Hi I am reading about the force of a coherent EM-beam acting upon an atom, and I have a question in this regard. It is regarding the explanation on page 150 of this book, starting from "The geometric approximation of atom optics is valid when": http://books.google.dk/books?id=SUB...phase gradient dissipative force atom&f=false. It is only the first part of that page. As far as I understand, what they try to tell us is that in order to treat the atom as a classical particle, the time it takes for the internal state to change (1/Gamma) has to be very short compared to the time it takes for the external dynamics to change. That is at least what the inequality says. Physically I don't see why this condition must be satisfied. Does it simply mean that the atom has to be in equilibrium at all times? Best regards, Niles.