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Equation 42 · Part 2 · How Fast Can a Horizon Learn Which Path You Took?

Symbol t_c

Dirr(tc)=inf⁡u∈A(tc)12∥ρ0,Hu−ρ1,Hu∥1.D_{\rm irr}(t_c)= \inf_{u\in\mathcal A(t_c)} \frac12\left\|\rho^{u}_{0,H}-\rho^{u}_{1,H}\right\|_1.
tct_c

What this part means

the become irrevocable at an intermediate cutoff.

Its job in the formula

tct_c is an argument of the function-like quantity on the left; its role is set by that function’s stated inputs.

Where the article explains it

It does not answer how much information has become irrevocable at an intermediate cutoff tct_c .

The passage around this formula

Define A(tc)\mathcal A(t_c) as a preregistered set of physically admissible future protocols after the cutoff. It must specify which controls Alice retains, their duration, energy, acceleration, localization, and boundary conditions. For each u∈\inA(tc)\mathcal A(t_c) , let ρ0,Hu\rho^{u}_{0,H} and ρ1,Hu\rho^{u}_{1,H} be the completed conditional states on the declared horizon algebra. Then define an irreducible distinguishability envelope Dirr(tc)=inf⁡u∈A(tc)12∥ρ0,Hu−ρ1,Hu∥1D_{\rm irr}(t_c)= \inf_{u\in\mathcal A(t_c)} \frac12\left\|\rho^{u}_{0,H}-\rho^{u}_{1,H}\right\|_1. The infimum asks how little path distinguishability remains after the best allowed attempt to erase it. For equal priors, an associated unavoidable decision information is

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