Does an event horizon happen abruptly, and if so, what would happen if I was floating in front of it and shined a flashlight in its general direction? Would it appear as if I was standing in front of a giant wall? For the sake of this question, let's say I tossed up some dust first, and then shined the flashlight.
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1$\begingroup$ Could you shorten the title a bit? $\endgroup$– nicaelCommented Apr 10, 2018 at 17:35
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A black hole's event horizon is a set of events which only has global significance. Locally, within a region of spacetime around some event on an event horizon, as long as the region of spacetime is small enough that tidal forces are negligible within that region, spacetime behaves like ordinary, flat spacetime. An event horizon isn't locally visible or otherwise detectable.
Suppose you're within such a sufficiently small region, and as you cross the event horizon you toss up some dust in the direction of the black hole, and shine a flashlight on it. What will happen will just be that you'll see the dust in your flashlight beam. You won't be able to tell if the light from your flashlight hit the dust while you were both outside of the event horizon, or while you were both inside of the event horizon, or whether the light hit the dust inside the event horizon while you were outside, but then the reflection hit your eyes after you crossed the event horizon.
