Sudden approximation

The sudden approximation is a concept in quantum physics that describes situations where a change occurs so quickly that the system's particles don't have time to adjust their positions or states during the change. Think of it like pressing a fast switch; the system's wavefunction (its quantum state) remains unchanged instantly, but afterward, it may no longer be an eigenstate of the new conditions, leading to a transition into different states. This approximation simplifies calculations by assuming the process is instantaneous, making it useful for analyzing rapid events like sudden perturbations or interactions in quantum systems.