Talk:Liouville's theorem (Hamiltonian)

State the theorem

You're staring right at the equation! What do you mean state the theorem? Phys 19:52, 5 Sep 2003 (UTC)

Firstly, the equation I am staring at is something you have called a corollary, and that means it's not the theorem itself. Secondly, nothing is actually stated! You've left everything undefined, and no assumptions are given. If that's your idea of stating a theorem, you'll get straight Fs in any theoretical math course. The equation is a differential equation in which the notation { ρ, H } is not given any definition. "How probability distributions evolve under time evolutions"? What does that mean? One may assign to each instant t in time a probability distribution, say P_t in many ways, and there is no differential equation that this assignment must satisfy merely as a result of this thing's being an assignment of probability distributions to instants in time. Michael Hardy 23:23, 5 Sep 2003 (UTC)

... and is ρ supposed to be (1) a probability density function, or (2) a cumulative probability distribution function, or (3) a probability measure, or (4) something else? That's one thing you'd obviously need to state here in order to have a statement of the theorem. Michael Hardy 23:26, 5 Sep 2003 (UTC)

Yes, something odd here about the formulation, anyway. A smooth measure on a manifold has a different variance property to a function, no? So, is the point that in the Hamiltonian setting the background top-level differential form (wedge all the dp and dq) is invariant under the flow, meaning that we can blur the difference?

Charles Matthews 08:52, 10 Sep 2003 (UTC)

Restored this - my fault it went before. The page needs to be brought in line with symplectic topology, amongst other things.

Charles Matthews 20:09, 19 Sep 2003 (UTC)

I think the point here is that they want to talk about 'smooth measures' on the phase space, whether or not those are probability measures. 'Smooth measure' is a measure absolutely continuous with respect to the background measure μ, which is like

|dp^dq^...|

in usual notations. Then H means Hμ really, and the 'base case' H = 1 of the theorem is saying that the background Liouville measure μ is invariant under the flow. Well, I'm not saying this very well, but I think that's the setting.

Charles Matthews 16:47, 13 Sep 2004 (UTC)

help

Could someone please expand this article to help explain the theorem more fully. What is it used for? How is it used? What year did was it brought to light? How did it change or effect physics? Why is it significant? What are the practical ueses? Kingturtle 02:49, 4 Jun 2004 (UTC)

The information paradox article points here to find out why information is never destroyed. But I cannot remotely figure it out. This idea about information seems so important it would be nice to find a detailed explanation. Manuel

"distribution function"?

The article says:

In a system with Hamiltonian H and distribution function ρ, the theorem states that ...

Does "distribution function" mean "probability density function"? If so, it should say that, because the term distribution function is often taken to mean cumulative distribution function, which is of course a different thing. Michael Hardy 00:42, 12 Sep 2004 (UTC)

"Distribution function" seems to be the standard usage in the literature my job entails reading, even though the functions under consideration certainly aren't the cumulative distributions. For example, Blum and Rolandi's Particle Detection with Drift Chambers (Springer-Verlag, 1993), says the following, on p. 81:

The principal approximation of Sect. 2.2 was to take a single velocity c to represent the motion between collisions of the drifting electrons. In reality, these velocities are distributed around a mean value according to a distribution function

f₀(c) dc

which represents the isotropic probability density of finding the electron in the three-dimensional velocity interval dc_xdc_ydc_z at c.

An interesting little paper by Blatt and Opie (J. Phys. A, Vol. 7, No. 15, 1997) derives the Boltzmann transport equation from Liouville's theorem, as given in this article. In the introduction, they say,

The distribution function is assumed to satisfy the Liouville equation

{\frac {\partial \rho }{\partial t}}=-[\rho ,H_{n}]

where H_n is the N-particle Hamiltonian, the particles being confined to a rigid box of volume V, and the bracket on the right-hand side denotes a Poisson bracket.

"Distribution function" also seems to be the preference among my quantum and stat. mech. professors, but I'd have to dig out my lecture notes to be sure. What can I say? Physics people are strange. (But they lead charmed lives.)

