  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.
