Ok so... It's been a while since I first saw this problematic scenario and I want to know how to deal with it. The question arises in the context of special relativity. Suppose 2 objects moving at the same speed. The floor is the rest frame 'A' and the front object is the moving frame 'B'. The contradiction occurs when I want to find the coordinates of the middle object just as A observes that B is at x=d. Assuming any distance 's' (seen by B) between the moving objects, the coordinates should be: From A's viewpoint: ( d-s/γ , d/v ) From B's viewpoint: ( -s , d/γv ) Transforming A, you get: ( -s , d/γv + vs/c2 ) Transforming B, you get: ( d-γs , d/v - γvs/c2 ) Why the transformation of A's viewpoint doesn't give me the same result I got for B's viewpoint? (and viceversa). How to interpret this?