Anville 15:43, 13 Sep 2004 (UTC)

This is one case where I'm inclined not to be tolerant of terminology differences between fields, and say simply that they've got it wrong. Michael Hardy 00:32, 19 Sep 2004 (UTC)

You can say that a single article got it wrong, but you can't say an entire field "got the terminology wrong" as there's no reason to prefer the terminology of one field over that of another. I can assure you that I've read dozens of articles in which this terminology is used. I would argue however, that what currently (before my edit) appears in the article is incorrect: What I call the quantum Liouville equation is certainly not THE master equation. Any linear 1st order differential equation is A master equation. Cederal 12:51, 17 April 2007 (UTC)[reply]

Anville's change 29/10/2004

Sorry I don't agree that determinism alone is sufficient to give the theorem!

The local density in phase space D=N/V. You need to argue that N is constant within the marked volume, and this I think is what your argument on non-crossing of trajectories does.

You also need to show that V doesn't change as you follow the test particle (total dV/dt =0).

I think in essence V doesn't change because the shrinkage of V in the q coordinate direction is exactly offset by an equivalent increase in the conjugate p direction, and this RELIES on Hamilton's relationships between the p's and q's and their time derivatives.

In addition, the relationship is between generalised coordinates and their conjugate momenta, so the use of q rather than x is usual.

I shall edit accordingly if you agree.

Linuxlad 17:20, 29 Oct 2004 (UTC)

To all who have corrected my mistakes, both major and minor:

Thank you. I completely agree with your objections, and I have very similar statements in my stat. mech notes. Why I didn't get things right the first time, I can't tell you—but hey, the article is much better now, so it's all okay in the end.

Gratefully, Anville 14:48, 27 Nov 2004 (UTC).

version w/o Darboux coords

I think it would be nice if we could have a version of the proof which does not rely on Darboux coordinates, which only happens to be local in some symplectic manifolds. Phys 23:16, 11 Nov 2004 (UTC)

Open note to Denevans

Can we have a discussion on your recent mods to this article? - whilst I'm sure many are useful, some of them seem to put the cart before the horse (if I were a 3rd year physicist I don't think I'd get past your early description in the phase-space section); and in one or two places seem to me to muddle things (eg in _my_ world the convective derivative is NOT a definition it's a deduction ; and again your statement on Hamiltonian and non-Hamiltonian systems is rather unclear).

Phase space (section 2) is in any case discussed elsewhere. Avogadro's number is an irrelevance - stat mechanics applies to much larger and smaller system's than the 1 gram-mol scale! (is this perhaps a typo?)

The equation (section 3) you call Liouville's appears to me be 'just' a conservation of D equation found by integrating system-point flows over a control volume (ie the RHS is div (D 'v') where 'v' is a p,q 'velocity') - it's NOT the equation (presently your second) which physicists and theoretical chemists look to to eg justify the weighting on the Boltzmann distro, - this has some real-world dynamics in it. This first equation is indeed used later on as the (nearly self-evident :-)) starting point for the 'fluids' proof of LT (this is not by any means original with me - I can point you to at least 2 similar proofs on the web, from Harvard and Oxford TP departments IIRC).

(In passing - Is there a subtle distinction between LE (which already has an article) and LT which we should strive to maintain?)

Can we avoid the use of the term symplectic system until later - it may well be idea du jour to mathematicians and advanced physicists - but Wiki still lacks an adequate succinct expostulation accessible to a graduand-physicist (of my generation.) At present all symplectic space refs spin off into ideas like 1-forms, tangent bundles etc., which may be simple but are not presently explained in terms accessible to those with only a standard 'maths-methods' background.

The dynamics of non-Hamiltonian systems and non-conservation of heat are of course important, but most physicists sidestep them, being interested in micro-systems, or treating the system as embedded in a larger system obeying conservation.

It may be that a dual approach combining the 'physics' and symplectic systems ways is doomed not to work, like the tensors fiasco, but we can at least try.

