The idea of the Laurie M. Brown

Citation: Today 31, 9, 23 (1978); doi: 10.1063/1.2995181 View online: https://doi.org/10.1063/1.2995181 View Table of Contents: http://physicstoday.scitation.org/toc/pto/31/9 Published by the American Institute of Physics

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To avoid anomalies of and statistics Pauli suggested in 1930 that a neutral particle of small mass might accompany the in nuclear beta decay, calling it (until Chadwick's discovery) the .

Laurie M. Brown

During the 1920's physicists came to ac- tron, and that both are created at their which conflict with data on the hyperfine cept the view that matter is built of only moment of emission. Perhaps because of structure of atomic spectra. two kinds of elementary particles, elec- the rapid acceptance of Fermi's theory • Although both alpha and gamma decay trons and protons, which they often and the tendency to rethink history "as it show the existence of narrow nuclear en- called1 "negative and positive ." should have happened," the true nature ergy levels, the electrons from a given A neutral atom of mass number A and of Pauli's proposal has been partly over- beta-decay transition emerge with a broad atomic number Z was supposed to contain looked and its radical character insuffi- continuous spectrum of energy. A protons, all in the nucleus, and A neg- ciently emphasized. Contrary to the The strong contrast between the suc- ative electrons, A — Z in the nucleus and impression given by most accounts, Pau- cesses and the failures of quantum me- the rest making up the external electron li's "neutron" has some properties in chanics applied to the nucleus are no- shells of the atom. Their belief that both common with the neutron James Chad- where more evident than in a book by protons and negative electrons were to be wick discovered in 1932 as well as with .3 In it, all the passages found in the nucleus arose from the ob- Fermi's neutrino. concerning electrons in the nucleus are set servations that protons could be knocked off in warning symbols (skull and cross- out of light elements by alpha-particle Flaws in the model bones in the original manuscript). bombardment, while electrons emerged By the end of 1930, when our story be- Some physicists (among them Niels spontaneously (mostly from very heavy gins, quantum had triumphed Bohr and Werner Heisenberg4) took these nuclei) in radioactive beta decay. Any not only in atomic, molecular and crystal difficulties to indicate that a new dy- other elementary constituent of the atom physics, but also in its treatment of some namics, possibly even a new type of would have been considered superfluous, nuclear processes, such as alpha-particle space-time description, might be appro- and to imagine that another might exist radioactivity and scattering of alpha priate on the scale of nuclear distances was abhorrent to the prevailing natural particles from nuclei (including the case and energies, just as philosophy. of helium, in which quantum-mechanical begins to be important on the atomic Nevertheless, in December 1930 interference effects are so important). scale. These physicists were impressed However, the situation regarding elec- by the similarity of the nuclear radius to suggested a new elemen- z 2 tary particle that he called a neutron, trons in the nucleus was felt to be critical. the value e' /mc , the classical electron with characteristics partly like that of the The main difficulties of the electron- radius of H. A. Lorentz. At this distance nucleon we now call by that name, and proton model of the nucleus were: it had been anticipated that electrody- partly those of the lepton that we now call • The symmetry character of the nuclear namics would probably fail (and maybe, neutrino (more precisely the electron depends upon A, not Z as with it, the special theory of relativity). antineutrino, but this distinction is not predicted by the model; when A — Z is Bohr was willing to relinquish the con- needed here). Pauli's neutron-neutrino odd the spin and statistics of the nucleus servation of energy, except as a statistical idea became well-known to physicists are given incorrectly. For example, ni- law, in parallel with the second law of even before his first publication of it, trogen (Z = 7, A = 14) was known from thermodynamics. At the same time Heisenberg was considering the intro- which is in the discussion section fol- the molecular band spectrum of N2 to lowing Heisenberg's report on nuclear have spin 1 and Bose-Einstein statis- duction of a new fundamental length into structure at the Seventh Solvay Confer- tics. the theory. It seemed that anything 2 ence, held in Brussels in October 1933. • No potential well is deep enough and might be considered acceptable as a way narrow enough to confine a particle as out of the dilemma—or perhaps anything Shortly after attending this conference, except a new elementary particle. published his theory of beta light as an electron to a region the size of decay, which assumes that a neutrino al- the nucleus (the argument for this is ways accompanies the beta-decay elec- based on the and Pauli's proposal relativistic electron theory). It was in this context of ideas that Pauli Laurie M. Brown is a professor in the Department • It is hard to see how to "suppress" the dared to suggest the existence of a new of Physics and Astronomy, Northwestern Uni- very large (on the nuclear scale) magnetic neutral particle. His proposal, intended versity, Evanston, Illinois. moments of the electrons in the nucleus, to rescue the quantum theory of the nu-

0031-9228/78/3109-023$0.50 © 1978 American Institute of Physics PHYSICS TODAY / SEPTEMBER 1978 23 GOUDSMIT COLLECTION, AIP LIBRARY 1957 Zurich lecture. After pointing out one of the major difficulties with the nu- clear model containing only protons and electrons (the symmetry argument men- tioned above), Pauli says: "I tried to connect this problem of the spin and statistics of the nucleus with the other of the continuous beta spectrum, without giving up the energy theorem, through the idea of a new neutral parti- cle."

