"Entropy can be reset to a previous or to an initial state"
Being a late-comer has its hazards. I apologize for repeating anything already coverred - my bad, I just didn't see it.

Is the title of the OP even coherent? Or the OP itself?

My contribution here is to refer to "Poincare recurrence." Easy enough to google. Very broad strokes: the idea is that in any system, wait long enough and some configuration of it will recur. I add this: if any will recur, then given enough time, all will recur. It just takes a really, really, really long time. So enjoy the popcorn before it gets cold!

Excellent point, Tim; and that should be an end to it. But doubtless these appallingly bad physics and maths threads will continue.

They are an embarrassment, really. Any curious scientist who passes by the forums would quickly and quietly move on.

doubtless these appallingly bad physics and maths threads will continue. — Banno

The comment that provoked the firestorm was that dark matter is a metaphysical conjecture. Terrible thing to say.

The principle of causal closure is about what counts as physical. Anything that has a physical effect counts as a physical thing. We observe physical effects (galaxies rotating faster than we would otherwise expect should be possible without flying apart, and galaxies accelerating away from each other faster that we would otherwise expect), and we don't yet know what is causing them, so we give whatever those things are placeholder names, "dark matter" and "dark energy". But since they have physical effects, whatever those things turn out to be count as physical things. — Pfhorrest

Which is the subject of Hempel’s dilemma.

Anyway, how did we get stuck on this topic of dark matter...? — Pfhorrest

From this remark:

Current physics do describe "dark matter" and "dark energy". These names describe physical phenomena that have been observed. — Echarmion

That's what the subsequent debate was about, culminating in me being repeatedly called a 'moron' by one of the Forum Moderators who has a history of abusive posts towards me. Anyway, I maintain that dark matter has not been observed, and from everything I've read about it, this is a fact.

I'm logging out for several weeks - I need to say this, otherwise I just tend to gravitate back here by force of habit, and I have some projects that need single-minded attention. So long for now.

Wayfarer.
Re-read my comment. There are no 'events' and there is no 'physicality' except with respect to the evolving perceptual needs of humans who consensually segment and re-segment what they call 'the world'. Ultimately all definition becomes subject to an infinite regress in which axioms like 'entropy' have ephemeral utility. Such is the basis of pragmatism versus naive realism.

The entire infinite space with infinite matter, the whole system is a closed system.
— god must be atheist

Well we don't really know that, do we? — Echarmion

Yes, there is a lot of assumptions thrown behind that claim of mine, among others, that there are no supernatural powers. Another assumption is that there are even more supernatural powers. And there are more assumptions, like there are even more supernatural powers than that.

However, if the laws of physics are different beyond a certain region, it still is a closed system. It would only be an open system if it received or lost some energy to the outside of the system, but since it encompasses the entire space, there is no "outside" room to lose to or to gain from any energy.

00000000000

Of course my claim assumes that energy can't be created or destroyed in ANY part of the universe. This is an invalid assumption in a philosophical sense, but not invalid in a physics sense. Physics never claimed this tenet (the indestructibility of energy) to be a universal truth. But it assumes it is true until disproved. This assumption we can carry over to any region of space, including the entire expanse of space.

"All bets are off" does not exclude assumptions. It excludes knowledge. Assumptions are not knowledge; they are presuppositions. In this sense, we can't know (and this is compatible with current knowledge of physics) whether energy is indestructible in our observed space. We assume it is.

So we don't know if the second law of thermodynamics applies to our observed world.
We don't know if the second law of thermodynamics applies to beyond our observed world.
We assume that the second law of thermodynamics applies to our observed world.
We assume that the second law of thermodynamics applies to beyond our observed world.

The only difference between the two sets of assumptions is the word "beyond". Since everything else is the same, we can conclude that the world beyond our observed world can be assumed to behave like our world. (A different assumption is just as valid to the world beyond our observed one. But the assumption I advocate is not invalid. That's my point.)

They are an embarrassment, really. Any curious scientist who passes by the forums would quickly and quietly move on. — Banno

As you are an embarrassment to logical thinking, @Banno. Your style and behaviour pattern has chased away at least one really smart and useful contributor, and I daresay he may not have been the only one.

I admit you are not the only one to blame for the quitting of the ex-member, but you display the flagship behaviour that made the smart contributor quit consistently and over and over again.

Your contributions, @Banno, are distasteful, meaningless and never backed up by reasoning. You seem to appeal to a select group of other users, and together you form a clique.

