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Comment by nullspace | Hacker News Reader

Related, here's an excellent lecture by Sean Carroll on the Arrow of Time: https://www.youtube.com/watch?v=hagHHV6RYZ8

As far as I understand, his central (layman-level) proposition, is that time is just an emergent property when you consider that entropy always increases.

Fascinating stuff.

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A book I enjoyed on this subject was Warmth Disperses and Time Passes: The History Of Heat By Hans Christian Von Baeyer (https://www.penguinrandomhouse.com/books/184266/warmth-dispe...)

How does this model accommodate systems that lower entropy locally (at expense of the environment), such as biological systems?

You'd draw the system boundary to include the entropy increases in the environment.

But if you insist on drawing the boundary such that the entropy of the system decreases, you could probably argue it behaves similarly to a system with time reversed.

It also matters that life only lowers entropy locally for an infinitesimal interval. In a way it's a kind of noise in almost monotonic march of entropy.

And that being the case... is time going forward and entropy increasing, or going backward and entropy decreasing? Cause leads to effect or effect leads to cause, you choose.

" is that time is just an emergent property "

This guy says, it is the other way around:

https://www.quantamagazine.org/were-stuck-inside-the-univers...

Ah, but is time not a parameter in much of fundamental physics? I think the claim is that the arrow of time emerges from increasing entropy.

I like to think of time as a surrogate key to the state of the earth. This lecture looks interesting, thanks!

But why does entropy increase?

This is only true for closed systems (having no entropy-adding or -subtracting flow of matter or energy across their boundaries), and within them, there are usually more high entropy states than low entropy ones, so, on average, its evolution is more likely to end up in a higher entropy state than the one it started from, than in a lower one.

There's no time-asymmetry in the statement "There's so many more higher entropy states than lower entropy states", so one can't deduce a time-asymmetric statement like "Entropy increases as time goes forwards" from it. If I call two states S1 and S2, with S2 at a later time than S1, then BOTH are likely to be high-entropy, and neither has any particular reason to be higher entropy than the other, just by virtue of this paucity of low-entropy states reasoning. See Loschmidt's paradox (https://en.wikipedia.org/wiki/Loschmidt%27s_paradox).

> There's no time-asymmetry in the statement "There's so many more higher entropy states than lower entropy states."

That is a good point, but this is only one of the premises of the full argument for the origin of time's arrow (and it is the one that answers gowld's question). The other premise is that the universe started in a low-entropy state. This establishes a contingent asymmetry in time, in that there happens to have been one special time when entropy was at or near a minimum, and then the asymmetry in the outcome of processes, from the first premise, puts a rachet on the time axis, so to speak, so that complex things will not make round trips in time.

That is my understanding of the argument as it is often presented (e.g. by Feynman, in the article here, and also in Carroll's talk, mentioned by nullspace above), and the article you link to presents the big bang as a valid resolution of Lochschmidt's paradox.

Thanks for bringing Lochschmidt's paradox to my attention, I had not heard of it before.

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