Neutrinos—ejected or created? It is often overlooked in discussing the history of the neutrino idea that Pauli suggested his particle as a constituent of the nucleus, with a small but not zero mass, together with the protons and the electrons. (Chien-Shiung Wu, for ex- ample, emphasizes the non-conservation of statistics that would occur in beta decay without the neutrino.67'9 However, Pauli refers rather to the spin and statistics of stable nuclei such as lithium 6 and nitro- gen 14.) This point is of some signifi- cance; had Pauli proposed in 1930 that were created (like ) in transitions between nuclear states, and that they were otherwise not present in the nucleus, he would have anticipated by PAULI ON THE WAY TO PASADENA, 1931 three years an important feature of Fer- mi's theory of beta decay. Pauli did not cleus from its contradictions, was pre- discrete quantum states of the nucleus (as claim to have had this idea when he wrote sented in good humor as a "desperate indicated by alpha and gamma emission), the Tubingen letter, but he did say (in his remedy," although it was a serious one. some workers, including Meitner, thought Zurich lecture) that by the time he was (The Viennese version would have been, that the beta rays were radiating some of ready to speak openly of his new particle, according to the old joke: desperate, but their energy as they emerged through the at a meeting of The American Physical not serious.) During the next three years strong electric field of the nucleus.5'67 Society in Pasadena, held in June of 1931, he lectured on what he called the "neu- This led C. D. Ellis and William he no longer considered his to be tron" at several physics meetings and he Wooster at the Cavendish Laboratory in nuclear constituents. It is for this reason, discussed it privately with colleagues. Cambridge, England, who did not believe he says, that he no longer referred to them Pauli's first proposal was put forward in the radiation theory, to perform a ca- as "neutrons"; indeed, that he made use only tentatively, as he recalled in a lecture lorimetric experiment with radium E of no special name for them. However, he delivered in Zurich in 1957, after re- (bismuth) as a source. Their result, later there is evidence, as we shall see, that ceiving news of the experiments con- confirmed in an improved experiment by Pauli's recollections are incorrect; that at firming violation in beta decay.5 Meitner and W. Orthmann,8 was that the Pasadena the particles were called neu- Invited to a physics meeting in Tubingen, energy per beta decay absorbed in a trons and were regarded as constituents Germany, which he was unable to attend thick-walled calorimeter was equal to the of the nucleus. (because of a ball to be held in Zurich, at mean of the electron energy spectrum, I have not been able to obtain a copy of which he declared he was "indispens- and not to its maximum (endpoint). Pauli's Pasadena talk or scientific notes able"), he sent a message with a colleague Furthermore, Meitner showed that no on it; he said later that he was unsure of as an "open letter," although it was in- gamma rays were involved. According to the matter and thus did not allow his tended mainly for Hans Geiger and Lise Pauli (in 1957), this allowed but two pos- lecture to be printed. The press, how- Meitner. An English translation of this sible theoretical interpretations: ever, took notice. For example, a short letter is given in the Box on page 27. • The conservation of energy is valid only note in Time, 29 June 1931, headed Pauli was anxious for their expert advice statistically for the interaction that gives "Neutrons?", says that Pauli wants to add as to whether his proposal was compatible rise to beta radioactivity. a fourth to the "three unresolvable basic with the known facts of beta decay. • The energy theorem holds strictly in units of the universe" (proton, electron In the 1957 lecture Pauli also