The arrow of time appears to be pointing in one very specific direction. Natural processes, from rivers flowing downhill, never uphill, to eggs always breaking, never spontaneously reassembling, to cups of coffee always cooling down, show us that direction.  Yet the laws of physics that govern the motion of all matter are time-symmetrical: they don’t distinguish between past and future. What give time’s arrow its direction is the initial conditions of each situation: an orderly state of affairs which then disintegrates into a disorderly state. The question then is, what was that original orderly state of the entire universe, giving the whole of time its direction, asks Paul Davies.

Take a movie of an everyday scene and play it backwards to an audience. People are sure to notice: rivers flowing uphill, broken eggs reassembling themselves, footprints washed into existence by the retreating tide… In daily life we have no trouble telling past from future, so it’s easy to spot the deception when sequences are reversed. The directionality inherent in natural phenomena is dubbed ‘the arrow of time.’ It is such a familiar part of experience that it often comes as a shock to learn that its origin is shrouded in mystery.