Perimeter Institute has lost a long-time visiting fellow, collaborator, mentor and friend with the passing of theoretical physicist Robert Leigh on April 18 at his home in Champaign, Illinois, at the age of 61.
Only a handful of physicists of his generation achieved both the breadth of contributions and the depth of understanding that Leigh achieved, especially on the problem of quantum gravity, one of the most difficult questions in fundamental physics.
“What set him apart is that while most theoretical physicists specialize in one domain and remain within it, Rob constantly returned to foundational questions. He refused to be confined to any single intellectual silo,” said Perimeter faculty member Laurent Friedel, who worked closely with Leigh.
Leigh’s work stands out for its ability to weave connections across disciplines, Freidel added. “I cannot think of anyone under the age of 70 who combined such breadth with such depth. His influence, while already significant, remains in my opinion substantially underappreciated and will continue to grow. Like all profound ideas, it takes time for its full impact to be recognized.”
Leigh was born in Chatham, Ontario and went on to become an eminent scientist in both the United States and Canada, holding a professorship in high energy physics at University of Illinois Urbana-Champaign.
His research lay at the heart of current efforts to build a fundamental theory of matter, including quantum gravity effects. He was known for his outstanding work in string theory, supersymmetric field theory, particle physics, and early universe cosmology.
He was one of the discoverers of D-branes, which are extended dynamical objects in string theory that are fundamental to modern string theory.
“Rob’s contributions to physics were both foundational and profoundly generative, opening paths that reshaped entire areas of the field,” said Perimeter postdoctoral researcher Luca Ciambelli.
“His discovery of D-branes during his PhD was a landmark moment in the string theory revolution, with consequences that continue to reverberate across theoretical physics. His work on conformal field theory and the renormalization group in quantum field theory was equally influential, providing insights that helped advance and connect these central domains.
“In more recent years, Rob brought the same depth and originality to quantum gravity, pushing the boundaries of knowledge through new and fertile directions of inquiry,” Ciambelli added.
Leigh was often seen at Perimeter where he served as a visiting fellow, colleague, collaborator and mentor to faculty, postdoctoral researchers, and graduate students.
He published numerous important papers, including recent collaborations with Ciambelli and Freidel.
Among these recent papers, published in the Journal of High Energy Physics in 2024, was Quantum null geometry and gravity which tackled the problem of time in physics and developed a possible solution on null hypersurfaces. They also published an earlier paper, Null Raychaudhuri: canonical structure and the dressing time.
With Freidel and other collaborators, Leigh also published numerous papers including On the inevitable lightness of vacuum and Quantum spaces are modular. The latter paper revisited the notion of Euclidian space from the point of view of quantum mechanics.
As a mentor, Leigh earned the respect and admiration of his students. He had high standards for what he considered good work and he taught them to be confident theoretical physicists.
Ciambelli was finishing his PhD when he met Leigh in Paris. “Rob became an important mentor to me, and his influence shaped not only my research but also my confidence as a scientist.”
Leigh also contributed to building the Perimeter Scholars International (PSI) Master’s program at Perimeter through many years of dedicated service as an evaluator, Freidel said. “His presence helped shape both the intellectual and human fabric of the institute.”
“Rob enjoyed coming to Perimeter as a place to think deeply. It offered him a rare intellectual refuge—a space where fundamental questions could be explored seriously and without distraction. In that sense, Perimeter was not only enriched by Rob; it was also a place where he felt at home,” Freidel said.
To many at the institute, he was a frequent and deeply valued visitor, not only contributing to the intellectual activity of the Institute, but also helping to shape the conversations that connected its different research communities.
“Rob had a rare ability to recognize deep connections where others might have seen separate subjects. He brought people, ideas, and techniques into contact with one another, creating a distinctive and lasting synergy,” Ciambelli said. “He helped make the Institute a more connected, more ambitious, and more intellectually adventurous place.”
On a personal level, Leigh was also a hockey fan and a stamp collector, and later in life he amassed sizeable collections of historical postcards, postmarks, stamps, and hockey cards. He held multiple positions in the Champaign-Urbana Stamp Club.
He also contributed significantly to postal history in Canada. He was heavily involved in the Postal History Society of Canada and was its webmaster from 2006 until shortly before he passed away.
Scientifically, academically, and for everyone whose lives Leigh touched, the loss is immense.
The field of theoretical physics “has lost a profoundly creative thinker: someone with the courage, imagination, and independence to pursue new directions and to reshape the questions around him,” Ciambelli said. Moreover, he was a friend who was “always attentive, always supportive, and always present.”
Freidel also said he will miss his friend who was “reliable, trustworthy, honest—always present when it mattered.”
He shared a fond memory of a day in Banff, where Freidel, Ciambelli, and Leigh had spent a long day working together in a room. Just as they finished an important calculation, a deer came to their window. It stood there for a minute, almost as if it understood what they had just done.
“Rob’s smile that day was immense.”
About PI
Perimeter Institute is the world’s largest research hub devoted to theoretical physics. The independent Institute was founded in 1999 to foster breakthroughs in the fundamental understanding of our universe, from the smallest particles to the entire cosmos. Research at Perimeter is motivated by the understanding that fundamental science advances human knowledge and catalyzes innovation, and that today’s theoretical physics is tomorrow’s technology. Located in the Region of Waterloo, the not-for-profit Institute is a unique public-private endeavour, including the Governments of Ontario and Canada, that enables cutting-edge research, trains the next generation of scientific pioneers, and shares the power of physics through award-winning educational outreach and public engagement.
