@marick Your description is mostly correct. A missing point is that measurement requires interaction and thus *changes* the quantum state. If you measure the momentum of a particle, it has a new quantum state after the measurement, and in that state, position is undefined. It's not just impossible to measure position and momentum at the same time. There is no possible quantum state in which position and momentum have well-defined values. That's what Heisenberg's uncertainty principle is about.