Indirect Contributions Are Essential To Physics

A few weeks back, MIT Press published a piece about Paul Ehrenfest by Eric Johnson, whose biography of Boltzmann I reviewed here a while back. It’s fitting that the same person should write about both, as they’re closely linked through their work on statistical mechanics, as noted in what may be the greatest opening to a textbook in history:

Ludwig Boltzman, who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics.

Perhaps it will be wise to approach the subject cautiously.

(From States of Matter by D. L. Goodstein.)

Ehrenfest is an interesting figure not just because of his sad end (read Johnson’s article above for the details) but because his career touches on something I’ve written about before, namely the different models of success in a scientific career. Ehrenfest made some notable contributions to quantum physics as it was coming together in the early 20th century, but his primary contributions were as an explicator and facilitator. He was admired, as Johnson describes, mostly for his ability to see through to the essential core of a theory and explain it clearly. He also excelled at identifying problems and points of confusion.

Ehrenfest was regarded in his time as an essential contributor to the developing field of quantum physics less because of his own direct contributions than because of what he enabled others to do. By clearly explaining known physics, and clearly expressing problems with new theories, he helped the pioneers of quantum mechanics find their way.

Another example of this kind of indirect success, from the same era of physics, might be Arnold Sommerfeld. Sommerfeld made some significant contributions during the period of the “old quantum theory,” the ten-ish years between Niels Bohr’s quantum model of hydrogen and the development of full quantum mechanics by Heisenberg and Schrödinger (and Born and Pauli and Jordan and…).

His most important contribution, though, was his students. Sommerfeld was a generation older than most of the great quantum pioneers (ten years older than Einstein, even), but had an admirable degree of foresight in recognizing the importance of quantum theory, and steering his students in that direction. That’s extremely unusual in science, where the usual process is closer to Max Planck’s famous description: “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” Sommerfeld saw the truth approaching and made sure his students grew up with it, and as a result the list of his Ph.D. students is a murderer’s row of great quantum physicists, headlined by Werner Heisenberg and Wolfgang Pauli and Hans Bethe.

By its very nature, this kind of scientific success is a little difficult to identify— physicists who are second-order famous are more or less by definition famous primarily within a limited community and little known to outsiders. I suspect the late Sidney Coleman probably belongs in this group, but I don’t know his field well enough to say how important his own work was relative to his legendary explanations of quantum field theory (which even a cloddish low-energy experimenter such as myself has heard of). Coleman’s student David Griffiths might be another modern example, though a step down the educational ladder, thanks to his widely-used undergraduate textbooks on quantum physics and electromagnetism.

It’s also striking that most of the examples that come to mind are theoretical physicists. There are a bunch of factors at work, here, starting with the fact that explication and education are inherently more theory-based. Most great experimentalists in the key historical period became great in their own right— the list of notable experimentalists who can trace their lineage to Ernest Rutherford (and through him to J.J. Thomson) is easily as long as the list of notable theorists descended from Sommerfeld, but it’s difficult to claim that any of them outshine Rutherford.

The best example of a well-known experimentalist whose own direct contributions are outweighed by those of their students and colleagues might be I.I. Rabi, who in addition to directly supervising Nobel laureates in multiple subfields (Norman Ramsey in experimental atomic physics, Martin Perl in particle physics, and Julian Schwinger in theoretical quantum electrodynamics) also excelled as a scientific administrator, helping to establish the Brookhaven National Laboratory on Long Island and indirectly helping launch CERN. Of course, Rabi had a Nobel of his own, so it’s not like he was lacking in direct accomplishments, either…

If pressed to name a modern example of a Nobel-less experimental physicist who is more known for their students than their own work, I’d probably go for Rabi’s academic “grandchild” Dan Kleppner (who, full disclosure is my own academic “grandparent”—the Ph.D. advisor of my Ph.D. advisor). Former students and post-docs from his lab are all over experimental atomic physics, and his dogged pursuit of Bose-Einstein Condensation over a period of many years pushed toward the creation of that field, with a revolutionary impact on atomic physics.

Anyway, the example of Ehrenfest is yet another reminder that discussions of physics and science in general tend to focus a bit too much on a very particular, narrow model of success through dramatic individual contributions. In reality, physicists contribute in numerous indirect ways as well, and some of those can be as crucial to the future success of the field as anything done by an individual “genius.”