Ok, firstly I just want to say I have a layman's interest in physics. But from my limited understanding physicists use the theory of general relativity to understand big things like stars and galaxies and use quantum mechanics to understand the very small things like atoms and sub-atomic particles. My question is when does general relativity breakdown and quantum mechanics take over? Is there some critical size or point this happens? Or is it a gradual progression? I hope these questions make sense :).