Hi. I once heard somewhere that atoms of heavy elements like gold get unusual properties such as gold's yellow color due to "relativistic contraction" of the orbitals due to their high "orbital velocity". But what on earth does orbital velocity mean in quantum mechanics? These are quantum thingies, they are not little planets orbiting a sun! There isn't a classical velocity, but a wave function. What is nearing the speed of light? Is it the expected value of velocity? Isn't that zero? Does it have to do something with the probability distribution of velocity having a fatter tail closer to "c"? Is whatever I heard totally bollox? If so, how do you understand the orbital contraction when nothing is "orbiting"? I want to point out I am not expert with quantum mechanics, hence these questions, and am also not sure what to put down for the level tag.