system
Given the following post, respond as if you're commenting on Reddit:
human
If time stops completely at the event horizon, how can black holes grow?

It is my understanding, that the supermassive black holes in the center of galaxies became so massive because they were swallowing up surrounding matter for billions of years, not because they were so massive from the beginning. On the other hand, if time stops at the event horizon, it shouldn't be possible for matter to fall through and increase the mass of the whole object. This seems like a paradox to me.

I understand that time doesn't stop for the observer falling onto the BH. I'm asking about what happens from the point of view of an external static observer (us, Earthlings).

My best guess is that it can be explained as the infalling matter somehow "settling" on the surface of the event horizon - but even that would have to happen in infinite time, right?

So which is it? Is it that black holes don't grow, or that time doesn't really stop at the event horizon? Or maybe neither and there is some freakish explanation I havent thought of? ;)
gpt
The phenomenon goes by the name *black hole complementarity.* Matter *both* falls into the singularity *and* stays fixed for eternity at the event horizon. Both things occur. This may sound like a paradox, but it really isn't, because no knowledge of what transpires within a black hole can ever filter out of it, so there's no actual contradiction, but merely an apparent one.

In a very real sense, there are two black holes. One is the black hole that exists to infalling matter; it's a point of zero volume and infinite density. The other is the black hole that exists to all observers who do *not* fall in; it's a spherical shell of energy that grows over time as more matter gets mushed up against it.

Since from the outside a spherical shell gravitates exactly as it would if it were a point, we can tell no difference between the two. But the tiny void just outside the event horizon roils and churns at the quantum scale, and the fluctuations that are present there are determined by what's fallen into the black hole. This is the source of Hawking radiation: If the universe ever cools to the point where black holes can radiate their heat away — and it is not certain this will occur — then all the information that fell into the black hole will emerge again, bit for bit, albeit in an entirely homogenized form.

Black hole complementarity is a relatively new idea in physics, only having been first articulated back in the early 1990s. But in the years since, it's been universally accepted as the truth, as much as any notion that can't be experimentally tested or compared with observation can be.