Situation from M1 frame of reference... Code: A2(.8c---->) A1 M1 B1 (4:00) (4:00) Situation from the A2 frame of reference... Code: A2 (<-----.8c) A1 M1 B1 (4:00) (4:15) M1 is at the midpoint of A1-B1. The time is flashed from both A1 and B1 simultaneously in the A1-M1-B1 frame, (say 4:00 for example), and reaches M1 in the middle simultaneously just as A2 reaches him. Now then, A2 also sees the flashes simultaneously, but according to him the flash from B1 occurred before the flash from A1. So, the clock at B1 is running ahead of the clock at A1 with 4:00 occurring at B1 before it occurred at A1. When A2 reaches B1, he looks at the time on the clock at B1. Now then, both A2 and B1 will agree on the time displayed on B1's clock, but they will disagree on the time at A1. We have the following two situations: according to B1: Code: A2(.8c---->)(The clock at A1 reads 4:45) A1 M1 B1 (5:00) (5:00) according to A2: Code: A2(The clock at A1 reads 5:00) A1 M1 B1 (<-----.8c) (5:00) (5:15) This is impossible because it requires A2 to be located in two different places simultaneously.