If we consider a system (say an electron) with respect his own reference frame, we find that it is placed in an exact position (let's say the origin) and it has an exact momentum (it is at rest with respect to his own reference frame); due the very definition of such reference frame. HEISENBERG O_O Quantum mechanics excludes the existence of proper reference frames?. If so, what happens with proper time?. Can we talk about simultaneity when dealing with quantum mechanics? Do 'simultaneous measurements' (of non-cunmuting observables or whatever) have sense?