Friedrichs's Principle

Friedrichs's principle states that when a physical system with discrete energy levels interacts with a continuum of states (like a bound state interacting with free particles), the discrete energy levels become slightly shifted and acquire a width, turning sharp lines into broadened resonances. This reflects the idea that stable states can become unstable when coupled to a continuum, leading to phenomena like decay or scattering. Essentially, the principle explains how the interaction with a continuum causes changes in energy levels, influencing the system’s stability and resonance behavior in quantum mechanics.