"danR This nicely puts what has been a problem for me: we've imbedded motion in a static 4-space geometry. Why should particles 'simply start following' the geodesics...? What does 'following' mean? Is it simply a description of the necessary direction of entropy? But isn't entropy in this case is driven by the influence of a gravitational 'force'? What is 'forcing' the system? I seem to come back to some kind of 'force'." Newtonwannabe: Well geodesic are curves of extremal length. One can derive the geodesic equation by applying the principle of stationary action so particles following geodesics are particles following extremal curves. All particles in classical mechanics seem to obey this principle (analogous to the statement that objects in free fall in flat space follow straight lines) as long as they are in free fall and I don't think we actually know WHY its just the way things are. ____________ The above, from another discussion, is a preamble to the following conjecture: A particle 'P' in a field, here gravity, has a wave-function describing the probability that its position will be here or there in time. Suppose the probability is skewed by the gradient of the field so that P's probability of being 'there' is more likely, or being there more often than not; then the centre of the probability distribution (which we will hold equivalent to P's centre of mass) will shift to regain symmetrical distribution of probability. This is a struggle it cannot win: the probability/mass centre constantly moves there-ward, or tries to. Naturally it has to answer many questions: why would P's behaviour be identical in an accelerated frame? How does an ensemble of particles behave? Does it explain the motion of a photon in a gravity field? Do all elementary particles behave identically? Perhaps this is already out there, or is similar to something, or too naive. Any merit? Is it fixable? Too dumb?