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Equation 22 · Part 9 · The Statistical Mechanics of Irreversibility at Molecular Scale

Denominator: langleSigma_τrangle

Var⁡(Jτ)⟨Jτ⟩2≥2⟨Στ⟩.\frac{\operatorname{Var}(J_\tau)}{\langle J_\tau\rangle^2} \ge \frac{2}{\langle\Sigma_\tau\rangle}.
⟨Στ⟩\langle\Sigma_\tau\rangle

What this part means

The complete quantity below the fraction bar; it must be nonzero for this division.

Its job in the formula

langleSigma_τrangle occurs below the fraction bar. The quantity above the bar is divided by this expression; zero is excluded as a denominator.

The passage around this formula

A molecular clock or motor is useful not only when its average current is nonzero but when that current is sufficiently precise. Let JτJ_\tau be an integrated current observed for time τ\tau , such as net molecular steps or transported ions. Define dimensionless mean entropy production ⟨\langleΣτ\Sigma_\tau⟩\rangle=⟨\langleΔ\Delta stots_{\mathrm{tot}}⟩\rangle/kBk_{\mathrm B} . A canonical steady-state thermodynamic uncertainty relation has the form Var⁡(Jτ)⟨Jτ⟩2≥2⟨Στ⟩\frac{\operatorname{Var}(J_\tau)}{\langle J_\tau\rangle^2} \ge \frac{2}{\langle\Sigma_\tau\rangle}. The relation says that suppressing relative current fluctuations requires dissipation. Barato and Seifert introduced this precision–cost connection for biomolecular processes [ 5 ] , and later large-deviation results established broad bounds…

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Learn the underlying idea

A fraction a/b means a divided by b. The top number is the numerator; the bottom number is the denominator, and it cannot be zero.

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Sources cited in the surrounding passage

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