Abstract
The manner in which the extension of a small wave packet moving in a circular orbit in a central potential V(r) changes with time is examined. It is shown that in general, the extension in the orbital plane increases indefinitely at large times unless rV"-V'=0 at the radius of the orbit. The question whether there exist any special 'coherent' wave packets which retain a finite size for arbitrary times in a Coulomb potential is investigated, and the answer is shown to be in the negative.