Relating the Lorentzian and exponential: Fermi’s approximation, the Fourier transform and causality

The Fourier transform is often used to connect the Lorentzian energy distribution for resonance scattering to the exponential time dependence for decaying states. However, to apply the Fourier transform, one has to bend the rules of standard quantum mechanics; the Lorentzian energy distribution must be extended to the full real axis −∞ < E < ∞ instead of being bounded from below 0 ≤ E < ∞ (“Fermi’s approximation”). Then the Fourier transform of the extended Lorentzian becomes the exponential, but only for times t ≥ 0, a time asymmetry which is in conflict with the unitary group time evolution of standard quantum mechanics. Extending the Fourier transform from distributions to generalized vectors, we are led to Gamow kets, which possess a Lorentzian energy distribution with −∞ < E < ∞ and have exponential time evolution for t ≥ t0 = 0 only. This leads to probability predictions that do not violate causality.