(sorry for quality of the drawing) There are 2 accelerating observers, A and B in the infinite and asymptotically flat universe. When accelerating both A and B observe Unruh radiation from their Rindler horizon. If there is a cold screen between observer B and his horizon, B detects less Unruh radiation. Now the question. Before they accelerate, observer B in unaware of the screen (as universe is dark, there is no light). However, when they start accelerating, B from having less radiation can instantly deduce the presence of the screen. Even more, varying the acceleration B can even calculate the distance to the screen. How B can instantly obtain the information about the screen far away?