From my understanding QM deals with small things in the universe I use the term "small" loosely when I refer to small I'm talking about sub atomic particles. Anyways back on to the question here it goes. Why can't QM be applied to bigger objects? I know that we have General relativity for planetary masses and galaxies to describe their behavior. Could the two be interchangeable, say we could use general relativity for things that are small and QM for stuff that is big?