My question is, if an object with no velocity is placed in a gravitational field, what causes it to accelerate? In Newton's theory of gravity, it was just accepted that if there is a force on an object, it accelerates. However, in general relativity, acceleration is now due to an object's motion through curved space. An object always travels in a straight light in the sense that if it is moving, it takes the shortest distance between two consecutive points in space. If space is curved then this distance is changed. An object moving in a straight line in a curved space would then seem to be, relative to an observer in flat space, moving in a curved path or, changing speed. So I can see why an object that is already moving with respect to another massive object would appear to change velocity...the curvature of space changes the way an object moves through space. But if the object is not already moving why would we see acceleration? I can see why curved space would change motion but not why it would cause it. There doesn't seem to be any reason for an object to "fall" or be impelled towards another massive object. Textbooks have illustrations of our planets "rolling downhill" the curved space surrounding the Sun. But of course this analogy doesn't really explain anything because the only reason objects roll down anything in the first place is because of gravity. So in effect we're saying gravity causes things to accelerate because of gravity. I've only had a brief introduction to general relativity in my modern physics class so perhaps I'm taking things too literally.