(b. Tokyo, Japan, 23 January 1907; d. Kyoto, Japan, 8 September 1981)
Hideki Yukawa, incentor of the meson theory of nuclear forces, was the fifth of seven children born to Takuji and Koyuki Ogawa. Both parents came from scholarly families of the samurai tradition. At the time of Hideki’s birth, Takuji Ogawa was a geologist on the staff of the Geological Survey Bureau in Tokyo. In 1908 he became professor of geography at Kyoto Imperial University, and Hideki received all of his formal education in Kyoto, which he regarded as his home. The Ogawa children were strongly influenced by their father’s broad cultural interests, which extended well beyond his scientific profession. Four sons became university professors: of metallurgy, of Chinese history, of Chinese literature, and (Hideki) of physics. The youngest son Masuki, died in World War II.
In 1923 Yukawa enrolled at the Third High School in kyoto. The future physicist Sin-itiro Tomonaga was his classmate there and also at Kyoto Imperial University (now Kyoto University), which both entered in 1926. After graduating in 1929, they stayed on at Kyoto until 1932, in which year Yukawa was appointed lecturer at Kyoto and Tomonago moved to Tokyo. In the same year, Hideki married Sumi Yukawa, assumed her family name, and went to live with the Yukawa family in Osaka. In 1933 Yukawa became a lecturer at Osaka Imperial University while continuing to lecture at Kyoto. He proposed the meson theory in his first published scientific paper (January 1935) and was appointed associate professor at Osaka in 1936. He received the Ph.D. at Osaka in 1938.
In the fall of 1939 Yukawa returned to Kyoto Imperial University as professor (a position he retained until his retirement) and also made his first journey abroad. During the war he played a minor consulting role in military research while continuing his scientific work. In the year 1948–1949 he was visiting professor at the Institute for Advanced Study in Princeton, then went to Columbia University, where, after receiving the Nobel Prize for physics for 1949, he became professor of physics. To honor him (and to bring him back to Japan), the Japanese government in 1953 established the Research Institute for Fundamental Physics at Kyoto University, with Yukawa as its first director. After his retirement in 1970, he remained active, writing essays, editing Progress of Theoretical Physics, a Western-language journal he founded in 1946, and working in international movements for peace and world federation.
The paper in which Yukawa proposed the meson theory was written in English and published in Proceedings of the Physico-Mathematical Society of Japan. The article attracted little attention for about two years, although it turned out to define a watershed in nuclear and elementary particle physics and proved to be a powerful influence on the development of the Japanese physics community. The most striking feature of the new theory was the prediction that new particles, serving as the “heavy quanta” of the nuclear force field, would be produced in high-energy nuclear collisions, such as those that occur naturally when cosmic rays enter the earth’s atmosphere, Particles that appeared to meet Yuwaka’s requirements were actually observed in 1937, and as a result Yukawa immediately acquired a worldwide reputation.
In 1949 Yuwaka received the first Nobel Prize in physics to be awarded to a citizen of Japan, “for his prediction of the existence of mesons.” Coming so soon after their disastrous defeat in World War II, Yuwaka’s international recognition gave the Japanese particular pride and encouragement.
With regard to the delay in the acceptance of Yuwaka’s theory, Nicolas Kemmer, one of the first Western physicists to work on the meson theory, wrote in 1965 that “Yukawa in 1935 was ahead of his time and found the key to the problem of nuclear forces when no other theoretical physicist in the world was ready to accept it.” In his introduction to Yukawa’s Scientific Works, Yasutaka Tanikawa calls the meson theory “a miracle in the history of Japanese physics.” But upon closer examination we can see that this “miracle” was preceded by arduous and intensive personal preparation. By ex-ning the unpublished record and Yukawa’s own account, we can follow the internal and external influences on his scientific thinking.
An account of Yukawa’s intellectual development up to 1935 is given in his popular autobiography Tabibito (The Traveler). In it he describes how he grew up in a large household that included the children, three grandparents, an affectionate mother, and a father who appeared to the young Yukawa as a cold and humorless intellectual. The grandparents were warmly attentive; his maternal grandfather, Komakitsu Ogawa, who had been a samurai teacher at the Tokugawa castle in Wakayama until the Meiji regime abolished the samurai class, taught him to read kanji (Chinese characters) before he entered school. Although he showed an early talent for mathematics, Yukawa had no interest in science until he reached high school, when he began to consider a scientific career.
Yukawa matriculated at Kyoto Imperial University in 1926, the year after Werner Heisenberg’s quantum mechanics. By his second year Yukawa was spending all his spare time reading Schrödinger’s papers in the physics library. He had already read some of Max Planck’s and Niels Bohr’s writings with profit; on the other hand, he found Heisenberg difficult to understand. Yukawa’s graduation thesis in 1929 was based upon Paul A. M. Dirac’s relativistic electron theory of 1928.
