What’s time?
In: Physics
Most of the laws of the physics are the same backwards and forwards. If you take a movie of a few seconds of one billiard ball hitting another [1], you can play it forwards or backwards. Either forwards or backwards looks physically reasonable, even if you carefully track each ball’s position and velocity and check all the math.
But obviously there’s something *different* about the past and future — we experience time “flowing”, we get older over time, lots of things in the natural world change over time, etc. Scientists have a couple different explanations for why this is (called the “arrow of time”).
Imagine putting something with color into water, like when making tea or coffee. Then play the movie forwards and backwards. You can easily tell which is the backwards one, because the color un-dissolving from the water looks super-unnatural — stuff dissolves all the time, but we never see it un-dissolve on its own [2].
This seems reasonable. But there’s a bit of a tricky problem, if the tea and the cup are all made of atoms, and atoms are like billiards, and billiards look the same backwards and forwards, how does it happen the tea looks different?
Think about it for a minute.
It turns out the answer has to do with probability. The *most probable* situation is for the tea to be totally mixed. The billiards analogy is like the triangle you put the balls in when you start the game — if you play a movie of someone hitting the *triangle* backwards and forwards, you can tell which one is backwards. Since while each collision is individually physically plausible, having the balls fly into a triangle and come to complete rest right next to each other is so unlikely, it just won’t happen.
[1] In space, so there’s no surface friction or air resistance to complicate things.
[2] Of course you can un-dissolve stuff by changing the conditions, which always boils down to using energy — for example boiling all the liquid away.
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