Two-Atom Model and Possible Resolutions

In order to check finite propagation speed Fermi, in 1932, had considered two atoms A and B separated by some distance R. At time t = 0, A is in an excited state, B in its ground state, and no photons are present. Fermi’s idea was to calculate the excitation probability of B. In a model-independent way and with minimal assumptions – Hilbert space and positive energy only – it is proved, not just for atoms but for any systems A and B, that the excitation probability of B is nonzero immediately after t = 0. Possible ways out to avoid a contradiction to finite propagation speed are discussed. The notions of strong and weak Einstein causality are introduced.