tells how each individual primary process, but at and ); adding, "He calls it the he became convinced of a crisis associated the same time there is emitted with the neutron." with beta decay. During the decade that electron another very penetrating radia- Upon examining the program of the followed the discovery by Chadwick in tion, consisting of new neutral particles. Pasadena Meeting, I discovered that 1914 of beta rays with a continuous energy To the above, Pauli adds, "The first pos- Samuel Goudsmit spoke at the same ses- spectrum, it became established that sibility was advocated by Bohr, the second sion as Pauli (and even upon the same these were the true "disintegration elec- by me." 5 announced subject—hyperfine structure). trons," rather than those making up dis- But although the conservation of en- I wrote to Goudsmit and received a most crete electron line spectra, which were ergy, and possibly other conservation laws interesting reply, from which I should like later shown to arise from such causes as in beta decay were very much in Pauli's to quote: photoelectric effects of nuclear gamma mind at this time, this was not his only "Pauli accompanied my former wife rays, internal conversion and Auger pro- reason for proposing the neutrino. He and me on the train trip across the cesses. Because a continuous spectrum makes this point (already obvious from US. I forgot whether we started in seemed to disagree with the presence of his Tubingen letter) quite explicit in his Ann Arbor or arranged to meet in Chi-

24 PHYSICS TODAY / SEPTEMBER 1978 GOUDSMIT COLLECTION. AIP NIELS BOHR LIBRARY cago. We talked little physics, more about physicists. Pauli's main topic at the time was that he could imitate P. S. Epstein and he insisted that I take pictures of him while doing that. We spent a couple of days in San Francisco, where we almost lost him in Chinatown. He'd suddenly rush ahead and around a corner while we were window shopping ... He may have talked about the "neutron" on that trip, but I am not at all certain

Goudsmit does not now recall exactly what Pauli said at Pasadena, except that he mentioned the "neutron"; however, he sent me a copy of his report at the Rome Congress on what Pauli had said four months earlier in Pasadena. To continue, then, with Goudsmit's letter: "Fermi was arranging what was prob- ably the first meeting. It was held in Rome in October 1931 ... It was the best organized meeting I ever attended, because there was very much time available for informal dis- cussions and get-togethers ... Fermi had arranged marvelous leisurely sightseeing trips for the group. There were about 40 guests and 10 Italians. "Fermi ordered the then 'young' participants, namely [Nevill] Mott, GOUDSMIT (MIDDLE) AND FERMI (RIGHT) WITH UNIDENTIFIED MAN [Bruno] Rossi, [George] Gamow (who mentum, it would make it very proba- ance of certain lightly ionizing cloud- could not leave Russia but sent a man- ble that another particle is emitted at chamber tracks from cosmic rays that had uscript) and myself, to prepare sum- the same time with the beta-particle. been reported. mary papers for discussion ... As you The mass of these neutrons has to be The complex problem of the energy loss know, I don't use and don't keep very much smaller than that of the of relativistic charged particles was crucial notes. But I have a clear picture of proton, otherwise one would have de- to the interpretation of the various com- Pauli lecturing [at Pasadena] and his tected the change in atomic weight ponents of the cosmic rays observed in the mention of the 'neutron'. . . Pauli was after beta-emission." atmosphere, and had attracted the at- supposed to attend the Rome meet- Goudsmit added that Pauli believed tention of many theorists. Carlson and ing, but he arrived a day or so late. In "neutrons may throw some light on the Oppenheimer were unable to account for fact, he entered the lecture hall the nature