How chaos drives the arrow of time

Time only moves forward — this is what we experience in our everyday lives. Often, people connect this to the fact that there are clearly processes in nature which cannot be undone: when you boil an egg you can not make the egg-white transparent again, your coffee is very unlikely to go back into the coffee powder, which will never become a full bean again itself.

On the left, many balls are assembled in one corner of the box — a state with very low disorder, i.e. low entropy. On the right, the opposite is the case. The second law of thermodynamics states that configurations like the left one will evolve to one like the right one and never the other way around — this is what some people call the arrow of time.
Reversible dynamics und Newton’s equations: A car is made to move along a road because of a force F acting on it. If we let it drive for some time t it will end up at the end of the road. But equally as well, we could then make the car move back to its initial place again by acting on it with the opposite force, or, put differently, let it evolve for the same amount of time, but “backwards”.

It took another discovery to enable full understanding why at least classical many body systems effectively forget their initial state: Chaos!

We have discussed Chaos before in this post, but the most important thing you have to know about it here is summarized in this picture:

Very slight changes in the position where the ball is dropped onto the slope mean enormous changes in its final position when it hits the ground. (adapted from our earlier post)

Trying to make sense of quantum physics with the help of green tea.