Unfortunately for you and your forum friends, logic and reason is not established on consensus. They are established on their own terms. This is your downfall: your bs and your negative judgement do not go beyond approval by your so-called friends in this forum. And perhaps beyond those users, who by default value style over content.

Re-read my comment. There are no 'events' and there is no 'physicality' except with respect to the evolving perceptual needs of humans who consensually segment and re-segment what they call 'the world'. Ultimately all definition becomes subject to an infinite regress in which axioms like 'entropy' have ephemeral utility. Such is the basis of pragmatism versus naive realism. — fresco

What comment? I cannot find. Is this a statement of your "beliefs"? Ever break a bone, your arm or leg perhaps? Or if not would you attempt seriously to tell someone so injured that there was no event, no physicality, and that the break was just a consensual slice-and-dice of "what they call" the world? Please make sense of this if you can. Or did I read you backwards?

And perhaps beyond those users, who by default value style over content. — god must be atheist

Style? @Banno? I always thought he was a beer-on-the-porch-in-the-afternoon Australian. "Anything on the barbie, mite? Toss me another, uhhn." Or whatever sounds they make when they communicate with each other.

Entropy is a concept that is useful on a microscopic scale, but has trouble applying itself to the macroscopic one. As such, there is no such thing as "the entropy of the world, or of the universe", or even heat death of the universe owing to entropy. Entropy is even problematic in the microcosm, as studies show.

Entropy is a concept that is useful on a microscopic scale, but has trouble applying itself to the macroscopic one. As such, there is no such thing as "the entropy of the world, or of the universe", or even heat death of the universe owing to entropy. Entropy is even problematic in the microcosm, as studies show. — Pussycat

Dear Pussycat, entropy is not present in the quantum level of existence (if that's what you mean by microscopic scale), but it is very much present in the macrophysical scale. This is very elementary physics. If you like to check for validity, please check Wiki, or the nearest high school's physics textbooks.

From wikipedia:

https://en.wikipedia.org/wiki/Heat_death_of_the_universe#Controversies

Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition. More recently, Walter Grandy writes: "It is rather presumptuous to speak of the entropy of a universe about which we still understand so little, and we wonder how one might define thermodynamic entropy for a universe and its major constituents that have never been in equilibrium in their entire existence." According to Tisza: "If an isolated system is not in equilibrium, we cannot associate an entropy with it." Buchdahl writes of "the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system". According to Gallavotti: "... there is no universally accepted notion of entropy for systems out of equilibrium, even when in a stationary state." Discussing the question of entropy for non-equilibrium states in general, Lieb and Yngvason express their opinion as follows: "Despite the fact that most physicists believe in such a nonequilibrium entropy, it has so far proved impossible to define it in a clearly satisfactory way." In Landsberg's opinion: "The third misconception is that thermodynamics, and in particular, the concept of entropy, can without further enquiry be applied to the whole universe. ... These questions have a certain fascination, but the answers are speculations, and lie beyond the scope of this book."

A recent analysis of entropy states, "The entropy of a general gravitational field is still not known", and, "gravitational entropy is difficult to quantify". The analysis considers several possible assumptions that would be needed for estimates and suggests that the observable universe has more entropy than previously thought. This is because the analysis concludes that supermassive black holes are the largest contributor. Lee Smolin goes further: "It has long been known that gravity is important for keeping the universe out of thermal equilibrium. Gravitationally bound systems have negative specific heat—that is, the velocities of their components increase when energy is removed. ... Such a system does not evolve toward a homogeneous equilibrium state. Instead it becomes increasingly structured and heterogeneous as it fragments into subsystems."

Also:

https://en.wikipedia.org/wiki/Non-equilibrium_thermodynamics

Another fundamental and very important difference is the difficulty or impossibility, in general, in defining entropy at an instant of time in macroscopic terms for systems not in thermodynamic equilibrium; it can be done, to useful approximation, only in carefully chosen special cases, namely those that are throughout in local thermodynamic equilibrium.

So, entropy cannot be defined for:

a) systems that are not in this thing called "thermodynamic equilibrium", the vast majority of systems in nature are like that.
b) the universe as a whole.

Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition. More recently, Walter Grandy writes: "It is rather presumptuous to speak of the entropy of a universe about which we still understand so little, and we wonder how one might define thermodynamic entropy for a universe and its major constituents that have never been in equilibrium in their entire existence1This is an assumption they can't substantiate.." According to Tisza: "If an isolated system is not in equilibrium, we cannot associate an entropy with it."2. Assumes the entire universe is not an isolated system. Buchdahl writes of "the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system". According to Gallavotti: "... there is no universally accepted notion of entropy for systems out of equilibrium, even when in a stationary state." Discussing the question of entropy for non-equilibrium states in general, Lieb and Yngvason express their opinion as follows:"Despite the fact that most physicists believe in such a nonequilibrium entropy, it has so far proved impossible to define it in a clearly satisfactory way."3. READ THE WORDS: DESPITE THAT FACT THAT MOST PHYSICISTS BELEIVE IN SUCH A NON-EQUILIBRIUM THEORY In Landsberg's opinion: "The third misconception is that thermodynamics, and in particular, the concept of entropy, can without further enquiry be applied to the whole universe. ... These questions have a certain fascination, but the answers are speculations, and lie beyond the scope of this book."4. MY OPINION IS NOT, REPEAT, NOT LANDSBERG'S BOOK.
A recent analysis of entropy states, "The entropy of a general gravitational field is still not known", and, "gravitational entropy is difficult to quantify". 5. i AM NOT TALKING GRAVITATIONAL ENTROPY. The analysis considers several possible assumptions that would be needed for estimates and suggests that the observable universe has more entropy than previously thought. This is because the analysis concludes that supermassive black holes are the largest contributor. Lee Smolin goes further: "It has long been known that gravity is important for keeping the universe out of thermal equilibrium. Gravitationally bound systems have negative specific heat—that is, the velocities of their components increase when energy is removed. ... Such a system does not evolve toward a homogeneous equilibrium state. Instead it becomes increasingly structured and heterogeneous as it fragments into subsystems."6 THIS HAS PATENTLY NOTHING TO DO WITH THE POINT. — Pussycat

I love it when a dilettante searches the Internet to disprove a point. They come up with pearls of wisdom that they can't fathom, and they actually help disprove their criticism with their quotes.

Thank you, Pussycat. Prrrr.

Another fundamental and very important difference is the difficulty or impossibility, in general, in defining entropy at an instant of time in macroscopic terms for systems not in thermodynamic equilibrium; it can be done, to useful approximation, only in carefully chosen special cases, namely those that are throughout in local thermodynamic equilibrium.7 READ: AT AN INSTANT OF TIME. OUTSIDE OF AN INSTANT OF TIME IT IS NOT DIFFICULT, IT IS NOT IMPOSSIBLE. — Pussycat

Again: the dilettante does not know how to read carefully, because it's above his or her head. So to speak. But they have a very strong opinion, and they will stick by it tooth and nail.

And claws. Prrr.

I don't need to do much searching, as I studied physics. And sorry man, I don't have time for guys like you, I used to have, but now it seems that I have run out of time. So believe what you like, no one cares anyway, it makes no difference.

, the search you have done proved you wrong in parts, and did not prove you right in other parts. If you studied physics, you should demand a refund of your tuition fees.

I am not battling with you, I'm battling with your ideas. The quotes you supplied prove beyond any reasonable doubt that you not only don't know physics, but don't know how to read text, either.

I am sorry. I really did not enjoy this either. Most likely even less than you have. I just refuse to give in to nonsubstantial, ill-gotten, unreasonable arguments. I wish I was doing something else instead, too, man, please don't feel it was only bad for you.

thanks for the summary. All I remember from high school was doing tests in chemistry that reduced local entropy at the cost of a shit load of energy.

My contribution here is to refer to "Poincare recurrence." Easy enough to google. Very broad strokes: the idea is that in any system, wait long enough and some configuration of it will recur — tim wood

Too broad, IMO. This is a result that requires a function that takes points in the space under consideration back into that space. However, with regard to a measure defined on the space, the function must preserve that measure. This is quite restrictive. If I examine systems in the complex plane, using the normal Euclidean measure, the function seems to be a simple linear translation. Extremely restrictive.

This is a result that requires a function that takes points in the space under consideration back into that space. — jgill

I'll take your word for it. My understanding was akin to shuffling a deck of cards a lot of times. Do it enough times and the particular shuffled orders will start to recur. And it's not clear to me (lots of things aren't clear to me, so this is not an invitation to argue, but if you've got the ambition, to educate) that a function has anything to do with it. Of course in the case of the universe, that make take until after dinner.

I encounter "Poincare recurrence" in this video, which I found pretty entertaining (actually in the extra video).
https://www.youtube.com/watch?v=3P6DWAwwViU

"Battling with my ideas" is your inline comments numbered 1-7, which is perfectly allright.

