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The Second Law of Thermodynamics (2011)

franklambert.net

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Re: The Second Law of Thermodynamics (2011)

#51

Earlier quoted context omitted.

you don't buy it? This is foundational. This isn't something I'm making up. It's the formal definition of entropy. https://www.labxchange.org/library/items/lb:LabXchange:ac117... .

I think his point is that it is not clear why every microstate is equally probable.

It's an assumption we make due to imperfect knowledge of a system. We assume all microstates have equal probability.

Just like rolling 6 dice. We assume all configurations of the 6 dice have equal probability.

What's the probability of rolling exactly 1,1,2,6,3,5? It's the exact same probability as rolling 6,6,6,6,6,6. We instinctively assign these probabilities based of of assumption because we also assume each number has a 1/6 probability so rolling exactly a certain number for each roll yields (1/6)^6

It's the macrostates that are subjectively grouping these microstates in various ways. If I pick the macrostate where all dice are the same there's only 6 microstates for that. If I make up the macrostate for at least one dice is different that's every possible microstate except 6 microstates where they are all the same.

Re: The Second Law of Thermodynamics (2011)

#52
post #44

Earlier quoted context omitted.

> Why does probability seem to follow an arrow of time, it doesn't seem symmetrical like the rest of physics. One cannot really oppose probability to "the rest of physics". Probability is not "a part of" physics. Probability is what we use to describe imperfect knowledge of a physical system - and we know more about the past than we do about the future.

No, probability is a mathematical game with axioms and theorems. Why the rules of this game happens to describe systems where we have imperfect knowledge... nobody knows. Another thing is all of these systems have probability travel in a single direction. Things with high probability are more likely to happen. If time were to go backwards, low probability events will start to spontaneously occur.

Ok, so we agree that probability is not a part of physics. I also agree that the question of how to apply probability in physics is interesting.

I’m not sure about the “nobody knows” though. I would say that statistical mechanics has been quite successful in “knowing”: https://arxiv.org/pdf/cond-mat/0501322

Re: The Second Law of Thermodynamics (2011)

#53

Earlier quoted context omitted.

Delimited continuations are to exponentials as convex conjugates are to implications?

I'm pretty sure I don't understand the possible meanings of what you said there either so let's try :) I meant that the tangent to the convex conjugate ("momentum") provides bounds on what the values returned by the dual step in a primal-dual algo should be. I don't know which meaning of "exponential" I should focus on here (the action perhaps? A power set? A probability distribution?), but "implications" seem to ref…

That makes much more sense than my flash, which had been following a spark in the other direction:

Delimited continuations are functions, and as such (in a world of algebraic types where we can take sums and products of types) exponentials of types, ran^dom.

[in particular, with the substitution of isomorphism for equality they follow the normal K-12 rules: C^(A+B) ~= C^A * C^B, etc.]

I'd just been glancing at https://en.wikipedia.org/wiki/Convex_conjugate#Examples and the pattern by which single-branched f(x) seems to often become a multibranch f(x) reminded me of how logic-reversing functions in general and logical implication in particular "adds branching": if we wish to establish x'y may be satisfied on one branch by not x but on the other requires y.

[and on the general topic: I like to think of temperature as tying together energy and entropy, where positive temperatures yield the familiar relationships but negative temperatures "unintuitive" ones]

Re: The Second Law of Thermodynamics (2011)

#54
post #52

Earlier quoted context omitted.

No, probability is a mathematical game with axioms and theorems. Why the rules of this game happens to describe systems where we have imperfect knowledge... nobody knows. Another thing is all of these systems have probability travel in a single direction. Things with high probability are more likely to happen. If time were to go backwards, low probability events will start to spontaneously occur.

Ok, so we agree that probability is not a part of physics. I also agree that the question of how to apply probability in physics is interesting. I’m not sure about the “nobody knows” though. I would say that statistical mechanics has been quite successful in “knowing”: https://arxiv.org/pdf/cond-mat/0501322

Haven't you heard of quantum mechanics. The probability wave makes it a foundational part and not just a macro phenomena.

Statistical mechanics is just a study of the application of probability to that macro phenomena. We still don't know why all of it works.

