For this we need a thought experiment: imagine you're on a gedanken planet manning a gedanken laser cannon, and it's pointing straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now let's keep you safe in a bubble of artistic licence, and make a few little changes. Let's make the planet denser and more massive. The laser cannon is still pointing straight up. The light still doesn't curve round, or slow down as it ascends, or fall down. No problem. Then let's make the planet even denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. Still no problem. Then let's make the planet denser still and more massive, so much so that we take it to the limit. Let's make that planet so dense and so massive that it's a black hole. At no point did the light ever curve round, or slow down as it ascends, or fall down. But Houston, we have a problem. Because that black hole is black, because the light can't get out. So the $64,000 dollar question is this: Why can't the light get out? PS: If you're tempted to invoke redshift, remember that when you send a 511keV photon into a black hole, the black hole mass increases by 511keV/c² only. The photon E=hf energy didn't actually increase, conservation of energy applies. And if you're tempted to invoke the waterfall analogy, please note that Einstein never ever modelled a gravitational field as a place where space is falling down.