Popularized treatments of quantum mechanics describe it as applicable to the behavior of submicroscopic particles, while relativity applies to the very large (i.e., astronomical). This seems totally arbitrary to me. Where is the boundary between the 2 domains? Atoms? Neutrons? Quarks? Or is it perhaps the nature/properties of the bodies whose behavior is being described rather than their absolute size which is critical? I realize this may be a meaningless question to the expert but what am I missing?