there are several questions i'm going to ask about scattering. however, first i got to say something on a plane wave, say, 'exp(ik*r)'. Here both k and r are vectors and the star means a scalar product. As is known, this state has a current density J proportional to 'k' but independent on 'r', which would indicate that a detector will have the same amount of counts during an interval, despite where it's placed. Is this right? If so, I'd like to go on. Usually, in scattering problems, the incident particle is assumed in a plane wave, and so is the outgoing one. But, both the incident particle and the outgoing one should have a well-defined position, viz, its uncertainty should be smaller than the resolution of the detector. So, they should be superpositions of plane waves. They have to be wave packets. The only compromise seems that, such wave packets have a principal wave-vector, which determines the energy of the particle. Is this right? Finally, In my opinion, scattering is a temporal event, whose treatment should involve the time-dependent perturbation theory. However, usually, these things are completely dealt with using the stationary Schrodinger equation. So, what's the relationship between these two jargons? I'm especially keen to hear a physical explanation. Thank you very much !