Answer to Question #90153 in Molecular Physics | Thermodynamics for Lo

Question

As we have seen, the second law of thermodynamics states that all systems tend toward disorder. Ordered systems tend to become disordered, and it takes work to reorder them.

Now suppose your room were extremely messy and you went in to clean it up. However, you decide to clean up with a broom. You shut the light, close the door and flail away with the broom blindly, in every direction. After a while you stop and turn on the light. Imagine your surprise if the room were perfectly ordered. Now this could happen by chance. It's by no means impossible, yet it never occurs in our experience. Why not?

Take into consideration a similar situation. Put a drop of blue ink in a fish tank full of clear water. The water turns a pale shade of blue as the ink disperses. Each collision between an ink molecule and a water molecule is reversible, so it is possible that the ink would not disperse but stay on a blue blob. This never happens, although it could. Why not?

Answer 1

Why not 1.

We are talking about the probability to see your room clean after cleaning chaotic. And the probability of this is very small, because we need an exact result in a chaotic way in a finite time. But, of course, if we have infinite time for this, it is possible.

Why not 2.

It is impossible that the ink is not dispersed, but remained on a blue blob. Now we are talking about the molecular world. But deeper, we have quantum laws, and any molecule has non-zero energy, so we can see the diffusion process. You can imagine this as a harmonic oscillator: a lot of balls with strings between them oscillate with great frequency. Everything that we see fluctuates. That is, there is not a single chance that diffusion will not happen as a result. This process can be slowed down, but not stopped.