For three years following graduation, Yukawa was an unpaid assistant of Kajuro Tamaki. From 1929 to 1931, the Japanese physicists Bunsaku Arikatsu, Yoshikatsu Sugiura, and Yoshio Nishina, all of whom had studied in Europe, lectured on quantum mechanics at Kyoto. Through them, especially Nishina, Yukawa became acquainted with the Copenhagen spirit. Nevertheless, he remained determinedly independent and resolved to seek out his own problems and their solutions, unlike the more practical-minded Tomonaga, who began to work on molecular problems suggested by Sugiura.
Two great and evident problems remained: the atomic nucleus and quantum electrodynamics. The latter, the quantum theory of the electromagnetic field, was introduced by Dirac in 1927. It was formulated in a relativistically covariant manner in a 1929 paper by Heisenberg and Wolfgang Pauli that has become a classic of physics. This careful work exposed a terrible defect in the quantum theory of fields, that of the so-called divergences, which meant that some measurable physical quantities, such as the electron mass, were predicted by the theory to be infinite when electromagnetic corrections were taken into account. Yukawa’s view agreed with the consensus that the divergence problems were related to the behavior of the fields near their source—or, to put it otherwise, to the singularity of the point charge.
In Yukawa’s introduction to an unpublished work of early 1933, “On the Problem of Nuclear Electrons, I,” he wrote [with our minor grammatical editing]:
The problems of the atomic nucleus, especially the problems of nuclear electrons, are so intimately related with the problems of the relativistic formulation of quantum mechanics that when they are solved, if they ever be solved at all, they will be solved together. But meanwhile, we can only attempt to solve one or the other problem on rather arbitrary assumptions, insofar as they do not contradict our experimental knowledge.
By “relativistic formulation of quantum mechanics,” Yukawa meant the problem of consistently quantizing the combined electromagnetic field and Dirac electron-positron field. He considered that to be a deeper problem than that of the nucleus, and he returned to it throughout his lifetime. Ironically, it was Yukawa who provided the key to the nuclear force problem but Tomonaga who, a decade later, showed how one could deal with the divergence problem of quantum fields.
Yukawa began to approach the nuclear force problem by studying (in his words) “what was probably the only organized book on the theory of the nucleus at that time,” George Gamow’s Constitution of Atomic Nuclei and Radioactivity (1931). Gamow’s book presented the then standard view that the nucleus was composed of protons and electrons. In that unified picture all matter, including nuclear matter, was considered to be electrical in nature, and the fundamental forces in nature were thought to be exclusively electromagnetic and gravitational. To explain the detailed behavior of specific nuclei—for instance, the radioactivity of heavy elements— certain nuclear substructures were introduced, such as the alpha particle and the “neutron” (proposed by Ernest Rutherford); but these, too, were considered to be composites of protons and electrons.
Systems composed of electrons and nuclei held together by electrical forces, such as atoms, molecules, and crystals, were being successfully treated by quantum mechanics. These systems are characterized by distances of the order of the Bohr radius, about 10−8 cm. Relativistic quantum dynamics, as shown by the success of the Klein-Nishina formula for Compton scattering, appeared to be successful at least down to the scale of the Compton wavelength, about 10−11 cm. Nuclei, however, are about 100 times smaller than that, and there were serious doubts (originating with the electron theories at the beginning of the century and never finally laid to rest) that the ordinary laws of physics, even the modern relativistic quantum electrodynamics, would apply at such short distances.
It was clear to many physicists that the presence of electrons within the nucleus would imply a failure of quantum mechanics. Gamow set forth the reasons in his textbook of 1931: violation of the uncertainty principle of Heisenberg; contradiction of observed nuclear spin angular momentum and statistical behavior; and nuclear magnetism as it affected atomic spectra. (The latter problem, the so-called hyperfine structure, was treated by Yukawa while he was a member of Tamaki’s research group in Kyoto. That work was never published, as it was similar to a paper by Enrico Fermi that appeared at about that time.)
After James Chadwick’s discovery of the neutron in 1932, the situation did not at first seem different, since the neutron was thought to be a composite of a proton and an electron. It occurred to some physicists (notably Dmitri Ivanenko) that the neutron could be an elementary particle and not a composite, but several physical processes seemed to demand that nuclei contain electrons. Perhaps the most compelling example was beta-decay, in which electrons appear to emerge from the nucleus, but other processes also seemed to demand them. Heisenberg wrote in 1932:
Such phenomena are the Meitner-Hupfeld effect, the scattering of γ-rays on nuclei; further all experiments which split neutrons into protons and electrons (an example is the stopping of cosmic ray electrons on their passage through nuclei).
This sentence is from the article in which Heisenberg proposed his neutron-proton model of the nucleus (“Über den Bau der Atomkerne,” in Zeitschrift für Physik, 77 , 1–11). In that work the neutron is treated (as regards nuclear structure) as an electrically neutral proton. In particular, it is assigned the same spin angular momentum as the proton. Such particles (fermions) obey Fermi-Dirac statistics, while particles of zero or integer spin (bosons) obey Bose-Einstein statistics, as do identical systems that contain even numbers of fermions. That meant the neutron could not be a proton-electron composite, unless the laws of quantum mechanics did not apply to such a small system; that was indeed what Heisenberg concluded. At the same time, he proposed his mechanism for the neutron-proton force: a neutron emits a negative electron, thus turning into a proton; that electron is absorbed by another proton, which is transformed into a neutron. Heisenberg also assumed a similar charge-exchange mechanism, analogous to homopolar chemical binding, to provide an attractive force between neutrons. Between protons he allowed only the repulsive electric Coulomb force. Thus he regarded the neutron as composite but the proton as elementary, even though at the same time he introduced a formalism that treated proton and neutron symmetrically (later known as the isospin formalism).
