There is an open question about what the source of "the arrow of time"[1] is, which is this apparent weirdness where at very small scales the laws of physics ar

There is an open question about what the source of "the arrow of time"[1] is, which is this apparent weirdness where at very small scales the laws of physics are symmetric in time but at larger scales there appears to be something dictating that they are asymmetric in time. It should be clear that at larger scales, most physical laws are still symmetric in time it's just that we generally see that the results are biased towards one direction (hence the "arrow" of time). Now, this is a more general and complicated thing than entropy. Entropy is a statistical effect (and has very little to do with cooking eggs -- other than its link to thermodynamics). I can give you a short primer on entropy with an example: You have 2 balls and push one so that it knocks the other one. Record it, then reverse the footage. In principle neither view of the event is more correct than the other (this is slightly untrue due to friction and the ball would appear to accelerate on the tabletop, but imagine it's a sufficiently frictionless surface for the purpose of this discussion). So motion is symmetric in time, right? As with most things in physics, the correct answer is "yes, but..." But what happens when we add more balls? Imagine you have 100 lined up in a grid and you knock one -- you end up with a completely jumbled mess. Now reverse the footage, and a jumbled mess becomes 100 perfectly lined up balls. Which is the correct direction of the footage? Obviously there is a difference and you can tell what it is without me needing to explain it -- it's almost impossible that knocking a jumbled mess would just sort it! Why is it so improbable -- after all, isn't the motion just as time-symmetric as in the 2-ball example? Yes, but the problem is the statistics. What is the probability that a given jumbled set of balls will be sorted when you knock them? It's astronomically small. In thermodynamics, these same statistics are the basis for what we consider entropy (imagine the 100 balls are countless numbers of particles). Effectively the idea is that because there are so many particles of matter involved in gas interactions, that the probability distributions are so skewed that you only ever see one macro-level result (that the ideal gas law is obeyed for instance). So while entropy may look like some fundamental force, it's a statistical effect that effectively guarantees that certain effects will happen. Entropy is the thermodynamic arrow of time (and was the first thing that spawned the idea of an "arrow of time"), but there are other arrows of time. For instance, the universe is clearly expanding and thus there is a cosmological arrow (if you reversed the motion the CMBR would be different). There is a "weak" quantum arrow of time where some reactions violate CP-symmetry and thus by CPT theorem they must violate T-symmetry (the T in CPT being time). [1]: https://en.wikipedia.org/wiki/Arrow_of_time https://en.wikipedia.org/wiki/Arrow_of_time