Malvin “Mal” Ruderman ’45, Physics Professor and Researcher

Mal Ruderman ’45, a popular professor whose contributions to physics and astrophysics were wide and deep, died on July 20, 2024, in New York City. He was 97. Ruderman, the Centennial Professor Emeritus of Physics, taught at Columbia for more than five decades — from 1969 until the Covid-19 epidemic started in 2020; he retired in 2022.

A man of warm nature and infectious enthusiasm, Ruderman was known to phone students at unusual hours to share a new idea. He also would frequently drop into a colleague’s office and ask, “Have you got 5 minutes?” and then spend several hours at the colleague’s blackboard explaining his latest idea on the superfluid dynamics inside a neutron star or on the geometry of a neutron star’s magnetic field.

For many years Ruderman taught the class “Order of Magnitude Physics,” known as one of the fun classes among Ph.D. candidates. Homework problems required students to estimate the number of piano tuners within 10 blocks of campus, or to calculate the time to cook a Thanksgiving turkey. Many homework sets were returned from grading with the comment, “You burned the turkey!”

Born on March 25, 1927, in New York City, Ruderman completed a Ph.D. in physics at Caltech in 1951 under the supervision of Robert J. Finkelstein; his thesis was titled Electron Decay of the Pion. He began his teaching career at UC Berkeley in 1953 before moving back to his hometown in 1964 to become a professor at NYU. He shifted to Columbia in 1969.

Ruderman chaired the Department of Physics 1973–75. He was named a Guggenheim Fellow in 1957, and in 1972 he was elected to the National Academy of Sciences, followed in 1996 by election to the American Philosophical Society. Ruderman was a founding member of the elite “JASON” group, whose goal is to provide independent scientific and technical expertise to the U.S. intelligence and defense communities.

Some of Ruderman’s earliest work came in the field of condensed matter physics, and he collaborated with many other noteworthy physicists. In 1954, with Charles Kittel, Ruderman discovered what is now called the RKKY interaction between nuclear magnetic moments in metals (independently discovered at the same time by Naohiro Kasuya and Naoki Yosida). This interaction is essential to our understanding of magnetic metals and is of fundamental importance to condensed matter physics as the first demonstration of how singularities associated with the Fermi surface could produce qualitatively new effects.

Ruderman’s research later turned toward astrophysics, particularly the collapsed objects known as neutron stars, which were discovered as radio pulsars in 1967. A year later, he was the first to realize that the outer layers of neutron stars were crystallized and formed a solid “crust.” In 1969 he was the first to propose that the sudden speed-up in rotation of the Vela pulsar was due to cracking of the star’s solid crust (a so-called “star quake”).

Later that year, with Gordon Baym, Christopher J. Pethick and David Pines, Ruderman was the first to interpret pulsar glitches as evidence of nuclear superfluidity in neutron star interiors. In 1975, with Peter Sutherland, he published his seminal work on pulsar emission, Theory of Pulsars: Polar Gaps, Sparks, and Coherent Microwave Radiation. This paper laid the theoretical framework for all future models of electron-positron pair production and coherent emission above pulsar polar caps and remains among the most influential and highly cited works on pulsar emission.

In 1977, with Elliott Flowers, Ruderman discovered an instability of magnetized stars, now known as the “Flowers-Ruderman Instability,” which has deep implications for the topology of magnetic fields in stellar interiors. In 1982, with Mehmet Ali Alpar, Meng Cheng and Jacob Shaham, he proposed that weakly magnetized neutron stars could be spun up to millisecond rotation periods by accretion from a binary companion. It remains the most widely accepted theory for the origin of millisecond pulsars.

Ruderman was a creative thinker who inspired generations of young astrophysicists and was deeply respected by his colleagues. He is survived by his wife, Paula SW’77; sons, Peter LAW’85 and Robert; daughter, Nina; three grandchildren; and brother, Neil ’56.

— Alex Sachare ’71