Jim Hartle and Murray Gell-Mann, longtime friends and colleagues, in discussion at SFI's Cowan Campus in 2011. (image: InSight Foto)

James Hartle, External Professor at the Santa Fe Institute and Professor of Physics Emeritus at the University of California, Santa Barbara, passed away on May 17 in Switzerland at the age of 83. Even though he had Alzheimer’s, he continued to work on the problems that were his life’s work: the origin of the universe, how can quantum mechanics be applied to cosmology, and how to ensure that theories accurately model physical observations. Until the very end, he was passionate about physics.

Often referred to as the father of quantum cosmology, Hartle published landmark papers with game-changers in the field of theoretical physics — people like Murray Gell-Mann, Stephen Hawking, and Kip Thorne. His relationship with Gell-Mann, one of SFI’s co-founders, led to Hartle become a frequent visitor at the Institute and, in 2006, an SFI External Faculty member.

Hartle earned his Ph.D. in particle physics in 1964 from the California Institute of Technology, where Gell-Mann was his advisor. They remained friends long after both had moved away from particle physics — Hartle into cosmology and Gell-Mann into complexity. Hartle joined the UCSB faculty in 1966, mentoring students and postdocs of his own.

Sean Carroll, now an SFI Fractal Faculty member, was one of those postdocs. Carroll recounts the huge influence Hartle had on his life. “He provided a role model of a thoughtful, creative, deep-thinking scientist. He always worked on problems that he thought were the important ones, regardless of what others might have thought. And he was able to make progress on seemingly intractable issues because of the enormous care with which he approached any issue. He was an exemplar of what a physicist should be, and the field is much poorer with his passing.”

Hartle also served as a Ph.D. adviser for David Wolpert, now an SFI Professor. “Jim was my adviser for a year when I was studying quantum gravity. He always knew more than he let on. A smart, insightful, clear thinker, his papers reflected that: they were easy to read,” says Wolpert. 

Throughout his life, Hartle pushed the boundaries of quantum mechanics, cosmology, astrophysics, and general relativity. In 1983, Hartle collaborated with Stephen Hawking to publish a paper on no-boundary wave function, postulating that the shape of the cosmos is similar to that of a shuttlecock. They derived a formula to describe this shape, or the “wave function of the universe,” as encompassing all of time —past, present, and future simultaneously. Their radical reconceptualization of time, which challenged existing hypotheses on the origin of the universe, has inspired physicists for many years.

Geoffrey West, Distinguished Faculty and past President of SFI, first met Hartle through Heinz Pagels, a fellow physicist and mutual friend, when they were postdoctoral fellows. Since they shared the same academic circles, they often ran into each other at different events over the years. When West decided to immerse himself in complexity and moved to the Santa Fe Institute, he reconnected with Hartle. It was during West’s time as SFI’s President that Hartle joined the External Faculty.

Hartle had been the director of the Kavli Institute of Theoretical Physics from 1995 to 1997. When West became SFI's president in 2005, the two often discussed the challenges that come with leading a scientific institute. 

“SFI was in a rough spot, and at one point, I had sent a note saying there would be a 30 percent cut to everything, except science, which would be capped at five percent. Jim wrote to me and was extremely supportive, something I really needed then,” says West.

Hartle’s summer visits to SFI made him a beloved figure around the campus. “Exceptionally generous and polite, Jim was one of my favorite SFI people,” says SFI Professor Jessica Flack. “I wrote to him about his unconventional information gathering and utilizing systems paper only a few weeks ago. His formulation was an outgrowth of his work on the role of observers and information in quantum cosmology. It is general enough that it overlaps considerably with my own ideas about collective computation, a connection he first perceived and shared with me after a talk several years ago and that we continued to discuss by email or in person as time allowed.” 

Hartle’s visits became even more important to SFI as Murray Gell-Mann advanced in years. Their collaborative work kept Gell-Mann actively engaged in deep, profound scientific questions. They had co-authored numerous papers when Gell-Mann was alive, and Hartle continued to recognize their collaboration even after Gell-Mann died in 2019. Hartle submitted their last project — a study on decoherent histories quantum mechanics and Copenhagen quantum mechanics — on arXiv in October 2021. 

David Krakauer, President of SFI, recollects that “Jim’s brilliance, humor, and imperturbability seemed to make him immune to the usual slings and arrows of outrageous fortune. I cannot think of anyone else who could have worked as closely and as long as Jim did with Murray and retain such good cheer. Jim’s science is recorded not only in the long list of his own remarkable papers and books, but in his extraordinary collaborations, that have contributed to a greater understanding of the universe.”

The pandemic shuttered SFI’s doors to visitors, but when the lockdown was lifted and the campus opened to summer researchers again, Hartle was back at SFI, a familiar face offering profound insights to conversations in his characteristically gentle manner. He will be immensely missed.