For an electron can I not do the following to determine both the position and momentum? I take a screen with a small hole and I eventually make the hole smaller and smaller. Cathode rays emitted will hence get diffracted after passing through the hole making momentum more and more uncertain. What if I weaken my cathode ray to such an extent that only one electron passes at an instant. I could calculate the electron's velocity by measuring precisely how much time it took from gun to detector(placed behind the wall with the hole). In the most ideal condition at least one electron passes through the hole and when it does I calculate the velocity(hence momentum) and I could keep doing this to arbitrary precision. I'm aware this won't work. What I want to know is the limit where this experiment fails to violate the principle.