The hypothesis that our universe is only one element among others within a larger reality, the multiverse, is being debated. Moreover, the scientific character of the hypothesis is questioned, and some compare it to a religious belief. The lecture will aim to distinguish and evaluate the different types of motivation for the hypothesis. I will take as a starting point a discussion on string theory and the “landscape problem”. String theory can be used to derive, depending on the geometrical assumptions adopted, a wide range of solutions. Some have been tempted to see this as a sign of the existence of a multiverse (a landscape of universes). I will argue that this argument is unconvincing because it exploits principles that are not strictly scientific. In a second part, it will be shown that some physical theories and cosmological models could nevertheless, in principle, associate the existence of a multiverse with an empirically detectable signature. Based on collaborative work with James Read (University of Oxford).
Baptiste Le Bihan is a philosopher with research interests in theoretical philosophy (metaphysics, philosophy of science, philosophy of physics), mainly on time and space, modality, mereology and material objects. After a PhD in metaphysics of time at the University of Rennes, he was a postdoctoral researcher in the Space and Time after Quantum Gravity project (conducted at the University of Illinois at Chicago and at the University of Geneva) and then a research associate in the project To and Fro: Scientific Metaphysics at Physics’s Frontiers in Geneva. He is now the PI of a Swiss NSF Ambizione project at the University of Geneva on the metaphysics of quantum gravity and a member of the Geneva Symmetry Group.