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Finite-size mixing and distinguishability conventions

Physical scope

Exact finite-N calculation for two equal ideal-gas compartments at equal temperature; distinguishes adopted indistinguishable-label and fixed-species conventions and reports no empirical observation.

Example

Take two initially separate equal-volume boxes, each containing N≥1 particles of the same mass m>0 at the same T>0, with ideal Hamiltonian K=∑pj2/(2m). Adopt the classical reference with N! counting for indistinguishable labels and fixed action reference a0>0. Removing the partition while fixing total particle number 2N gives ΔSidenticalkB=log⁡ ⁣(4N(N!)2(2N)!)>0, whereas for two distinguishable species with fixed N of each, ΔSspecieskB=2Nlog⁡2. The probability under the final identical-particle positional law of finding exactly N particles in each half is (2NN)/4N, the reciprocal factor inside the logarithm above. The identical-particle entropy change is O(kBlog⁡(N+1)), hence sublinear in N; no exact zero at finite N is claimed. For the distinguishable-species comparison, initially confine species A (exactly N particles) to the first box and species B (exactly N) to the second; after removing the partition, each species can occupy the full volume 2V.

Facts & Assumptions

Given: The two ideal-gas preparations, fixed action reference, equal temperature and the stated particle-counting convention.

[A1]

Each ideal gas uses the adopted classical 1/N! quotient-counting reference when labels are operationally indistinguishable (Classical particle counting and physical observables).

[A2]

Both states are canonical at the same β=(kBT)−1>0 with the SI temperature convention SI quantities and unit conventions, and the position volumes are V initially per side and 2V after partition removal.

[F1]

The Gaussian integral ∫−∞∞e−x2 dx=π proves ∫Re−x2dx=π. For each a>0, y=a x is a linear diffeomorphism with Jacobian a, so ∫Re−ax2dx=a−1/2∫Re−y2dy=π/a. Applying this scaling separately in each momentum coordinate gives the stated product of Gaussian factors.

[F2]

For a canonical Gibbs density, the entropy identity is S=kB(log⁡Z+β⟨H⟩), the equality case of the reference-relative Gibbs identity (Reference-relative entropy and Gibbs variational inequality).

Verification

Given: [A1]–[A2] and the counting, Gaussian and canonical entropy interfaces [F1]–[F2].

1.1A1A2F1F2algebra

For r particles in position volume W, separate the r position integrals and use [F1] in all dr momentum coordinates. For one coordinate put a=β/(2m); integrating ddp(pe−ap2)=e−ap2−2ap2e−ap2 over R gives ⟨p2⟩=m/β, as the boundary term vanishes by Gaussian decay. Thus the mean kinetic energy is dr/(2β). The position law factorizes into r uniform position laws. With λT=a0β/(2πm), the partition function is Zr(W)=Wrr!a0dr(2πmβ)dr/2=(W/λTd)rr!. For equal total particle count 2N before and after, the βU terms in [F2] cancel, so the entropy change is kB times the logarithm of the partition-function ratio.

2.1A1A2step 1.1algebra

Initially the ratio is Z2N(2V)/ZN(V)2 =4N(N!)2/(2N)!; for two fixed distinguishable species it is [ZN(2V)/ZN(V)]2=4N. The first ratio exceeds one because (2N)!/(N!)2=(2NN)<∑j=02N(2Nj)=4N. Under the final positional law, the 2N particles independently fall in either half with probability 1/2, so the central binomial probability is (2NN)/4N, the reciprocal of the identical-particle ratio.

3.1A1step 2.1algebra∎

For each integer n≥1, monotonicity of log⁡x on unit intervals gives nlog⁡n−n+1≤log⁡(n!)≤nlog⁡n−n+1+log⁡n. Indeed the lower bound integrates log⁡x≤log⁡j on [j−1,j] for j=2,…,n, and the upper bound integrates log⁡x≥log⁡j on [j,j+1] for j=1,…,n−1, then adds log⁡n. Substitution for n=N and n=2N cancels the terms linear in N in Nlog⁡4+2log⁡(N!)−log⁡((2N)!), leaving a quantity bounded in absolute value by a constant multiple of log⁡(N+1). Therefore ΔSidentical/N→0 as N→∞.

Remarks

This is a finite mathematical consequence of the two declared counting conventions. It is not a mixing experiment, and the macroscopically sublinear correction does not make the finite-N entropy change vanish.

Depends on

Dependency tree · 2 levels

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