If this flowing time is not contained within our fundamental description, where does it come from? Most modern efforts to understand it begin with the second law of thermodynamics. This says that the entropy of the universe – its overall disorder, essentially – always increases. The argument is superficially convincing: you can’t unscramble an egg or unspill a glass of wine emptied onto your beautiful Afghan carpet, because to do so is to decrease entropy. Many physicists interpret entropy increase as time’s arrow: a one-way street for the processes of the universe. If relativity gives us the outline of the stage on which reality plays out, thermodynamics explains why the play always follows the same plot line.

If this flowing time is not contained within our fundamental description, where does it come from? Most modern efforts to understand it begin with the second law of thermodynamics. This says that the entropy of the universe – its overall disorder, essentially – always increases. The argument is superficially convincing: you can’t unscramble an egg or unspill a glass of wine emptied onto your beautiful Afghan carpet, because to do so is to decrease entropy. Many physicists interpret entropy increase as time’s arrow: a one-way street for the processes of the universe. If relativity gives us the outline of the stage on which reality plays out, thermodynamics explains why the play always follows the same plot line.