So, yesterday’s blog version of my March Meeting talk ended by highlighting four events in 1948 that launched quantum electrodynamics (QED) into the world and thus make it a very important year in the history of physics. Those were:
Schwinger’s presentation at the Pocono Conference
Feynman’s presentation at the Pocono Conference
Tomonaga’s letter to Oppenheimer
Dyson’s summer vacation
(If you want those fleshed out more, click on the link above and read the original post.)
I wrapped up the talk with four messages (because I believe in the power of parallel construction) to take away from the launch of QED in 1948 regarding the history and nature of physics as a science. Yesterday’s post was running long, so I saved those for today:
This is a reminder of the importance of the interplay between theory and experiment. QED is a triumph of theoretical physics, and a highly mathematical theory, but it comes into being as it does, when it does in large part because of technology. The key problems that Tomonaga, Schwinger, and Feynman solved— the apparently infinite self-energy of the electron—were well known in the early 1930’s, but didn’t become urgent until after WWII. That happened because advances in microwave technology driven by the wartime programs to develop better radar systems allowed Lamb and Retherford and Kusch and Foley to do measurements that absolutely could not be explained by waving off the problematic terms.
I used a big photo of Willis Lamb on the title slide (seen above) because he works as a kind of exemplar of this. Lamb was trained as a theorist, but there’s a funny story in at least one of the books about this period about how when he joined the radar effort during the war, Rabi made him build a magnetron source, basically as a form of mild hazing. And while he continued to mostly do theoretical work, he ended up achieving scientific immortality for a bit of experimental work, the 1947 discovery of what’s now called the Lamb shift in his honor. That was one of the first results definitively showing the need for a better theory of QED, so both sides of Lamb’s career are essential.
The renormalization method of QED is ultimately a conservative solution to the problem. In the end, the core idea of QED as formulated by Tomonaga and Schwinger and Feynman is a bit of a complicated mathematical hack. The infinities that show up in calculations starting with the Dirac equation are removed by realizing that what we actually measure are differences. We never see an electron without the effects of self-energy and vacuum polarization, we can only compare the energy of an electron with just those things to the energy of an electron with those things plus something else. The difference between them is all that we can observe, so you end up with infinity minus infinity equaling the Lamb shift.
This kind of weird-sounding operation can work, but it requires the infinite terms to meet very stringent conditions, and showing that they do involves a lot of tedious and complicated mathematics. Which is basically what Tomonaga and then Schwinger and Feynman did, each in their own particular way.
On some level, though, that’s not a very exciting solution, nor an elegant one, and that’s part of why it took so long to get around to it. The pioneers of quantum physics— folks like Niels Bohr and Werner Heisenberg and Wolfgang Pauli and to some extent Paul Dirac— wanted the solution to involve the kind of revolutionary leaps that they had taken in order to produce quantum mechanics, introducing bold new concepts or discarding core principles of older physics that had become outmoded.
That expectation held back development of the theory, particularly in Europe. The basic idea of renormalization popped up a few times, but it’s really hard to do right, and the inevitable early false steps were taken by the core quantum community as proof that renormalization was the wrong path, so they never got corrected. And the one guy who did work the whole thing out in the 30’s, Ernst Stueckelberg, was basically ignored: he was isolated in Switzerland, publishing in obscure journals using idiosyncratic notation, and his only conduit to mainstream quantum physics was Wolfgang Pauli, who was relentlessly negative about the whole business. Stueckelberg had more or less everything needed for modern QED by the mid-1930’s, but nobody realized it until years later.
The events of 1948 highlight physics as a trans-national enterprise. Tomonaga’s letter to Oppenheimer is maybe the most pivotal of all the moments I highlighted, and the one that speaks the best of all the people involved. Tomonaga was in Japan, and Oppenheimer had famously just finished leading the Manhattan Project to develop the atomic bombs that were used on Japan to conclude what was undoubtedly one of the nastiest and bloodiest wars in human history. But Tomonaga was willing to reach appeal to Oppenheimer through their shared interest in physics, and Oppenheimer both recognized the quality of the work and exerted his influence to see to it that Tomonaga was properly credited for his accomplishments. (Probably to the detriment of Freeman Dyson, though he dined out on not getting a share of that Nobel for a good many years…)
It’s not hard, particularly given the nasty and racist rhetoric directed at Japan through the war years, to imagine that exchange turning out very differently. Oppenheimer could easily have buried or disparaged the Japanese contribution, relegating Tomonaga to the same footnote-to-history status as Stueckelberg, but he didn’t. There are a lot of complicated factors going into that— Oppenheimer felt some level of guilt about the bomb project, and was in part making a deliberate political point— but I think it deserves to be recognized as a landmark moment. Physics (and science more broadly) is bigger than any particular nation or culture, and can help to cross even the bitterest divisions, and that’s a good lesson to take away.
Quantum physics happened astonishingly fast. I mentioned this in the previous post, but the field goes from not even existing— because I am Of A Certain Age I had a slide for 1898 that just read “ERROR 404: THEORY NOT FOUND”— to the full relativistic theory of QED in under fifty years. If you prefer to date it from the launch of “real” quantum mechanics in 1925, it’s only 23 years. That’s shockingly fast for a change of that magnitude.
By way of comparison, Newton’s Principia Mathematica, introducing his three laws of mechanics, was published in 1687 (give or take). The next great advance in the field, Lagrangian mechanics, was a full century later, in 1788. The next refinement after that, Hamilton’s reformulation of the Langrangian approach, was in 1833. And there are probably arguments to be had about whether what Hamilton and even Lagrange did are comparable in scale to the change from “old quantum theory” to matrix mechanics, let alone QED. The time scale is similarly slow in electricity and magnetism: Coulomb’s Law is in 1785, Maxwell’s Equations are in 1862. Thermodynamics is muddier, but its development also spans more or less the entire 19th century.
The revolutionary fervor felt by that first quantum generation really was justified, even if it did lead them a bit astray when it came time for the next step, because they were genuinely part of an unusual moment in the history of science. At the same time, this should also probably serve to temper some of the more hyperbolic laments about the current “crisis” in fundamental physics— it’s really just not reasonable to expect the field to continue to advance at the pace it did between Planck and Dyson/Feynman/Schwinger/Tomonaga.
So, those are what I think of as the take-away messages from the quantum milestone year of 1948. It’s not quite as revolutionary as 1905 or 1925, but does mark a significant turning point, and has a lot to teach us if we take the time to look into it.
Shockingly, I actually carried through with yesterday’s promise of a follow-up. This is not normal behavior for me, but if you would like to encourage it, here’s a button:
And if you want to argue with or elaborate on any of the points I made here, the comments will be open:
Is the talk available to watch online anywhere?
I'm not really a "comment on Substacks" guy, but these history-of-science posts are great.