Abstract

Late in the nineteenth century, physics noticed a puzzling conflict between the laws of physics and what actually happens. The laws make no distinction between past and future—if they allow a process to happen one way, they allow it in reverse.1 But many familiar processes are in practice ‘irreversible’, common in one orientation but unknown ‘backwards’. Air leaks out of a punctured tyre, for example, but never leaks back in. Hot drinks cool down to room temperature, but never spontaneously heat up. Once we start looking, these examples are all around us—that’s why films shown in reverse often look odd. Hence the puzzle: What could be the source of this widespread temporal bias in the world, if the underlying laws are so even-handed? Call this the Puzzle of Temporal Bias, or PTB for short. It’s an oft-told tale how other puzzles of the late nineteenth century soon led to the two most famous achievements of twentieth century physics, relativity and quantum mechanics. Progress on PTB was much slower, but late in the twentieth century cosmology provided a spectacular answer, or partial answer, to this deep puzzle. Because the phenomena at the heart of PTB are so familiar, so ubiquitous, and so crucial to our own existence, the achievement is one of the most important in the entire history of physics. Yet it is littleknown and underrated, at least compared to the other twentieth century solutions to nineteenth century puzzles. Why is it underrated? Partly because people underestimate the original puzzle, or misunderstand it, and so don’t see what a big part of it is addressed by the new cosmology. And partly for a deeper, more philosophical reason, connected with the view that we don’t need to explain initial conditions. This has two effects. First, people undervalue the job done so far by cosmology, in telling us something very surprising..