Aatish Bhatia wrote a really fun and interesting explanation of entropy – check it out here!
Let’s start with a puzzle.
Why does this gif look totally normal…
…but this one look strange?
The second gif is just the first one played in reverse. But something about it immediately seems off. This just never happens. Ice melts on a warm day, but a glass of water left out will never morph into neatly-stacked cubes of ice.
But here’s the weird thing. Imagine you could zoom in and see the atoms and molecules in a melting cube of ice. If you could film the motion of any particle, and then play that film back in reverse, what you’d see would still be perfectly consistent with the laws of physics. It wouldn’t look unusual at all. The movements of the atoms and molecules in the first gif are every bit as ‘legal’ (in the court of physical law) as those in the second gif. So why is the first gif an everyday occurrence, while the reverse one impossible?
This isn’t just about ice cubes. Imagine you dropped an egg on the floor. Every atomic motion taking place in this messy event could have happened in reverse. The pieces of the egg could theoretically start on the floor, hurtle towards each other, reforming into an egg as it lifts off the ground, travel up through the air, and arrive gently in your hand. The movement of every atom in this time-reversed egg would still be perfectly consistent with the laws of physics. And yet, this never happens.
The Origin of Irreversibility
So there’s a deep mystery lurking behind our seemingly simple ice-melting puzzle. At the level of microscopic particles, nature doesn’t have a preference for doing things in one direction versus doing them in reverse. The atomic world is a two-way street.
And yet, for some reason, when we get to large collections of atoms, a one-way street emerges for the direction in which events take place, even though this wasn’t present at the microscopic level. An arrow of time emerges.
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