Besides violating the principles of quantum statistics, Heisenberg’s picture violated the conservation of angular momentum in the elementary process of neutron breakup and formation, which he supposed also provided the mechanism for beta-decay; in the latter case it also violated the conservation of energy.
All that was not strange at the time, for many phenomena at high energy and small distances seemed unexplainable without either new dynamics or new particles. The former was favored by the Copenhagen school, to which Heisenberg belonged, but the explanation turned out to be new particles. These particles were the neutron (an elementary particle) and the positive electron (positron), both discovered in 1932, and the neutrino, proposed in print by Pauli for the first time in 1933.
Heisenberg’s article appeared in three parts in Zeitschrift für Physik during 1932 and early 1933. Yukawa read them eagerly and prepared a summary in Japanese of the first two parts, which he published together with his own critical introduction in Journal of the Physico-Mathematical Society of Japan (his first publication). On 3 April 1933, Yukawa read a paper entitled “A Comment on the Problem of Electrons in the Nucleus” at the annual meeting of the Physico-Mathematical Society at Tohoku Imperial University, Sendai; in it he tried to make a fundamental quantum field theory of Heisenberg’s phenomenological charge-exchange force.
Meanwhile, Ettore Majorana in Rome and Eugene Wigner in Princeton had pointed out that Heisenberg’s forces, in their original form, did not lead to good agreement with the known properties of the lightest nuclei. For example, if only the Heisenberg forces acted, then the deuteron spin would be given incorrectly, and the small binding energy of the deuteron would be incompatible with the large binding energy of He4. Majorana and Wigner each proposed different modifications of Heisenberg’s nuclear forces and both, especially Majorana, stressed the essentially phenomenological nature of theories of the Heisenberg type.
Yuwaka, on the other hand, wanted to construct a formalism analogous to the Heisenberg-Pauli quantum electrodynamics of 1929, which is an intrinsic expression of the wave-particle duality, since in it field and particle are simply different representations of the same physical entity. He wanted the electron to play a role analogous to the light quantum, while the source, analogous to the electric current, was to be composed of proton and neutron, which he regarded as the charged and neutral components of a single field (today called the nucleon).
Among the Yukawa papers in Kyoto, there are four sets of calculations along these lines and three unfinished manuscripts from 1933, including (in English) “On the Problem of Nuclear Electrons, I,” the opening sentence of which was quoted above. In the abstract published in advance of Yukawa’s talk at the Sendai meeting in 1933, he says that a nuclear electron acts “as a kind of field inside the nucleus.” Its equation of motion, however, is not derivable from any Hamiltonian function, so that “we may not apply the concept of energy in the usual sense.” Since Yukawa, like Heisenberg, wanted electron emission to account for beta-decay as well as for nuclear binding, and since beta-decay appeared to violate the energy principle, the lack of energy conservation in the theory might have been seen as a boon, not a defect.
Yukawa writes further in the Sendai abstract: “From the fact that the electron has a finite rest mass, we expect the interaction energy to decrease rapidly as the distance between neutron and proton becomes large in comparison with h/2πmc.” (Here h, m, and c are, respectively, Planck’s constant, the electron mass, and the velocity of light.) That combination is a distance about 200 times the range of nuclear forces. Nevertheless, the statement has a prophetic ring, for a more massive electron would produce a suitably short range.
In reminiscences, Yukawa recalled that at the Sendai talk, Nishina asked him why he did not just assume that the nuclear electron, unlike the atomic one, obeyed Bose-Einstien statistics and had zero or integer spin. In his recorded recollections, Yukawa failed to note that on that occasion, he had already expressed the key relationship between the range of a force field and the rest mass of its quantum. Perhaps his memory lapse occurred because in the lecture he delivered, he withdrew the statement in the abstract about the range of the force. On the back of one page of his lecture notes, Yukawa wrote the wave equation satisfied by the electron field, and next to it, an exponentially decreasing (damped) solution. But beside that he wrote “mistaken conjecture,” and in a colloquium he gave a bit later at Osaka Imperial University, the solution he presented was propagating and undamped.
Up to that point, Yukawa was not aware that Pauli, as early as 1930, had suggested that the electron in beta-decay might be accompanied by a light (perhaps massless) natural particle of spin 1/2, eventually called the neutrino. Such a particle, for which there was no direct experimental evidence, would allow all conservation laws to be satisfied. Pauli’s first publication of the idea was as a comment to the report “Structure of the Nucleus,” given by Heisenberg at the Solvay Conference held at Brussels in October 1933. Enrico Fermi, who was present at that conference, returned to Rome, where he incorporated Pauli’s neutrino idea in a new field theory of beta-decay.
