There is no momentum without saying relative to what. A train is moving relative to the people not on the train. The earth is moving relative to the sun. But if you are on earth the sun is moving and you are stationary and your viewpoint is not wrong! The reason is, that there is no underlying space we are moving relative to. Maybe this helps: Imagine a train in a train. Driving with the same speed but in the opposite direction. Now from the outside the inner train is not moving at all, but the outer train is. From the inside the inner train is moving and the world around them is. If there is now a portal inside the outer train and the inside train is passing through. What happens. Nothing because the inside train is not moving or it shoots out because it is moving?