Place: United States of America
Subject: biography, physics
US theoretical physicist who was awarded the 1969 Nobel Prize for Physics for his work on the classification and interactions of subatomic particles. He is one of the originators of the quark hypothesis.
Gell-Mann was born in New York City on 15 September 1929. He showed strong scholastic aptitude at an early age and entered Yale University when he was only 15 years old. He received his bachelor's degree in 1948 and proceeded to the Massachusetts Institute of Technology, where he earned his PhD three years later. He spent the next year as a member of the Institute for Advanced Studies at Princeton University. In 1952 he joined the faculty of the University of Chicago as an instructor to work at the Institute for Nuclear Studies under Enrico Fermi, becoming an assistant professor in 1953. In 1955, Gell-Mann moved to the California Institute of Technology at Pasadena as associate professor of theoretical physics, becoming professor a year later and, in 1966, Robert Andrews Millikan Professor of Theoretical Physics (emeritus from 1993), a position that he still holds. He is also on the faculty of the Santa Fe Institute, which is concerned with the interdisciplinary field of complexity theory.
From the early 1930s, the relatively simple concept of the atom as being composed of only electrons and protons began to give way to more complex models involving neutrons and then other particles. By the 1950s, the field was in a state of complete chaos following the discovery of mesons, which have masses intermediate between a proton and an electron, and then hyperons, which are even more massive than the proton. One of the most puzzling things was that the hyperons and some mesons had much longer lifetimes than was predicted by accepted theory at that time, although still only of the order of 10−9 seconds.
In an endeavour to bring order to the subject, Gell-Mann proposed in 1953 that the long-lived particles, and indeed other particles such as the neutron and the proton, should be given a new quantum number called the strangeness number. The strangeness number differed from particle to particle. It could be 0, −1, +1, −2, etc. He also proposed the law of conservation of strangeness, which states that the total strangeness must be conserved on both sides of an equation describing a strong or an electromagnetic interaction but not a weak interaction. Similar ideas were also proposed independently by Nishijima Kazuhiko in Japan.
The law of conservation of strangeness formed an important theoretical basis for the subsequent theory of associated production, which was proposed by Gell-Mann in 1955. This model held that the strong forces that were responsible for the creation of strange particles could create them only in groups of two at a time (that is, in pairs). As the partners in a pair moved apart, the energy that would be required for them to decay through strong interactions would exceed the energy that had originally gone into their creation. The strange particles therefore survived long enough to decay through weak interactions instead. This model explained their unusually extended lifespan.
Gell-Mann used these rules to group mesons, nucleons (neutrons and protons), and hyperons, and was thereby able to form predictions in the same way that Dmitri Mendeleyev had been able to make predictions about the chemical elements once he had constructed the periodic table. Gell-Mann's prediction of the existence of a particle, which he named xi-zero, to complete a doublet with xi-minus was soon rewarded with experimental verification.
The law of conservation of strangeness was also important in the formulation of SU(3) symmetry, a scheme for classifying strongly interacting particles that Gell-Mann proposed in 1961. This classification system itself formed part of the basis for yet another classification scheme entitled the eightfold way (named after the eight virtues of Buddhism). This model was devised by Israeli physicist Yuval Ne'emann (1925-2006) and Gell-Mann, and published in 1962.
The eightfold way was intended to incorporate all the new particles and the new quantum numbers. It postulated the existence of supermultiplets, or groups of eight particles that have the same spin value but different values for charge, isotopic spin, mass, and strangeness. The model also predicts the existence of supermultiplets of different sizes. The strongest support for the theory arose from the discovery in 1964 of a particle named omega-minus, which had been predicted by the eightfold way.
In 1964 Gell-Mann proposed a model for yet a further level of complexity within the atom. He proposed that particles such as the proton are not themselves fundamental but are composed of quarks. Quarks differ from all previously proposed subatomic particles because they have fractional charges of, for instance, +2/3 or −1/3. A similar theory was proposed independently by George Zweig, who called the hypothetical particles ‘aces’.
Quarks always occur in pairs or in trios and can never be detected singly, exchanging gluons that bind them together. Gluons are the quanta for interactions between quarks, a process that goes by the exotic name of quantum chromodynamics, and their behaviour is in some ways analogous to the exchange of photons between electrons in electromagnetic interactions (quantum electrodynamics). Six quarks have been predicted, and all, by 1994, had been indirectly detected. The names of the quarks are up, down, strange, charm, bottom, and top, and they have been ascribed properties of electric charge, strangeness, charm, bottomness, topness, baryon number, and so on.
Gell-Mann's work has been characterized by originality and bold synthesis. His models have been useful not only for their predictive value, but also for the work they have spurred others to do.
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