Leon Cooper

A Bronx childhood

Leon Neil Cooper was born Leon Kupchik in the Bronx, New York City, on February 28, 1930. His father, Irving Kupchik, had come to America from Belarus after the Russian Revolution of 1917; his mother, Anna Zola, came from Poland. She died when Leon was seven. When his father remarried, he changed the family name to Cooper.

Leon was one of the bright city children who won places at the Bronx High School of Science, a public school that would produce an astonishing number of Nobel laureates. He graduated in 1947 and went on to Columbia University, taking his bachelor’s degree in 1951, his master’s in 1953 and his doctorate in physics in 1954. His thesis dealt with muonic atoms, exotic atoms in which a heavy cousin of the electron orbits the nucleus, and what they reveal about the size of the nucleus.

A forty-year-old mystery

After a year at the Institute for Advanced Study in Princeton, Cooper was recruited by John Bardeen of the University of Illinois, co-inventor of the transistor, who was looking for a young theorist to help him attack one of the great unsolved problems of physics. In 1911 the Dutch physicist Heike Kamerlingh Onnes had found that mercury, cooled close to absolute zero, lost all electrical resistance. Other metals did the same. A current started in a ring of superconducting wire could flow for years without fading, and superconductors expelled magnetic fields. For more than four decades, no one could explain why.

Cooper moved to Illinois in 1955 as a research associate, joining Bardeen and his graduate student J. Robert Schrieffer. The puzzle was that electrons repel one another, and in an ordinary metal they constantly bump into the vibrating atoms of the crystal, which is what causes resistance. Something had to organise them into a state where they moved together without losing energy.

Cooper pairs

In 1956 Cooper found the key. In a paper in Physical Review, “Bound Electron Pairs in a Degenerate Fermi Gas,” he showed that even a very weak attraction between two electrons, of a kind that the vibrations of the crystal lattice can provide, is enough to bind them into a pair when they move among all the other electrons of a metal. One electron slightly distorts the lattice as it passes, and the distortion attracts a second electron. These partnerships became known as Cooper pairs.

The next year Bardeen, Cooper and Schrieffer built a complete theory on that idea. In 1957 they showed that, below a certain temperature, vast numbers of Cooper pairs condense into a single collective quantum state. Breaking up one pair would cost a definite amount of energy, called the energy gap, so ordinary bumps with the lattice cannot slow them down. The current flows with no resistance at all. The theory, known by their initials as BCS, explained the vanishing resistance, the expulsion of magnetic fields, the energy gap and the way the critical temperature depends on the mass of the atoms. Cooper was in his twenties when the work was done.

What superconductivity does for people

BCS theory explains the conventional superconductors, such as mercury, tin and the niobium–titanium alloy used in most superconducting magnets. Those magnets are at the heart of the MRI scanners found in hospitals around the world, which produce detailed images of soft tissue without X-rays, and they are also used in NMR machines that chemists and drug designers use to work out the structure of molecules, and in particle accelerators such as the Large Hadron Collider. By one estimate, MRI systems account for about four fifths of the roughly six billion dollars of economic activity that depended on superconductivity in 2014.

The theory reached far beyond metals. The idea of pairing was taken up in nuclear physics to explain the behaviour of protons and neutrons in nuclei, and the concept of a symmetry spontaneously broken by a condensed state helped inspire ideas in particle physics. The Josephson effect, used in ultra-sensitive magnetic sensors and in superconducting quantum computers, was predicted using the framework that BCS provided. Bardeen, Cooper and Schrieffer shared the Nobel Prize in Physics in 1972.

Fifty years at Brown

After a year at Ohio State University, Cooper joined Brown University in Providence, Rhode Island, in 1958, and stayed for the rest of his life. He became Henry Ledyard Goddard University Professor in 1966 and Thomas J. Watson Sr. Professor of Science in 1974, and he retired from teaching only in 2014, at 84, while continuing his research.

He was a devoted teacher. He wrote an introductory textbook, An Introduction to the Meaning and Structure of Physics, and taught an unusual course built around Michael Frayn’s play Copenhagen, with a historian and a theatre professor. A former student remembered his quantum mechanics course as an example of how physics ought to be taught. Colleagues said he wore his fame lightly and went out of his way to help graduate students from abroad settle into American campus life.

From superconductors to the brain

After BCS, Cooper turned to a question he found even more fascinating: how does the brain learn and remember? In 1973 he founded Brown’s Center for Neural Science, later the Institute for Brain and Neural Systems, and served as its first director, bringing together mathematicians, biologists, linguists and physicists. It helped lay the foundation for the university’s present Carney Institute for Brain Science.

In 1982, with his doctoral students Elie Bienenstock and Paul Munro, he published what became known as the BCM theory, one of the first mathematical models to show how the strengthening and weakening of connections between neurons could produce learning. It was designed to explain how the visual cortex develops as an animal learns to see. In 1975 he and Charles Elbaum co-founded Nestor, a company that applied artificial neural networks to recognising patterns, and in 1994 Nestor worked with Intel on an early neural-network chip.

Jewish roots

Cooper was the son of Jewish immigrants: his father came from Belarus and his mother from Poland. Like many children of that generation he grew up in the Bronx, then one of the great Jewish neighbourhoods of New York, and rose through its free public schools to Columbia. The change of the family name from Kupchik to Cooper was typical of immigrant families seeking to fit into American life. He is listed among Jewish Nobel laureates by the Jewish Virtual Library.

He did not write publicly about religious practice, and the sources describe his outlook as broadly humanist: he thought of science and art as kindred creative enterprises and liked to compare building physics to building a cathedral.

The man

Cooper had wide interests. He spoke excellent French, read widely in French and English literature, and loved French culture; he lectured at the Collège de France in 1977. On campus he was known for driving a sporty 1968 Chevrolet Camaro convertible. He published a collection of essays, Science and Human Experience, in 2014. According to The New York Times, the character Sheldon Cooper in the television series The Big Bang Theory was named in part after him.

He married first Martha Kennedy, with whom he had two daughters, Coralie and Kathleen, and in 1969 Kay Allard. He received the Comstock Prize, the Descartes Medal and Brown’s highest faculty honour, the Rosenberger Medal, and was a member of the National Academy of Sciences, the American Philosophical Society and the American Academy of Arts and Sciences.

Legacy

Leon Cooper died at his home in Providence on October 23, 2024, at the age of 94. His idea of the Cooper pair solved one of the longest-standing puzzles in physics and gave the world a working theory of superconductivity, the phenomenon behind MRI scanners, research magnets and the quantum devices now being built for computing. His second career helped turn the study of learning and memory into a mathematical science. When John Hopfield received the Nobel Prize in Physics in 2024 for work on neural networks, Brown noted that Hopfield had cited Cooper’s work on how neurons store information.

Sources & further reading

Leon N. Cooper, Biographical, NobelPrize.org · Passages: Nobel Prize winner and Brown professor of physics Leon Cooper, Brown University · Superconductivity theorist Leon Cooper dies aged 94, Physics World · Leon Cooper, Wikipedia · Superconducting magnet, Wikipedia · Leon Cooper, Jewish Virtual Library · Researched from the sources above and checked against them. Spotted an error? Tell us.

A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.