Uncertainty principle
The original equations dealt with a kind of vibrating body that would produce a wave, a little like the way a reed in an organ would produce a sound wave of a characteristic frequency. So there was motion back and forward (like the vibrating of a reed) and there was an emitted wave that could be graphed as a sine wave. Much of what had earlier been figured out about physics on the atomic level had to do with electrons moving around nuclei. When a mass moves in an orbit, when it rotates around some kind of a hub, it has what is called "angular momentum." Angular momentum is the way that something like a merry-go-round will continue to rotate after people have stopped pushing it. The math used for phase calculations and angular momentum is complicated. On top of that, Heisenberg did not show all of his calculations in his 1925 paper, so even good mathematicians might have trouble filling out what he did not say.