Simple question. So the energy of a particle is observed to be E_1 (for example) at time t=0. At time t=0 the wavefunction psi(x) collapses to phi(x)exp(-i(E_1)t/h). At time t>0 the wavefunction is also in this state (right?). Is it in this state until it interacts with another particle or what? Or does it go into the state phi(x)exp(-i(E_1)t/h) when you observe it but then after it goes back into the state psi(x)? As far as I am aware it collapses into the state phi(x)exp(-i(E_1)t/h) for t=0 and stays in that state (but until when?) for t>0.