LWN: Comments on "Development quote of the week"

Black Hole Hunting

nix — Wed, 19 May 2021 17:23:04 +0000

The fun thing about this degree of spacetime distortion is that the topology of these things is demented: from some reference frames (not those outside the hole, obviously), the spacetime inside a black hole's event horizon expands constantly. *Fast*. Ludicrously fast.

It's a bit hard to find papers that describe this in comprehensible fashion to a layman, but they usually say things things like every (ridiculously tiny space interval) inflates e-fold times per (ridiculously tiny time interval) for the entire lifetime of the hole. When I say ridiculously tiny, I mean bigger than the Planck scale but not by much. So the inside of a black hole of any age is going to be an *immense* region, likely vastly bigger than the entire observable universe for almost any hole of reasonable age, and growing more immense all the time -- not that you can ever tell because anyone who can tell is on a one-way trip and cannot communicate outwards... and probably spaghettified as well.

Black Hole Hunting

hummassa — Sat, 08 May 2021 11:59:29 +0000

As I understand the theory it's not really appropriate to talk about the "size" of something after it is closer to the center than Rs. By general relativity, either you consider that the radius of the star is Rs (directly proportional to its mass) or you consider it's zero (independent of its mass).

But IANAA (astrophysicist).

Black Hole Hunting

Wol — Fri, 07 May 2021 19:49:43 +0000

And wikipedia says the Schwarzchild radius is the event horizon - which is what I said.

As a star collapses and gets smaller (and more massive) so the Schwarzchild radius grows, and as the star shrinks inside the Schwarzchild radius it turns into a black hole.

Hence my statement that the black hole itself gets smaller as it gets more massive. After all, relativity assumes it collapses into a singularity (yes I know this is where relativity collides with Quantum Mechanics, and we don't actually know how to reconcile it).

Cheers,
Wol

Black Hole Hunting

hummassa — Fri, 07 May 2021 18:10:25 +0000

> the more massive they are, the smaller they get.

?! ... The Schwarzchild radius is directly proportional to the mass...

Rs = 2GM/c²

Black Hole Hunting

mathstuf — Fri, 07 May 2021 16:39:13 +0000

Thanks for the info, but I'm going to be a bit pedantic here :) . I think "active black holes" is more accurate :) .

Also, yes in layman terms, "black holes are bright". But in astronomical terms, they are still dark. Their environment might be "excited" by their presence into being bright, but the hole itself isn't "bright". I guess it's the "moonlight" versus "the Moon is really reflective" distinction :) .

Black Hole Hunting

zlynx — Fri, 07 May 2021 16:37:54 +0000

Because of the theoretical time dilation at the event horizon you really can't say anything about the "actual size" of the black hole. It's surface is frozen in time just below that horizon from our point of view.

Possibly, in theory, from the black hole's reference frame it continues to collapse and the entire universe outside collapsed into it instantly.

I remember reading about some wild theories that our universe is the inside of a black hole from another universe and the Big Bang is the collapse of several galaxies worth of mass/energy from the outside.

Black Hole Hunting

Wol — Fri, 07 May 2021 15:52:23 +0000

Another fun fact about black holes - the more massive they are, the smaller they get. I very much doubt Sagitarius A* is 30 times the size of the sun, seeing as a neutron star is about the size of the earth.

I think you're talking about the event horizon - the actual black hole will be MUCH smaller - the size of the moon maybe?

Cheers,
Wol

Black Hole Hunting

calumapplepie — Fri, 07 May 2021 12:18:02 +0000

Fun (but utterly useless) space fact: Black holes are very, very bright. Black holes tend to emit a boatload of light. The matter falling into them is superheated as it reaches obscene speeds. This causes intense X-ray radiation, which is the way we use to spot them. Black holes in isolation don't do that, as there's nothing falling into them, but a "where light isn't" technique isn't very powerful.

If you look for black holes by looking for areas without light, you'll have a very bad day, because space is full of areas without much light. The actual radius of a black hole is quite small: Sagitarius A*, a supermassive black hole at the center of our galaxy, is only about 30 times the radius of the sun. The only reason we know this is because of how much it emits in the radio spectrum. Finding similarly-sized objects that emit no light would be virtually impossible.

A more interesting metaphor would be to spot black holes by looking at how they distort reality around them. The measurements we have of Sagittarius A*'s mass stem from looking at the stars around it. That's also why we know there aren't any large black holes that are very close to us, and how I know that the black sphere flying towards me is a bowling ball, and not a black ho-