If a classical object changes direction it is done due to a force, but I'm much less certain about quanta. A quantum's position and momentum and uncertain until we measure it. After we measure it it becomes uncertain again until we measure it a second time. Between measurements its direction and momentum is uncertain and there is no law, except for probabilistic, statistical laws that can predict its next position between measurements. What causes this? Is it just a property of quanta that their position and momentum are uncertain? I'm pretty sure that's the answer but it's always good to double check. One other thing, does a quantum's velocity fluctuate? If it slows down does it just convert that lost velocity and hence lost kinetic energy into potential energy?