# Talk:Ergodic hypothesis

Active discussions

## average over time

Why does this article not mention anything about the fact that the Ergodic Hypothesis is what allows us to relate ensemble averages to time averages?

In response to this unsigned and undated comment: This is actually a valid point, but I'm not sure what should be said.--guyvan52 (talk) 13:22, 2 April 2014 (UTC)

## Untitled

"The quick definition of ergodic is that given sufficient time, a system will return to states that it has previously experienced. The text below explains this basic premise in detail."

is simply not correct. I am removing it. The next paragraph of the article is a good introduction. Johnfranks (talk) 22:16, 11 April 2009 (UTC)

## deleted incorrect interpretation

The lead read, "[...], i.e., that all accessible microstates are equiprobable over a long period of time." This is incorrect, and I've deleted it. Equal probability of every microstate is sufficient but not necessary. All that matters is whether, after fixing a coarse-graining, the probability of every cell is approximately proportional to its volume.--75.83.64.6 (talk) 15:31, 23 December 2012 (UTC)

## Troublesome sentence

The following sentence is weak. It is also important, but I don't quite know how to fix it:

Liouville's theorem ensures that the notion of time average makes sense, but ergodicity does not follow from Liouville's theorem.

--guyvan52 (talk) 15:17, 9 February 2014 (UTC)

I rewrote the introduction, added two figures, and will remove the "cleanup" template if nobody objects within the next few weeks.--guyvan52 (talk) 16:43, 9 February 2014 (UTC)

## What does the picture have to do with anything?

Does the picture have anything to do with an ergodic process??!! It's been around since 2012.

I put the picture in because it is an example of how the microscopic world of atoms is so different from the macroscopic world. There is a deep connection between the ergodic hypothesis and the second law of thermodynamics which is difficult to put into words. But the fact that the shape of the funnel cannot cause a pressure differential is an example of both the ergodic hypothesis and the second law of thermodynamics. I will put the figure back in, but will agree to a consensus when other editors voice opinions. In my four months of editing, I have understood conflict resolution. The figure does neither any great harm nor is essential. Let's just get a consensus and move on.--guyvan52 (talk) 20:11, 28 March 2014 (UTC)
I suggest a using caption that clearly explaining that, while the flies superficially seem to violate the second law, the gas knows how to behave, and how this relates to the ergodic hypothesis. Paradoctor (talk) 20:53, 28 March 2014 (UTC)
Yes, but the phrase "the gas knows how to behave" has a wonderful ring but does not belong in WP, I would think. On the other hand, I can't think of a better caption, so I will modify your caption and see if anybody can improve it. Thanks for your input --guyvan52 (talk) 00:04, 29 March 2014 (UTC)
You (or someone else who understands this stuff) need to fix the caption, because as it stands today it is not a sensical English sentence. I'd fix it myself except I don't understand this stuff. :-) — Preceding unsigned comment added by 68.100.229.3 (talk) 01:44, 2 April 2014 (UTC)
The ergodic hypothesis says that all states are equally possible, or more precisely, that all regions of phase space are equally probable provided phase space is measured using what are called canonical variables. The vagueness of all this to someone unfamiliar with Hamiltonian mechanics inspired me to include the fruit fly analogy. In this case, ordinary three-dimensional space is an example of a space where all accessible locations are equally probable. Fruit flies congregate near the fruit because they smell the fruit and can find the hole. Why they don't leave is a mystery to me -- maybe they like it there, or maybe they can't find the hole. Either way, such a trap could never cause a higher percent of atoms to be inside the jar, because if that were possible we could use that compressed air to operate a motor and create a certain type of perpetual motion machine. (I think its called a perpetual motion machine of the second kind -- it obeys energy conservation but violates the second law of thermodynamics regarding using spontaneous heat transfer to do useful work.) Does that help? Thanks for your interest in this.--guyvan52 (talk) 13:18, 2 April 2014 (UTC)
The fruit flies don't leave because they have a bias. They'd much rather go up, whether flying or crawling up the walls, and so will get preferentially get stuck at the top where there isn't an exit. Also flying/crawling toward the light, so therefore preferring the walls. If gas molecules had biases who knows what mischief they'd do to theorists! Shenme (talk) 05:41, 6 April 2014 (UTC)
As they say, all analogies limp. Paradoctor (talk) 12:51, 6 April 2014 (UTC)
One more comment: When I was much younger and more foolish I invested some time trying to understand why perpetual motion machines are impossible. This "fruit fly" trap is one of the few situations where we have concrete proof that a certain class of perpetual motion machines are impossible. No matter how you design the shape that "atom trap", we have a mathematical theorem (Liouville's) that says it won't work. --guyvan52 (talk) 13:29, 2 April 2014 (UTC)

## Eh? Sentence missing a phrase (or maybe a phase?)

"If it were possible to construct a sort of tunnel whereby specular reflections cause atoms to move from a less populated container to an identical one with greater density, thereby allowing the direct conversion of random thermal energy into useful work in a way that does not require a heat bath."

