Talk:Microcanonical ensemble

message: reason for a recent revert:
some edits are of dubious nature:
1. the symbol Z is used before ever defining ( it is in fact the partition function, in this case, multiplicity. and that's explained in the current version.)
2. there is no need to used generalized function here. one can just simply take that symbol, in this case, to mean the density function is 1 on the level set of interest and zero elsewhere.
3. the phase space measure is the Lebesgue measure. if you'd like to add something, don't remove what's correct.
4. Liouville's thm states that that measure is invariant under the flow. again, welcome to add the Poisson bracket, don't remove what's correct.
5. it's already obvious from the discussion that \; \Omega is the multiplicity. Mct mht 14:56, 23 May 2006 (UTC)
replies: text: Can we at least keep some of the matrix formalism which the previous posts alluded to? --HappyCamper 17:03, 23 May 2006 (UTC)

text: if you're referring to the density operator of a MCE, it's already in the article. Mct mht 17:07, 23 May 2006 (UTC)
replies: text: Ah yes, I see it now. Still rather technical of an article though. I think later I might add some stuff when I get a chance. --HappyCamper 17:13, 23 May 2006 (UTC)
thread_title:  reason for revert 

message: an ensemble of quantum mechcanical harmonic oscillators (or some other system that lends itself easily to both treatments) can be added as a good example comparing the semi-classical approach and the density matrix formulation. Mct mht 19:03, 23 May 2006 (UTC)
thread_title:  suggestion for addition to article 

message: I think my (Top.Squark) revision is correct and should be restored. Explanation:
1. Z is defined when first used: "Z is a normalization constant" I.e. Z is such a constant
that the integral of rho over the phase-space is 1. You are right that it is the same as
\; \Omega, which should be made explicit.
2. Rho cannot be "1 on the level set of interest and zero elsewhere" because:
a. Integral of rho would be _zero_ rather than 1 as suitable for a probability distribution
since it's supported on a set of _zero measure_.
b. The expectation value of an obserable A is integral of A rho and for your
function it would be _always zero_.
This is precisely the kind of thing generalized functions were invented for!
3. The phase space (Liouville) measure_is not_ the measure induced by the microcanonical
ensemble since the later is supported on a level submanifold of H and is a probability
measure while the former is neither.
4. Liouville theorem states the _Liouville measure_ is invariant, not the microcanical
ensemble measure.
5. It was not stated that \; \Omega is the multiplicity earlier (in my revision).
6. What do you mean? The constant energy surface _is_ embedded in the phase space.
7. What it has to do with my changes?
Top.Squark 10:18, 24 May 2006 (UTC)
thread_title:  reply to "reason for revert" 

message: 1. In my revision it wasn't explicit and I agreed it should be explicit. This is the part where I'm agreeing with you.
2. "he support of ρ has Lebesgue measure 0? then the ρ is essentially the zero measure?! clearly nonsense." Right, nonsense!! What you describe is nonsense _because_ it leads to rho being the zero measure, which it shouldn't be! The correct function is rho = Omega^-1 delta(H-E), _not_ "1 on the level set of interest and zero elsewhere".
A generalized function, to your information, is a functional on a function space is context-dependant in general, but is usually taken either as the Shwartz space (smooth functions that decay faster than polynomial in infinity together with all derivatives) or the space of compactly supported smooth functions. In this case the functional is integral over M_E divided by M_E's hyperarea (Omega).
3. In the discussion you mentioned XaosBits correctly says that dp dq (in the simplest case, i.e. a point particle in 1 space dimension). If you intented that dp dq is invariant under the flow according to Liouville's, that's correct and can be integrated into the article. However, your statement "this is also the measure induced by the microcanonical ensemble density function" is incorrect since the later is Omega^-1 delta(H-E) dp dq, not just dp dq.
4. Not at all (see above)
5. It _was_ stated in my revision, but not earlier!! First you claim that "it's _already obvious_ from the discussion that Omega is the multiplicity" and now you say "that's right, it wasn't stated in you edits. that's why _it's not clear_"??
6. The constant energy surface _is_ a submanifold. What's the problem?
7. I don't see any contradiction between ergodicity and generalized function. What are you getting at?
Top.Squark 19:05, 24 May 2006 (UTC)
replies: text: arguably, this could be related to the "degeneracy" of the microcanonical ensemble, referred to in the 2nd sentence of the article. Mct mht 03:47, 25 May 2006 (UTC)
thread_title:  Reply to Mct mht 

