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Equation 3 · Part 5 · Decoherence: The Quiet Selection That Makes the World Look Solid

=

ρ(x,x′,t)=ρ(x,x′,0) exp⁡ ⁣[−Λ(x−x′)2t]\rho(x, x', t) = \rho(x, x', 0)\, \exp\!\left[-\Lambda (x - x')^2 t\right]
=

What this part means

The expressions on both sides represent the same quantity under the stated assumptions.

Its job in the formula

The equals sign connects the complete expression on the left with the complete expression on the right. Both sides must have compatible units.

The passage around this formula

Joos and Zeh made this quantitative in 1985 by deriving a genuine, non-phenomenological master equation for the reduced density matrix of an object’s centre-of-mass position under repeated recoil-free scattering [ 7 ] . For the off-diagonal elements of that density matrix — the very quantity whose survival or destruction is the entire question of macroscopic superposition — their result, in the short-time, many-collisions regime, takes the form of a simple exponential decay in the separation between the two positions being superposed: ρ(x,x′,t)=ρ(x,x′,0) exp⁡ ⁣[−Λ(x−x′)2t]\rho(x, x', t) = \rho(x, x', 0)\, \exp\!\left[-\Lambda (x - x')^2 t\right]. Here Λ\Lambda is what Joos and Zeh call the localization rate: a single number, with units of inverse length squared per unit time, built…

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Learn the underlying idea

An equals sign says that the expression on its left and the expression on its right have the same value under the stated definitions and assumptions.

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Sources cited in the surrounding passage

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