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Oral History Transcript — Dr. P.M.S. Blackett

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Interview with Dr. P.M.S. Blackett
By John L. Heilbron
At Blackettís Office, Imperial College of Science and Technology, London, England
December 17, 1962

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P>M.S. Blackett; December 17, 1962

ABSTRACT: Part of the Archives for the History of Quantum Physics oral history collection, 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: Niels Henrik David Bohr, Paul Adrien Maurice Dirac, James Franck, Nevill Francis Mott, Giuseppe Occhialini, and Ernest Rutherford.

Transcript

We shall not be able to immortalize Mr. Blackett in our archives; he would neither let me use the recorder, nor take any notes during the short half hour he devoted to our activities. Itís of value to record Blackettís objections since unfortunately some seem quite reasonable, and it might help avoid a similar unfortunate occurrence in the future. His ostensible objection was that he hadnít enough time; he could only give us half an hour at this busy period during the term, etc. Actually the difficulties were much deeper and involved our whole approach to prospective interviewees.

His main objections, I think, were the semi-autobiographical approach used in the outline. He felt that he could not give any information about himself on so impersonal a basis; in fact he said -- and this is almost a direct quotation -- ďWhy should I tell about my personal life?Ē And, although I made every effort to persuade him that it is very difficult to separate some aspects of personal biography from the story of modern physics, he was unimpressed. In fact he told me that if biography was what I wanted, I should approach the family, look over documents, and generally take a great deal of time. He thought it pointless to give absolute stranger biographical information whose future application is uncertain. This brings up the second difficulty Blackett felt in our biographical approach; namely, that whatever information he might have been willing to give us would be dropped into our files for use at some future unspecified occasion. [It seems that Blackett was unhappy at the prospect that the information might never be used -- rather the reverse of the usual objection.)

He did say he was sympathetic to the aim of our undertaking -- that is in the collection and preservation of relevant material; but he will be sympathetic only at no great inconvenience to himself. I asked him whether we could obtain copies of his notes of lectures by C.T.R. Wilson which, according to some published biographical fragment, he consults frequently. He said that if I wrote him a formal letter, he might consider it. I think nothing will come of it. He did provide me with copies of his Rutherford Memorial Lecture which is supposed to contain anecdotes, recollections, stories, etc.

After discussing his reservations at about the same length at which I have already spoken, Blackett proceeded to dash through the outline as quickly as possible. It was apparent he was only seeing me because he had somehow talked himself into it, had accepted an appointment by letter, and was simply discharging an obligation. This experience suggests an important modification in our general approach scheme. We must make quite explicit how much time we expect, our intention to use a tape recorder, etc., in our first written contact with a prospective interviewee. The yield is sparse enough when one obtains a full two hour, but a half hour talk is just absurd.

With fifteen minutes to go Blackett began to run through the outline. He made short work of the opening paragraphs which were to have been devoted to a discussion of his family background, ear1y education, and the rise of his interest in science. Only under II did he begin to make a few remarks in answer to specific questions. Of course, he said nothing about the curriculum, nor about the questions asked under ďThe State of Physics.Ē It is interesting that the question, ďHow did people react Ö particularly irritated him. He said something like, ďI donít know how people reacted to this or that,Ē with some vehemence, and it might be that we should try to remove that set of questions from our future outlines. I think itís true that we have obtained nothing of value from these questions in the past and that when answers are given to questions implied under this heading in our outline they usually arise in connection with other parts of the discussion and so would enter the record in another way.

Blackett began to make specific remarks in answer to the questions under C of Part II, and I will try to record his answers as closely as I can remember them.

II.C.I. About the first paper involving the question of the validity of the inverse square law at small distances and of the conservation of energy and momentum in close collisions, Blackett said that it was generally believed that these laws held at the distances he investigated which, in fact, were not very close; in very close collisions it was known that there were deviations from the inverse square law, no doubt due to the influence of the external electron rings.

II.C.2. About paper No. 2, the natural curvature of alpha ray tracks, Blackett remarked that, as far as he knows, no explanation has yet been offered for this effect. He still feels that there is such an effect and he continued to experiment with it, at least up until 1930 (see below). The trouble with the curvature of the alpha ray tracks was that it was only an observation for which, of course, there was no theory, and the statistics were not sufficiently convincing to make the investigation attractive. The connection with the Bohr theory, however, is perfectly obvious; according to Blackett, the natural asymmetry of the Bohr atom would immediately suggest itself.

II.C.3. Concerning question No. 3, the origin of his interest in questions of excitation of atoms by electronic impact was simply the papers of Franck. Blackett merely applied the classical laws of scattering to the collision in order to determine the cross section available for scattering in which the atom undergoes a transition between different angular momentum states. The only interesting remark in connection with this paper was that, according to Blackett, he felt his treatment to be in the spirit of Bohrís Correspondence Principle. He did not apply his considerations to other collision phenomena such as the Ramsauer effect.

II.C.4. About question No. 1 he said that Rutherford had noticed that in collisions between alphas and nitrogen atoms the proton was occasionally ejected with kinetic energy greater than that which could be expected in an elastic collision; Rutherford put him on to investigating this collision in order to discover why.

II.C.5. About question No. 5 Blackett was unwilling to make the slightest comment. He felt that the sources of support for research were of no importance for a history of quantum physics, a curious opinion which is perhaps not without some relation to his political views.

We then proceeded to discuss in the same brief fashion certain of the topics listed under III.