Can we talk? Your place or mine, or in the appropriate talk page? Linuxlad 15:14, 13 Feb 2005 (UTC)

Informal Proof & Symplectic Geometry equivalent

Having struggled through several hundred pages of Penrose (ref. 1) I find that the 'informal proof' (taught IIRC on Natural Sciences Tripos) is essentially the same as the symplectic geometry statement given in 20.4 ('Hamiltonian dynamics as symplectic geometry') of that book . Since one of the main problems for eg physicists is in reading across terms like '2-form', I have added a paragraph to 'guide' those who follow. Please feel free to amend or correct, but remember the intended audience for this section is from the physical sciences. Linuxlad 09:23, 8 Mar 2005 (UTC)

revision

I tried to revise this completely. There was a lot that was redundant on this page, and it wasn't ever clear where the main statement of the theorem was, or what it had to do with physics, so I tried to clean it up a little, and removed the excessive subsectioning. I don't think it is perfect, but I hope you'll agree its an improvement. –Joke137 22:47, 4 Jun 2005 (UTC)

My first impression is that it's mathematically top-heavy for my taste (Though I agree that it was in urgent need of a cleanup).

From a pedagogical point of view I think that there should a clear and early statement that the theorem is 0) that the phase space density around a state point (ie convective derivative) is constant, and that this follows from 1) if you follow a set of points, the stretch in dp is offset by the contraction in dq and/or 2) viewed as a fluid in phase space, the flow has zero divergence. The basic statement of the theorem and the key reasons should be accessible to a competent final-year physics or chemistry 'major' Linuxlad 09:40, 5 Jun 2005 (UTC)

Well, I tried to not introduce any unnecessary formalism, but of course a better writer could still simplify it. Liouville's theorem is a mathematical statement, so it is hard to imagine stating it in a non-mathematical way. I think that the statement (1) you make is the statement about volume, in particular the Jacobian of the time evolution, in "other statements." It seems no easier to formulate precisely than the usual way, in terms of Liouville's equation. In particular, I don't think that dq_i always compensates for dp_i: it could be that dp_j compensates, or even dq_j... (but see below - linuxlad)

(2) By zero divergence, I assume you mean in the static case. Otherwise the statement is that the total derivative of the flow vanishes. (see below - linuxlad) –Joke137 03:30, 6 Jun 2005 (UTC)

Later:- No, the effects must cancel in (p,q) pairs (because of the Hamilton relations - see my original para, which you've cut) - isn't this what you'ld expect? (the 'system' could comprise two pendulums at opposite ends of the universe, so p1,q1 and p2,q2 have no coupling) It's essentially the divergence statement (where the cancellation is also in p,q pairs) in a Lagrangian frame (as I think You're saying above), I think. The 'proof' is standard - I was given it in 3rd year physics many years ago, (and found it helpful and memorable) - and it's also on the web. :-) Linuxlad 08:01, 6 Jun 2005 (UTC)

That is only true for infinitesimal times. Of course in your example of seperated pendulums, it holds for infinite times, but for a general system there is mixing between all the variables. –Joke137 15:48, 9 Jun 2005 (UTC)

Incidentally I think you need care when using for single particles (as when you introduce Mr Vlasov) - since the theorem is about 'system points', and classically there could be any number of those, provided you choose an appropriate relative density.

_Your_ last step (using the Hamilton's relations) _is_ showing that the divergence is zero.

Linuxlad 08:01, 6 Jun 2005 (UTC)

I don't know exactly what you mean by "system points." The theorem is about a phase space distribution function, which could either, physically, represent one particle (with some uncertainty about its position in phase space) or several, non-interacting particles, as in the case of the Vlasov equation. This can then be generalized to the Boltzmann equation, in which a collision operator is added. –Joke137 15:48, 9 Jun 2005 (UTC)

I have removed a couple of typos - in the equation of continuity, (which says nothing about convective derivatives being zero), and the role of N. Added a brief reference to the 'fluids analogy' (which is essentially what your proof is) because it's easier not to make mistakes (for me anyway) that way. And also on the motion of of a cloud of points, because I think it gives a feel for what's going on in phase space.

