Me and a friend at school have been thinking about this question for a while and have come up with a few perspectives, I'm sure there's a 'proper' answer but I haven't managed to find one even after scouring numerous web pages. We are aware that the electron doesn't 'move' in a sense, only the probability of it being in certain positions. On one hand, I say that upon being 'impacted' by a photon, the electron changes orbital instantly because the photon holds a specific quanta of energy and just as 4+4=8, the adding of extra energy to the system does not take any time. But on the other hand, my friend argues that it's impossible for no time to have elapsed because the smallest measure of time is the planck time - which is non zero. So, exactly, the question is: How long does it take for the probability cloud of an electron to 'jump'? And from that: Is the time elapsed exactly the same for different consecutive clouds, or different? And why is it not instant (if it isn't)? Thanks! - Newbie Edit: Sorry for the inaccuracy of the use of planck time, had realised just after about zero being 'a number'!