1. The problem statement, all variables and given/known data Einstein developed much of his understanding of relativity through the use of gedanken, or thought, experiments. In a gedanken experiment, Einstein would imagine an experiment that could not be performed because of technological limitations, and so he would perform the experiment in his head. By analyzing the results of these experiments, he was led to a deeper understanding of his theory. In each the following gedanken experiments, Albert is in the exact center of a glass-sided freight car speeding to the right at a very high speed v relative to you. Albert has a flashlight in each hand and directs them at the front and rear ends of the freight car. Albert switches the flashlights on at the same time. In Albert's frame of reference, which end, front or rear, is struck by light first, or are they struck at the same time? Which end is struck first in your frame of reference? 2. Relevant equations None (conceptual question) 3. The attempt at a solution So I can easily figure that since the train is at rest in Einstein's frame, and the speed of light is a constant in all frames, both ends will be struck at the same time for him. The textbook reading elaborated on the relativity of simultaneity, and I figured from the textbook's thought experiment (involving a car with firecrackers at each end) that both observers must agree that the light hits each end of the train at the same time, and that this is made possible by having Einstein turn on the flashlight on the right before he turns on the flashlight on the left in the reference frame of the stationary observer. It turns out that this is incorrect, and that instead of when Einstein turns on the lights being relative, it is relative when the light reaches the ends of the car. My question is, how do I know which "kind" of relativity of simultaneity is the correct solution? I approached the problem the way I did because the example of a car with firecrackers at each end, given in the book, involved the assumption that both observers must agree on when the light from either firecracker reaches the center of the car after they explode simultaneously relative to a stationary observer, and yet here, I am instead supposed to assume that both observers agree on when the light is sent, rather than when it is received. I have considered that it might not make sense for the observers to disagree on when Einstein turns on the lights, since Einstein would seem to be executing a different set of actions to each observer, but I figured that the situation is no different from if we were to replace the lights with exploding firecrackers that Einstein was simply observing. It would be greatly appreciated if anyone could clear this up to me!