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Friedman explains role of quarks, in Killian talk

After more than a decade of disagreement and doubt in the scientific community, the quark is finally accepted as a basic building block of nature, says Institute Professor Jerome I. Friedman, Nobel laureate, professor of physics and the 2001 James R. Killian Jr. lecturer.

Professor Friedman, one of a team of physicists who proved these tiny components of the proton are real, said now that the battle of the quark is over, the next step is to learn more about the particle's structure. Does it have a measurable size? And if so, does it have internal constituents? It is conceivable, he said, that the next generation collider, the CERN Large Hadron Collider, which will be completed in 2005, could uncover radically new information about this quirky subatomic particle.

He delivered the Killian lecture, "Are We Really Made of Quarks?" to a packed Wong Auditorium on March 20.

"What brought me into science was a great curiosity about how the universe works," Professor Friedman said. "I must say, if I look back over my 40 years in the field, I find that I am as fascinated as ever, if not more so. I wish I were 20 years younger so I can keep participating in this great project."

"Jerome Friedman is one of the giants of physics and, in his self-effacing manner, one of the gentle giants of MIT. His extraordinary accomplishments make him a worthy recipient of the James R. Killian Jr. Faculty Achievement Award," wrote the selection committee chaired by John de Monchaux, professor of urban studies and planning.


Ever since the modern era of atomic theory began in the 18th century, researchers have been unearthing clues about tinier and tinier constituents of the atom.

By the 1960s, technology such as high-energy particle accelerators and bubble chambers, which enabled scientists to observe particle collisions in great detail, helped unearth dozens of new particles.

Nobel laureate Murray Gell-Mann's "eightfold way" theory, also proposed by Yuval Ne'eman, brought order to the chaos created by the discovery of some 100 particles. Then he and (independently) George Zweig found that this successful classification scheme implied that all of those particles, including the neutron and proton, are composed of fundamental building blocks that Gell-Mann named "quarks."

The hunt was on. Scientists searched for them at acclerators, looked in seawater, air, cosmic rays and in the Earth. Not a quark was found. Even Gell-Mann admitted it was difficult to believe in quarks, and wrote that even if quarks were not real, they were still a useful mathematical tool.

Professor Friedman, the late Professor Henry Kendall of MIT, Professor Richard Taylor of the Stanford Linear Accelerator Center (SLAC) and a team of researchers from MIT and SLAC performed a series of electron-scattering experiments over seven years that provided the first direct evidence that there are point-like objects inside the proton.

But these objects were smaller than could be measured, and still there was disagreement. Sure, there were things inside protons, but were they quarks?

The comparison of electrons and neutrinos scattering off these point-like particles unequivocally demonstrated that these particles have the fractional charges assigned to quarks. The physics community was finally forced by inescapable and compelling evidence to accept quarks.

There are six kinds of quarks: up, down, strange, charm, bottom and top. All decay into up and down quarks, and these comprise matter as we know it.


Scientists are now exploring the almost inconceivably powerful fields that hold these tiny bits together.

"The quark, like the electron, is point-like, so it occupies almost no space," Professor Friedman said. "We are made up of quarks, so we are mostly empty space, but this empty space is filled with fields.

"If you added up the weight of the up quarks, the down quarks and the electrons that make up an 150-pound individual, it would come to around three pounds. What accounts for the rest of the weight?

"The quarks are moving around so fast that they give us weight. As Einstein pointed out in E=mc2, energy is equivalent to mass." About half of our mass comes from the motion of the quarks, and about half from the force field that holds the quarks together in the proton. Fifteen tons of force hold two adjacent quarks together.

"If I push on this table, the electrons in my hand are being repelled by the fields in this table. Solidity comes from fields," Professor Friedman said.

The Killian faculty achievement award was established in 1971 "to recognize extraordinary professional accomplishments by full-time members of the MIT faculty." It is the greatest honor the faculty can bestow on one of its members.

A version of this article appeared in MIT Tech Talk on April 4, 2001.

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