Re: The Second Law of Thermodynamics (2011)

#55

> Don't put me down. I could have snowed you with differential equations and diagrams instead of what you see everyday. We're being practical and visual rather than going the math route, essential as that is in chemistry. > The big deal is that all types of energy spread out like the energy in that hot pan does (unless somehow they're hindered from doing so) They don't tend to stay concentrated in a small space. I am…

This is what confuses people. There is this universal law, but you already know about it. It's probability. Increasing Entropy is a result of probability. That's all it is. When you have a bunch of particles and you jostle the particles it is MORE probable for the particles to become spread out then it is to become concentrated in one corner. That probability is what is behind this mysterious force called entropy. Wh…

this is great and it does make perfect sense to me, someone with no stat thermo background. but it makes me wonder

how is it then, that things are becoming more spread out over time?

what is the property about the past, that it seems have a lot of uncommon states and not a lot of the common ones?

if common states are mathematically more likely to be common, why is it that the future has them and the past does not, in general?

like, why hasn't heat death happened? the probability thing seems to be almost proof-like, you cannot really argue against it. but clearly, the universe did, to a great degree, at some point in the past. why?

Re: The Second Law of Thermodynamics (2011)

#56
post #44

Earlier quoted context omitted.

> Why does probability seem to follow an arrow of time, it doesn't seem symmetrical like the rest of physics. One cannot really oppose probability to "the rest of physics". Probability is not "a part of" physics. Probability is what we use to describe imperfect knowledge of a physical system - and we know more about the past than we do about the future.

No, probability is a mathematical game with axioms and theorems. Why the rules of this game happens to describe systems where we have imperfect knowledge... nobody knows. Another thing is all of these systems have probability travel in a single direction. Things with high probability are more likely to happen. If time were to go backwards, low probability events will start to spontaneously occur.

> Another thing is all of these systems have probability travel in a single direction. Things with high probability are more likely to happen. If time were to go backwards, low probability events will start to spontaneously occur.

If I shoot a pool ball to strike a heavier second ball which is at rest they will end up moving in opposite directions. If "time were to go backwards" - whatever that means - they would approach at the end one would be at rest. That's seems indeed unlikely with "time going forwards" because we wouldn't be able to do that if we tried (at least not systematically).

I don't think there is a conceptual problem or anything surprising there: we know how balls move given their initial conditions but we cannot control those initial conditions with the precision required to obtain a precise outcome.

There are also cases were we can prepare systems in the right configuration and produce "low probability" events even with "time going forwards": https://en.wikipedia.org/wiki/Spin_echo

Re: The Second Law of Thermodynamics (2011)

#57
post #55

Earlier quoted context omitted.

This is what confuses people. There is this universal law, but you already know about it. It's probability. Increasing Entropy is a result of probability. That's all it is. When you have a bunch of particles and you jostle the particles it is MORE probable for the particles to become spread out then it is to become concentrated in one corner. That probability is what is behind this mysterious force called entropy. Wh…

this is great and it does make perfect sense to me, someone with no stat thermo background. but it makes me wonder how is it then, that things are becoming more spread out over time? what is the property about the past, that it seems have a lot of uncommon states and not a lot of the common ones? if common states are mathematically more likely to be common, why is it that the future has them and the past does not, in…

>how is it then, that things are becoming more spread out over time?

It's more likely for things to spread out then to concentrate in one corner when you randomly move all particles in a box. If all particles moved to one corner of a box you would assume there's an intelligence at work moving the particles because such movement is too low of a probability to happen without intelligent intervention.

>if common states are mathematically more likely to be common, why is it that the future has them and the past does not, in general?

Common states are called "common" because their are more of them in general. Think in terms of things with a few states like rolling dice. You have a machine that rolls rice continuously and checks the result. Use this as an analogy of particles of gas moving around in a box and then use that as an analogy of the universe.

Nobody knows why probability works this way. Probability is what differentiates the arrow of time.

>like, why hasn't heat death happened? the probability thing seems to be almost proof-like, you cannot really argue against it. but clearly, the universe did, to a great degree, at some point in the past. why?

Entropy is just a phenomenon of probability. Don't let the concept of entropy rule your brain and override what's going on. Heat death hasn't happened because entropy can be frozen. You freeze ice then the particles stop moving.

Additionally all of what I said above doesn't apply to things with gravity like black holes. With gravity things begin to automatically self organize into circular orbits or spherical planets. Why? It's because entropy isn't measuring disorder. That's a mistake. Entropy is just saying that systems drift towards high probability macrostates. In systems with gravity, spherical shapes and circular orbits ARE a higher probability macrostate then one where the particles disordered. In this system a higher entropic state is actually MORE ordered then a lower entropic macrostate.

