Is there a banana for scale or does Lemmy use a different model for scale? Beans?
I think all the bananas (and beans) are already in the picture
Well, even the picture is in the picture…
And it has a density of only about 3g per cubic meter. It’s not much denser than a vacuum made with a mechanical pump.
That’s the thing about black holes that always blows my mind. I don’t understand how the larger a black hole is, the less dense that it is. In my mind, I always think of black holes as super dense objects containing so much matter in such a little space that the gravity is crazy strong. How can something so not dense be a black hole? It doesn’t make sense to me!
To be fair, the density is calculated from the event horizon, which is a somewhat arbitrary boundary. All the mass is still concentrated at the singularity which is still infinitely dense, just… a bit more so.
Ah, I didn’t realize that. I guess that’s a little more terrifying. Sounds like you could pass the event horizon and not be instantly crushed, but would have no way of ever escaping. You’d just eventually get sucked into the singularity.
Hiw stable is this kind of density? Is it going to shrink over time?
Not really. If more material falls in, its mass and size increases (the volume increases faster than the mass, which is why it’s so unexpectedly low density in the first place), but otherwise it just sort of sits there.
Over the very long term, it will evaporate away by Hawking radiation. But that’s a very very slow process. Like, long after everything else in the universe has ended.
And then what’s left?
Heat death
…and thennnn??
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Supposedly.
They can still rotate, so that means they’re not just unidimensional points in space.
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Technically couldn’t a ray emanating from a singularity rotate? Like, the point could have a sort of orientation without having any real measure?
Not saying that’s the case here, but I think it could happen.
Let me stop you right there: a ray can’t emanate from a black hole, that’s why it’s a black hole, not even zero-mass light-speed photons can get out.
We know about black holes rotating, because we can detect frame dragging around them, which means whatever mass is in there, has an average rotation.
The thing is, an absolute “singularity” doesn’t even make sense for a black hole. From what we know of how they get formed, they’re just a bunch of star that gets compressed so tight that its own gravity doesn’t let anything escape… but that doesn’t mean every particle goes straight to the center of mass. Forming a singularity would require the initial star core to be kind of perfectly symmetrical, at absolute rest and 0K, which definitely is not the case. What’s more likely, is that at the center of a black hole, there is a star worth of particles “orbiting” the center of mass at speeds close to the speed of light, sometimes bumping into each other, but since not even mass-less photons can escape the black hole, nothing can get bumped out like it would in normal stars, atmospheres, and so on.
From a mathematical point of view it makes sense to say there is a “singularity”, since for most purposes it behaves like one… but it really isn’t one. It’s also easier to think of the event horizon to be “empty” inside… but it also really is not, it’s going to be full of recently trapped particles on decaying orbits, with a lot of them being still right on the other side of the event horizon (more particles will be entering at a shallower than a steeper angle).
Also, being an actual singularity would make evaporation due to Hawking radiation kind of impossible.
When I said “ray” I just meant an imaginary line that could be drawn to extend in a given direction, not a literal particle escaping. It was mostly to think of a way you might conceptualize an orientation of an object that may not have any dimension. As in, if the matter just outside a singularity rotates, perhaps you could consider it to rotate? But I’m not sure that would be accurate to say anyway. My grasp of the physics of black holes is obviously pretty loose. :)
Thanks for taking the time to explain!
an orientation of an object that may not have any dimension
The thing is, if it had no dimension, then there would be no way for it to have any orientation in some dimensions, it would have to be perfectly identical regarding all dimensions.
if the matter just outside a singularity rotates
It’s a bit more fun, because it would be normal for matter to orbit around before falling in, but “frame dragging” means that not just matter, but also light outside it rotates with the black hole, and time gets stretched.
Normally black holes are considered to be everything up to the event horizon. E.g., from the Wikipedia page:
The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is proportional to the mass, M
The term “black hole” derives from the fact that beyond a certain point light can’t escape, that point being the event horizon.
That’s actually smaller than I would have thought. I wouldn’t have expected our solar system to even be visible in comparison.
What the hell are you talking about, that thing is beyond comprehension.
We shouldn’t downvote people when they realize they have been thinking about something the wrong way and admit it.
Did I miss something? I didn’t down vote them
Not you I’m sure, but they were at 0 when I posted, so thought I’d note it.
Lucky for us, it is to far away.
How big is this, in real numbers?
More than 1 AU.
That’s technically correct.
About 1600 AU, according to wikipedia.
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