Linuxlad 14:18, 5 Jun 2005 (UTC)

Yes, thanks for this. The total derivative popped up in the equation of continuity – I must have accidentially copied it from the Liouville equaton. –Joke137 03:30, 6 Jun 2005 (UTC)

joke137 - I'm sorry I don't agree with your last few points - the phase space is system points of potentially quite complex entities - any thing you can write a Hamiltonian for. (Eg a million flying bedsteads tied together with string) Mixing of variables is by the by - the cancellation clearly arises from the relation between pairs of ps & qs.

Classically, the use of the theorem is to justify some version of the ergodic hypothesis ie in a time-stationary system the probability of a system being in a small phase space volume is proportional to the phase space volume. .

Lets take this off-line rather than slog it out here - my email is fizziks@n-cantrell.demon.co.uk Bob

clarification

from the article:

which was Gibbs's name for the theorem

err...what was? Deepak 00:38, 31 May 2006 (UTC)[reply]

'Conservation of density in phase (space)', as it says. Linuxlad 08:27, 31 May 2006 (UTC)[reply]

Liouville - Louiville

We should note somewhere (not least in the keywords!) that Joseph's name and the eponymous theorem are extensively spelled/misspelled as Louiville in the literature. This includes not only the scientific literature but even an item in the existing Wikipedia entry! (try searching the page). I assume that this is because the latter spelling is more natural (indeed I only assume that the former is actually correct!)

I think that would belong in the article on the man himself. Deepak 16:08, 31 May 2006 (UTC)[reply]

Noether's theorem

Curiously, the article now points out that the tuplet $(\rho ,\rho {\dot {q}}^{i},\rho {\dot {p}}_{i})$ is a conserved current; this is a corollary of the proof. But this implies that Noether's theorem applies. What, exactly, in this context, is the set of symmetries implied by Noether's theorem? This article should make this connection explicit; Noether's theorem is a major landmark in physics, so this is just begging the question. linas 05:37, 13 June 2006 (UTC)[reply]

Never mind. linas 14:17, 13 June 2006 (UTC)[reply]

Noether's Theorem states that symmetry implies conservation. In the article it is said "Equivalently, the existence of a conserved current implies, via Noether's theorem, the existence of a symmetry. The symmetry is invariance under time translations, and the generator (or Noether charge) of the symmetry is the Hamiltonian." This needs to be changed, conserved quantities do not, at least by Noether's Theorem, imply symmetry. I'm guessing this should read something like "The symmetry of invariance under time translations implies, via Noether's Theorem, that current is conserved." However, since I'm not sure this is true, I didn't want to change this myself. 208.124.63.250 (talk) 14:03, 7 April 2008 (UTC)[reply]

The statement that "the conservation of rho comes from symmetry in time" confuses me, because time symmetry leads to conservation of the Hamiltonian, which seems completely different than rho.Chris2crawford (talk) 02:34, 21 August 2020 (UTC)[reply]

Also I don't see how "as well as conservation of the Hamiltonian along the flow have been used." AFAIK all you need is Hamilton's equations and equality of mixed partials.Chris2crawford (talk) 02:41, 21 August 2020 (UTC)[reply]

Hmm. Unstated assumptions are that (1) the number of p,q variables is constant in time (this assumption is violated in chemical reactions) and that (2) the total energy aka Hamiltonian is constant in time. The conventional, standard derivation of Hamilton's equations is to write an expression for the energy, and then to minimize the energy over all possible trajectories, which then gives Hamilton's equations as providing that minimum. This is, for example, how one finds geodesics in Riemannian manifolds, in general: geodesics have the lowest energy. Similar ideas for symplectic manifolds. For this reason, Hamiltons eqns are not considered to be "fundamental", but rather follow from the variational minimization of energy. Not sure how to shoe-horn this additional information into this article. I'll look. Hmmm. 67.198.37.16 (talk) 22:33, 19 May 2024 (UTC)[reply]

Probable false claim requires proof

I believe the following claim to be false:

It is straightforward to show that as the cloud stretches in one coordinate – $p_{i}$ say – it shrinks in the corresponding $q_{i}$ direction