of cosmic rays." cloud-chamber tracks that appeared very moment that I mentioned his It does appear clear from this passage thinner than those of an "ordinary ra- name! Like magic! I remarked (to which Pauli evidently made no objec- dioactive" . Their calcula- about it and got a big laugh from the tion at the time) that at Pasadena the tions of energy loss (which agreed in a audience." neutron was intended to be a particle that general way with independent calcula- could be bound in the nucleus by mag- tions by Heisenberg and Hans Bethe, and Goudsmit's Rome report netic forces. In his letter to me Goudsmit with an older classical estimate by Bohr) At Fermi's request, then, Goudsmit also said, "It was who showed that charged particles should have reported at the Rome Conference on some time ago pointed out that I was the a relativistic increase of ionization with Pauli's talk in Pasadena. Here is what he first to put Pauli's idea on paper and in energy. The particles leaving light tracks said:10 print." were very penetrating (and thus probably "At a meeting in Pasadena in June After leaving Pasadena Pauli remained relativistic) and it was concluded that 1931, Pauli expressed the idea that in the United States until the fall, when they could not be electrons or protons. there might exist a third type of ele- he went to Rome. He gave a seminar at (These quarklike tracks have not, to my mentary particles besides protons and the Summer Session of the University of knowledge, been explained. Perhaps electrons, namely 'neutrons.' These Michigan at Ann Arbor (probably at one they were examples of old and "faded" tracks, which often plagued cloud cham- neutrons should have an angular mo- of their Symposia on Theoretical Physics, bers of the untriggered variety.) mentum V2 h/2-K and also a magnetic where Fermi had, the previous summer, moment, but no charge. They are given his famous lectures on the quantum Carlson and Oppenheimer decided kept in the nucleus by magnetic forces theory of radiation). At the seminar therefore to make a theoretical investi- and are emitted together with beta- Pauli spoke, according to the Berkeley gation, as they said,11 of the "ionizing theorists J. F. Carlson and J. Robert Op- rays in radioactive disintegration. 11 power of the neutrons which were sug- This, according to Pauli, might re- penheimer, about "the elements of the gested by Pauli to salvage the theory of move present difficulties in nuclear theory of the neutron, its functions and its the nucleus. These neutrons, it will be structure and at the same time in the properties." remembered, are particles of finite proper explanation of the beta-ray spectrum, mass, carrying no charge, but having a in which it seems that the law of con- Tracks in the cloud chamber small magnetic moment. . ." servation of energy is not fulfilled. If Carlson and Oppenheimer wondered Could thin tracks, like those in the one would find experimentally that whether Pauli's "neutrons" could be used cosmic rays, be seen from beta decays? there is also no conservation of mo- to solve yet another puzzle: the appear- "If they were found, we should be cer-

PHYSICS TODAY / SEPTEMBER 1978 25 AIP NIELS BOHR LIBRARY

The participants in the Seventh Solvay Conference, where Pauli pre- A. Kramers, E. Stahel, E. Fermi, E. T. S. Walton, P. A. M. Dirac, P. Debye, sented his neutrino idea, included, in the first row, E. Schrodinger, I. Joliot, N. F. Mott, B. Cabrera, G. Gamow, W. Bothe, P. M. S. Blackett, M. S. N. Bohr, A. Joffe, M. Curie, 0. W. Richardson, P. Langevin. Lord Ruther- Rosenblum, J. Errera, E. Bauer, W. Pauli, J. E. Verschaffelt, M. Cosyns ford, T. De Donder, M. de Broglie, L. de Broglie, L. Meitner, J. Chadwick, (in back), E. Herzen, J. D. Cockcroft, C. D. Ellis, R. Peierls, A. Piccard, and in the second row, E. Henriot, F. Perrin, F. Joliot, W. Heisenberg, H. E. 0. Lawrence, L. Rosenfeld. The photograph is by Benjamin Couprie.

