I have a question about the electrons in atoms. This question is from quantum mechanics, but it is really a situation of classical electrodynamics. When the electron goes around the nucleus, it is said that the electron will emit electromagnetic wave and then fall to the nucleus. But in fact all the atoms we meet in the world are stable. So it is necessary to develop the new theory---quantum mechanics. My question is that why should it emit something and then fall. How can we describe it? Are there any formulas or equations for it? Another similar question, if the electron moves in an magnetic field, you know, it goes around and follows a cycle and the velocity changes all the time just like it goes around the nucleus, will it emit something and then fall? If it does, where fall?