1. The problem statement, all variables and given/known data How does the blackbody paradox argument show that the electromagnetic field cannot be classical while electrons and atoms are quantum mechanical? Should the same arguments apply to treating gravity classically and electrons quantum mechanically? 2. Relevant equations 3. The attempt at a solution What is the blackbody paradox argument anyway? Is it the ultraviolet catastrosphe? If it is the ultraviolet catastrophe, then the quantum mechanical behaviour of the electrons and atoms - the discrete spectrum of energy levels - implies that light cannot be classical. In other words, light must be absorbed or emitted by the electrons and atoms in individual units of photons. This absorption and emission of light in discrete units of photons and quanta is not a classical behaviour, but rather a quantum behaviour. Is this what's required of the first part of the question?