tain that the neutrons not only played neutron to distinguish it from Chadwick's tron, containing an additional term (Zu- a part in the building of nuclei, but neutron and Fermi's neutrino. Carlson satzglied) called the "Pauli anomalous that they also formed the cosmic and Oppenheimer state that the neutral magnetic moment" term.14 This equa- rays." particle of spin lli, satisfying the exclusion tion describes a spin-1^ particle that may The calculations of Carlson and Op- principle, was introduced by Pauli not be either charged or neutral; the extra penheimer were published12 almost a year only to resolve the difficulties in nuclear term makes a contribution to the later, in September 1932; by that time theory, but "on the further ground that charge-current four-vector, which need they no longer believed that "neutrons" such a particle could be described by a not vanish for a neutral particle. might leave observable cloud-chamber wave function which satisfies all the re- tracks. In addition, the situation in nu- quirements of quantum mechanics and Fermi is positive clear physics had changed profoundly, as relativity ... The experimental evidence Carlson and Oppenheimer derived a it also was about to in cosmic-ray physics: on the penetrating beryllium radiation general formula for the collision cross Chadwick's study of "the penetrating suggests that neutrons of nearly protonic section of magnetic neutrons and exam- radiation produced in the artificial dis- mass do exist; and since our calculations ined the result for small velocities. (They integration of beryllium" had revealed the may be carried through without specify- were well aware of the perils involved in existence of the neutron, announced the ing the mass or magnetic moment of the pushing this highly singular interaction to previous February; Anderson's discovery neutron, we shall consider the most gen- excessive energies.) For the collision of of the in cosmic rays was an- eral particle which satisfies the wave a neutron against a particle of equal mass, nounced in August.13 Certainly one equation proposed by Pauli. It is im- they found a large probability, nearly in- could no longer speak of the proton as portant to observe that there may very dependent of velocity and proportional to synonymous with positive electricity, and well be other types of neutral particles, the square of the magnetic moment. The one might suppose that now a new parti- which are not elementary, and to which average energy loss per collision was rel- cle like Pauli's would be acceptable; but our calculations do not apply ..." atively large, and they deduced that such this was not the case: Thus we find, surprisingly, that there a particle "will never produce ion traces For one thing, the positron was thought were thought to be also purely theoretical in a cloud chamber, since it tends to lose to be only the absence of a negative elec- grounds for considering a neutral particle an appreciable fraction of its energy, and tron of negative energy, a hole in the with a magnetic moment; it is one of the suffer an appreciable deflection at every vacuum. For another, the neutron of few simple types of elementary particles impact." For targets much lighter or Chadwick, the heavy neutron, was gen- that are allowed by relativistic quantum heavier than the neutron, smaller energy erally regarded as a composite object (it theory. In the wake of Chadwick's neu- losses occur; cloud-chamber tracks might was not thought to be unstable when free), tron discovery, Carlson and Oppenheimer result in this case, but the collision a kind of tightly bound hydrogen atom or in 1932 redefined Pauli's particle to be one probabilities are small unless the mag- neutral nucleus made of a proton and an whose wave function obeys a certain rel- netic moment of the neutron is assumed electron, like other nuclei. It was perhaps ativistic wave equation. We should not, to be improbably large. The concluded thought to be elementary only by Ettore however, assume that the Berkeley theo- (correctly) that there is no evidence for Majorana in Rome who (according to rists were soft on new particles. On the magnetic neutrons. (The heavy neutron, Emilio Segre) called it the neutral pro- contrary, the final paragraph of their with a magnetic moment only one thou- ton. lengthy article reads, "We believe that sandth of a Bohr magneton, leaves no For our purposes, the Carlson-Op- these computations show that there is no tracks.) At the Seventh Solvay Confer- penheimer article is significant in what it experimental evidence for the existence ence in 1933, Pauli no longer felt the tells us about the view held by Pauli, in of a particle like the magnetic neutron." magnetic neutron to be "well-founded." that summer of 1931, about his neutral Pauli's wave equation for the neutral Let us return now to the Rome Con- particle, which, following the Berkeley particle, given at Ann Arbor, is a variant gress of 1931, which Pauli considered authors, we will now call the magnetic of the linear Dirac equation for the elec- important in the development of the

