I am not an expert on cosmology, merely an interested layman, so I hope my question is not either too stupid or obvious. As I understand it from reading books (currently Brian Greene's "Fabric of the Cosmos"), general relativity (GR) is used for analysing large massive objects whereas quantum mechanics (QM) works for small light objects and that the 2 theories have not yet been successfully unified. But this poses a question, What about the middle ground? At what point in the spectrum from large/ massive to small/ light does GR not function and vice versa for QM. Is it sudden or gradual fall off in effectivness for either theory. Greene refers to the centre of a black hole (massive and tiny) where both GR and QM are needed but can't be used because they don't fit together.