Time’s Almost Reversible Arrow
This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time.<p>Our direction of time is defined by the observation that entropy (or &quot;chaoticness&quot;) always increases with time. If you mix orange juice with water, you will not see the result unmix itself again, even though the interactions between the individual molecules are perfectly reversible.<p>In fact, you can build a toy-universe in your head, where all interactions are time-reversible, and you would still observe that the chaotic-ness increases with time, unless you start with a completely random initial state (in which the chaoticness stays roughly constant). This increase in chaoticness is comletely unsurprising: It&#x27;s a simple consequence of the fact that there are much, much more &quot;states&quot; in which a system looks chaotic, than states in which it looks ordered. Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic.<p>The real mystery, for me, is: Why was the initial state of our universe so completely non-chaotic? Why did the universe not start out in a state that is essentially equal to the heat-death [1]? The microscopic processes that are not time-reversible might have something to do with this, but it&#x27;s not clear at all how they can solve this mystery.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Heat_death_of_the_universe" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Heat_death_of_the_universe</a>