Sometime in 1934, Yukawa saw Fermi’s paper and considered the possibility that the strong nuclear binding force could be mediated by the exchange of the electron and neutrino as a pair, to give a unified treatment of binding force and beta-decay. That idea, called the Fermi field, was taken up by others as well, including Heisenberg. In particular, the Russians Igor Tamm and Ivanenko each published a letter in Nature, in which they gave the results of calculating the force of the Fermi field and claimed it was impossible to reconcile the strength and the range of the nuclear force with the strength of the beta-decay interaction. When Yukawa saw this work (as he says in Tabibito), he resolved not to look among the known particles, including the neutrino, to find the quantum of the nuclear force field. The result was the meson theory of nuclear forces.
Yukawa’s theory was proposed at meetings in Osaka and Tokyo in October and November 1934, and described in the article “On the Interaction of Elementary Particles, I,” published in February 1935. Its characteristic features are retained in current theory, although certain additions and alternative forms have been considered from time to time. The presentation of the theory begins thus: “In analogy with the scalar potential of the electromagnetic field, a function U(x,y,z,t) is introduced to describe the field between the neutron and the proton”. The paper then develops the properties of the U-field in a compelling manner, using electromagnetism as a model.
The full theory of electromagnetism requires a vector potential in addition to the scalar one, but the U-field is primarily intended to describe the forces in nuclei, where the nearly nonrelativistic motion of the source particles (neutrons and protons) allows their description by nonrelativistic wave functions and also implies the dominance of the scalar potential over the vector. When the classical U-field is quantized, it is represented by U-quanta that are analogous to electromagnetic light quanta; later the U-quanta were called by many names and eventually became known as mesons. Like light quanta, U-quanta obey Bose-Einstein statistics, but unlike them, they are massive (their mass being inversely proportional to the range of the nuclear force) and have zero spin. The source of the U-field, analogous to electric charge and current, is the neutron-proton transition current, which had appeared in the articles of Heisenberg and Fermi to which Yukawa refers. The strength of the interaction is described by a new coupling constant, g, the analogue of the electronic charge.
The exchange of U-quanta is intended to provide a theoretical basis for Heisenberg’s phenomenological charge-exchange force, so the U-quanta carry electric charge, either plus or minus, in addition to g. Thus the U-field is complex, as opposed to the electromagnetic field, which is real. In order to incorporate the beta-decay process in a unified way, Yukawa gives the U-quanta an additional (weak) interaction with the electron-neutrino interaction current, its strength characterized by a second chargelike constant, g′, In this way he distinguishes clearly, and for the first time, two nuclear forces, one strong and one weak. The U-quantum is the carrier for both the strong and weak interactions.
The meson theory replaced Heisenberg’s phenomenological exchange potential, J(r), with an explicit function,
J (r) = + or − (g/r) exp (−ar),
with a=Mc/h, M being the mass of the U-quantum. (The reciprocal of a is the range of the nuclear force). Heisenberg’s J(r) was an arbitrary function, and he chose the plus sign to give the deuteron zero spin. But by 1934 the spin of the deuteron was known to be 1, leading Yukawa to prefer the minus sign. (Since his theory is a fundamental one, it actually fixes the sign. With later and more elaborate versions of meson theory, the duteron spin is shown to be given correctly.)
From an estimate of the range of the nuclear force, Yukawa calculated the mass M to be about 200 times that of the electron. To explain why such particles had not yet been observed, he noted that they could not be produced in “ordinary nuclear transformations”, where the energy available was insufficient, but that they might be present in the cosmic rays. Curiously, he did not at first realize the relevant fact that the U-quantum’s weak interaction would make it radioactive, with a short mean life, so that it could not be found in ordinary matter. After the Indian physicist H.J. Bhabha pointed out that the meson should decay. Yukawa and Shoichi Sakata calculted its lifetime and found it to be about one hundred-millionth of a second.
For two years the impact of Yukawa’s meson theory, both in Japan and abroad, was nil. Yukawa pursued other scientific activities, publishing ten papers in English, most of them in collaboration with Sakata, including an important calculation of the inverse beta-decay process: the absorption of an orbital electron by a nucleus with the emission of a neutrino. That article (submitted in July 1935) was noteworthy, not only because it was the first to call attention to a new effect but also because it was the first additional application of the meson theory and thus showed that Yukawa and Sakata had faith in it. Although that is the only published reference to the meson during those two years, the Yukawa archive in Kyoto contains a number of incomplete versions of a second meson paper begun in 1936, as well as a letter submitted to Nature on 18 January 1937, calling attention to the theory of the U-field. The latter was probably stimulated by a paper in Physical Review of 15 August 1936 by Carl D. Anderson and Seth H. Neddermeyer, reporting anomalous cosmic-ray tracks, observed in a cloud chamber exposed in a magnetic field atoppike’s Peak, that were not easily classifiable as either electrons or protons; Yukawa’s letter to Nature states that “it is not altogether impossible” that they were mesons.
