Hi, I have been asking friends with physics degrees this question, but they cannot answer it, or give conflicting answers. When two reference systems move relative to each other, space becomes shorter in them, as seen from the other system. Gravity and acceleration are indistinguishable. The gravity field moves at the speed of light. Therefore the effect of acceleration should also move at the speed of light. This means that when I start accelerating an object, the part furthest away from the point where I apply the force will start moving slightly later than the part that I'm pushing against. Is this what causes the spatial contraction?