Equation 36 · Part 2 · The Entry a Relabeling Cannot Write
Symbol ρ
What this part means
ρ is an argument of the function-like quantity on the left; its role is set by that function’s stated inputs.
Its job in the formula
ρ is an argument of the function-like quantity on the left; its role is set by that function’s stated inputs.
Full expression→Symbol ρ→Article meaning
The passage around this formula
Before asking what a genuine coarse-graining exposes, the construction has to survive the case where nothing at all is discarded. Take = — no reduction in the space of states — and let the “coarse” description be nothing more than the same physics seen through a fixed, time-independent relabeling: a rotated spin basis, a renamed pair of levels, a change of which linear combination of states a lab calls “up.” Model this as := W for a fixed unitary W , and, because nothing has been thrown away, define the coarse Hamiltonian self-consistently as the same physical generator carried through that same relabeling, := W .
Learn the underlying idea
A variable is a named place for a value. Its letter is a local label: x can mean position in one formula and a data point in another.
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