Yukawa’s letter was rejected, but in July 1937 he published a note in Proceedings of the Physico-Mathematical Society of Japan entitled “On a Possible Interpretation of the Penetrating Component of the Cosmic Rays”. It had the same opening paragraph and essentially the same content as the rejected Nature letter. However, during the first half of 1937, several cloud-chamber groups had confirmed the existence of anomalous tracks, and while interpretations differed, Neddermeyer and Anderson (and also J. C. Street and E. C. Stevenson) asserted that they had observed positive and negative particles intermediate in mass between the electron and the proton. A clear example was also given by a Japanese cloud-chamber group led by Nishina. In June, E. C. G. Stueckelberg, from Geneva and J. R. Oppenheimer and R. Serber from Pasadena sent letters to Physical Review that called attention to the meson theory. From that point on, Yukawa’s international fanme was assured.
With his students at Osaka University, Yukawa completed three additional parts of the series entitled “On the Interaction of Elementary Particles”, part I being the original paper on the U-quantum. With Sakata as co-author, part II was submitted in November 1937; it includes the material in Yukawa’s unpublished manuscript of 1936, subtitled “Generalization of the Mathematical Scheme”, and it reformulates meson theory as that of a relativistic scalar field, using a method developed by Pauli and V. F. Weisskopf in 1934. Forces between neutrons or between protons required the exchange of two oppositely charged mesons. By 1937, however, the equality of like-particle and unlike-particle nuclear forces (charge independence) had been inferred from nuclear scattering experiments, and a corresponding “field theory” (of Fermi-field, not meson, type) had been proposed by Nicolas Kemmer. For that reason, part II contains a speculation on the possible existence of an additional electrically neutral “heavy quantum”.
Part III of the series added Mituo Taketani as a third author, and part IV had Minoru Kobayashi as a fourth. Both of these papers, worked on simultaneously with part II, appeared in 1938; both used a quantized generalization of Maxwell’s electromagnetic theory involving “two four vectors and two six vectors, which are complex conjugate to each other respectively”. Neither of the two previous formulations was judged to be “ample enough” for the description of the broad spectrum of processes to which the theory was to be applied; nuclear forces and nuclear scattering, meson-nuclear scattering, magnetic moments of the neutron and proton, and weak interactions, among others. At the same time, essentially the same problems were being attacked by similar methods by Stueckelberg, Bhabha, Kemmer, H. Fröhlich and W. H. Heitler. The last two theorists, refugees from the Nazis, were in England at that time; together with Kemmer, they produced the first charge-independent meson theory of nuclear forces.
The publications of Yukawa and his school from 1939 until after World War II became increasingly concerned with resolving a disturbing discrepancy between the meson regarded as the carrier of nuclear forces and the “meson” observed in the cosmic rays. For example, in 1943 Sakata and Takesi Inoue wrote “On the Correlations Between Mesons and Yukawa Particles”. At issue were the mean lifetime, where there was a discrepancy of a factor of about 100, and the apparent lack of strong nuclear interaction of the cosmic-ray meson, evidenced by its very small absorption in matter. Both of these problems were resolved by the discovery of the existence of two mesons, the cosmic-ray meson being the daughter arising from decay of the short-lived nuclear-force meson. This solution was proposed theoretically as early as 1942 by Sakata and Inoue and by Yasutaka Tanikawa, and it was experimentally confirmed after the war.
Although meson theory was Yukawa’s greatest accomplishment, throughout his scientific career he regarded the nuclear force problem as of less importance than that of formulating a mathematically consistent relativistic quantum theory, free of “infinities” like those brought to light in the Heisenberg-Pauli quantum electrodynamics of 1929. One of Yukawa’s earliest unpublished manuscripts (1934) approached the problem of a relativistic quantum theory from the standpoint of the theory of measurement. He continued to examine epistemological questions, such as the nonseparability of cause and effect in quantum mechanics, and fundamental issues of the theory of quantum fields, even while he was working most actively on meson theory. Around 1940 he introduced the idea he called “maru” [circle], representing a finite region of an elementary particle within which relativistic causality is not valid.
Beginning in 1950, Yukawa developed the idea of nonlocal quantum fields, an idea strongly influenced by Heisenberg’s concept of a fundamental universal length. Like some older physicists, notably Heisenberg and Dirac, Yukawa never fully accepted the renormalization method of quantum electro-dynamics, regarding it as a mere calculational device, a door that concealed the difficulty but at the same time blocked the road to progress.
The idea of nonlocal fields (which is to be distinguished from the idea of local fields having nonlocal interaction) gradually became a theory of elementary particles with internal structure. By the late 1960’s it was superseded by Yukawa’s concept of “elementary domain”, based upon the quantization of the classical continuously deformable body. These fundamental ideas do not play a major role in current theoretical physics but may well be vindicated in a future physics.
Yukawa’s emphasis in the early 1940’s on the importance of formulating quantum field theory in a closed space-time region helped Tomonaga to develop his covariant quantum electrodynamics, according to the latter’s testimony. It is not possible to discuss here Yukawa’s original ideas on creativity, history and philosophy of science, and the differences between Eastern and Western modes of thought. These ideas, carefully presented and ranging over subjects as diverse as Epicurus, Taoism. The Tale of Genji, and the nature of creative scientific thought, have been influential in Japan as well as (in translations) on the international level.
