Einstein's Fridge: How the Difference Between Hot and Cold Explains the Universe

by Paul Sen

Entropy increases with time, therefore, because the chances of its decreasing are tiny. In fact, and this is the mind-blowing part of Boltzmann’s logic, only by observing entropy increase can we tell the direction of time. We differentiate the future from the past because in the future the overall entropy is greater. So, by trying to understand heat in terms of atoms, Boltzmann had uncovered what lay behind William Thomson’s discovery of time’s arrow. Imagine you’re watching a film that shows the heat in a kitchen flowing back into an oven, or a film showing the milk in a teacup separating back out. You would rightly surmise that the film was running backward. Time’s arrow is simply a reflection of the inexorable march from statistically unlikely ordered arrangements to more likely disordered ones. There’s a subtle point here: a movie showing heat flowing back into an oven isn’t showing something that couldn’t happen; it’s showing something that’s extremely unlikely to happen. But it’s so unlikely that, if we see it, we know something is awry.