26 PHYSICS TODAY / SEPTEMBER 1978 neutrino concept, for there he had the opportunity to discuss it with Bohr and especially with Fermi, with whom he had Pauli proposes a particle a number of private conversations. While The letter in which Pauli proposed the neutrino, translated from the German of reference 5, Fermi's attitude toward the neutrino was reads as follows: very positive, Bohr was totally opposed to it, preferring to think that within nuclear Zurich, 4 December 1930 at rest is a magnetic dipole of a certain mo- distances the conservation laws were Gloriastr. ment fi. Experiment probably requires that 15 Physical Institute of the the ionizing effect of such a neutron should breaking down. not be larger than that of a 7 ray, and thus /u Federal Institute of Technology (ETH) 13 "From the empirical point of view," Zurich should probably not be larger than e.10~ said Pauli, "it appeared to me decisive Dear radioactive ladies and gentlemen, cm. whether the beta spectrum of the elec- As the bearer of these lines, to whom I ask But I don't feel secure enough to publish trons showed a sharp upper limit" or, in- you to listen graciously, will explain more anything about this idea, so I first turn con- stead, an infinitely falling statistical dis- exactly, considering the "false" statistics of fidently to you, dear radioactives, with the tribution. Pauli felt that if the limit were N-14 and Li-6 nuclei, as well as the contin- question as to the situation concerning ex- sharp, then his idea was correct, and uous /3-spectrum, I have hit upon a desperate perimental proof of such a neutron, if it has remedy to save the "exchange theorem" * something like about 10 times the pene- Bohr's was wrong. of statistics and the energy theorem. trating capacity of a 7 ray. In mid-1933, Ellis and Mott suggested Namely [there is] the possibility that there I admit that my remedy may appear to that the beta-ray spectrum has indeed a could exist in the nuclei electrically neutral have a small a priori probability because sharp upper limit, corresponding to a particles that I wish to call neutrons, which neutrons, if they exist, would probably have 1 unique energy difference between parent have spin /2 and obey the exclusion princi- long ago been seen. However, only those and daughter nucleus.16 Furthermore, ple, and additionally differ from light quanta who wager can win, and the seriousness of they added, in that they do not travel with the velocity of the situation of the continuous /3-spectrum light: The mass of the neutron must be of can be made clear by the saying of my hon- "According to our assumption the j3- the same order of magnitude as the electron ored predecessor in office, Mr. Debye, who particle may be expelled with less en- mass and, in any case, not larger than 0.01 told me a short while ago in Brussels, "One ergy than the difference of the ener- proton mass.—The continuous /3-spectrum does best not to think about that at all, like gies ... of the two nuclei, but not with would then become understandable by the the new taxes." Thus one should earnestly more energy. We do not wish in this assumption that in /3 decay a neutron is discuss every way of salvation.—So, dear paper to dwell on what happens to the emitted together with the electron, in such radioactives, put it to the test and set it excess energy in those disintegrations a way that the sum of the energies of neutron right.—Unfortunately I cannot personally in which the electron is emitted with and electron is constant. appear in Tubingen, since I am indispensable Now the next question is what forces act here on account of a ball taking place in less than the maximum energy. We Zurich in the night from 6 to 7 of Decem- may, however, point out that if the en- upon the neutrons. The most likely model for the neutron seems to me to be, on wave ber.