I. Original Works. Scientific Works, Yasutaka Tanikawa, ed. (Tokyo, 1979), contains all of Yukawa’s scientific papers not in Japanese as well as English translations of some of his scientific papers and essays in Japanese. His books include Creativity and Intuition, John Bester, trans. (Tokyo, 1973); and Tabibito (The Traveler), L. Brown and R. Yoshida, trans. (Singapore, 1982). There are many works in Japanese only.
The bulk of Yukawa’s extensive unpublished notes, manuscripts, and letters are in the Yukawa Hall Archival Library, Kyoto University, Kyoto 606, Japan; parts have been cataloged. Unpublished manuscripts, trans. Rokuo Kawabe, and other material on Yukawa, are in L.M. Brown, R. Kawabe, M. Konuma, and Z. Maki, eds., Proceedings of the Japan-USA Collaborative Workshops on the History of Particle Theory in Japan, 1935–1960 (Kyoto, 1988).
II. Secondary Literature. Laurie M. Brown, “Yukawa’s Prediction of the Meson,” in Centaurus, 25 (1981), 71–132; L.M. Brown, M. Konuma, and Z. Maki, eds., Particle Physics in Japan, 1930–1950 (Kyoto, 1980), I and II; Satio Hayakawa, “The Development of Meson Physics in Japan,” in Laurie M. Brown and Lillian Hoddeson, eds., Birth of Particle Physics (Cambridge, 1983), 82–107; N. Kemmer, “Hideki Yukawa,” in Biographical Memoirs of Fellows of the Royal Society, 29 (1983), 661–676; Viśvapriya Mukherji, “History of the Meson Theory of Nuclear Forces from 1935 to 1952,” in Archive for History of Exact Sciences, 13 (1974), 27–102; and Shigeru Nakayama, David L. Swain, and Eri Yagi, eds., Science and Society in Modern Japan (Cambridge, Mass., 1974).
Laurie M. Brown
Hideki Yukawa, the first Japanese Nobel Laureate in physics and the originator of the meson theory of nuclear forces, was born in Tokyo on January 23, 1907. During most of his life, he lived in Kyoto, Japan's ancient capital, and he died there on September 8, 1981. He was the fifth of seven children of Koyuki and Takuji Ogawa. Both of his parents stemmed from scholarly families of the samurai tradition. The family moved from Tokyo to Kyoto in 1908 when Yukawa's father, a geologist, was appointed Professor of Geography at Kyoto Imperial University. Besides Hideki (who changed his name to Yukawa when he married Sumi Yukawa and was adopted by her family in 1932), three other Ogawa sons became university professors and renowned scholars in their respective fields.
Early Education and Research
As a youth, Yukawa's interests were mainly literary, and he read widely in world literature and philosophy. In 1923 he entered the Third High School, actually a junior college, one of only eight then existing in Japan, and found himself strongly attracted to physics. One of his classmates (from elementary school through university) was Sin-itiro Tomonaga, who shared a Nobel Prize in Physics in 1965 for his contributions to quantum electrodynamics. Both young men became acquainted with modern atomic and nuclear physics from their high school physics teacher.
Yukawa and Tomonaga enrolled in the physics program at Kyoto Imperial University in 1926 and graduated in 1929. They stayed on as unpaid assistants at the university due mainly to the worldwide depression. As a postgraduate, Yukawa first tried to solve the so-called divergence problems of quantum electrodynamics (QED): the theory predicted infinite electron charge and infinite mass, both predictions being absurd. Failing in this attempt (in which Tomonaga and others succeeded in the 1940s), Yukawa turned to what seemed to be an "easier" problem, that of nuclear forces.
The Meson Theory of Nuclear Forces
Beginning in 1933, Yukawa tried to improve a theory that had been proposed in 1932 by the German physicist Werner Heisenberg to explain the forces holding the nucleus together. In this theory, the main attractive force arose through the exchange of an electron between a neutron and a proton in the nucleus. That is, a neutron would emit an electron and become a proton. Normally the electron would be absorbed by a proton in the same nucleus, turning into a neutron. Heisenberg's theory attributed the
radioactive process known as beta decay, in which a nucleus emits an electron, to the occasional escape of an electron from the nucleus.
However, Heisenberg's theory had a number of serious difficulties. As a theory of the forces holding the nucleus together, it violated several principles of the theory of quantum mechanics. For one thing, Heisenberg's own uncertainty principle would not allow an electron to be confined in a space as small as the nucleus. Other laws were violated as well, including the conservation law of angular momentum. As for radioactive beta decay, while the nucleus lost a definite amount of energy, the emitted electron had lesser, and variable, amounts of energy, and Heisenberg's theory did not account for this missing energy. Accounting for this energy would require that a new particle be emitted with the electron; this particle was called the neutrino, proposed in 1930 by Wolfgang Pauli. One way out of these contradictions was to assume that entirely new laws of physics held in the nucleus.
