I'm reading Hartle's "Gravity," for the general relativity class i'm taking; chapter 12-Gravitational Collapse and Black Holes says, "A horizon's locaton at any one moment depends on the geometry of spacetime to the future of that moment." (pg. 168) That statement is vague and prozaic -- but a figure on the next page elaborates: it shows a star which has collapsed to a singularity; followed by a spherical shell of mass that some time later falls into the black hole. The figure shows the event horizon beginning to expand BEFORE the shell enters it; i.e. the shell expands as the shell compacts towards it - with them eventually meeting at what becomes the final event horizon radius. What the hell?! And it doesn't explain why this is; nor alleviate the inevitable terrors that arise after reading it. Can anyone explain or elaborate at all? The radius of the event horizon is directly proportional to the mass INSIDE --> so how can it expand before the mass is accreted? If we say that at infinity, there is an infinitely large (and infinitely massive) spherical shell--> would the black hole "know" of it ahead of time, and be of infinite size already? Im really confused... Thanks!