Uncertainty principle
He had no idea that he was going to stumble over a hidden secret of nature when he set off to discover the explanation for the intensities of the colored lines characteristic of each of the elements. The study of quantum mechanics had already shown why hydrogen has four bright lines in the part of the spectrum that humans can see. It must have seemed that the next thing to learn would simply be how to calculate their brightness. Hydrogen seemed to be the obvious place to start since hydrogen has only one electron to deal with, and only four lines in the visible part of the spectrum. Surely there must be a good reason for their not being equally bright. The explanation for the brightness of the different-colored lines of neon and the other elements could wait.