Van der Waals interaction at short and long distances: a pedagogical path from stationary to time-dependent perturbation theory
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Abstract
The van der Waals interaction between neutral atoms is typically studied using stationary perturbation theory for the short-distance (London) limit, while long-distance (Casimir-Polder) results are usually derived via semiclassical, time-dependent approaches. In this pedagogical article, we demonstrate that reformulating stationary perturbation theory calculations in terms of time-ordered correlation functions significantly simplifies the mathematical treatment. This reformulation is particularly advantageous for higher-order calculations required in the long-distance regime, where retardation effects become important. Our approach provides a unified framework that connects both limiting cases while offering a clear conceptual picture suitable for advanced quantum mechanics courses.