum electrodynamic perturbation series that involve branching and looping within the framework of the basic interaction.

The range of effects due to different photon energies run from Thomson Scattering, though the photoelectric effect, Bremsstrahlung (the case where the electron emits a soft photon while interacting with a field), double scattering (where an “extra real” high-energy photon occurs), and pair production (the case where a photon’s energy takes the form of an electron and a positron). When Compton started his work in 1920, only Thomson Scattering was generally well understood. Even the photoelectric effect was still viewed with skepticism while pair production was completely unimaginable.

Moreover, there was another whole area of possibilities involved in radiative interactions that was really not well understood until the breakthroughs in quantum electrodynamics in the late 1940s. While these higher order possibilities would have had relatively little impact on what Compton was observing in the early 1920s, they are worth mentioning to suggest the possible complexities of what he was encountering. For the moment, here are the Feynman diagrams for the lowest order interactions involved in the Compton Effect. The next post (Compton Effect Three) will be a detour into the higher order possibilities of the interactions. These diagrams are from the CERN study in 1998 that described how to use the whole range of possible permutations of the interactions in the perturbative series, where the effect gets more elaborate at higher energies, to calibrate polarization detectors in high energy particle accelerators (the squiggly lines are photons and the other lines are electrons):