This is tantamount to saying that the diffeomorphism induced by every Hamiltonian flow preserves the symplectic leaves of the canonical coordinates. It's total phase volume that's preserved, not phase volume projected down to individual leaves. The condition stated is sufficient, but it's certainly not necessary. It might even be worthwhile to state a counterexample. A four-dimensional phase space is minimal. You can have Hamiltonians both where $q_{0},p_{0}\rightarrow 0$ and where $q_{0},q_{1}\rightarrow 0$ . Anybody want to illustrate the Hamiltonian functions? --Eh9 (talk) 19:12, 21 March 2008 (UTC)[reply]

The relationship holds because Hamilton's relations apply to pi, qi pairs, and so this ensures the

\Delta p_{i}

\Delta q_{i}

products (no summation convention) stay constant as the volume flows through phase space. Go back to say March 2005, or look on the web - not anything clever, but something for us simple physicists, or chemists. Bob aka Linuxlad (talk) 20:17, 21 March 2008 (UTC)[reply]

The double-linked pendulum provides a counterexample. In general, there are always periods of counter-phase amplitude oscillations in each pendulum as seen against the other. This corresponds to an quasi-oscillating transfer of phase volume between the two.

Mathematically, if all the mixed partials of the Hamiltonian are zero, that is, if

{\tfrac {\partial ^{2}H}{\partial q_{i}\partial q_{j}}}={\tfrac {\partial ^{2}H}{\partial q_{i}\partial p_{j}}}={\tfrac {\partial ^{2}H}{\partial p_{i}\partial p_{j}}}=0

when

i\neq j

, then the Hamiltonian can be expressed separably on 2-dimensional symplectic leaves, that is,

H=\sum _{k=0}^{n}H_{k}(q_{k},p_{k})

. In this case phase volume is preserved in leaves. If not, however, the mixed partial derivatives mean that cross-leaf dependencies appear in Hamilton's equations.--Eh9 (talk)

There's already a Wikipedia article on the Double pendulum.--Eh9 (talk) 02:51, 24 March 2008 (UTC)[reply]

Ta - (Lets continue the discussion of the double pendulum on your user page first). On the main issues, as I've written on your userpage, I think the statement on dpi v dqi is true to first order, for small times, for the faces of a suitably aligned small hypercube in phase space. Which is all that's really being claimed. The longer term gyrations and twists are by the by. Bob aka Linuxlad (talk) 08:47, 24 March 2008 (UTC)[reply]

Footnote - we've been here before, in both my physicists' 'baby notation' and Penrose's 2-forms - see March 2005 versions and p484 in the Penrose tome. Bob aka Linuxlad (talk) 13:58, 26 March 2008 (UTC)[reply]

There's a good treatment of Liouville's theorem in V.I.Arnold's Mathematical Methods of Classical Mechanics (in the 2nd edition, it's in section 16). It does not use differential forms but only basic calculus. The version with the Hamiltonian flow and the canonical symplectic form comes later in that book. The language of this proof can be extended to provide a disproof of the challenged assertion.
Penrose simply used 2-forms. Élie_Cartan invented differential forms, and, as I understand it, he did so for Einstein to do GR with (although I don't believe he ever switched), and, again as I recall, this was in the 1920's. Darboux's theorem in its original form is from 1882, and recognized as a 2n-form shortly after those objects were invented. It's not like this stuff is particularly recent. —Preceding unsigned comment added by Eh9 (talk • contribs) 15:52, 27 March 2008 (UTC)[reply]

I just now removed the troublesome sentence. I don't know why y'all didn't remove this 16 years ago. Yes, I suppose if you are able to find suitably diagonalized conjugate coordinates, this statement would be true (it is generally true in symplectic geometry, if I'm not mistaken. but ...). But that statement occurs before any discussion of being able to diagonalize, or more generally to find foliations of the Hamiltonian system that factorize it appropriately, a la symplectic geometry. At any rate, it is not "straightforward", and not something that is "simply found with a web-search."

