Hello. In oral exams my professor likes to ask if Alice and Bob can communicate, if Alice ist just above the event horizon of a schwarzschild black hole and Bob ist just below. He wants to hear: Communication is possible, because the event horizon is observer dependent. Only an observer resting at infinity distance couldn't see something cross the event horizon. Is this correct? I can't understand it. Doesn't you draw diagrams like this https://faculty.etsu.edu/gardnerr/planetarium/relat/eventho2.gif exactly to see, that this is not possible? I read about apparent horizons, but the lecture is the introduction to general relativity. We talk about an ideal schwarzschild black hole, not a perturbed black hole. Would you please help me to understand?