Yukawa sought to retain the principles of quantum theory and the conservation laws and to eliminate the difficulties of Heisenberg's theory by relating the exchange force to a new field that he called the U-field. He took for a model the electromagnetic field that, according to quantum field theory, results from the exchange of massless light quanta (photons). Allowing his new U-quanta to be massive, Yukawa discovered in 1934 an important relation between the mass of the exchanged heavy quanta and the range of the force in question. From this relation he inferred that the quanta of the nuclear force field must be about two hundred times as massive as the electron, based upon the known short range of the strong nuclear forces. He assumed that the new heavy quanta have charges of the same size as the electron's, either positive or negative
To account for beta decay, Yukawa assumed that his heavy quanta, interacting with the nuclear particles with high probability (that is, they were strongly coupled), could also decay with small probability (weak coupling) into an electron and a neutrino. Yukawa and his students later calculated that the decay of the free heavy quantum should take about 10-8seconds. Yukawa was the first physicist to make a clear distinction between the two kinds of nuclear force: the strong force to give binding and the weak force for beta decay.
Yukawa's theory involving the new heavy charged quanta (now called mesons) was published in an English-language Japanese journal at the beginning of 1935. However, it attracted worldwide notice only in 1937, after American physicists Carl Anderson and Seth Neddermeyer, studying the cosmic rays, found particles that appeared to fit Yukawa's requirements.
Beginning in 1937 many theorists, Yukawa and his students, and several groups working in Europe and America worked out versions of the meson theory to account for phenomena occurring in nuclear physics as well as in the cosmic rays. However, there was only rough qualitative agreement with the experiments which were simultaneously undertaken. For example, the absorption of the new cosmic ray particles by nuclei was much weaker than expected.
In 1947 cosmic ray workers in England demonstrated, with a new technique that used sensitive photographic emulsion, that the particles that had been observed, mainly at sea level, were not Yukawa mesons as was believed, but rather they were decay products of Yukawa mesons. These decay products were later called muons, and the Yukawa mesons were called pions. Pions are produced by collisions of cosmic rays with air molecules higher in the atmosphere, at mountain altitude or higher. The pion is an example of a class of particles having strong nuclear interaction called hadrons (including the proton and the neutron), while the muon belongs to the weakly interacting class called leptons (including the electron). After 1947, new particles of both classes were found in experimental searches in the cosmic rays and at new laboratories where large particle accelerators were constructed to create and study these particles. Among the new particles forming part of the particle explosion was a neutral pion. Its existence had been predicted in the 1930s by the Yukawa group in Japan and by Nicholas Kemmer in England, who had claimed that it was required by the known charge independence of nuclear forces, that states that the nuclear forces between any pair of nuclear particles is the same.
Yukawa's Academic Career
In 1933, Yukawa taught at Osaka Imperial University, and in 1939 he returned to Kyoto as a full professor. From 1948 to 1949 he was a member of the Institute for Advanced Study in Princeton, New Jersey. He was then a visiting professor at Columbia University in New York City until 1951, the year in which he was awarded the Nobel Prize in Physics, when he was made a full professor. In 1953, he returned to Kyoto to become the director of a new interuniversity research institute founded (and later named) in his honor. He wrote many essays on cultural and scientific subjects and participated in international movements for world federation and peace, such as the Pugwash Movement.
Brown, L. M. "Yukawa's Prediction of the Meson." Centaurus25 , 71–132 (1981).
Hayakawa, S. "The Development of Meson Physics in Japan" in Birth of Particle Physics, edited by L.M. Brown and L. Hoddeson (Cambridge University Press, Cambridge, UK,1983).
Mukherji, V. "History of the Meson Theory of Nuclear Forces from 1935 to 1952." Archive for the History of Exact Sciences13 , 27–102 (1974).
Yukawa, H. Creativity and Intuition, translated by J. Bester (Kodansha, New York, 1973).
Yukawa, H. Tabibito (The Traveler), translated by L.M. Brown and R.Y Yoshida (World Scientific, Singapore, 1982).
Laurie M. Brown
The Japanese physicist Hideki Yukawa (1907-1981) was one of the world's most highly-respected theoretical physicists. His most visible contributions to science were in the field of particle physics.
Hideki Yukawa was born in Tokyo on Jan. 23, 1907. His father was a professor of geology at Kyoto University, and Yukawa grew up in an academically-oriented household which focused his attention on science from his early years. He entered Kyoto University in 1926, and, showing his intelligence early, graduated only three years later with a master's degree. Setting out on the long journey to academic achievement, Yukawa spent the next ten years continuing his education and teaching. First came three years of research, which was followed by a 1934 appointment as a physics lecturer at Kyoto University. Next, there was a move to a lecturer's post at Osaka University, where Yukawa completed his doctorate in physics in 1938.
By this time, Yukawa was already immersed in the study of sub-atomic particles that be his focus for the rest of his life. His first paper, "On the Interaction of Elementary Particles," met with a lukewarm reception when he presented it in Osaka at the 1934 meeting of the Physico-Mathematical Society. Nevertheless, he chose to publish it the following year, in the Society's Proceedings, Yukawa's paper postulated that, as an analogy to the way in which a particle of light may be exchanged between two charged particles in the "electromagnetic interaction," a new particle (later termed the meson) might be exchanged between two nucleons in the "nuclear interaction." The meson was envisioned by Yukawa to be a particle providing the "glue" for holding together the various other particles making up the nucleus of the atom.
