I thought of yet another simultaneity paradox that I find even more confusing: We're just in space with spaceships. There's a ship moving past us at some relativistic speed. On the front and back of the ship there are thrusters at the top that can be activated by a pulse of laser light striking them. As the ship flies past me, I shine two lasers like in the original scenario at the same time (for me) such that they hit the front and back sensors simultaneously (for me), thus turning on the engines. From my perspective they ignite at the same time, thus the ship should start adding a downward component to its velocity relative to me. From the ship's perspective, the front should ignite first, then the back. Now, we've established that the orientation can be different in different frames, but here's what I find puzzling. because the ship will spend some time with an unbalanced force at the front, there will be a torque creating angular momentum. Then, the force is balanced when the back fires, eliminating that torque, but the angular momentum remains. Furthermore, due to the rotation, the engines are firing along the same vector as observed from the other frame. The ship should continue to rotate as it accelerates, which would have it move on a completely different trajectory compared to what would be observed in the other frame. I'm sure there's something I'm missing here, so I hope someone can shed some light on this one.