But I fail to see how the rest of your comments is "battling with my ideas" and not "battling with me": "I love it when a dilettante searches the Internet to disprove a point. They come up with pearls of wisdom that they can't fathom, and they actually help disprove their criticism with their quotes", "Again: the dilettante does not know how to read carefully, because it's above his or her head. So to speak. But they have a very strong opinion, and they will stick by it tooth and nail".

dante, dante, diledante

dilettante kse-dilettante, I won't trouble myself with such bad attitudes ever again, I had my share. I'm done. After all, there are other fish in the sea, other fish to fry. :naughty:

My understanding was akin to shuffling a deck of cards a lot of times. — tim wood

Yes, I'm not sure what further if any conditions would be placed on that experiment. Combinatoric calculations in probability. Here is a comment from Geology Wiki:

"The Poincaré recurrence time of certain systems is the time for them to revert to a state almost identical to their current state. The system should satisfy the following properties:

1. All the particles in the system are bound to a finite volume.
2. The system has a finite number of possible states.

The universe might not satisfy these properties."

Mathematical theory of PR is pretty strict. :cool:

1. All the particles in the system are bound to a finite volume.
2. The system has a finite number of possible states. — jgill

Which boundaries are crossed iff either or both of 1) or 2) is violated, yes? I'm thinking the universe conforms unless it's infinite. As to the states, I see the "almost identical." Seems unlikely, maybe not before dinner?

Assuming equal probability, the finite number of states simply means that the fewer the states the higher probability one re-occurs over a lengthy series of experiments. And as the number of states increases without bound ("goes to infinity"), the probability of a particular state shrinks toward zero. Roughly speaking.

The function has to do with the mixing of the particles, say, from moment to moment. Without preservation of "area" the distance between two points might shrink each iteration, and the configuration one would like to see re-emerge would not be possible. Roughly speaking.

If matter has existed from infinite past, then entropy is such that it can be reset to a previous state.

If this was not true, the world would be approaching much closer to a fully entropic state than what we experience right now. Or else perhaps we'd be in a fully entropic state. — god must be atheist

Entropy is a spectrum ofcourse. To say which stage of entropy we should be in right now would be hard for any scientist to claim considering the tremendous amount of unknown variables that modern science has to figure out. The user Devans99 would agree with you on this and I would have to say you are very much right in the first part of this OP.

The conversation has drifted into ergodic theory rather than entropy. :roll:

They are closely related topics.

You're welcome, although what I posted had to do with entropy in its broadest sense, and not as it is used in laboratory experiments.

Anyways, a good book on entropy is "Understanding Non-Equilibrium Thermodynamics" by Georgy Lebon, David Jou and Jose Casas-Vazquez.

https://b-ok.cc/book/508021/aad3be

From the preface:

Besides being an introductory text, our objective is to present an overview, as general as possible, of the more recent developments in non-equilibrium thermodynamics, especially beyond the local equilibrium description. This is partially a terra incognita, an unknown land, because basic concepts as temperature, entropy, and the validity of the second law become problematic beyond the local equilibrium hypothesis. The answers provided up to now must be considered as partial and provisional, but are nevertheless worth to be examined.

Right, so non-equilibrium thermodynamics is a terra incognita, a no man's land, well a no woman's land as well, to be politically correct, and not to be accused of sexism.

From chapter 2:

An important question is whether a precise definition can be attached to the notion of entropy when the system is driven far from equilibrium. In equilibrium thermodynamics, entropy is a well-defined function of state only in equilibrium states or during reversible processes. However, thanks to the local equilibrium hypothesis, entropy remains a valuable state function even in non-equilibrium situations. The problem of the definition of entropy and corollary of intensive variables as temperature will be raised as soon as the local equilibrium hypothesis is given up.

By material body (or system) is meant a continuum medium of total mass m and volume V bounded by a surface Σ. Consider an arbitrary body, outside equilibrium, whose total entropy at time t is S. The rate of variation of this extensive quantity may be written as the sum of the rate of exchange with the exterior d^eS/dt and the rate of internal production, dⁱS/dt:

dS/dt = d^eS/dt + dⁱS/dt (2.7)

So, the total entropy of the system under consideration is the sum of its internal entropy production, plus the entropy that it exchanges with/due to its surroundings.

