An electron in an orbital of an atom has some energy and some momentum. In some ways it can be considered "orbiting" but it is not really moving in a classical sense. I've heard more than once the explanation that electrons in high orbits move close to the speed of light and that this effects the properties of the material (e.g. the distinctive color of gold), but this doesn't ring true to me. The optical properties have to do with the difference in energy levels, and that's it. Right? But that got me thinking... what about time dilation? Does the electron experience less time because it has momentum, even though the wave is stationary? A more concrete example might be an exotic atom where a muon replaces an electron. The proper time of the muon is reflected in the decay time. Does a bound muon decay slower than a free stationary one, with the effect increasing with energy level?