# Why physical understanding should precede the mathematical formalism - conditional quantum probabilities as a case-study

## TL;DR

This paper investigates a paradox in quantum conditional probabilities related to degeneracies, emphasizing the importance of physical principles over mathematical formalism, with a case study on the Zeeman effect.

## Contribution

It highlights the necessity of considering physical measurement limitations before applying quantum formalism, resolving a paradox involving degeneracies in conditional probabilities.

## Key findings

- Identified a paradox in quantum conditional probabilities with degeneracies.
- Resolved the paradox by incorporating finite measurement resolution.
- Illustrated the importance of physical principles in quantum formalism.

## Abstract

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.