Oral History Transcript — Dr. John Wheeler
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John Wheeler; March 24, 1962
ABSTRACT: This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Carl Anderson, and Niels Henrik David Bohr.
I had arrived in Copenhagen in September of ‘34 and didn’t leave until June of ‘35. The center of discussion was the limits of electrodynamics. The reason for that subject being of vital importance at that time was the great penetrating power of the cosmic rays. It had become clear that the cosmic rays were not protons, and the question was: Could they be electrons, positive and negative? The positive electron had been discovered of course, and could these penetrating particles be electrons, positive or negative? It was perfectly clear that some of them were positive and some were negative, in almost exactly equal numbers. Therefore, it came down to a question, could an electron penetrate 10 centimeters of lead? And to get through with the sort of energies that you saw cosmic rays emerging with? The work of Bethe on the radiation by fast moving electrons based on the Dirac theory of the electron indicated that an electron could not get through. But the question was: ‘Was the Dirac theory of the electron sufficiently valid?’ Bohr has this probing approach to everything, wanting to get down to the brass tacks and just test the uttermost limits to which a thing can be defended. So he raised the question: “Could it not be that the quantum mechanics failed at small distances in such a way that actually the electrons had lost radiating power compared to what theory demanded.
Well, this is where E. J. Williams, who was there in Copenhagen, figured in such an important way. He was able to take over the method that Bohr had used long ago in the ionizing power of the fast-moving electron, to analyze the effect of the nucleus on the electron by going to the frame of reference in which the electron was at rest and the nucleus going by. Then the field of force of the nucleus appeared to the electron as a pulse of radiation. The electric and magnetic field due to fast-moving proton or nucleus, going past the electron — the strength of the electric and magnetic field are so nearly equal and at right angles to each other — is a very good approximation to a radiation pulse. So the poor electron doesn’t know it isn’t radiation. Therefore the question of the effect of the nucleus on the electron as it zooms by could be carried back to this question of the effect of the radiation on the electron in the electron’s frame of reference. And the wonderful thing about it that the radiation — although the electron might be of fantastic energies — 100 million volts, a 1000 million volts, a 100,000 million volts — extremely relativistic — still, if the electron goes by the nucleus at a substantial distance, which is the distance at which most of the radiation loss occurs, the apparent frequency spectrum of the nuclear field of force as felt by the electron is low frequency.
There you know, by George, from good experiments, what the response of the electron is in the Compton effect (for a full-time zone). So from this you could predict precisely the scattering power — how many photons will be scattered by the electron and low energy photons. Then you transfer back to the other frame of reference and these look like high energy photons. And this is responsible for the stopping property. So you could say, by George, although you think that this high-powered quantum electrodynamics treatment of Bethe comes to questions of deep theory at short-range distances, actually you aren’t relying on anything that isn’t firmly tested at quite lower energies. Therefore there is this faintest shadow of a doubt that the electron must loose energy, and therefore it must be a new particle that you see. Now to be sure, Carl Anderson is the person who gets the credit for having discovered the meson, but this work set the philosophical background for believing that his work meant what it purported to show, that you could trust it. So that you could talk about Bohr having questioned Dirac’s theory of the electron, but he questioned it to a good end — to establish more firmly than ever before that it gave sound predictions, so that you could rely on it for the meson.