In the past century, however, physicists and philosophers have begun trying to unite the thermodynamic and psychological arrows. Many researchers note that real-world objects that store memories — such as human brains and computer hard drives — often heat up as they operate. Heat generation increases entropy and is an irreversible process, so the laws of thermodynamics require that such objects can only run in one direction: from past to future.

But memories don’t have to generate heat, point out Brun and physicist Leonard Mlodinow of the California Institute of Technology. For example, ripples on a pond record a rock falling into the water, and yet could, in principle, travel in reverse. Could such a memory remember the future instead of the past, the researchers wondered?

To answer this question, Mlodinow and Brun conducted what is known as a thought experiment. They imagined a system in which a chamber full of bouncing particles is connected to a nearly empty chamber of the same size by a tunnel through which particles can pass one at a time — a low-entropy configuration similar to the concentration of matter in the early universe. Although particles can cross from either chamber to the other, thermodynamics ensures that eventually both chambers will contain roughly equal numbers of particles.