Problems about Causality in Fermi's TwoAtom Model and Possible Resolutions
Abstract
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 modelindependent 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.
 Publication:

arXiv eprints
 Pub Date:
 July 1997
 DOI:
 10.48550/arXiv.quantph/9707016
 arXiv:
 arXiv:quantph/9707016
 Bibcode:
 1997quant.ph..7016H
 Keywords:

 Quantum Physics
 EPrint:
 13 pages, Latex