Foundations of Physics

, Volume 43, Issue 10, pp 1252–1256 | Cite as

A Conservative Solution to the Stochastic Dynamical Reduction Problem

Case of Spin-z Measurement of a Spin-1/2 Particle
  • T. Halabi


Stochastic dynamical reduction for the case of spin-z measurement of a spin-1/2 particle describes a random walk on the spin-z axis. The measurement’s result depends on which of the two points: spin-zħ/2 is reached first. Born’s rule is recovered as long as the expected step size in spin-z is independent of proximity to endpoints. Here, we address the questions raised by this description: (1) When is collapse triggered, and what triggers it? (2) Why is the expected step size in spin-z (as opposed to polar angle) independent of proximity to endpoints? (3) Why does spin “lock” in the vertical directions? The difficulties associated with (1) are rooted, as is Bell’s theorem, in the time-asymmetric assumption that the present distribution over hidden variables is independent of future settings. We believe, a priori of any of the experiments of modern physics, that such a time-asymmetric assumption is dubious when probing the microscopic scale. As for (2) and (3), they are simultaneously resolved by abandoning the fundamental distinction drawn between spin and spatial angular momentum, and by appealing to very tiny (in both magnitude and spatial extent) but numerous patches of magnetic noise in the Stern-Gerlach’s field.


Gambler’s ruin Arrow of time Quantum measurement Spin lock Random walk 



I wish to thank professors Anthony Leggett for pointing out the gambler’s ruin background, Michael Weissman for reminding me to be mindful of the gyromagnetic ratio issue, and Lawrence Schulman for providing the probability formula for first incidence in a random walk (later matched with the same formula in the gambler’s ruin description).


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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  1. 1.Institute for Condensed Matter Theory, Department of PhysicsUniversity of Illinois at Urbana-ChampaignUrbanaUSA

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