I don't know if there's going to be a "heat" death, but for sure we are moving towards higher entropy as the law says. But this does not necessarily mean more disorder or things getting spread out.

That's it. I think the word entropy just confuses everyone. It's just someone observed these weird phenomenon with heat and called it entropy. Then we realized it's just a bunch of particles moving into high probability patterns.

Re: The Second Law of Thermodynamics (2011)

#58
post #56

Earlier quoted context omitted.

No, probability is a mathematical game with axioms and theorems. Why the rules of this game happens to describe systems where we have imperfect knowledge... nobody knows. Another thing is all of these systems have probability travel in a single direction. Things with high probability are more likely to happen. If time were to go backwards, low probability events will start to spontaneously occur.

> Another thing is all of these systems have probability travel in a single direction. Things with high probability are more likely to happen. If time were to go backwards, low probability events will start to spontaneously occur. If I shoot a pool ball to strike a heavier second ball which is at rest they will end up moving in opposite directions. If "time were to go backwards" - whatever that means - they would app…

>If I shoot a pool ball to strike a heavier second ball which is at rest they will end up moving in opposite directions. If "time were to go backwards" - whatever that means - they would approach at the end one would be at rest. That's seems indeed unlikely with "time going forwards" because we wouldn't be able to do that if we tried (at least not systematically).

in a vacuum no ball will ever go to rest if it's moving. It will move forever because there is no resistance.

What's happening in the pool table is that the ball is losing it's movement energy to the table. Table is absorbing it, the air is resisting it and slowly that vibrational energy becomes more and more spread out until it's basically imperceptible heat (which is also technically atoms vibrating).

What happens when time goes backwards is a bunch of tiny low probability events start happening. Heat from the background vibrating atoms by pure random luck happen to align and happen to produce motion that's noticeable. This happens from several places and by pure luck all of this vibrational motion concentrates on one place, the pool table and the ball. The kinetic vibrations just happen to push the ball slowly in one direction more and more with all kinetic vibrations by pure luck speeding up the ball. The ball being picking up speed until you with the tip of the pool cue catch the ball and ease it into a perfect stop, absorbing all the kinetic energy into the stick and your body.

All within the laws of physics but all extremely low probability events.

>There are also cases were we can prepare systems in the right configuration and produce "low probability" events even with "time going forwards": https://en.wikipedia.org/wiki/Spin_echo

When you put an intelligence in the system it's sort of cheating as you can manipulate random events to be non random and thus violate the laws of probability by intelligent choice. There's some computational theory here that states that the act of thinking itself produces entropy thus it's sort of conserved in a way but that' some other theory stuff that's another rabbit hole to dive into.

Re: The Second Law of Thermodynamics (2011)

#59
post #52

Earlier quoted context omitted.

Ok, so we agree that probability is not a part of physics. I also agree that the question of how to apply probability in physics is interesting. I’m not sure about the “nobody knows” though. I would say that statistical mechanics has been quite successful in “knowing”: https://arxiv.org/pdf/cond-mat/0501322

Haven't you heard of quantum mechanics. The probability wave makes it a foundational part and not just a macro phenomena. Statistical mechanics is just a study of the application of probability to that macro phenomena. We still don't know why all of it works.

> Haven't you heard of quantum mechanics.

Haven't you looked at the article I sent which says "quantum" in every other page. Haven't you heard of its author Roger Balian and his (two-volume) book From microphysics to macrophysics: methods and applications of statistical physics.

> Statistical mechanics is just a study of the application of probability to that macro phenomena. We still don't know why all of it works.

"We" may "know" different things - and have a different view on the relative importance of what is known and what it isn't.

Re: The Second Law of Thermodynamics (2011)

#60
post #56

Earlier quoted context omitted.

> Another thing is all of these systems have probability travel in a single direction. Things with high probability are more likely to happen. If time were to go backwards, low probability events will start to spontaneously occur. If I shoot a pool ball to strike a heavier second ball which is at rest they will end up moving in opposite directions. If "time were to go backwards" - whatever that means - they would app…

>If I shoot a pool ball to strike a heavier second ball which is at rest they will end up moving in opposite directions. If "time were to go backwards" - whatever that means - they would approach at the end one would be at rest. That's seems indeed unlikely with "time going forwards" because we wouldn't be able to do that if we tried (at least not systematically). in a vacuum no ball will ever go to rest if it's movi…

> In a vacuum no ball will ever go to rest if it's moving.

When a moving ball hits another massive ball that was in its way it’s pretty safe to assume that it was not moving in a vacuum.

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