message: at first glance the main issue (of the conversation above) is resolved by a recent edit by 62.0.156.172(Top quark?). whoever editted it, can you provided a reference on the way that measure is obtained? i didn't see anything incorrect, just would like a reference. also, the measure theoretic language regarding Liouville's theorem should be added back, as it's not immediately clear one can talk about Poisson's bracket in a weak(distribution) sense. (if you wanna be fully rigorous, as you seem to, better to use the measure you constructed in detail.) Mct mht 16:45, 25 May 2006 (UTC)
thread_title:  comment on recent edit 

message: Is there a way to incorporate both teaching methodologies into the article? --HappyCamper 19:31, 24 May 2006 (UTC)
thread_title:  Compromise 

message: Can we add a brief comment about the p's and q's in the article? --HappyCamper 20:35, 24 May 2006 (UTC)
thread_title:  p and q 

message: This is one of several articles in this area that can easily loose a large part of its target audience. Can undergraduate physicists and chemists read the first 3 paragraphs and get a clear idea of the ensemble, and browse the remaining paragraphs without getting jargon overload?
Bob aka Linuxlad 20:51, 24 May 2006 (UTC)
thread_title:  target audience 

message: I submitted a new revision which hopefully combines both point of view.
The old revision refers to the constant energy hypersurface as the "phase space" (which is confusing) and works w.r.t. the natural measure there without stating so explicitely (even using notation which suggests the contrary, e.g. dp dq which only correctly describes the measure on the _full phase space_). That "local" point of view is the most natural in the context of ergodic theory, but is less favorable in some other contexts, for instance when one wishes to compare the microcanonical ensemble to other ensembles used in thermodynamics (e.g. the canonical ensemble).
The new verision uses accurate notation (using the "global" phase space as a starting point) but does not de-emphasize the local point of view like my previous revision. I added a paragraph explaining how the natural measure on the constant energy hypersurface is defined in a rigorous manner (it of course follows from the Dirac delta-function expression for the density on the full phase space).
Also, I noticed there have been complaints about the level of technicality of the article. I personally do not think technical jargon should or can be avoided in a satisfactory presentation. If one would integrate the detailed explanation of every technical term (e.g. the Dirac delta-function, the Liouville measure etc.) into the article it would become oversized and unfocused. On the hand, avoiding the use of such notions altogether would make the article unrigorous, lower its information content and render it completely unsatisfactory for the more sophisticated reader. The reader unfamiliar with one of these notions should address the separate Wikipedia article on the subject (especially given most of those are linked to this one).
Top.Squark 16:36, 25 May 2006 (UTC)
replies: text: overall i think it's a good edit, see comment above. Mct mht 16:56, 25 May 2006 (UTC)

text: I think the Liouville stuff will stay -- it just means that this article needs more development. Most articles on Wikipedia do not mature until well over a year or two of continuous editing. Well, unless there's an active push for it, if we're lucky, a few months. --HappyCamper 19:49, 25 May 2006 (UTC)
replies: text: I'm quite happy with Liouville's theorem being in there (It's pretty fundamental to the way 3rd year physicists were taught it years ago - and it clarifies that the phase space units dp.dq occur as the appropriate weightings in Boltzman etc), but we didn't need the Liouville or Lebesgue measures, and even now I'm not too sure what a sub-manifold is...  Bob aka Linuxlad
per Top.Squark's comment, as far as technical sophistication goes, this article is just fine. these are essentially rigorous versions of presentations found in graduate (possibly undergraduate as well) physics texts, see, for example, Statistical Mechanics by Pathria. if one would like, one can maybe add a note saying, for example, in case the reader is not familiar with the mathematical machinery used, some notions can be interpreted naively and article can still be understood. also, the title "deeper relations..." is rather misleading. it's also misleading to say "There are many different approaches to deriving the ensemble properties..." Mct mht 20:56, 25 May 2006 (UTC)
thread_title:  New version 

message: Well, the title explains it all.. --HappyCamper 21:09, 25 May 2006 (UTC)
thread_title:  "Deeper" for want of a better word... 

