Hideki Yukawa and the meson theory Some 50 years ago, in his first research contribution, a young Japanese theoretical physicist explained the strong, short-range force between neutrons and protons as due to an exchange of "heavy quanta." Laurie M. Brown A little over 50 years ago, Hideki cist who had never even traveled Yukawa's papers at Kyoto University. Yukawa, a young Japanese theoretical abroad, but instead by one of the many (These documents have been organized physicist at the University of Osaka, Western physicists, some of them of and cataloged in the Yukawa Hall proposed a fundamental theory of nu- great reputation, who were working on Archival Library, and I am grateful to clear forces involving the exchange of the theory of nuclear forces. Further- Rokuo Kawabe and Michiji Konuma massive charged particles between neu- more, Yukawa's paper was totally ne- for translations and comments.) trons and protons. He called the ex- glected for more than two years, al- changed particles "heavy quanta" to though it was written in lucid English The problem of nuclear forces distinguish them from the light quanta and published1 in a respected journal of The meson theory was the result of a of the electromagnetic field, but now rather wide circulation. The meson powerful creative act. It incorporated they are known as pions, the lightest theory has turned out to be an impor- a number of ideas that are common- members of a large family of particles tant paradigm for the theory of elemen- called mesons. These days, the meson tary particles, as seminal as Ernest O. theory seems to be a straightforward Lawrence's cyclotron has been for its Hideki Yukawa, at right, with (from left) his father-in-law Genyo Yukawa, his mother-in- application of quantum field theory to experimental practice. In this article I law Michi and his wife Sumi at their home the nuclear forces, but it could not have will trace the intellectual trail Yukawa in Osaka in 1932. (Photos illustrating this appeared so 50 years ago. Otherwise it followed to arrive at his theory, using article courtesy Michiji Konuma; from would not have been invented by an unpublished documents that have re- reference 9, reproduced with permission of obscure, unpublished Japanese physi- cently been discovered among Sumi Yukawa.) 0031-9228 / B6 / 1200 55- 0B / $01.00 1986 American Institute ol Physics PHYSICS TODAY / DECEMBER 1986 55 Manuscript page containing Yukawa's wave equation, based on the Dirac equation, for describing the intranuclear electron field having the nucleon-exchange current as a source. Equation 1 in the manuscript corresponds to equation 3 in the this article. (The documents illustrating this article come from the Yukawa Hall Archival Library, Kyoto University, courtesy of Ziro Maki.) would rapidly escape the nucleus. An- other difficulty was that certain nuclei, those with an odd number of intranu- clear electrons, should have had mag- netic fields about a thousand times larger than that inferred from the hyperfine structure of atomic spectra. One such nucleus—nitrogen-14, with 14 protons and 7 electrons—should have had half-integral spin and obeyed Fermi-Dirac statistics, but was known to have integral spin and to obey Bose- Einstein statistics. As early as 1920, Ernest Rutherford had suggested that there might exist a neutral nuclear constituent that he called the "neutron," consisting of a proton and an electron much more tightly bound than in the hydrogen atom. James Chadwick, working in the Cavendish Laboratory in Cambridge in 1932, assumed, when he discovered the neutron, that he had found Ruther- ford's composite neutron, which had been sought at the Cavendish for a place today, but which were novel and remember the time when man's decade. Chadwick did consider the surprising 50 years ago. These are chief purpose was to hunt some possibility that his neutron was a new some of the original features proposed animals, when he did not have elementary particle, but said4 that the by Yukawa: chairs to sit in, did not have beds, idea had "little to recommend it." • The nuclear binding force is trans- did not have covers, did not have a Within a few weeks after Chadwick's mitted by the exchange of massive jacket, did not have anything, not announcement, Heisenberg wrote the charged particles (the heavy quanta). to mention that he did not have a first part of a three-part paper present- • The range of the force is inversely radio to speak into. It is hard to ing5 a theory of nuclear structure in proportional to the mass of the quan- remember that. which protons and neutrons were the tum. The physicist's view of matter before principal constituents (Bausteine) of • There are two nuclear forces, one the discovery of the neutron was that the nucleus, instead of the protons and strong and one weak, and hence two everything, including the nucleus, con- electrons (and sometimes alpha parti- coupling constants. sisted of protons and electrons and was cles) of the earlier models. (See