We sat on the train and the train quickly moved forward. At this time, the objects outside the window were moving backwards. Why?
In essence, this is all due to the reference. It’s easy to explain why things that are close to us move in the opposite direction—because they do. With background objects as reference objects, they move at a rapid speed in the opposite direction.
For farther objects—usually on the horizon, or at least where you can see them—these objects are undoubtedly moving backwards because the train is moving forward, but what we see is not the case. Our brains rarely see the real existence of objects, but tend to rely on references to make judgments.
If you look outside the window in the office, you can probably see a house 50 meters away. There is a tree in front of the house. Can’t see anything farther than this house. When it comes to the window, the tree is obviously moving backwards, but the house seems to move in the same direction. If the handle sticks out and points to a window in the house, the arm does start moving, and when moving forward, the finger is pointing at the back of the body. With the target you are pointing to, you can give yourself a reference point through which you can see that the window is moving backwards. This is not normally seen because there is no reference point to compare the relative changes in the position of the house. However, if the house is transparent, there will be other visible objects behind it that change in position, so that the house will move backwards like the tree.
But you can’t do this on the train, because even if the handle is extended, the horizon cannot be accurately pointed because it is too far away.