Lee Smolin (; born June 6, 1955) is an American theoretical physicist, a founding member of the Perimeter Institute for Theoretical Physics, an adjunct professor of physics at the University of Waterloo, and a member of the graduate faculty of the philosophy department at the University of Toronto. Smolin's 2006 book The Trouble with Physics criticized string theory's viability. He has made contributions to quantum gravity theory, in particular the approach known as loop quantum gravity. He advocates that the two primary approaches to quantum gravity, loop quantum gravity and string theory, can be reconciled as different aspects of the same underlying theory. He also advocates an alternative view on space and time that he calls temporal naturalism. His research interests also include cosmology, elementary particle theory, the foundations of quantum mechanics, and theoretical biology.
Smolin was born in New York City to Michael Smolin, an environmental and process engineer and Pauline Smolin, a playwright. Smolin said his parents were Jewish followers of the Fourth Way, founded by George Gurdjieff, an Armenian mystic. Smolin describes himself as Jewish. His brother, David M. Smolin, became a professor at the Cumberland School of Law in Birmingham, Alabama.
Smolin dropped out of Walnut Hills High School in Cincinnati, Ohio. His interest in physics began at that time when he read Einstein's reflections on the two tasks he would leave unfinished at his death: 1, to make sense of quantum mechanics, and, 2 to unify that understanding of the quanta with gravity. Smolin would take it as his "mission" to try to complete these tasks. Shortly afterward, he browsed the Physics Library at the University of Cincinnati, where he came across Louis de Broglie's pilot wave theory in French. "I still can close my eyes," Smolin wrote in Einstein's Unfinished Revolution, "and see a page of the book, displaying the equation that relates wavelength to momentum." Soon after that he would "talk his way into" Hampshire College, find great teachers, and get lucky in his applications to graduate school. As to his mission of solving Einstein's two big questions, by Smolin's account, he did not succeed; "Very unfortunately, neither has anyone else."
He held postdoctoral research positions at the Institute for Advanced Study in Princeton, New Jersey, the Kavli Institute for Theoretical Physics in Santa Barbara, and the University of Chicago, before becoming a faculty member at Yale, Syracuse, and Pennsylvania State Universities. He was a visiting scholar at the Institute for Advanced Study in 1995 and a visiting professor at Imperial College London (1999–2001), before becoming one of the founding faculty members at the Perimeter Institute in 2001. In 2026, he agreed to "pause" his relationship with the Institute in light of revelations that he had maintained contact with Jeffrey Epstein after the latter's initial conviction.
Smolin contributed to the theory of loop quantum gravity (LQG) in collaborative work with Ted Jacobson, Carlo Rovelli, Louis Crane, Abhay Ashtekar and others. LQG is an approach to the unification of quantum mechanics with general relativity which utilizes a reformulation of general relativity in the language of gauge field theories, which allows the use of techniques from particle physics, particularly the expression of fields in terms of the dynamics of loops. With Rovelli he discovered the discreteness of areas and volumes and found their natural expression in terms of a discrete description of quantum geometry in terms of spin networks. In recent years he has focused on connecting LQG to phenomenology by developing implications for experimental tests of spacetime symmetries as well as investigating ways elementary particles and their interactions could emerge from spacetime geometry.
Background independent approaches to string theory
Between 1999 and 2002, Smolin made several proposals to provide a fundamental formulation of string theory that does not depend on approximate descriptions involving classical background spacetime models.
Experimental tests of quantum gravity
Smolin is among those theorists who have proposed that the effects of quantum gravity can be experimentally probed by searching for modifications in special relativity detected in observations of high energy astrophysical phenomena, including very high energy cosmic rays and photons and neutrinos from gamma ray bursts. Among Smolin's contributions are the co-invention of doubly special relativity (with João Magueijo, independently of work by Giovanni Amelino-Camelia), and of relative locality (with Amelino-Camelia, Laurent Freidel, and Jerzy Kowalski-Glikman).
Foundations of quantum mechanics
Smolin has worked since the early 1980s on a series of proposals for hidden variables theories, which would be non-local deterministic theories which would give a precise description of individual quantum phenomena. In recent years, he has pioneered two new approaches to the interpretation of quantum mechanics suggested by his work on the reality of time, called the real ensemble interpretation and the principle of precedence.
Cosmological natural selection
Smolin's hypothesis of cosmological natural selection, also called the fecund universes theory, suggests that a process analogous to biological natural selection applies at the grandest of scales. Smolin published the idea in 1992 and summarized it in a book aimed at a lay audience called The Life of the Cosmos.
Black holes have a role in this natural selection. In fecund theory, a collapsing black hole causes the emergence of a new universe on the "other side", whose fundamental constant parameters (masses of elementary particles, Planck constant, elementary charge, and so forth) may differ slightly from the universe where the black hole collapsed. Each universe gives rise to as many new universes—its "offspring"—as it has black holes, giving an evolutionary advantage to universes in which black holes are common, which are similar to our own. The theory thus explains why our universe appears "fine-tuned" for the emergence of life as we know it. Because the theory applies the evolutionary concepts of "reproduction", "mutation", and "selection" to universes, it is formally analogous to models of population biology.
When Smolin published the theory in 1992, he proposed as a prediction of his theory that no neutron star should exist with a mass of more than 1.6 times the mass of the sun. Later this figure was raised to two solar masses following more precise modeling of neutron star interiors by nuclear astrophysicists. Smolin also predicted that inflation, if true, must only be in its simplest form, governed by a single field and parameter.
Contributions to the philosophy of physics
Smolin has contributed to the philosophy of physics through a series of papers and books that advocate the relational, or Leibnizian, view of space and time. Since 2006, he has collaborated with the Brazilian philosopher and Harvard Law School professor Roberto Mangabeira Unger on the issues of the reality of time and the evolution of laws; in 2014 they published a book, its two parts being written separately.
A book length exposition of Smolin's philosophical views appeared in April 2013. Time Reborn argues that physical science has made time unreal while, as Smolin insists, it is the most fundamental feature of reality: "Space may be an illusion, but time must be real" (p. 179). An adequate description according to him would give a Leibnizian universe: indiscernibles would not be admitted and every difference should correspond to some other difference, as the principle of sufficient reason would have it. A few months later a more concise text was made available in a paper with the title Temporal Naturalism.