Based at the Perimeter Institute for Theoretical Physics, Smolin has challenged some of the tenets of theoretical physics, particularly the shortcomings, as he sees them, of string theory. He followed up his 2013 book. Time Reborn, with The Singular Universe and the Reality of Time. Co-authored with Brazilian philosopher Roberto Mangabeira Unger, the book argues that time is real, and that the mathematical models of physics have their limits. His critics say he argues by metaphor.