In my thread titled "What does cold hydrogen gas emit at 1420 MHz?", physicsforums member blueleaf77 told me the following: "There are three main types of interaction between bound state electrons and photons: absorption, stimulated emission, and spontaneous emission. The first one involves annihilation of a photon to raise the electron state to a higher level. The last two, opposite to the first one, involve creation of a photon by decreasing the state of an electron to a lower level, and eventually to end up in the ground state." I just watched the Cosmos: A Spacetime Odyssey episode Hiding in the Light. In Hiding in the Light, the host Neil Degrasse Tyson explained how photons of light and the electrons of an atom interact with each other. Tyson said that when an electron absorbs a photon, the energy of the photon will cause the electron jump up to an orbital with a higher energy level. I've known that for over a decade. But then Tyson said that nobody knows why electrons drop down to lower energy levels. From what blueleaf77 told me, I thought that what causes the electron to drop down to a lower energy level is the electron's emission of a photon. What am I missing here? I learned about what blueleaf77 told me about the interactions between photons and electrons many years ago, but I forgot that information when I made the thread "what does cold hydrogen gas emit at 1420MHz". I thought that what blueleaf told me about an electron's emission of a photon causing the electron to drop to a lower energy level is correct. Who is correct, Neil Tyson or blueleaf77? Or are they both correct and i'm just missing something?