This paper, authored by V. Weisskopf and E. Wigner, discusses the calculation of natural line width based on Dirac's theory of light. The authors approximate the Dirac equations governing the interaction between an atom and radiation, providing solutions that describe the intensity profile of emission lines over the entire emission time. They start by analyzing the time-dependent wave packet of a swinging oscillator, derived from classical theory, and derive an expression for the intensity of light at a given frequency. This expression is then adapted to quantum theory, where the constant \(\gamma\) is replaced by the reciprocal of the average lifetime \(\tau_A\) of the initial state \(A\). This leads to a formula for the probability density of emitted light frequencies. The paper also explores alternative views on line width, including the assumption that energy levels are not infinitely sharp and that the probability of an atom's energy lying between \(E\) and \(E+\Delta E\) is given by a specific formula. These formulations are consistent with the results derived from Dirac's theory and are supported by other works in the field.This paper, authored by V. Weisskopf and E. Wigner, discusses the calculation of natural line width based on Dirac's theory of light. The authors approximate the Dirac equations governing the interaction between an atom and radiation, providing solutions that describe the intensity profile of emission lines over the entire emission time. They start by analyzing the time-dependent wave packet of a swinging oscillator, derived from classical theory, and derive an expression for the intensity of light at a given frequency. This expression is then adapted to quantum theory, where the constant \(\gamma\) is replaced by the reciprocal of the average lifetime \(\tau_A\) of the initial state \(A\). This leads to a formula for the probability density of emitted light frequencies. The paper also explores alternative views on line width, including the assumption that energy levels are not infinitely sharp and that the probability of an atom's energy lying between \(E\) and \(E+\Delta E\) is given by a specific formula. These formulations are consistent with the results derived from Dirac's theory and are supported by other works in the field.