Here's a thought experiment; Suppose we could isolate one single electron in space and there are no external forces acting upon it (I don't think this is even possible, is it?) and we fired a single photon at the said electron, which direction would the electron move in? here's my thinking; the alternating EM field will cause the electron to jiggle a little bit, but as the interaction ceases, the electron will travel in the last direction it was travelling prior to the end of the interaction, i.e in a transverse direction to the incoming photon. ....but, the photon also has momentum right? so if it hits the electron head on would we see the electron move in the direction the photon was travelling? any thoughts?