Quote Originally Posted by Diamond
No. No. And No.

There is no such thing as "what happens in both frames at the same time" - that is covered in the chapter about the "relativity of simultaneity". Light waves do not travel as c+v or c-v but at c. There is no preferred frame of reference - each observer correctly describes the phenomena consistently with the same laws of physics.
I don’t mean they will both “see” the same events “at the same instantaneous moment”. They see the flashes at different moments, but the whole process of the flashes and the movement of the light and the observers’ individual “seeing” of the flashes at different instantaneous moments takes place in both frames “at the same time”, ie during the course of the thought experiment. When one sees a flash, something is going on in both frames at the same time, whether they both “see” the flashes at the same moment or not.

Einstein says in Chapter 9 that the moving train observer encounters the two flashes of light at c + v and c – v. That’s why that observer sees the B flash first. If the moving observer saw both flashes arrive at him at the same time, then he’d be seeing the fronts of both beams travel toward him at c. But if that happened, the embankment observer would see the A flash first at the speed of c + v.

The “relativity of simultaneity” was noticed by way back in history. Soldiers on the battlefield in Newton’s time knew that they heard cannon sound later than the actual firing of the cannon and delayed by more time the further they were away from the cannon. Renaissance scientists knew that simultaneity of “seeing” light and “hearing” sound was just relative and depended on the position and state of motion of the observer. Doppler wrote papers about this in the 1840s. In 1676 Romer knew that when he saw the moons of Jupiter, he was seeing them on a delayed basis.