Yoichipo Nambu

(b. 1921)

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(1921–) Japanese physicist

Nambu was educated at the university in his native Tokyo, serving (1945–49) as a research assistant there before being appointed professor of physics at Osaka City University. He moved to America in 1952 and, after a two-year spell at the Institute of Advanced Studies, Princeton, he joined the University of Chicago and was appointed professor of physics in 1958, a position he held until his retirement in 1991.

In 1965 Nambu, in collaboration with M. Y. Han, tackled a major problem arising from the supposed nature of quarks. Baryons, i.e., is particles that interact by the strong force and have half-integer spin, were composed of three quarks. Thus the proton consists of two up and one down quark and consequently has a configuration written uud. But some baryons are composed of three identical quarks. The omega minus (Ω) particle, for example, is composed of three strange quarks with an sss configuration. Quarks, however, are fermions and are thus governed by the Pauli exclusion principle – i.e., no two identical particles can be in the same quantum state. As three s quarks will have the same quantum number, and as their spins can be aligned in only two ways, it seemed that at least two of the s quarks of the Ω particle occupy the same state.

Nambu proposed that quarks have an extra quantum number, which can take one of three possible values. The quantum number was arbitrarily referred to as ‘color’, and the varieties equally arbitrarily as red, green, and blue. In this manner three up (uuu), down (ddd), or strange (sss) quarks could coexist without violating any quantum rules, as long as they had different colors. Nambu's work has been confirmed experimentally and is part of what is known as the standard model.

Nambu went on to consider the problem of quark confinement. How could it be, he asked, that free quarks were never encountered? When baryons decay they do not break down into quarks, but into different baryons and other particles. In response to this problem Nambu introduced string theory into physics in 1970. Particles were seen not as small spheres, but as massless rotating one-dimensional entities about 10–13 centimeters long, with an energy proportional to their length. The quarks are located at the string's ends. In the simplest case, a meson, a quark is located at one end and an antiquark at the other.

The quarks that make up a meson cannot be separated by stretching the string because the energy required rapidly increases with length. Nor would cutting the string suffice, for at the breaking point a newly created quark–antiquark pair would be created, yielding not a free quark but a further meson.

Though Nambu's string theory had its attractions as a theory of elementary particles it soon ran into other difficulties. Nonetheless, it has been revised by such theorists as John Schwarz in the form of superstring theory.

Subjects: Science and Mathematics.

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