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For Richard Feynman's 100th birthday: smaller than his myth

Hocus-pocus against infinity

However, the drawings alone did not solve the basic problem with QED: to determine the sum of all possible courses of the reaction between the initial and final state. The creation of three, four or five virtual particles was much less likely than the simple exchange of a photon, but it was not excluded. Where should physicists draw the line when adding up the radiation corrections? Finally, it was also possible that the virtual particles have practically any amount of energy - and if the sum to be formed at the end included such contributions, it could no longer be finite and physically meaningful itself.

Specifically, it was about the energy levels in the second main shell of hydrogen and the magnetic moment of the electron, for which precise new measurements were available in spring 1947. The theorists could not understand it. Feynman set about it with his methodology and finally came up with a trick. He later called it "hocus-pocus" himself, but got the correct results. He argued that only the finite contributions to the radiation corrections were physically meaningful. The infinite parts of the sum belong, so to speak, to the electron mass and charge, they are even the real reason that the particles have a rest mass at all.

At least that's how Feynman described it in his Nobel Lecture. And there he admitted that the procedure called renormalization for dividing finite and infinite proportions only serves to "sweep the difficulties of the theory under the carpet". Julian Schwinger from Harvard University and Sin-Itero Tomonaga from the University of Education in Tokyo came up with the infinity trick at the end of the 1940s. From the external form of the derivations it was not evident that the three physicists were saying the same thing in different ways. It took the mathematical skills of the Briton Freeman Dyson to show the equivalence of the approaches - and to make the discovery worthy of a Nobel Prize. All three renormalizers received the award in 1965 in equal parts.

The “Feynman Lectures” had been published a year earlier. These textbooks, bound in red material, established his reputation for being a brilliant teacher, because he took on a completely different tone in them than the authors of earlier works. The volumes came with simple language and everyday examples, but above all they reflected his worldview that physics as he saw it could explain the whole world. This perspective is "an essential part of the true culture of modern times," Feynman put it in the epilogue - and if professors in other subjects saw it differently, they would simply be wrong.

Failed as a teacher

The problem with the Feynman Lectures, however, was that they overwhelmed most of the students. The author himself confessed at the end that perhaps two or three dozen students out of a total of 200 could have followed him - those who hardly needed any instruction anyway. He apologized to the rest: "If you hate the subject because of me, I'm sorry." An assistant from then later recalled that over the two years more and more students had dropped out of their first semester. Doctoral students and teachers flocked to the lecture hall to experience their subject in a completely new way. Measured against the original purpose, the lectures should therefore be rated as a failure. But as an example of how you think about problems in physics, they have been spectacularly successful.

The biographer James Gleick confirms this in his Feynman book: The lecture was a »tour de force«, both as a university course and as a textbook. Many universities first introduced the three volumes and withdrew them a few years later. "Feynman didn't have the patience to lead a student through a research problem," summarizes Gleick. The influence of the Caltech researcher was felt more indirectly, because the three red volumes affected the established physicists and colleagues at universities. Some therefore describe the books as "introductory courses for doctoral students".