Let me define an action A which occurs only when action B and action C occurs simultaneously ( for eg. Let ball 1 falling on the rod at distance x from the centre be action B and ball 2 falling on the other end of the rod at distance x from the centre be action C and the rod being in equilibrium be action A ). As you explained that the notion of simultaneity is different for different frames of references, this also means that the particular action A might happen in one frame of reference and might not happen at all in another frame. How does special relativity explain this? You also mentioned a part where the alien sweeps into the human’s past or future. Let’s say the alien’s frame of reference sweeps into the human’s future and the action A does not happen at all in its frame. But it is quite possible that it might happen in the Human’s future when it eventually arrives at it right? How does special relativity explain this?