You can use the Schrödinger equation in a classical, non-relativistic, space-time, do calculations and get a lot of the chemistry explained more or less correct

You can use the Schrödinger equation in a classical, non-relativistic, space-time, do calculations and get a lot of the chemistry explained more or less correctly. There is no speed of light, and electric fields have immediate effects over long distances. But to explain the electron transitions that make gold yellow, you need to use the Schrödinger equation in a relativistic space-time. (But just special relativity suffices here, no need to go to general relativity.)