I am studying "Introducing Einstein's Relativity" (D'Inverno, Oxford University Press) and I am trying to understand Fig.2.12, page 23. There it goes:
Observer B sends:
a light ray towards Q at time t(R) which bounces back at t(U)
a light ray towards P at time t(S) which bounces back at t(V)
Observer B concludes that P and Q are not simultaneous, but that anyhow they are equidistant (because light takes equal time intervals RU and SV to bounce back).
The doubt I have is about the need for active illumination of targets for which we want to measure the distance.
Clearly, our observer B is using a radar, or a flashlight, or some other sort of emitting device. I can see that using a radar is radically different from using a binocular, and that telescopes require parallax considerations in order to calculate distances.
But if active illumination is the only way to produce distance estimations then in Einstein's train carriage experiment the observer on the train (who's only receiving light beams) won't be able to judge his own position inside the carriage. Is this correct?