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Equation 69 · Part 5 · The Entry a Relabeling Cannot Write

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ΔEΛ(ρ)=Eb\Delta E_\Lambda(\rho) = E_b
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What this part means

Capital delta attached to a quantity marks a difference between two values of that quantity; the article’s sign convention determines the order.

Its job in the formula

Capital delta attached to a quantity marks a difference between two values of that quantity; the article’s sign convention determines the order.

The passage around this formula

Two limits inside this one formula answer different questions about how confidently the ledger can be interpreted. First, an uncorrelated, non-interacting bath: set HSBH_{SB} = 0 and let ρ\rho = ρS\rho_S ⊗\otimes |b⟩\rangle⟨\langle b| for any system state ρS\rho_S and any bath eigenstate |b⟩\rangle with HBH_B|b⟩\rangle = EbE_b|b⟩\rangle . Then Δ\Delta EΛ(ρ)E_\Lambda(\rho) = EbE_b for every choice of ρS\rho_S whatsoever — a pure constant, entirely independent of what the system is doing, unambiguously assignable to a named, located pile of energy sitting in a part of the world the coarse description agreed in advance not to look at. Nothing is missing in any troubling sense here; the coarse observer is simply declining…

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Learn the underlying idea

Addition combines quantities; subtraction measures the signed difference between them. Parentheses show what is combined before the rest of the expression is evaluated.

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Sources cited in the article section

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