What, pray tell, was this sentence supposed to say? Because it doesn't have an end as yet. Shenme (talk) 05:09, 6 April 2014 (UTC)

Done Paradoctor (talk) 05:21, 6 April 2014 (UTC)

"In macroscopic systems, the timescales over which a system can truly explore the entirety of its own phase space can be sufficiently large that the thermodynamic equilibrium state exhibits some form of ergodicity breaking."

I think that a macroscopic system can NEVER explore its own phase space on usual time scales. If I am right about that, then this should not be a reason for ergodicity breaking.

Sigh. The article as currently written is quite reasonable, and sticks to the facts as generally accepted. Of course, once could expand considerably on what is being said here, but a "complete rewrite" is a bit derogatory. 67.198.37.16 (talk) 04:16, 5 November 2020 (UTC)

## Please use the Ergodic hypothesis and Information theory to explain black holes and dark matter

Make pages. Wikipedia mustn't create new knowledge (communists always respect the retarded rules and never say something new). We can be as commies and as silly as you want, but create new pages to reveal ideas that exist.

## Adding Some Accessibility (Technical Template suggestion)

This article could use a little more laymen's terms. I've written and read a great deal of technical material but this subject is feeling a little opaque to me after reading fairly extensively on wikipedia's various articles immediately surrounding the subject. I feel the differentiation of Liouville's theorem from the presumption of Ergodicity is an excellent place and discussion mechanism for adding clarity. Perhaps contrasting Liouville's theorem and Ergodic hypthesis could be its own section? Please bear with me as I ask some questions in hope of stimulating another author. For instance, at this point:

"But Liouville's theorem does not imply that the ergodic hypothesis holds for all Hamiltonian systems."

...my interpretation seems to lead to the idea that ergodic hypothesis presumes all microstates are equiprobable over time but Louisville's theorem only presumes that all microstates accessible at a given specific moment in time are only equiprobable if that was the initial distribution of probability, but given the evolution of the system over time each individually accessible microstate does not necessarily inhabit the same probability "slot" per se.

It feels unclear in my mind whether Liouville's theorem says anything about the equiprobability of microstates of a given energy level, but I'm presuming that's a requisite of "ergodic" therefore the quote above does imply different probabilities of accessible states at the same energy. After pulling this quote from liouville's theorem page:

" that is that the density of system points in the vicinity of a given system point traveling through phase-space is constant with time. This time-independent density is in statistical mechanics known as the classical a priori probability"

...it seems like "with time" means "over time" instead of at a given time. This seems to lead toward the question. "How can time independent probability density NOT be a presumption of ergodicity?" It seems that it's being presumed, without being directly said, that some equal energy states are not available at all times. The idea of travelling through phase space seems a bit obscure but I'm presuming it means considering all the available states over a given set of moments.

Overall this my stab at explaining it in terms I can grasp:

It seems as though what is being said is that Liouville theorem asserts that, assuming equiprobability for any state accessible at a given moment, that probability distribution will always remains the same, even though, in consideration of the full set of the states accessible over time, the microstate members of the immediately accessible set may be subject to change from one moment to the next and therefore the longer-term probability of any given possible state (out of the full set over time) is not necessarily equiprobable with the others. IE: The set of currently available microstates only remains the same over time in its probability relationships, but the members of the set can change. This ability to change members allows for the possibility of different probabilities between members of the larger total set of micro-states available to the system over time. ( apparently even at the same energy level)

Obviously my shaky grasp of the subject makes it impossible for me to edit the article so I'm hoping someone with a better grasp can use my confusion and attempt to straighten it out as a basis for making this article more accessible to a broader audience. (I also look forward to understanding it better personally) Nemesis75 (talk) 14:53, 21 September 2016 (UTC)

The article on ergodicity now has a long, informal introduction that is perhaps more readable? Of course, it covers a much larger range of topics than that here. 67.198.37.16 (talk) 04:20, 5 November 2020 (UTC)