Pablo Arrighi: The complete story of a toy universe

Speaker: Pablo Arrighi, Inria Saclay Title: The complete story of a toy universe Abstract: The laws of Physics are time-reversible---yet we can only go towards the future. This apparent contradiction is known as the "arrow of time problem". Its resolution is that the future is the direction of increasing entropy. But entropy can only increase towards the future... if it was low in the past, and past low entropy is a very strong hypothesis to make, because low entropy states are so improbable! Some authors suggest that we can do away with this "past hypothesis" within reversible dynamical laws featuring expansion à la GR. We prove that this is indeed the case in principle, within a toy model. It consists in just graphs upon which particles circulate and interact according to local reversible rules. Some rules locally shrink or expand the graph. We prove that almost all states expand and that entropy always increases as a consequence. Interestingly, the toy universe can easily be run backwards, until "before the Big Bang". We can therefore tell the story of "the beginnings of times" in this model. It's enlightening... and mind-bending at the same time. Joint work with Gilles Dowek and Amélia Durbec QISS Virtual Seminars General Info Seminar topics range from fundamental theory to experiments and the philosophy of science, in particular on questions at the intersection of Quantum Gravity, Quantum Information and Quantum Foundations. The goal is to stimulate discussion across disciplines. The presentation is short (less than 40 mins) and speakers are encouraged to make it accessible and provocative. Questions and comments are then open to all participants for one hour. Seminars are announced ahead of time at qiss.fr/virtual-seminars. The zoom link is sent a few days before through this mailing list. You can join the list through qiss.fr/join-mailing-list. Talks become available for later viewing at    / qissproject  .