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This ever-increasing movement towards messiness is called entropy.

But here’s the thing. We wouldn’t say ‘the house fell down due to the gradual movement of its component elements from a low-entropy state to a high-entropy state’ (at least, most people wouldn’t). We’d say – the house fell down over time.

Entropy is what drives time forwards, and only forwards. It’s the reason you can’t un-stir the milk from your coffee, the reason you can smash but can’t un-smash a glass vase, and the reason that if you did smash a glass vase then fixed it really well, somebody might say, ‘it looks as good as new’. People get old and die over time, things get lost over time, stuff gets broken over time. Entropy is the inevitable sliding of all things from an ordered state towards disorder and meaninglessness; ice cubes melting, tea cooling, roofs caving in, glass vases smashing, people ageing, all manner of things we might casually associate with time passing.

And as everything gets messier, more and more things end up in the wrong places, until – eventually – there are no wrong places any more, because the bigger things that the smaller things were once a neat part of have also completely fallen apart. The more our universe moves towards a maximum entropy state of affairs – from a confusing jumble of broken and out-of-place things, to a heap of barely recognisable bits, to a heap of interchangeable particles – the less can actually happen, and the more slowly the arrow of time can move forward. To begin to get an idea of this, think about stirring a pile of sand with a stick. No matter how much you stir, it doesn’t make the pile of sand any more disordered, because there’s no order to the sand grains anyway – it doesn’t matter where the individual grains go, it’s still just a pile of sand. Because things can’t get any messier within the pile of sand, entropy cannot increase and so there cannot be a discernible before or after, nothing can happen, nothing can change or, to put it another way, time does not pass. ssss1