 So here we have Alice standing in the center of a train, which is stationary in her reference frame. And at either end of the train are two of Bob's friends, who we'll call friend one and friend two. So above each friend is something strange. We have a light sensor attached to a bucket of water. So if light were to hit, say, the left light sensor, it would dunk water all over friend one. So how is light going to reach any of these sensors? Well, Alice is carrying a light bulb, and as soon as she turns it on, it will send photons in either direction, activating the light sensors, and dunking water onto Bob's friends. So let's suppose the entire train has lent L, in which case the distance from Alice to either friend is L over two. So in Alice's frame, what are T1 and T2? Where T1 is the time taken in between Alice switching on the light bulb and friend one getting dunked, and similarly for T2.