Once entropy is defined, it is necessary to formulate the second law, i.e. to specify which kinds of behaviours are admissible in terms of the entropy behaviour. The classical formulation of the second law due to Clausius states that, in isolated systems, the possible processes are those in which the entropy of the final equilibrium state is higher or equal (but not lower) than the entropy of the initial equilibrium state. In the classical theory of irreversible processes, one introduces an even stronger restriction by requiring that the entropy of an isolated system must increase everywhere and at any time, i.e. dS/dt ≥ 0. In non-isolated systems, the second law will take the more general form

dⁱS/dt > 0 (for irreversible processes) (2.10a)
dⁱS/dt = 0 (for reversible processes or at equilibrium) (2.10b)

It is important to realize that inequality (2.10a) does nor prevent that open or closed systems driven out of equilibrium may be characterized by dS/dt < 0; this occurs for processes for which d^eS/dt < 0 and larger in absolute value than dⁱS/dt. Several examples are discussed in Chap. 6.

Therefore, equations 2.10a and 2.10b, which, as the text says, is the 2nd law of thermodynamics in a more general form, refer to the internal entropy of the system: the internal entropy of a system will always increase or remain constant. If the system is isolated, which means that there is no exchange whatsoever with the surroundings, then the term d^eS/dt of equation 2.7 is zero and therefore, dS/dt = d^eS/dt + dⁱS/dt = 0 + dⁱS/dt = dⁱS/dt >= 0. So, dS/dt = dⁱS/dt >= 0. This is the form of the 2nd law of thermodynamics for isolated systems: its entropy equals its internal entropy, and remains constant (at equilibrium) or increases with time (when not in equilibrium).

For systems, however, whether open or closed, that are nonetheless driven out of equilibrium, their total entropy may as well decrease with time, the 2nd law has no say in this, if the rate of external entropy exchange d^eS/dt is negative and larger in absolute value than the internal entropy production. In other words, the entropy of a non-isolated system can do whatever it pleases, when not in equilibrium.

It is also important to note that all of the above can be said for systems where the local equilibrium hypothesis holds, so what does this hypothesis state? Again from the text:

According to it, the local and instantaneous relations between thermodynamic quantities in a system out of equilibrium are the same as for a uniform system in equilibrium. To be more explicit, consider a system split mentally in a series of cells, which are sufficiently large for microscopic fluctuations to be negligible but sufficiently small so that equilibrium is realized to a good approximation in each individual cell. The size of such cells has been a subject of debate, on which a good analysis can be found in Kreuzer (1981) and Hafskjold and Kjelstrup (1995). The local equilibrium hypothesis states that at a given instant of time, equilibrium is achieved in each individual cell or, using the vocabulary of continuum physics, at each material point.

And then they go on to give a more technical description of the hypothesis, as well a justification for doing so. The local equilibrium hypothesis is therefore a rather good approximation for describing, thermodynamically and in terms of entropy, a system which is out/known to be out of thermodynamic equilibrium, by assuming that at each instant of time the system behaves like it is in fact in equilibrium.

But it just so happens that there are systems where this hypothesis has to be given up, due to the fact that fluctuations from equilibrium are just too great, as well as the time scales where anything takes place are too small for even definining a local entropy per unit time. By giving it up, the 2nd law of thermodynamics becomes highly problematic, up to the point that we are not even able to ascribe a temperature, or say that heat flows from hot to cold anymore, a fundamental tenet of this law. And so physicists have to devise new concepts, and to reformulate this 2nd law in terms of a more general "transport law":

...As a consequence, when working at short timescales or high frequencies, and correspondingly at short length scales or short wavelengths, the generalized transport laws must include memory and non-local effects. The analysis of these generalized transport laws is one of the main topics in modern non-equilibrium thermodynamics, statistical mechanics, and engineering. Such transport laws are generally not compatible with the local equilibrium hypothesis and a more general thermodynamic framework must be looked for. — chapter 7

And all this happens in the laboratory, for well known chemical and biological processes that exhibit such out-of-equilibrium behavior. What is there is to say for the thermodynamics of the universe, where gravitational phenomena kick in, comprising of hypothetical dark matter and dark energy, of which we know absolutely nothing with regards to entropy? I mean, how on earth do you extrapolate ignorance that you have, that you know that you have, on a local level to a global one, to be able to produce certain and definite conclusions, beyond a reasonable doubt, about the fate or the state of the universe?? That's .. that's just mad! Why do that thing? Why put yourself in such a position? Oh, I guess it's just the need to mythologize, like the mythical beings that we are, to tell you the truth, I have the same urge. But I think it's better to be more practical and fight the 2nd law instead, this "law" of decay and decadence, rather to embrace it.