Hideki Yukawa first postulated the existence of mesons, which later became known as pi-mesons or pions, as fundamental carriers of the nuclear force. His prediction of the particles’ existence was confirmed in 1948, when they were discovered by Cecil Frank Powell. Moreover, although the particles later proved not to be fundamental, they were still intimately connected with the nuclear force, largely confirming Yukawa’s other prediction.
The existence of positively charged nuclear states was experimentally confirmed by physicists working between 1910 and 1920. They found that atomic nuclei contained several positive charges, which seemed to indicate that the centers of atoms should fly apart as a result of electromagnetic coulomb repulsion. The unexplained failure of atomic nuclei to explode became a foremost concern of theoretical physicists.
In 1932, three new particles—the neutron,
At the age of twenty-two, Hideki Yukawa set two goals for himself: to investigate the quantum mechanics of the atomic nucleus and to develop relativistic quantum mechanics. Quantum mechanics
By 1932, Yukawa had arrived at the notion that the nuclear force (that is, the force holding nuclei together despite their electromagnetic repulsion) was a primary force and not derivable from electromagnetism or gravity. This was a break with most thinking in theoretical physics, which was attempting to explain nuclear structure as some combination of positive charges (protons) and negative charges (electrons). Unfortunately, there were a considerable number of problems associated with Yukawa’s theory. First, electrons and protons each have a “spin” of ½, as does the neutron. If one takes as an example the deuteron, the simplest compound nucleus in nature, one finds that it has a charge of +1 and a spin of 1. If it were composed of two protons and one electron, it would have a charge of +1, but its spin would be either ½ or 3/2, because spins, like charges, are arithmetically additive.
In October, 1934, Yukawa became convinced that a meson was responsible for nuclear forces. Mesons (“middle ones”) originally were defined as being midsized or middleweights, as compared with lightweight leptons (“light ones”) or heavyweight baryons (“massive ones”). Yukawa realized that the small range of nuclear forces required a force carrier of a particular mass. Nuclear forces, as was known at the time, had a range of the order of only 0.02 trillionth of a centimeter, or 2 × 10
–15
meters. Yukawa saw that there was an inverse relationship between mass and distance or range. A small-mass particle such as the electron acted over distances hundreds of times greater than the diameter of the nucleus. Protons and neutrons, being relatively massive, acted over distances of the order of 1/10 that diameter.
In November, 1934, Yukawa presented his initial paper on the existence of such mesons to the Osaka branch of the Physico-Mathematical Society of Japan. Yukawa’s paper, which later earned him the Nobel Prize in Physics,
Yukawa viewed the nuclear force as a quantum effect whereby a sizable quantum of energy was exchanged between the proton and the neutron in the nucleus. His original paper called for only two such quanta, the pi-plus and the pi-minus. He did not predict the existence of the pi-naught. These pions, or nuclear quanta, have 0 spin and thus contribute nothing to the nuclear spin states of the particles they compose. This 0 spin feature makes them “bosons,” obeying Bose-Einstein statistics, which in turn allows two or more to exist in the same quantum states within a specific nucleus.
In 1937, two American physicists, Seth Neddermeyer
In 1948, Cecil Frank Powell and his coworkers at Bristol, England, found pi-plus and pi-minus meson tracks in photographic emulsions left for several months on mountain tops. Only the year before, in 1947, the American physicist J. Robert Oppenheimer
With Powell’s discovery confirming his prediction, Yukawa had fulfilled his first research objective in bringing quantum theory into nuclear physics to explain nuclear structure. That he had done so in such a spectacular way earned for him the 1949 Nobel Prize in Physics and elevated the reputation of Japanese physics in the worldwide scientific community. Powell himself won the award in 1950 for developing the photographic emulsion technique of detecting subatomic particles. Yukawa was able to enter the scientific elite by predicting theoretically the existence of a clearly detectable family of subatomic particles whose mass, spin, and charge he was able to describe. Seldom if ever has such a clear, incisive, and elegant prediction been verified in physics. Pions proved to be universal carriers of nuclear force at low energies (or residual strong force) and largely responsible for holding nuclei together.
