: : Why should there be a graviton? There is no need for a messenger particle if the "force" of gravity is simply a property of inertia (i.e. acceleration on a curve that bends into a higher dimension). In this case..... is there a messenger particle for centrifugal force?? I should think not.

In purely macroscopic mechanics there is no need for the graviton just as there is no need for the photon in that scale. The graviton is not a prediction of relativity. It comes from an attempt to quantize gravity. Force carrier particles are quantum mechanics predictions. Its only in that scale that the graviton is a need for such messenger particles.