—With many greetings to you, also to ergy merely disappears, implying a mechanical grounds (more details are known Mr. Back, your devoted servant, breakdown of the principle of energy by the bearer of these lines), that the neutron W. Pauli conservation, then in a 0-ray decay energy is not even statistically con- * In the 1957 lecture, Pauli explains, "This reads: exclusion principle (Fermi statistics) served. Our hypothesis is, of course, and half-integer spin for an odd number of particles; Bose statistics and integer spin for an also consistent with the suggestion of even number of particles." Pauli that the excess energy is carried off by particles of great penetrating CHADWICK power such as neutrons of electronic mass." The question of the upper limit of the beta spectrum, although not easily re- solved, is of some importance, for the shape of the upper end of the spectrum is sensitive to the neutrino mass. This was discussed again by Ellis at an interna- tional conference in London, held in the fall of 1934, where he referred to accurate magnetic spectrograph measurements of W. J. Henderson that strongly suggested a neutrino of zero mass.17 Fermi's theory of beta decay had already been pub- lished,18 and Ellis assumed it in his anal- ysis, but an energy-nonconserving theory, that of Guido Beck and Kurt Sitte, shared equal time with Fermi's at the confer- ence. Fermi spoke at the London Conference, but his subject was the neutron-activation work of the Rome experimental nuclear physics group. He also had attended the Seventh Solvay Conference, held in Oc- tober, 1933, where he heard Pauli present his first suggestion for publication of the existence of a neutrino. The complete Solvay remarks of Pauli are given in En- glish translation in the Box on page 28; we leave it to the reader to decide whether Pauli still thought that the neutrino or the

PHYSICS TODAY / SEPTEMBER 1978 27 electron were constituents of the nucleus. (That a massless neutrino could be created at the moment of its emission Pauli becomes bolder with the electron was clearly proposed 19 that year by Francis Perrin, who also The discussion comments in which Pauli presented the idea of the neutrino at the Seventh attended the Seventh Solvay Conference.) Solvay Conference, ref. 2. The text is based on the translation from the French original There was, in any case, no doubt that a by Chien-Shiung Wu, ref. 9, with corrections by Laurie Brown noted in brackets. light or massless neutral particle of spin V2 has to be emitted with the beta-decay The difficulty coming from the existence of diation of neutral particles, which has not electron in order to save the conservation the continuous spectrum of the 0-rays con- been observed yet. The sum of the energies laws, and that is surely the idea of neu- sists, as one knows, in that the mean life- of the beta particle and the neutral particle trino! times of nuclei emitting these rays, as that (or the neutral particles, since one doesn't of the resulting radioactive bodies, possess know whether there be one or many) emitted Fermi's theory of beta decay is in many well-determined values. One concludes by the nucleus in one process, will be equal ways still the standard theory. Called by necessarily from this that the state as well as to the energy which corresponds to the upper Victor Weisskopf "the first example of the energy and the mass, of the nucleus limit of the beta spectrum. It is obvious that modern field theory,"20 it eventually which remains after the expulsion of the /3 we assume not only energy conservation but caused Bohr to withdraw21 his doubts particle, are also well-determined. I will not also the conservation of linear momentum, concerning "the strict validity of the persist in efforts by which one could try to of angular momentum and of the character- conservation laws." A radical generali- escape from this conclusion for I believe, in istics of the statistics in all elementary pro- zation of quantum theory was not re- agreement with Bohr, that one always cesses. quired, though new particles and new in- stumbles upon insurmountable difficulties in With.regard to the properties of these explaining the experimental facts. teractions were. Within a few months of neutral particles, we first learn from atomic Fermi's theory, positron beta decay was In this connection, two interpretations of weights [of radioactive elements] that their the experiment present themselves. The mass cannot be much larger than that of the seen (the first example of artificial ra- interpretation supported by Bohr admits that electron. In order to distinguish them from dioactivity); and beta decay was to be the the laws of conservation of energy and mo- the heavy neutrons, E. Fermi proposed the prototype of a larger class of weak inter- mentum do not hold when one deals with a name "neutrino." It is possible that the actions. nuclear process where light particles play an neutrino proper mass be equal to zero, so The neutrino can be regarded as one of essential part. This hypothesis does not that it would have to propagate with the ve- the first (if not the first) of the new par- seem to me either satisfying or even plau- locity of light, like photons. Nevertheless, their penetrating power would be far greater ticles that made the new physics of the sible. In the first place the is conserved in the process, and I don't see than that of photons with the same energy. 