The classical equations of motion for particles that move in a conservative force field do not change when the sense of progression of time is reversed. For the spinless* particles considered in this chapter the corresponding quantum equations also possess time-reversal symmetry. After reversing the arrow of time the wave packet returns to its initial state and will further evolve according to the laws of quantum mechanics.
*The concept of spin is introduced in the chapter on the Stern-Gerlach experiment.