John David Reppy was born February 16, 1931, in Lakehurst, New Jersey. His father was stationed at the US Naval Air Station, where he worked with helium as a lifting gas for naval lighter-than-air aviation. The family moved almost every year to follow his military placements, including an assignment to Pearl Harbor prior to World War II. In 1943 he was sent to the western Pacific and the rest of the family settled in Haddam Neck, Connecticut.

In Connecticut John Reppy became interested in herpetology, geology and rock climbing, exploring local quarries. He graduated from high school in 1950.^[1]

Reppy immediately enrolled at the University of Connecticut, beginning with summer school. He majored in mathematics but also began working for his thermodynamics instructor Charles Reynolds. At this time Reppy became friends with David M. Lee, who was also a student. Reppy received a bachelor's degree in math and physics in 1954 at University of Connecticut and a master's degree from the same school two years later.^[2]

In 1956, Reppy joined Cecil T. Lane's Yale Low Temperature group at Yale University. As part of his Ph.D. work, Reppy adapted a design by Jesse Beams and built an apparatus for rotating a container of liquid helium in vacuum and measuring the helium's angular momentum. He completed his dissertation in 1960,^[1] and received his Ph.D from Yale University in 1961.^[3] Reppy spent 1961 working with Nicholas Kurti in Oxford on a National Science Foundation (NSF) Fellowship.^[1]

In the 1950s and 60s, Reppy was active on Ragged Mountain in Connecticut where he collaborated on many first ascents and on publishing an area guidebook with Sam Streibert. The well-known Reppy's Crack on Cannon Mountain in New Hampshire bears his name and he has made first ascents as of number of routes in the Shawangunks and elsewhere. He made early attempts on Armadillo, a long alpine rock climb on Mount Katahdin, Maine. In addition, he has climbed extensively in England, the Alps and western America.

Reppy has said his earliest interest in rock climbing as a very young teenager was related to the re-opening of small open-pit mica mines in eastern Connecticut during World War II in response to increased war-time demand for electronics materials.^[4][5]

Reppy was among the first climbers in the United States to practice so-called "clean climbing" techniques, which he learned in England. While most climbers of the day were hammering their way up the cliffs with pitons, Reppy helped introduce the use of nuts, which at the time consisted of hex nuts from truck wheels, strung with nylon webbing. Unlike pitons, nuts are placed and removed without the use of hammers and save the rock from permanent damage. The technique often makes placements easier and faster to achieve, and thus may make difficult climbs easier to complete. Many advances in rock climbing were made possible by this method. Among a substantial number of others deserving credit for this trend in the U.S. are Yvon Chouinard and John Stannard.^[6]

Reppy returned to New Haven, Connecticut, in 1962, and spent four years as an assistant professor at Yale University. He joined the Cornell University Physics Department in 1966, becoming the John Wetherill Professor of Physics in 1987.^[3] As physics professor at Cornell University, he studies quantum properties of superfluids with an emphasis on boundary conditions and phase transitions in systems of reduced dimensionality.

Reppy's research group has close associations with David M. Lee and Robert C. Richardson also of Cornell, who shared the 1996 Nobel Prize in Physics with Douglas D. Osheroff for discoveries related to super fluidity in helium-3 ice. Lee, in his Nobel Prize speech, credited Reppy's "extraordinary technical ingenuity" in experiments leading to the discovery. In the speech, Lee made other references to Reppy, noting that his work helped confirm related insights.^[7]

Reppy also figured at least on the fringes of the 2001 Nobel Prize for Physics which was awarded to Eric Cornell and Carl Wieman of the Joint Institute for Laboratory Astrophysics, or JILA, in Boulder, Colo., and Wolfgang Ketterle of the Massachusetts Institute of Technology. Bose–Einstein condensation was predicted in 1924, and was seen decades ago in liquid helium, according to Ketterle, who acknowledged a controversial earlier claim by Reppy. Ketterle says that Reppy brought this finding to his attention, and that the priority claim was fair. "I think the results appeared conclusive", Ketterle reportedly said. Co-laureate Wieman reportedly said that Reppy's claim is "really a stretch" and that "Ketterle is being gracious".^[8]

Physicists generally agree that the atoms in superfluid helium-4 is not a Bose–Einstein condensate in the original sense of the term because its atoms interact too strongly. Reppy studied an exception: tiny amounts of helium trapped in nanometer-sized pores of a spongelike glass called Vycor. Even though the pores keep its atoms too far apart to jostle one another much, the helium still behaves like a three-dimensional fluid. In 1983 Reppy and colleagues reported results that suggested the helium was sloshing through the glass as a true Bose–Einstein condensate.

Separately, work from Cornell physics laboratories has been used to test a theory of cosmic strings, hypothetical objects, which may have been important in the formation of galaxies, and may have arisen through "phase transitions" in a fraction of a second after the Big Bang.^[9]

Reppy is the recipient of numerous awards and honors, including the Fritz London Memorial Prize in 1981 and the NASA Distinguished Public Service Medal for leadership and support to the NASA microgravity fundamental physics program in 2000. Reppy is also a member of the National Academy of Sciences (1988).^[2][10]

• "Supersolids reliant on disorder, say physicists" PhysicsWorld.com