the • The weak interaction is also mediat- held together by electric and magnetic article by Arthur Miller, PHYSICS TO- ed by the exchange of the heavy quanta forces.3 In that picture, the nucleus of DAY, November 1985, page 60.) Heisen- (charged intermediate bosons). mass number A contained A positive berg did not challenge the idea that the • The heavy quanta are unstable, protons and A — Z negative electrons; neutron was composed of a proton and decaying via the weak interaction. the neutral atom had, of course, Z an electron, but by suppressing the It was a great achievement to pro- extranuclear electrons. Thus, matter electron degrees of freedom in his pose such a bold solution for the nu- was said to be made of pure electricity. nuclear Hamiltonian he was able to clear-force problem, for it was difficult Within this framework, quantum me- construct the first genuinely quantum- at that time even to grasp and to chanics was scoring impressive suc- mechanical nuclear theory. (Dmitri formulate the dimensions of the prob- cesses in explaining phenomena that Iwanenko in Leningrad, at about the lem. At a symposium held in Minne- involved the extranuclear electrons, in same time, suggested6 treating the apolis in 1977 on the subject of nuclear fields such as spectroscopy, chemistry neutron as an elementary particle in physics in the 1930s, Eugene Wigner and magnetism, but the behavior of the 2 the nucleus, but he does not seem to remarked: intranuclear electrons was quite an- have developed a detailed theory of this The disorientation in physics other matter. The mere presence of type.) So far as nuclear systematics which existed . [around 1932] is electrons in the nucleus seemed to violate important principles of physics. was concerned, Heisenberg treated the hard to imagine. We can remem- proton and the neutron as the charged ber it hardly more than we can One problem, apparently insurmount- able within the conventional frame- and neutral states of a single entity, work of quantum mechanics, was that our present-day nucleon, and he intro- Laurie Brown is professor of physics and an electron, having low mass and duced operators that changed the astronomy at Northwestern University, Evan- confined within a space as small as a charge of the nucleon, turning neu- ston, Illinois. This article is based upon a nucleus, would according to Werner trons into protons or vice versa. The paper presented at an international sympo- Heisenberg's uncertainty principle force that he introduced between the sium on the golden jubilee of the meson have a kinetic energy so large that it proton and neutron had charge-ex- theory, Kyoto, Japan, 15-17 August 1986. change character, analogous to that in 56 PHYSICS TODAY / DECEMBER 1986 + the hydrogen molecular ion H2 ; the senberg regarded the proton and the the Great Depression and a difficult force between neutrons was analogous neutron as sisters, so similar that the time in which to find a job. He to the exchange force in the hydrogen exchange currents in terms of which he therefore continued to live with his molecule, but between protons, which expressed his charge-exchange forces family in Kyoto, where his father, Heisenberg regarded as elementary were formulated in terms of raising Takuji Ogawa, was a professor of geog- particles, there was only ordinary Cou- and lowering operators adapted from raphy at the university. For three lomb repulsion. Thus in assigning the Pauli spin matrices. Those are years, together with Sinitiro Tomon- forces Heisenberg clearly differentiat- precisely the modern isospin operators, aga—his classmate, the son of a Kyoto ed the "elementary" proton from the although Heisenberg employed them philosophy professor, and a future fel- "composite" neutron. Nevertheless, without the accompanying idea of the low Nobel laureate—Yukawa worked despite its unsymmetric treatment of charge independence of nuclear forces. as an unpaid assistant in the theoreti- what we now call the nucleon doublet, Nevertheless, he insisted that electrons cal-physics research room of Professor Heisenberg's theory, especially as of both the bound and loose varieties Kajuro Tamaki. Intensely ambitious, modified during the following year by were present in the nucleus. This view he was afraid that he had arrived upon Wigner and Ettore Majorana, is cor- so characterized Heisenberg's nuclear the physics scene too late to make a model that in 1933 Wigner, listing8 fundamental contribution to quantum rectly regarded as the foundation of the 9 modern phenomenology of nuclear "three different assumptions possible theory. Much later, Yukawa wrote in structure. concerning the elementary particles" his autobiography, "As I was desperate- While Heisenberg had seized the in the nucleus, began his list: "(a) The ly trying to reach the front line, the opportunity presented by the discovery only elementary particles are the pro- new quantum physics kept moving of the neutron to formulate a pheno- ton and the electron.