As I understand it black holes are infinitely dense. Yet some are larger because they’ve consumed more material than others.

So I’m confused because if they are infinitely dense shouldn’t they all be the same size? Or is infinity + a few hundred solar systems larger than just infinity? Maybe I just don’t understand infinity that well…

In: Planetary Science

Black holes most likely are not infinitely dense. There’s a point at the middle where the math breaks down and the density trends toward infinity but we don’t think that’s what *actually* happens in there. We just don’t have any way to observe it and don’t know what math to use to describe it. If a black hole really did achieve infinite density it would suck in the entire Universe, and we’re pretty sure that’s not happening.

A black hole’s size is based on the event horizon – something which depends exclusively on the mass in the hole. A larger one has more stuff in it and so more gravity.

The size of the matter inside of the hole is often quoted as infinitely small but the question “how big is the matter inside of the black hole” itself makes no sense.

So, a couple of things.

1. when we talk about the size of a black hole, we’re measuring its event horizon, which is larger or smaller (farther or closer from the center) depending in the black hole’s mass. There’s no real-life hard boundary there, like a wall of black you’d see approaching you or whatever. It’s just that closer than that, the speed required to fully escape its gravity is faster than the speed of light.

2. *According to our math*, the entire mass of the black hole, of any size, is entirely in a single point of 0 volume and infinite density. But another way of saying that is that… when we use our math to try to calculate anything within the event horizon, our equations return 0s and ∞s. I’ve only done college-level physics and engineering, but in my experience, whenever something spits out infinity, that means you’re doing something wrong and you’ve broken something. I’m not qualified to say much about black hole physics, but it really seems like “a singularity with infinite density” is a way of saying “our models of how the universe works break down and no longer produce real results here”. That is to say, I wouldn’t be shocked if the whole idea of a singularity is just a big clue that we need something other than Einstein’s relativity to model what a black hole even is. Which isn’t crazy, I mean relativity replacing Newton’s equations must have felt insane and impossible at the time too.

There are a couple different parts of a black hole being described here.

The singularity: This is like the “bottom” of the hole. Everything that falls in ends up here and gets crushed into a single point (hence the name). Since all that stuff is crushed into something with no dimensions, density is infinite.

The event horizon: The boundary between “light can escape” and “light cannot escape”. It’s like the edge of the hole. This is based on the gravity created by the singularity, so it actually has a size. The more stuff is crammed into the singularity the stronger its gravity, and the bigger the event horizon.

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