I see an example from book stated that a muon that are created at high altitudes can travel to the ground because of time dilation about its lifetime. In the view of muon, it travels shorter distance compared with the view of the Earth. Suppose there are two muons. In view of muon A, muon B is moving to it. And vice versa. Is there still shorter distance? I am really confused with the shorter distance. One of the reasons is length contraction is a reciprocal effect. And length of an object is supposed to measure two points at one instant. For the distance, it is not a reciprocal effect in the example above. And distance is measured by two points at different time. To find out "Is there still shorter distance?", how do I solve it? Using Spacetime?