4 ms·

Well the second law of thermodynamics really isn't a law of physics but of probability, since it basically states that it is very improbable for a macroscopic s

Well the second law of thermodynamics really isn't a law of physics but of probability, since it basically states that it is very improbable for a macroscopic system to go from a state with more disorder to a state with less. An example: If you take a glass chamber that is partitioned by a removable wall and where one side initially contains a given number N of non-interacting particles with random velocities, whereas the other side is empty. When you remove the wall between the two halves, the particles will start flying around freely in the chamber and will distribute roughly equally in the two halves. Now, the probability that they will again concentrate in only one of the halves of the chamber decreases exponentially with their number (as 2^N), so for macroscopic particle numbers (typically > 10^20) you would have to wait a VERY long time for this to happen. For a small number of particles, or a single nanosphere like in the article, it might well happen though.

darkmighty 13y ago
Well there's a little more physics to it, namely that biases towards certain distributions are small -- i.e. that systems always have paths to a large number of states. The laws of physics has to be such that they exhibit asymptotic equipartition. In your example, assuming that particles positions are iid over the entire container generates the second law, but only because the dynamics of the system allow such approximation -- imagine instead that particles liked to almost irreversibly "stick together", then the system would converge to most particles on a single side.
- ThePhysicist 13y ago
  Yeah that's true of course and why I assumed they are non-interacting. It's of course a gross simplification of the real theory but it explains quite well why entropy increases in a closed system and as an explanation is not conceptually wrong.

darkmighty 13y ago

Well there's a little more physics to it, namely that biases towards certain distributions are small -- i.e. that systems always have paths to a large number of states. The laws of physics has to be such that they exhibit asymptotic equipartition. In your example, assuming that particles positions are iid over the entire container generates the second law, but only because the dynamics of the system allow such approximation -- imagine instead that particles liked to almost irreversibly "stick together", then the system would converge to most particles on a single side.

ThePhysicist 13y ago
Yeah that's true of course and why I assumed they are non-interacting. It's of course a gross simplification of the real theory but it explains quite well why entropy increases in a closed system and as an explanation is not conceptually wrong.