Einstein Wrote Back by John W. Moffat

Author:John W. Moffat
Language: eng
Format: epub
Tags: BIO026000
ISBN: 9780887626159
Publisher: Thomas Allen Publishers
Published: 2010-10-15T16:00:00+00:00

Invariably, at each Thursday seminar, Dirac would sit in the first row, a slim, nondescript figure in his tweed suit. Early on in the talk, he would look at the speaker in his rather distant way and ask, “Is this a three-vector or a four-vector?” This referred to a vector field in quantum field theory either being in three-dimensional space or in the four-dimensional spacetime of special relativity theory. The question really had no relevance to the talk, but Dirac always asked it because, being the senior professor, he probably felt obliged to ask a question. We students had decided that one week the answer would be “three-vector,” and the next week whoever was speaking would reply “four-vector.” Dirac seemed completely unaware of this conspiracy, and after the question was answered, he would promptly fall asleep, with his chin sagging onto his chest and his mouth open.

Uncannily, however, very near the end of the talk, he would rouse himself, stare intently at the speaker, and, whatever the subject of the lecture, would ask his second question:“Can you fit the electron into your scheme?” Again, most often we considered this to be a totally irrelevant question because the talk would be about some technical subject such as dispersion relations in the scattering of elementary particles, in which the electron did not play a role at all. Again we had a conspiracy going, with three possible answers. One answer would be: “I’m not sure, Professor Dirac.” The second one would be, “No, I don’t think it can be fitted into this scheme.” And the third one would be, of course: “Yes, you can fit it into this scheme.” Dirac seemed unmoved by whichever answer we gave, and after some other desultory questions were asked, the seminar would end.

Dirac was particularly concerned about the electron because he did not keep up with the parade of new particles being discovered in accelerators at that time, and so could not ask a question about any of them. And of course the electron figured very prominently in his celebrated work on quantum mechanics, in his famous Dirac equation and in his early development of quantum field theory.

At the beginning of my second year of research, at one of the theory group seminars, Hamilton announced that the half-dozen or so students who had completed their first year of graduate work had to speak at each successive seminar on an original research topic of their choice. This was a daunting assignment, for finding an original research topic was a serious challenge so early in our careers.

When my turn came, I talked about quantum field theory and Hilbert space. This was a technically difficult corner of quantum field theory that had caught my interest. At my talk, everybody was surprised to see that Dirac did not fall asleep, and instead of asking his standard question about whether the field was a three- or four-vector, he asked me a specific question about the notation I was using. In fact, I

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