III.A. Blackett went to work with James Franck at Gottingen because he had become interested in Franckís work through his earlier investigation in paper No. 3. Blackett made the interesting remark that when he told Rutherford of his intentions to go to Gottingen, Rutherford was decidedly unenthusiastic; he felt that deserting the Cavendish for Gottingen was bad enough, but to desert the nucleus of the atom for the atom itself was inexcusable. Yet Blackett felt that the future break-throughs must come from studies of processes whose origin was in, the interior of the atom, and not in the nucleus.

IV.B. 1 and 2 Under IV.B.1. Blackett said nothing, but under IV.B.2. he did discuss his early exposure to the theory of the de Broglie waves. It seems that he first found out about do Broglieís paper at the Kapitza Club some time in l924, before he went to Gottingen. He recalls that in the minutes of the Club the unanimous opinion was recorded that de Broglieís theory was nonsense. [Blackett thinks that the minutes of the Kapitza Club have been destroyed but that if anything does remain, Sir John Cockcroft would be the person most likely to know.] He recalls that discussions arose in Gottingen from the reading, he thinks, of Einsteinís paper in the Berlin Academy. (He was quite clear that Einsteinís paper was in the Berlin Academy, but he had it a bit confused with Einsteinís 1909 fluctuation paper.) The discussions at Gottingen over the possibility of observing diffraction phenomena were perhaps participated in by Elsasser, whose paper Blackett remembers, and apparently Dyson was also in Gottingen at the same time. Blackett himself decided to try to obtain diffraction effects, but he is not sure whether this was immediately after his return from Gottingen. In any event, he was unsuccessful. I believe (although at this point my memory is somewhat hazy) that Blackett said he tried to obtain diffraction patterns with very slow electrons using a grating made of very thin wires. He gave up after three months or so.

III.C. About his own work at Gottingen, Blackett couldnít say much, although his remarks might help us to understand this peculiar paper. He said he and Franck observed that when hydrogen molecules were bombarded by electrons with just sufficient kinetic energy to ionize the molecule [I believe that that was the experimental arrangement) the Balmer lines were produced not only in the direct path of the beam, but also by atoms scattered throughout the gas. The conclusion was that the ionizing electron disrupted the equilibrium conditions so that the natural repulsive forces between the protons and the electrons were able to force the molecule apart. After 10-8 seconds one observed decays at whatever positions the split atoms then found themselves.

IV.A. and B Under IV.A. Blackett was unwilling to say anything except that he wanted to forget all about the curriculum. Under B also Blackett would say nothing, since he was as unwilling to give biographical details about others as about himself. However, he did indicate that he hoped after he retired to go through his papers and to write some memoirs. It might be that this intention in some way has prevented him from cooperating as fully with the project as he otherwise might.

IV.C.1. and 2. IV.C.1. and 2. were easily disposed. No useful answer was given to No. 1. Under No. 2 Blackett said that he hadnít worked with any theoretician before because they didnít have anything to say, but that when it came to him with such clear calculations, he was willing to undertake their experimental verification.

IV.C.3. Under No. 3 we did get some information. He first became interested in cosmic rays about 1928 when their existence was clearly demonstrated by (???).

At that time in the Cavendish the younger people were more or less restricted to a fairly well-defined course of research, but I gather that Blackett had become free of such restrictions and. was able to undertake other investigations. He was in Berlin in 1930 working, curiously enough, on the question of the curvature of alpha ray tracks, when he met a student or co-worker of Botheís named Rossi. Since Blackettís wife is Italian, he easily became quite friendly with Rossi who sent him Occhialini to work at the Cavendish. Occhialiniís intention was to come for three weeks, but he actually stayed for three years. The advantage which Occhialini brought to the laboratory was knowledge of the mysterious workings of the Geiger counter and certain circuitry which Rossi had designed. According to Blackett, at that time the Geiger counter was a very delicate instrument. As he put it: ďIn order to make it work you had to spit on the wire on some Friday evening in Lent.Ē One had to be initiated into all the mysteries in order to get any results at all. Blackett himself contributed his great knowledge of the workings of cloud chambers, with which he had been engaged for ten years. They decided to pool their mutual instrumental knowledge and investigate the cosmic rays. Blackett had to invent a new type of cloud chamber with some automatic piston devices and rubber seals and. so forth which has since become a standard design, and in combination with Occhialiniís Geiger counters it worked well enough to provide them with a large number of photographs of cosmic ray particles. Their work began long before Andersonís results were first published in Science, and as soon as they saw Andersonís article [which, incidentally, first appeared without any plates], they immediately looked for such events on their own plates and found them in great abundance.

According to Blackett, Bohr was at first unconvinced by Andersonís evidence, but was persuaded by the extensive evidence offered by the photographs of Blackett and Occhialini. Dirac worked very closely with them; in fact, he was often at the laboratory. When asked how long they had known about Diracís theory, Blackett replied he wasnít quite certain, but that it didnít matter anyway because nobody took Diracís theory seriously. As an illustration, he said that many problems have negative solutions, for instance, the square root; [an argument that doesnít seem entirely relevant.] Thus, although Diracís theory involved such a difficulty that was hardly evidence enough for a convincing demonstration of the existence of a new particle. In fact Dirac himself at first identified the negative energy particles with protons, and so in a certain sense did not have the courage of his own convictions either.

Iím afraid that at this point I was dismissed with what little information the foregoing will show.