If Sean Carroll is right, this interview has already happened. Not just once, but infinitely many times. In an earlier cycle of the cosmos, they met in Dublin, asked and answered these exact same questions, and discussed the unsettling possibility that the universe is destined to repeat itself exactly.
Carroll, a quantum cosmologist and philosopher at Johns Hopkins University in Baltimore, Maryland, investigates one of physics' deepest questions: how gravity and the classical universe we know emerge from the strange rules of quantum mechanics. Unlike most cosmologists, who begin with the universe we observe and try to work backwards, Carroll starts from the vast landscape of all quantum possibilities, essentially taking quantum theory itself as his guide.
Against his better judgement, this approach has led him toward a cosmos that continuously expands and then contracts before a new cycle begins. Cyclic universes are nothing new, but Carroll's proposal has a peculiar twist. In most of these models, each cycle differs from the previous one. In contrast, his cycles are exactly the same: each galaxy reforms, each life develops anew, and each conversation repeats word for word.
This bizarre picture also offers a possible solution to an uncomfortable problem involving so-called Boltzmann brains. In many models of universes that persist long enough, self-aware brains with false memories should arise, to the point where they would outnumber ordinary minds. Carroll recently conducted research on this topic with his collaborators at Johns Hopkins University, Nadiia Diachenko and Saakshi Dulani.
Carroll's approach begins with quantum mechanics rather than the universe described by general relativity. According to Carroll, quantum mechanics is the most fundamental perspective we have on the world. His strategy consists of beginning with quantum mechanics and deriving the observed world from it, rather than starting with the observed world and trying to quantise it, as is done traditionally in physics classes.
