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Conditional probabilities in quantum systems which have both initial and final boundary conditions are commonly evaluated using the Aharonov–Bergmann–Lebowitz rule. In this short note, we present a seemingly disturbing paradox that appears when applying the rule to systems with slightly broken degeneracies. In these cases, we encounter a singular limit—the probability “jumps” when going from perfect degeneracy to negligibly broken one. We trace the origin of the paradox and solve it from both traditional and modern perspectives in order to highlight the physics behind it: the necessity to take into account the finite resolution of the measuring device. As a practical example, we study the application of the rule to the Zeeman effect. The analysis presented here may stress the general need to first consider the governing physical principles before heading to the mathematical formalism, in particular, when exploring puzzling quantum phenomena.


This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in:

Yakir Aharonov, Eliahu Cohen, and David H. Oaknin, "Why physical understanding should precede the mathematical formalism—Conditional quantum probabilities as a case-study," Am. J. Phys. 87, 668–673 (2019).

and may be found at DOI: 10.1119/1.5115980.

This article was originally published in American Journal of Physics, volume 87, in 2019.

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American Association of Physics Teachers



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