As time is considered the 4th dimension can we say the 3-d world to be floating(advancing forward in time) in the 4D. If so is every inertial frame has is own velocity rate(moving in the time dimension) in the 4-D. When you have 3-D world if you want to bring 2 objects in contact you can bring them to the same point. But when dealing with 4-d consider this let A and B be start at the same point at t=0 now let B go for a trip and come back. As you can see A and B have their own 3-d layers advancing in time. Because of time dilation after B's return its 3-d layer would be below that of A's isn't it?how can they ever interact again?as they will continue to maintain a distance in the 4-d spacetime(as time rate is the same)? i m just trying to imagine 4-d spacetime. apologize if the question is non-sensical.hopefully someone can clarify me?