Black Holes

[sr.vinay]

Is there an end to a black hole? That is, what would happen if it died? Is there a limit to the amount of mass a black hole can suck in(so as to say)?

[timo]

There is no known reason/mechanism for an upper mass of a black hole, as far as I know. There is a trivial limit on the lower mass (zero). Black holes are often believed to lose mass over time by radiating off stuff, even to mass=0 where they then cease to exist. In that sense, there is an end to a black hole, but not in the sense that you seemed to have in mind: The reason stars have "an end" is that there's a process going on (fusion of whatever stuff) that creates a pressure countering the gravitational attraction. This process has a limited runtime (at some point you are out of stuff for the process) and after that runtime gravitation will do what it was prevented to do before. No such process stabilizing a black hole against gravitational collapse -a black hole is already as collapsed as anything can be- exists.

[Mr Skeptic]

There's a limit to the amount of mass a black hole could have in practice given what we know of cosmology. Not so much a limit to what it can ingest, but to what it can reach in a given amount of time since the start of the universe.

[sr.vinay]

There is no known reason/mechanism for an upper mass of a black hole, as far as I know. There is a trivial limit on the lower mass (zero). Black holes are often believed to lose mass over time by radiating off stuff, even to mass=0 where they then cease to exist. In that sense, there is an end to a black hole, but not in the sense that you seemed to have in mind: The reason stars have "an end" is that there's a process going on (fusion of whatever stuff) that creates a pressure countering the gravitational attraction. This process has a limited runtime (at some point you are out of stuff for the process) and after that runtime gravitation will do what it was prevented to do before. No such process stabilizing a black hole against gravitational collapse -a black hole is already as collapsed as anything can be- exists.

 

Black holes can radiate? They have so much gravity that they suck in everything in their event horizon right? Even if they do radiate stuff, it would just be sucked in again!

[Klaynos]

Black holes can radiate? They have so much gravity that they suck in everything in their event horizon right? Even if they do radiate stuff, it would just be sucked in again!

 

Outside the event horizon there is nothing special gravitationally about a black hole, as long as the radiation starts just outside the event horizon it is free to escape just like it would be from any other gravity well.

[sr.vinay]

The radiation won't be sucked back in due to the immense gravity? I mean, light gets sucked in!

[Sisyphus]

The radiation won't be sucked back in due to the immense gravity? I mean, light gets sucked in!

 

As Klaynos just said, that light can't escape is only true inside the event horizon.

[sr.vinay]

That's what I meant. I know that the gravity acts within the horizon. So, how would the radiation escape it in the first place?

---

Merged post follows:

Consecutive posts merged

And, can someone elaborate on how black holes disintegrate? What are the after effects of such a disintegration, if any?

[Sisyphus]

That's what I meant. I know that the gravity acts within the horizon. So, how would the radiation escape it in the first place?

---

Merged post follows:

Consecutive posts merged

And, can someone elaborate on how black holes disintegrate? What are the after effects of such a disintegration, if any?

 

I don't really understand the (quantum) mechanics of it, but the idea is that the photons are created just outside the event horizon.

[timo]

And, can someone elaborate on how black holes disintegrate?

I cannot but the mechanism by which black holes are believed to lose mass is called "Hawking Radiation". Maybe you'll have some luck with Wikipedia or Google.

[sr.vinay]

I did a readup on wikipedia. There it said that a particle-antiparticle pair is produced near the event horizon due to the gravitational effects. One of the pairs tunnels into the black holes, creating an illusion to the observer that a particle has also been emitted. This is what they refer to as the radiation referred to in the earlier posts?

[timo]

I did a readup on wikipedia. There it said that a particle-antiparticle pair is produced near the event horizon due to the gravitational effects. One of the pairs tunnels into the black holes, creating an illusion to the observer that a particle has also been emitted. This is what they refer to as the radiation referred to in the earlier posts?

What you say is roughly a typical description of Hawking Radiation (never heard "illusion" in that context, though), so in that sense it indeed is what I called radiation above. But this explanation does not make sense to me (or I just don't understand it) so I expect that it is an explanation that is simplified to the point of being wrong. That's why I simply said that black holes are assumed to radiate off stuff.

[allo4]

Hi, I don't know much about physics but I was thinking about black-holes gravity and antimatter. This will probably be easy to disprove but is it possible that a black-hole just contains antimatter and so stops feeding when it eventually runs out?(I imagine there is the same amount of matter and antimatter in a 'perfect' galaxy so when the black hole finally runs out of antimatter the galaxy is canceled out) Also this is a bit silly but could gravity be caused by antimatter and not matter, this would explain why it is so weak because there isn't much of it(apart from in black-holes).

 

Sorry if this is just rubbish and I've wasted your time

[sr.vinay]

I don't think the anti-matter explanation is possible. Because enormous amounts of energy needs to be expelled when matter and anti-matter cancel each other out. We don't see this phenomenon.

[insane_alien]

I don't think the anti-matter explanation is possible. Because enormous amounts of energy needs to be expelled when matter and anti-matter cancel each other out. We don't see this phenomenon.

 

1/ the process is slow. the signal would be lost in the thermal radiation given out by the acretion disk.

 

2/ it doesn't happen very fast until the black hole is very small, also lessening the signal.

 

a kilo of antimatter annihilating is only going to be equivalent to a tsar bomba(largest nuke ever detonated) which, in astronomical terms is an incredibly tiny event.

 

3/ the annihilation happens INSIDE the blackhole. the energy released wouldn't get to us anyway.

[sr.vinay]

1/ the process is slow. the signal would be lost in the thermal radiation given out by the acretion disk.

 

2/ it doesn't happen very fast until the black hole is very small, also lessening the signal.

 

a kilo of antimatter annihilating is only going to be equivalent to a tsar bomba(largest nuke ever detonated) which, in astronomical terms is an incredibly tiny event.

 

3/ the annihilation happens INSIDE the blackhole. the energy released wouldn't get to us anyway.

 

Annihilation of matter by anti-matter happens very quick. The time black holes take to 'die out' is far greater than what one would expect from that by annihilation.

[ajb]

But this explanation does not make sense to me (or I just don't understand it) so I expect that it is an explanation that is simplified to the point of being wrong.

 

I don't think the anti-matter explanation is possible.

 

From the calculations of Hawking (using semiclassical gravity) it is not easy to see if this interpretation is correct. I mean, the calculation does not obviously look like matter/antimatter pairs falling in or not.

 

I am not aware of anyone really making the connection between this interpretation and a calculation. It would require a nice local description of the mechanics. Maybe there is such a description and I am just ignorant.

[insane_alien]

Annihilation of matter by anti-matter happens very quick. The time black holes take to 'die out' is far greater than what one would expect from that by annihilation.

 

well that depends on how its getting annigilated doesn't it.

 

i mean, sure i could hit a block of antimatter with a matter sledge hammer and make a BIG mess very quickly but alternatively i could feed the antimatter in very very slowly a few picograms per second or so and take years to annihilate 1kg of matter.

[sr.vinay]

Well, from what I've read on wikipedia, black holes have something weird happening near their event horizon. Wiki says that there are pairs of particle and anti-particle generated near the event horizon, thus giving the illusion of radiation and absorption!

Maybe this is what is referred by you as the anti-matter effect.

Edited April 29, 2010 by sr.vinay
Grammar error