Richard P. Feynman on the Distinction Between Past and Future | Hacker News Reader

Richard P. Feynman on the Distinction Between Past and Future

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100 pointsvkramnik29 comments

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.

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

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...

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.

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.

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...)

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.

Then why is it that time doesn't run backwards in some volume, where the entropy decreases, e.g. a fridge?

See https://physics.stackexchange.com/questions/289628/why-does-...

http://archive.is/y74WU

I'm a little bit confused. My understanding (from General Relativity) is that time does not exist and the universe (or reality) is a 4D construct; and that our perception of the arrow of time is coming from our consciousness?

Do you have a source for that? And did you mean special relativity? In spacetime, time is different from space -- the metric tensor has opposite sign for time dimension.

https://www.quantamagazine.org/a-defense-of-the-reality-of-t...

has a brief argument that you can rotate around in space because it has 3 dimensions, but time is only 1 dimension so you can't turn around. There are two directions but you can't pass between them.

Just use an unmixing machine to make the entropy decrease -> https://www.youtube.com/watch?v=LrmK9kDc5SM

Which was created by work and the by-product of that work is more entropy if not locally but elsewhere in the system. The laws of thermodynamics yield for no one. At least not yet.

>yet

Time is this funny thing the system subject to itself.

From the outside perspective of the simulator you can sample interesting histories. If you want something to happen, you can set it up and back-propagate the phase space through time so that then when run in order it reaches the desired state.

From inside the system you could be on your way toward a cosmological convergence without knowing it until suddenly all colors sort themselves out. And once they do, the rules for your system "aren't" changed at all, but you are left perplexed on how your universe was fine-tuned for color alignment.

Things may reverse in an unlikely fashion in the case of the water separation, but your shattered glass will never jump back into your hand and become whole again no matter how long you wait.

Some things are truly irreversible because death is a one way door.

It's no wonder we have different psychological feelings about past and future. Some aspects of the future are potential which have yet to crystallize, and so they can be changed before they set. Some things in the future are inevitable. Some aspects of the past are set in stone and you must accept that they cannot be changed. Some aspects of the past can change with new found understanding of past events. Hence the distinction in emotions. It doesn't take a scientist to figure that out, although I appreciate the line of thinking.

I think you are missing the point. There's no difference between the water separation and a shattered glass. Both are based on physical laws that, on a molecular scale, will run backwards just as well as they run forwards. But in practice entropic processes only run one way.

Time is the non-spatial fourth dimension along which things travel in only one direction. Even if in the three physical dimensions the process could go in reverse, has it really reversed in time? The history of it breaking and being fixed has persisted, and so now it's not the same glass that it was before. It's now a glass which had been broken but is now fixed. It's a non-material distinction, but time is non-material. And in that way, it hasn't reversed at all.

Am I the only one who just doesn’t understand what there is to be reverent about? As I see it, time is simply an imperative, without it there wouldn’t even be a reality for nothing to exist in as existence itself is dependent upon the arrow of time. I dunno what I’m missing, but I feel similarly about this as I do when people try to explain things like spirit/spirituality. I just don’t get it. Am I missing something?

Seems like thoughts or ideas themselves are inherently not time reversible.

Hasn’t Time Parity been shown to be violated? There are a number of quantum effects which behave differently when going forwards vs backwards in time.

Yes but only in special cases. That's not enough to explain increasing-entropy in general.

though lower in probability to recall/remember future event but I "believe" when the reversal happen, people call them "dejavu"

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