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Comment by marcosdumay | Hacker News Reader

The Universe was once in a very low probability state. Entropy growing is that phenomenon of it evolving into the most likely possible state at each time, leading into the most probable state of them all.

That explanation uses some weird definition of "time" that depends on stuff happening, not on that thing we keep going through independently of what we are doing. But it's a valid definition, it's just not usual.

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This doesn't yet explain why, given the time symmetry of the laws of physics, entropy would in general change in the forward direction differently than in the backward direction.

If you consider a box of gas that just recently was in the improbable state where all gas molecules were concentrated in one corner of the box, and the current situation is such that you can trace back from the current trajectory of the molecules the prior improbable state with high confidence, then yes, entropy is obviously increasing. On the scale of the universe, this can be taken as an analogy to the low-entropy state right after the big bang and the current higher-entropy state.

However, I don't see how this explains an arrow of time in terms of breaking eggs and the human brain's perception of time.

I agree, I've never found the entropy explanation compelling even if it is ultimately mathematically sensible - but that applies to a lot of quantum physics, and the universe certainly isn't beholden to my whims.

Entropy "feels" more like an emergent property of any system of probability then a fundamental force - i.e. build a rule set, and anywhere you have state transitions you'll have entropy emerge as a term you can calculate and measure.

The thing is the other candidates are kind of equally mysterious. For example if you go with the idea that all matter is 4D vectors with a magnitude of C in spacetime, it's not really clear why we can affect our motion through the "time" vector (by acquiring physical velocity in other directions) but not completely invert it, or why there should be a speed limit at all.

The big bang explains it on a cosmological level, the sun drives it on a local level. Because basically all our energy is harvested in the process of entropy relaxing from the sun, via earth, to the coldness of outer space, we are in a local entropy gradient that is very steep and has one direction down.

The really implausible thing about reversing a broken egg is not that it works itself back into the "original" state, but that it takes the same energy back from the environment (thrown off as heat in the forward direction) and converts it perfectly into the needed work. Which goes against physical law and also is something it's hard to even visualize. The environment shoves heat at the thing in some very specific way and then it jumps back into nothing more or less than a whole egg. Hopefully it won't start oscillating.

This can be expressed in the language of entropy but it is essentially a thermodynamic principle. Carlo Rovelli expressed it in a way that I like:

> In order to leave a trace, it is necessary for something to become arrested, to stop moving, and this can only happen in an irreversible process -- that is to say, by degrading energy into heat. In this way, computers heat up, the brain heats up, the meteors that fall into the moon heat it; even the quill of a medieval scribe in a Benedictine abbey heats a little the page on which he writes. In a world without heat, everything would rebound elastically, leaving no trace.

Cool. So the fact that I can remember the past means some traces of energy dissipation are in my brain. Without energy dispersion we the people could not experience "time", whatever that "is".

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