Also, p.s. the two-form you are looking for is the symplectic form. This is generally covered in textbooks on classical mechanics. 67.198.37.16 (talk) 23:48, 19 May 2024 (UTC)[reply]

Missing premise

The Liouville equation as stated is only true when ${\frac {\partial H}{\partial t}}=0$ .--Eh9 (talk) 11:28, 26 March 2008 (UTC)[reply]

non-equilibrium system

Under "remarks" it reads "The Liouville equation is valid for both equilibrium and nonequilibrium systems." Can anyone make this more clear? Because I have textbooks here, which say that the equation only holds for equilibrium systems.. —Preceding unsigned comment added by 130.60.5.218 (talk) 09:17, 19 October 2009 (UTC)[reply]

Add more complete proof?

The previous two posts have me convinced that we need the actual proof. The article does outline the proof but I think we need a wee more. Here's why:

One commenter felt confused by the statement that the theorem is true for non-equilibrium cases. I believe that the statement in Wikipedia is true, but must confess that I could not define non-equilibrium without giving that word a great deal of thought. A simple algebraic proof would allow me to stop wondering what a word means.
The second comment, that a condition of the theorem to be valid is, $\partial H/\partial t=0$ , is something I do not recall. It might be true, but a proof would clarify the question without ambiguity; it would be nice to see what goes wrong if the partial derivative does not vanish.
The fact that proofs are found in physics textbooks is not helpful for those without convenient access to those books. For better of for worse, Wikipedia is the prime source of information for lots of people.
It is a beautiful proof, as I recall.--guyvan52 (talk) 15:52, 22 December 2013 (UTC)[reply]

I agree, if possible, a clean proof ought to be added. In general I'm not such a big fan of tedious proofs on wikipedia (especially not if they involve many tedious steps or sneaky approximations), however short and elegant ones can be illuminating more than words. Exactly as you say, I think it would clear up the range of validity. Nanite (talk) 21:59, 17 February 2014 (UTC)[reply]

I added a proof as a link to Wikiversity a while back. Looking at I realized I made two mistakes, both due to my inexperience with such issues: First I made it a Permalink, which would have served to suppress edits to that link. Second, I should created a Wikiversity page devoted exclusively to that link. I am now attempting to rectify both errors.

Here is an internal link to the sister wiki.

I will now insert this new link into the article.--guyvan52 (talk) 02:06, 18 February 2014 (UTC)[reply]

Questionable reference

The questionable reference is http://www.lecture-notes.co.uk/susskind/classical-mechanics/lecture-7/liouvilles-theorem/

I hate to remove it because it is well-written and because it takes a fresh approach to showing the incompressible nature of flow in Hamiltonian phase space. But one sentence bothers me:

Notice that, in many dimensions, we don't require that for each i,
${\frac {\partial v_{qi}}{\partial q_{i}}}+{\frac {\partial v_{pi}}{\partial p_{i}}}=0$
meaning that there isn't conservation of points in a particular region, but we can say that if the velocity increases in one direction, then it must decrease in another direction by the same factor.

Am I missing something here?-- guyvan52 (talk) 14:48, 8 January 2014 (UTC)[reply]

who did it first

I am not so sure Gibbs was the first. Two points of evidence:

Gibbs says in 1884 in the abstract of a paper titled "On the Fundamental Formula of Statistical Mechanics, with Applications to Astronomy and Thermodynamics.", the first time he used a primitive form of LT: "The object of the paper is to establish this proposition (which is not claimed as new, but which has hardly received the recognition which it deserves) and to show its applications to astronomy and thermodynamics."
Gibbs in his 1902 book, third chapter, cites the entire chapter (which relies on LT) as a follow up to a paper of Boltzmann from 1871. Cercignani's Ludwig Boltzmann: The Man Who Trusted Atoms interprets this as meaning that Boltzmann was already using LT in 1871. Can't confirm this one unfortunately, I can't read German.

Probably it's true that Gibbs was the first to appreciate the full LT application to thermodynamics, and very likely we can say Gibbs was the first to formulate it in the modern way (in 1902). I've updated the language in the text a bit to reflect this. Probably would be good to have one more citation (from a modern stat mech textbook) on the origin of LT. Nanite (talk) 21:55, 17 February 2014 (UTC)[reply]

Nolte has a good historical paper on this.^[1] It is probably Jacobi who should be credited with recognizing LT in a mechanical sense.

