James Hartle 1939–2023

9 August 2023
Jim Hartle

James Burkett Hartle passed away on 17 May in Zurich at the age of 83. Known as the father of quantum cosmology, Jim made landmark contributions to our understanding of the origin of the universe.

Born in Baltimore, Maryland, Jim obtained his undergraduate degree in physics at Princeton University, where he was mentored by John Wheeler. He attended graduate school at Caltech where he worked under Murray Gell-Mann, earning his PhD in 1964 with a dissertation entitled The complex angular momentum in three-particle potential scattering.

After graduating, Jim briefly taught at Princeton before joining the faculty at the University of California, Santa Barbara (UCSB) in 1966. Excited by the discoveries of pulsars, quasars and the cosmic microwave background radiation, Jim turned away from particle physics. In the late 1960s he wrote a series of influential papers, one with Kip Thorne, on the dynamics of rotating neutron stars. The pair organised regular gatherings between their research groups, which turned into the Pacific Coast Gravity Meetings that still run today.

In 1971 Jim used a Sloan Fellowship to go to the University of Cambridge, where he was immersed in the emerging fields of relativistic astrophysics and cosmology. There he met Stephen Hawking, with whom he developed a remarkable long-term collaboration. Two of their papers became classics: one, in 1976, introduced the Hartle–Hawking quantum state for matter outside a black hole, which is fundamental to black-hole thermodynamics and inspired the so-called Euclidean approach to quantum gravity; the other, in 1983, put forward the Hartle–Hawking “no-boundary” wave function of the universe, showing for the first time how the conditions at the Big Bang could be determined by physical theory.

Except for a brief appointment at the University of Chicago, Jim spent his entire career at UCSB, an environment he found congenial, supportive and inspiring. Jim was a wise and caring mentor to countless young scientists and, though reluctant to venture into the public arena, he also did much to forge a strong physics community. In 1979 he cofounded the Institute for Theoretical Physics (now the Kavli Institute for Theoretical Physics) at Santa Barbara, a mecca for physicists ever since.

The Hartle–Hawking wave function not only revolutionised quantum cosmology but also raised tantalising new questions. Jim began to think more deeply about what it entails to apply quantum mechanics to the universe as a whole. Throughout the 1990s, he and Gell-Mann developed the consistent-histories formulation of quantum mechanics, which clarified the physical nature of the branching process in Everettian quantum mechanics and was sufficiently general to describe single closed systems.

While part of some extraordinary collaborations, Jim was also an independent thinker. About one-third of his publications are beautifully written single-author papers often touching on seemingly intractable questions, far from current fashions and approached with enormous care and open-mindedness. In 2003 Jim published Gravity: An Introduction to Einstein’s General Relativity, a textbook gem with a minimum of new mathematics and a wealth of illustrations that made Einstein’s theory accessible to nearly all physics majors.

Jim retired in 2005, to focus on physics. In 2006 he became an external professor at the Santa Fe Institute, collaborating with Gell-Mann during summer visits. That year also marks the start of my own collaboration with Jim. We took up quantum cosmology again and became immersed in some of the field’s heated debates. Unperturbed, Jim set out the beacons. Often, we would be joined by Hawking, who by then had great difficulties communicating, to flesh out the predictions of the no-boundary wave function. Studying the role of the observer in a quantum universe, we were led to a top-down approach to cosmology in which quantum observations retroactively determine the outcome of the Big Bang, thereby realising an old vision of Wheeler’s.

Few scholars ventured as deeply into the fundamentals of physics as Jim did. A selection of his reflections on the deeper nature of physical theory were published in 2021 in The Quantum Universe: Essays on Quantum Mechanics, Quantum Cosmology, and Physics in General. With characteristic humility, however, Jim reminded us that he didn’t have a philosophical agenda.

Despite suffering the devastations of Alzheimer’s disease, physics remained the driving force in Jim’s life until the very end. Yet his intellectual curiosity stretched much further. He was a polymath and an eclectic reader whose interests ranged from Middle Eastern and Mayan archaeology, to American colonial history, Russian literature and eccentric 19th-century religious female figures. Above all, Jim was an exceptionally generous, wise, humble and gentle man.

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