The theory caused great interest in scientific circles, especially after such a particle was discovered in cosmic radiation by Carl Anderson, a 1936 Nobel laureate from the California Institute of Technology. Used together by researchers, the work of Yukawa and Anderson provided a noteworthy overture to the 1939 discovery of nuclear fission. Now an eminent scientist, Yukawa took his place in 1939 as Professor of Physics at Kyoto University. He taught there during World War II, which ended abruptly for Japan in 1945 with the shattering atomic destruction of Hiroshima and Nagasaki.
The Scientist's Real Responsibility
In 1948 he was invited to spend a year at Princeton University's Institute for Advanced Study. It was here that Yukawa met Albert Einstein, with whom he remained friendly for the rest of the older scientist's life. Quoted in Yukawa's own obituary in The Bulletin of the Atomic Scientists was an excerpt from his graceful epitaph for Einstein: "I feel very strongly that we have to take up his search and striving for world peace," a mission which Yukawa himself took extremely seriously.
His feeling that at least some of the responsibility for preventing war must rest with the scientists who produce its technology was strongly expressed once again in 1962, at the Kyoto Conference of Scientists. "The results of physics are inevitably connected with the problems of humanity through their application to human society," he warned. In 1975, at the 25th Pugwash International Symposium held in Kyoto, he took this theme even further. "Usually it has been thought that, particularly in pure science, it is desirable for its progress not to include any value criterion other than true-or-false. We physicists, by experience, have realized that the advent of nuclear weapons dealt a great blow to the above-mentioned way of thinking," he noted.
Yukawa's strong feelings on science's duty to humanity had begun to crystallize by 1949, when he won the Nobel Prize. He chose to donate most of his award money to several institutions in Japan, including the Research Institute for Fundamental Physics at Kyoto University, to which he returned in 1953 after three years at Columbia University. It was time, he said, to return to Japan to train "new faces." He remained there until 1970, when he retired.
Yukawa died at his home in Kyoto in 1981.
Yukawa discussed his contributions to meson theory in his Nobel lecture, reprinted in Nobel Lectures in Physics, vol. 3 (1964). Niels H. de V. Heathcote, Nobel Prize Winners, 1901-1950 (1953), contains important biographical material. For additional insights into his work see Rudolf Ernst Peierl's, The Laws of Nature (1956); Richard M. Harbeck and Lloyd K. Johnson's, Physical Science (1965); and Samuel A. Morantz's, Physics (1969).
Bulletin of the Atomic Scientists, October 1982.
New York Times, September 10, 1981. □
Hideki Ogawa (he changed his surname upon marrying Sumi Yukawa) was born on January 23, 1907, in Tokyo. A year thereafter his family moved to Kyoto, where he was raised and attended school. The fifth of seven children of Takuji and Koyuki Ogawa, Hideki came from a family of scholars. Although not inclined to science as he grew up, Hideki happened upon some books on modern physics while in high school and soon found quantum mechanics (which was still a rapidly developing field at the time) very intriguing. As a result of that interest, he entered Kyoto University to study physics in 1926. He received his M.S. from that institution in 1929 and a Ph.D. from Osaka University in 1938.
In the 1930s the English physicist James Chadwick had discovered the neutron, and scientists were struggling to determine how protons and neutrons interacted inside a nucleus. A theory known as quantum electrodynamics explains electricity and magnetism by assuming that the force is caused by the interaction of photons with charged particles. Scientists tried to create a similar theory of nuclear forces based on the interaction of protons and neutrons with some particle analogous to photons. Yukawa developed a theory for the interaction of massive force carriers, the so-called Yukawa potential, and predicted that, since the nuclear force only acts over distances of 10−15 meters (3.281 × 10−14 feet), these unknown force carriers should have a mass about two hundred times as heavy as an electron. Yukawa published his theory in 1935, but since such a particle was unknown at the time, his results were largely ignored.
This situation changed in 1937 when a new particle was discovered in a cosmic-ray experiment. It had the correct mass, and Yukawa's theory was thought to be vindicated as a consequence. However, the details of the theory did not correspond with the measured properties of this particle. In a confusing cosmic coincidence, it turned out that particle was a muon (a heavier electronlike particle), and it was not until 1947 that the pion (as the force carrier came to be known) was discovered. Finally, all the pieces of the theory of nuclear force fell into place, and in 1949 Yukawa received the Nobel Prize in physics.
Yukawa had left Osaka in 1948 to work in the United States. However, in 1953 he returned home to Kyoto to become director of a new interuniversity research institute housed in an academic building named for him. He retired from this position in 1970 and died in Kyoto on September 8, 1981.
see also Chadwick, James.
Michael J. Fosmire
Yukawa, Hideki (1982). Tabibito: The Traveler, tr. L. Brown and R. Yoshida. Singapore: World Scientific.