> that is making an assumption that the dynamics of an entire system could be thrown into reverse No, that's not an assumption, that's a mathematical feature o

> that is making an assumption that the dynamics of an entire system could be thrown into reverse No, that's not an assumption, that's a mathematical feature of all known laws of physics. An it's not that "the dynamics could be thrown into reverse", it's that for every initial state, there is a corresponding initial state where the system runs in reverse, and hence, for every state from which entropy increases there is a corresponding state where it decreases. For a Newtonian system, it's a state where all the velocities have opposite sign. (For quantum systems it's a little trickier to describe.) So if you choose a state uniformly at random from among all possible states, the odds that you will end up with one where entropy is increasing is exactly 50%. And it gets even worse than that. In a universe that obeys certain conservation laws (which as far as we can tell ours does) a time-reversible dynamic is unitary, which is to say, there is a one-to-one correspondence between an initial state and its successors. Therefore, for any initial state, the time evolution of that state must eventually loop back to its initial state [1], and so it must eventually enter an entropy-reducing state to get there. [1] https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theorem https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theor...