References

^ Nolte, David D. (2010). "The tangled tale of phase space". Physics Today. 63 (4): 33–38. doi:10.1063/1.3397041. ISSN 0031-9228.

Yes, this article sorely needs a history section. 67.198.37.16 (talk) 00:09, 20 May 2024 (UTC)[reply]

Liouville's theorem

What is given in the article are only applications of the theorem, but not the theorem itself. The original Liouville's theorem states that the phase space volume remains invariant upon canonical transformations of the generalised coordinates and momenta of a given mechanical system. In its proof a couple of properties of the Jacobian are used. In formulas:

\int d\Gamma ={\text{const}}

where

d\Gamma =dq_{1}\cdots dq_{s}dp_{1}\cdots dp_{s}

is the volume element of the phase space. This general form can be used in many more fields that just statistical physics, where it has been used by Gibbs. Nothing of this is in the article. The true (general) Liouville theorem and its proof are given in vol. 1 Classical Mechanics of the Landau & Lifshitz series. In the original Liouville paper from 1838, the proof uses the same idea but realizes it in a more complicated and roundabout way (the Jacobian was not yet invented then). Lantonov (talk) 12:18, 27 October 2020 (UTC)[reply]

Huge omission

The article never describes what kind of dynamical system Liouville's theorem applies to.

Instead, it just plunges into describing the equations relevant to Liouville's theorem.

It would be a very good idea if the article specified what type of dynamical system these equations apply to.

Certainly there are many dynamical systems that do not preserve a measure that is positive on all non-empty open sets. 2601:200:C000:1A0:494:DE11:8C09:9B16 (talk) 16:24, 30 September 2021 (UTC)[reply]

Good question. A measure-preserving dynamical system, for starters. But it would have to be one defined on a symplectic manifold, I guess. However, both of these are rather abstract concepts; I don't know if there is a simpler way to put it. I also don't know if there are examples that are not both measure-preserving and also symplectic. Perhaps there would be examples from sub-Riemannian geometry but I don't know.

The lede does link to conservative system (aka "incompressible dynamical system") which then invokes the ergodic decomposition theorem (red link), which in simple terms says that Liouville's thm applies only after you've waited long enough for all friction effects to wander away (dissipative system, wandering set). The original intent was to also exclude chemical reactions, which would change the number of particles, and/or transfer energy to internal degrees of freedom (grand canonical ensemble). I don't know enough about chemical thermodynamics to state all this correctly. 67.198.37.16 (talk) 22:47, 19 May 2024 (UTC)[reply]

I changed the lede to say the above. I punted on the chemistry angle, though. A single sentence at the end of the article mentions this in passing. It needs expansion. In the meanwhile, I think the current lede will do. 67.198.37.16 (talk) 00:05, 20 May 2024 (UTC)[reply]

Energy conservation requirements

The `Damped Harmonic Oscillator' section makes the claim that for Liouville's theorem to hold energy must be conserved, but this is at best misleading. The only true requirement is that the phase space evolves following Hamilton's equations (which the damped harmonic oscillator doesn't), but if we have a time-dependent hamiltonian Liouville's theorem will still work. --J B 14:56, 13 December 2022 (UTC)[reply]

Missing treatment of statistical systems

This article is missing a discussion & treatment of the general statistical/ergodic setting. All we've got is just one sentence, at the end:

The Liouville equation is integral to the proof of the fluctuation theorem from which the second law of thermodynamics can be derived. It is also the key component of the derivation of Green–Kubo relations for linear transport coefficients such as shear viscosity, thermal conductivity or electrical conductivity.

The primary issue here is that these topics all require discussions of internal (microscopic) degrees of freedom, whereas the theorem as stated applies only to conservative systems, where friction is absent. This is a huge gap in the current article. 67.198.37.16 (talk) 00:02, 20 May 2024 (UTC)[reply]

[Nolte2010-1] Nolte, David D. (2010). "The tangled tale of phase space". Physics Today. 63 (4): 33–38. doi:10.1063/1.3397041. ISSN 0031-9228.

[1]