Stephen M. Barr (born 1953) is a professor in the Department of Physics and Astronomy at the University of Delaware, and a member of its Bartol Research Institute. He does research in theoretical particle physics and cosmology. In 2011, he was elected Fellow of the American Physical Society, the citation reading "for original contributions to grand unified theories, CP violation, and baryogenesis." His notable work includes co-discovering the much studied Flipped SU(5) scheme of unification, identifying the Barr-Zee diagram as an important source of electric dipole moment for basic particles such as the electron and neutron in many theories, and proposing the so-called Nelson-Barr mechanism as a solution to the Strong CP problem. He is the author of the article on Grand Unified Theories for the Encyclopedia of Physics.
He obtained his Ph. D. in physics from Princeton University in 1978. Princeton awarded him the Charlotte Elizabeth Proctor Fellowship "for distinguished research". He went on to do research at the University of Pennsylvania as a post-doctoral fellow (1978–80), the University of Washington as a Research Assistant Professor (1980-5), and Brookhaven National Laboratory as an Associate Scientist (1985-7), before joining the faculty of the University of Delaware in 1987. He was elected Director of the Bartol Research Institute of the University of Delaware in 2011.
Barr writes and lectures frequently on the relation of science and religion. Since 2000, he has served on the Editorial Advisory Board (now the Advisory Council) of the religious intellectual journal First Things, in which many of his articles and book reviews have appeared since 1995. His writing has also appeared in National Review, The Weekly Standard, Modern Age, The Public Interest, and Commonweal. In 2002, he gave the Erasmus Lecture, sponsored by the Institute on Religion and Public Life (previous lecturers included Cardinal Ratzinger, Cardinal Lustiger, and Justice Clarence Thomas.) In 2007, he was awarded the Benemerenti Medal by Pope Benedict XVI. In 2010, he was elected a member of the Academy of Catholic Theology.
He is married to Kathleen Whitney Barr. They have five children.