Yukawa’s paper did not catch the attention of the physics community until the discovery of muons two years later. Yukawa wrote a letter to Nature, a prestigious science journal in England, stating that this discovery might be the particles that he had predicted. The editors rejected his paper and claim, but he published a note in a Japanese journal that established precedence for his claim. The discovery of muons at several places throughout the world in the next few years motivated physicists to look for explanations of what they might be, and Yukawa’s mesons seemed close enough to focus the attention of many theorists.
Yukawa’s paper clearly delineated nuclear forces from the electromagnetic and gravitational forces of nature and laid the theoretical groundwork for accounting for the behavior of protons in the nucleus by recourse to a separate fundamental force. Later physicists would complicate Yukawa’s model, especially once it came to be believed that pions are not themselves fundamental particles but are made up of such particles, known as quarks. A model then developed in which “strong nuclear force” referred to a force—carried by particles called gluons—that holds quarks together, while the force holding protons and neutrons together came to be seen as a side effect of strong force, known as simply “nuclear force,” or sometimes “strong residual force.” However, even within this more complicated model, it was still believed that pions were largely responsible for binding protons and neutrons together, making Yukawa’s prediction all the more impressive.
Physicists now recognize hundreds of mesons, of which Yukawa’s pions were the first predicted types. Newer and higher-energy particle accelerators have caused the number of known mesons to rise almost exponentially every decade since Yukawa proposed them. More than two hundred mesons are firmly established, and more have given experimental hints of existing. Most experimental physicists expect that many more mesons will be discovered in the future as newer and more energetic particle accelerators are built. Many of the newer mesons are very heavy, much heavier than protons and neutrons; therefore, the word “meson” has lost its middleweight meaning and now designates any integral spin particle that undergoes the strong nuclear interaction, as defined by Yukawa in 1935. Modern elementary physics owes its existence to Yukawa’s efforts in predicting pions.
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Böhm, Manfred, Ansgar Denner, and Hans Joos. Gauge Theories of the Strong and Electroweak Interaction. Stuttgart: Teubner, 2001. Details the place of pions within the complex set of subatomic interactions carrying the fundamental forces of nature. Bibliographic references and index. -
Brown, Laurie M. “Hideki Yukawa and the Meson Theory.” Physics Today 39 (December, 1986): 55-62. This brief article describes how Yukawa’s ideas contrasted with and were received by physicists of the time. -
Fuchs, Walter R. Physics for the Modern Mind. Translated by M. Wilson and M. Wheaton. New York: Macmillan. 1967. Chapter 7 is devoted to elementary particles but gives a series of models of how Yukawa’s meson exchange works between protons and neutrons in nuclei by considering pions as balls in a ballgame. Excellent descriptions and graphics for precollege-age students. -
Nobel Foundation. Physics. Vol. 3. New York: Elsevier, 1964. Contains Yukawa’s brief Nobel acceptance speech and a good motivation essay on his work. The Yukawa article is followed by one on Powell, the experimental discoverer of the pions; it sheds considerable perspective on Yukawa’s insights. -
Stuewer, Roger E., ed. Nuclear Physics in Retrospect. Minneapolis: University of Minnesota Press, 1979. A good reference for physics students and those interested in majoring in physics in college. Gives a good view of Yukawa’s contribution as seen by a fellow nuclear theorist. -
Weinberg, Steven. The Discovery of Subatomic Particles. Rev. ed. New York: Cambridge University Press, 2003. Discusses the discovery and importance of pions alongside muons and W and Z particles (the carriers of weak nuclear force). Bibliographic references and index. -
Yukawa, Hideki. Tabibito (The Traveler). Translated by L. Brown and R. Yoshida. Singapore: World Scientific, 1982. This book is a personal testament of Yukawa from his early childhood until he published his mesonic theory of nuclear forces in 1935. Includes an excellent description of how working Japanese academic scientists lived and worked before World War II.
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