If I were to hold a tennis ball out at arm's length, and release it, we all know it would fall due to gravitational attraction from the Earth's mass. In Newton's physics, gravity was a force that was created by and acted on all mass, causing two objects to accelerate toward one another, at a rate proportional to the sum of the masses and the inverse square of the distance between them (I think that's right). This is easily grasped even by schoolchildren. When Einstein came along, he removed the 'force' from gravity by describing it as a curvature in space-time, caused by the presence of the mass. Analogous to an acceleration, but not one, this concept is somewhat more difficult to wrap the brain around. My question is this. If gravity acts from curvature in space-time, what is the mechanism pushing the acceleration of the masses? The tennis ball in my hand, from the example, is at rest relative to the center of mass of the system. Why do the masses accelerate toward one another rather than remaining relatively motionless, when there is no energy being added to the system? Please don't say that holding the ball off of the ground adds 'negative energy' to the ball, and the mass loses that negative energy by falling. This is not satisfactory because the existence of 'negative energy' is questionable, and not really confirmed by any experiment that I know about, except in the case of considering gravity itself to be a form of negative energy. Please if I am off base with any of my information or assumptions, don't call me stupid. I'm here to get answers from those who know better than I do.