message: There is a slight technicality about the natural measure on the constant energy hypersurface. The Liouville measure on the full phase is just the Lebesgue measure on R^2n (given the phase space is R^2n). However, the induced measure on the constant energy hypersurface is not hyperarea in the sense of Euclidean geometry. For example, given a 1-dimension particle with phase space R^2, and Hamiltonian
H = p^2 / 2m + V(q), the measure on the p^2 / 2m + V(q) = E curve is not just arc-length. The correct measure of an arc is the time for the system to transgress it. For instance, if two plateaus (V = const) with different height are present, the measure on each will be m/sqrt(2m(E-V)) dq, hence normalized differently (due to different V). In the harmonic oscillator example, the measure will equal (up to a constant) arc-length only for a special choice of coordinates (sqrt(k/2)q, sqrt(1/2m)p) and for most systems no natural choice of coordinates will do the trick. Therefore I removed a paragraph stating dA is "just" the natural measure up to a constant: the definition of "natural measure" is precisely the definition of dA I wrote, nothing less.
Top.Squark 12:01, 27 May 2006 (UTC)
replies: text: Can we add a bit more material relating to Gibbs work? --HappyCamper 14:26, 27 May 2006 (UTC)
thread_title:  The natural measure on the constant energy hypersurface 

message: To understand the meaning of the microcanonical ensemble it is instructive to understand the distinction between a microstate and a macrostate:
The term macrostate indicates the overall thermodynamic state in a system. In an isolated system, the macrostate is defined by the type of substance considered, the amount of substance, N, its internal energy, U, and its volume, V. A macrostate thus refers to the bulk properties of a given system.
A microstate on the other hand is the most accurate and precise (without violating Heisenberg's uncertainty principle) description of a physical system. If a system is characterised by a certain macrostate (N,U,V), there is an enormous number of possible microstates. For example, if you know the position and the momentum of every particle in your system, you know its microstate. It therefore becomes apparent that the microstate of a physical system that contains a reasonable number of constituents (eg particles or strings), say N=10^{23}, changes at a rate of about 10^{23} times every second...  Clearly then some kind of averaging procedure is needed and this is where the microcanonical ensemble comes into play. The microcanonical ensemble, \Omega, equals the total number of microstates of a given system. For example, for the whole observable universe
:\Omega\sim e^{10^{86}}.
As one would then expect, quantum mechanics comes to our rescue because it is (at least conceptionally) far easier to count a discrete number of states than it is to count a continuum of states.
</blockquote>
while i don't dispute its veracity (for the most part), may i suggest considering relocating it to microstate (statistical mechanics). if it's insisted that the section be in the article, perhaps retool it a bit (the last three sentences in particular)? Mct mht 17:39, 10 April 2007 (UTC)
thread_title: Distinguishing Microstates from Macrostates

message: I think this text needs to be rewritten:
'Notice that, for the microcanonical ensemble, Ω plays the role of the partition function in the canonical and grand canonical ensembles.'Paranoidhuman (talk) 21:59, 11 March 2009 (UTC)
thread_title:  In the Entropy Section 

message: haned out my homwework of SM. Thanks god.
Tomorrow,QM h.w.
remains only one problem in it.  —Preceding unsigned comment added by 140.123.72.3 (talk) 05:17, 27 October 2009 (UTC)
thread_title: I have already

message: As it stands I'm not sure the content of this section is appropriate for a wikipedia article, as it reads more like a homework solution than a general overview of the microcanonical ensemble. I would propose deleting this section, but otherwise the section needs a cleanup regardless as it is full of spelling mistakes and formatting problems. 
AwkwardWhale (talk) 14:05, 9 December 2020 (UTC)
thread_title:  Delete section "Perfect gas entropy"?  

message: This is my first time here so I apologize if I am doing this wrong. But I have a few questions and I hope that this is the correct place to ask them.
Under the subsection Ideal gas within Examples section:
# Regarding the Sackur–Tetrode equation, what exactly is the energy E here? (Following the link on this page gives a slightly different, and to me more familiar and explicit, form of it.)
# Furthermore, is the E in the derivation of chemical potential the ideal gas' internal energy (sometimes denoted by U, like on the Sackur–Tetrode equation page)? Is there a equation for it in some simple case?
# Finally, I think there might be a missing minus sign in said chem. potential equation. Am I missing something in the derivation?
Thank you! Ivaya007 (talk) 14:47, 17 August 2023 (UTC)
thread_title:  Questions regarding ideal gas example 

message: As far as I can tell, $\omega$ is only vaguely defined as the 'energy' spacing. It is mentioned in the expressions for the entropy and is therefore important to at least know the definition. 131.211.54.68 (talk) 10:17, 22 April 2024 (UTC)
thread_title:  How is $\omega$ defined?