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Equation 3 · Part 1 · How Fast Can a Horizon Learn Which Path You Took?

Symbol nu

ν(T)=⟨h0(T)∣h1(T)⟩,V(T)=∣ν(T)∣.\nu(T)=\langle h_0(T)|h_1(T)\rangle, \qquad V(T)=|\nu(T)|.
ν\nu

What this part means

nu is part of the quantity the equation computes from the expression on the right.

Its job in the formula

nu is part of the quantity the equation computes from the expression on the right.

The passage around this formula

Tracing out the field multiplies the off-diagonal element of the path density matrix by the conditional-state overlap ν(T)=⟨h0(T)∣h1(T)⟩,V(T)=∣ν(T)∣\nu(T)=\langle h_0(T)|h_1(T)\rangle, \qquad V(T)=|\nu(T)|. Under balanced arms and otherwise ideal readout, V is the remaining fringe visibility. That statement belongs to ordinary complementarity, not black-hole mysticism. Englert derived a quantitative visibility–which-way bound for a two-path interferometer, and atom-interferometer experiments later measured visibility and distinguishability independently in agreement with the relation [ 11 , 12 ] .

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A function assigns an output to each allowed input. The expression f(x) means “apply f to x”.

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