It's Official: Black Holes Have Lots Of Mass

KewlPC writes "Spaceflight Now reports in this article that some scientists have been able to measure the "weight" (yeah, yeah, it's actually mass, not weight) of a black hole that is (or was, 13 billion years ago) eating up the most distant known quasar, some 13 billion light years away."

Comments

Neat

[Anonymous Coward]

This is neat, I'd never heard of this before:

The extreme brightness of this quasar also shows that the black hole in its core is swallowing matter at the maximum rate possible. This maximum rate is called the "Eddington Limit". If the black hole were accreting matter any faster, it would shine even brighter, and the intense luminosity would actually exert enough pressure to stop any more material falling in.

So there's a limit / "max throughput" to how much matter a black hole can suck in? Very interesting.

interstellar dust does red shifting too

[ndevice]

at least that's what I remember from astronomy classes. The article doesn't say if they take that into account or not - and if it's really so far away, that would be a lot of dust that light travelled through. If they do, they would have to assume some uniform amount of dust?

Does this say anything about its size?

[mcgroarty]

Do we know the physical size or the particle density of black holes?

I'm curious as to whether black holes are compacted so much that most of the space between atoms (and even subatomic particles?) is gone, or whether the repulsions keeping them apart are even stronger than the force of the black hole's gravity.

Now that they have a measure of the weight, if they know anything about the density or the size, they've got the other value as well.

Re:Does this say anything about its size?

[Smidge204]

I'm right in saying "the Schwarzschild radius of a black hole is proportionate to its mass", but more properly it's directly proportional; i.e., the proportionality constant is 1.

Something about that seems... counterintuitive?

You're saying that if I have a black hole with a mass of x, it has radius y. Then you say if it has mass 2x, it has radius 2y?

If a black hole is a sphere, doubling it's radius increases it's volume by a factor or about 33 1/2! Since mass only doubled, it's density just dropped by a factor of 17?

I admit I'm not very experienced with black holes, but if anything it seems a black hole would condense to some maximum possible density, and it would maintain that maximum possible density regardless of how much mass you add to it... so it just seems strange that doubling it's mass would actually double it's radius.
=Smidge=

Re:Does this say anything about its size?

[taliver]

Actually, a quick googling found this:

r0=2GM/c^2 (Eqn 10.1.5)

So it is directly proportional. However, I didn't look closely at the units that they are using here, but thta shouldn't matter to the solution at hand.

Re:Does this say anything about its size?

[maraist]

While I acknowledge that the geometry radically alters in the presence of intense gravity (in fact, String theory suggests that the number of dimensions collapses inside a black hole to an effective singularity (though protected by minimum plank-radius)), I'm not comfortable using the phrase circumference to describe it's externally apparent dimensionality.

At the least, you'd have to consider the black hole as a 2D surface (or shell). Which is still related to r^2.

Only speaking as a well read lay person, I've heard black holes being described as 2D planes; all the mass exists within the event-horizon (since time slows down as you approach it, approaching the speed of light).

That being said I can almost comprehend space being warped enough such that the r^2 flatens out to r, but It's still beyond me.

eddington limit and black hole evaporation

[ndevice]

Just speculating, but since black holes do evaporate, and the smaller they are the faster they evaporate, I wonder what the implications of evaporation would be in the presense of an acretion disk.

Given that in the process of evaporation, a black hole emits radiation, at some point the radiation pressure from the evaporation would balance out the force of gravity pulling matter into the black hole so then the black hole might stabilize in size.

Surely they'll have named that limit already, but I don't think it's the same as the eddington limit.

Or perhaps there won't be a limit here because the cross section area of the acretion disk would be so small compared to the surface area of the event horizon. (yes, I think that incoming matter would have to form a disk and not form an acretion shell)

Re:eddington limit and black hole evaporation

[barakn]

Only large black holes will have accretion disks. The radiation coming from a black hole is negligible until the black hole itself is tiny. It is my guess that the radiation pressure from a black hole would never be enough to prevent a net gain of mass.

Re:eddington limit and black hole evaporation

[KewlPC]

It is my understanding that black holes emit a very small amount of radiation, and do it very slowly. Therefore, the black hole would have to be very small (thereby having very little gravity) for this to take place.

Besides, a black hole would have to swallow all the matter within its reach before it could shrink to the size necessary for the effect you describe to take place, because the more matter it takes in, the bigger it gets. And since it would have already taken in all the matter it can get ahold of, the effect you mention wouldn't happen.

I hope you got some sleep...

[barakn]

You need it. The nearest star is Proxima Centauri at 4x10^13 km (also, your numbers for Alpha Centauri are erroneous). The 9 billion solar mass black hole's radius is thus .0225% of that distance.