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Gray lines are parallel mirrors. The red observer emits two lights colored orange and magenta. According to red observer, orange light moves vertically and magenta light moves diagonally. The green observer was located next to the red observer when the lights were emitted and he's moving to the left at a high speed so that he sees the magenta light move vertically and the orange light moves diagonally according to him. After the two lights were reflected by the top mirror, they reach the bottom mirror and then they are reflected in the horizontal opposite directions as shown. Now, according to red observer, the orange light will reach the center point (blue observer) before the magenta light, since the path is shorter. According to green observer, the opposite is true. But one of them must be wrong, because the blue observer can tell which light reached him before, and he can take a certain action based on the color he sees first. Hence the casuality problem. What's the resolution?