1930's, even though it took two more why conservation of charge would be more It seems to me admissible that neutrinos decades to observe the first neutrino- 1 fundamental than conservation of energy and possess a spin /2 and that they obey Fermi capture event. The weak interactions momentum. Moreover, it is precisely the statistics, in spite of the fact that experiments have been notorious for their capacity to energy relations which govern several do not provide us with any direct proof of this flout the expectations of physicists with characteristic properties of beta spectra hypothesis. We don't know anything about regard to symmetries and conservation (existence of an upper limit and relation with the interaction of neutrinos with other ma- laws. Although Bohr was too willing, in gamma spectra, Heisenberg stability crite- terial particles and with photons: the hy- his 1931 Faraday Lecture,15 "to renounce rion). If the conservation laws were not pothesis that they possess a magnetic mo- the very idea of energy balance," the valid, one would have to conclude from these ment, as I had proposed once (Dirac's theory relations that a beta disintegration occurs induces us to predict the possibility of neutral conclusion of that lecture is probably still always with a loss of energy and never a magnetic particles) doesn't seem to me at all appropriate today: "... notwithstanding gain; this conclusion implies an irreversibility well founded. all the recent progress, we must still be of these processes with respect to time, prepared for new surprises." In this connection, the experimental study which doesn't seem to me at all accept- of the momentum difference [read balance] able. in beta disintegrations constitutes an ex- In June 1931, during a conference in tremely important problem; one can predict Pasadena, I proposed the following inter- that the difficulties will be quite insur- This work was supported in part by a grant pretation: the conservation laws hold, the mountable [read very great] because of the from the National Science Foundation. I emission of beta particles occurring together smallness of the energy of the recoil nucle- would like to express my sincere appreciation with the emission of a very penetrating ra- to Arthur L. Norberg of The Bancroft Library, University of California, Berkeley, and to Judith Goodstein of the Robert A. Millikan Memorial Library of the California Institute 6. C. S. Wu, S. A. Moszkowki, Beta Decay, 13. C. Weiner, PHYSICS TODAY, May 1972, of Technology. I am much obliged to Samuel Interscience (1966). page 40. Goudsmit for his letter and for his kind per- 7. C. S. Wu, in Trends in Atomic Physics (0. mission to quote from it. 14. W. Pauli, in Handbuch der Physik, Band R. Frisch et al, eds.), Interscience (1959), 24/1 (1933), page 233; ref. 12, page 778. page 45; C. S. Wu, in Five Decades of Weak References 15. N. Bohr, in ref. 10, page 119; J. Chem. Soc. Interactions (N. P. Chang, ed.), New York (London), page 349 (1932). 1. R. A. Millikan, in Encyclopedia Britan- Acad. Sciences, New York (1977), page 37; 16. C. D. Ellis, N. F. Mott, Proc. Roy. Soc. nica, 14th edition, volume 8, page 340 A. Pais, Rev. Mod. Phys. 49, 925 (1977). (London), A 141, 502 (1933). (1929). 8. C. D. Ellis, W. A. Wooster, Proc. Roy. Soc. 17. C. D. Ellis, in International Conference on 2. Rapports du Septieme Conseil de Phy- (London) A 117,109 (1927); L. Meitner, W. Orthmann, Zeit. f. Phys. 60, 413 (1930). Physics, London, 1934, Vol. I, Nuclear sique Solvay, 1933, Gauthier-Villars, Paris Physics, Cambridge (1935); W. J. Hen- (1934), page 324. Pauli's remarks are in 9. C. S. Wu, in Theoretical Physics in the derson, Proc. Roy. Soc. (London) A 147, French. Twentieth Century (H. Fierz, V. F. Weis- 572 (1934). 3. G. Gamow, Constitution of Atomic Nuclei skopf, eds.), Interscience (1960), page 18. E. Fermi, Z. f. Phys. 88,161 (1934); English and Radioactivity, Oxford U.P. (1931). 249. translation: F. L. Wilson, Amer. J. Phys. 4. J. Bromberg, Hist. Stud. Phys. Sci. 3, 307 10. S. A. Goudsmit, in Convegno di Fisica 36,1150(1968). (1971). Nucleare, Reale Accademia d' Italia, Atti, 19. F.Perrin.Compt. Rend. 197,1625 (1933). 5. W. Pauli, Aufsdtze und Vortrdge u'ber Rome (1932), page 41. Physik und Erkenntnistheorie, Braun- 11. J. F. Carlson, J. R. Oppenheimer, Phys. 20. V. F. Weisskopf, in Exploring the History schweig (1961); Collected Scientific Pa- Rev. 38, 1737 (1931). of Nuclear Physics (C. Weiner, ed.), Amer. pers, volume 2, Interscience (1964), page 12. J. F. Carlson, J. R. Oppenheimer, Phys. Inst. of Physics, N.Y. (1972), page 17. 1313. Rev. 41,763(1932). 21. N. Bohr, Nature 138, 25 (1936). •

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