Everything I have read about emissivity simply says that in thermal equilibrium, emitted radiation has to be equal to absorbed radiation. That's a glib explanation. What about an object that is not in thermal equilibrium with its surroundings? What about an object that is in a vacuum that extends to infinity in all directions? Say it starts at some temperature; it will radiate away energy until it reaches absolute zero. Why would it radiate energy faster if it is a reflective material versus, say, carbon black? What is going on at the atomic level that makes emissivity equal to absorption? What is it about carbon, for example, that makes it very good both at absorbing and emitting radiation? What is it about a reflective material that makes it not emit as much radiation for a given temperature? Is it that the photons emitted by the thermally vibrating atoms are reflected back or otherwise not allowed to leave the atom?