Please, could someone here teach me, in a no-pun-intended crash course , of how relativity plays into the speed of electrons flying around an atomic nucleus as well as thermal energy vibrations. Ok - I know it's wrong to think like they did in the 19h century, making the analogy of electrons around atomic nuclei to planets revolving around a sun. Nevertheless, (average) particle speed is what temperature is. Also, I never understood: does the "speed" of electrons "flying around atomic nuclei" have anything to do with temperature? At sufficiently high temperatures, depending upon the element or compound, electrons do get stripped away from atomic nuclei. So, I assume a plasma is some sort of "soup" of electrons and atomic nuclei. Hence, it does not seem to me that there is any difference between temperature associated with electrons "flying around a nucleus" versus interatomic Brownian motion, since both contribute to the kinetic energy of the ensemble. Finally, the Nova program on the race for absolute zero asked if there is an "absolute hot". Wouldn't that be fixed by the "speed of light" of these electrons and nuclei?