Quantum Motion from Local Transition Susceptibility
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Abstract
We characterize quantum motion based on the susceptibility of a quantum state to weak local state conversion. The inverse susceptibility can be interpreted as a passage speed characterizing local motion independently of probability transport. For individual WKB branches, it reproduces the magnitude of the local dispersion velocity in both propagating and evanescent regions. A relativistic extension yields a passage speed bounded by the speed of light, reaching this bound at the finite energy corresponding to the center of the mass gap, where the evanescent decay length equals the reduced Compton wavelength. This identifies the reduced Compton wavelength as the shortest stationary evanescent length scale compatible with relativistic quantum motion.