Are Black Holes Infinite and Do They Emit Jets?

[stever19]

From what I understand a black hole is the result of a tremendous amount of matter being pulled together to a finite point in space and this point creates some kinds of a dip in space. Please correct me if I am wrong?

Questions..

How big can a black hole get?

I've heard something about jets coming off of black holes, is this a real phenomena and if so what is happening in this process?

 

[Passionflower]

From what I understand a black hole is the result of a tremendous amount of matter being pulled together to a finite point in space and this point creates some kinds of a dip in space. Please correct me if I am wrong?

The amount of matter is not relevant, it is the ratio between area and mass that matters.

A non-spinning object becomes (or is already) a black hole if the ratio between the area it occupies and the area that represents its mass is smaller than 4. As soon as this happens the object's occupied area will shrink to zero.

E.g.:

<br /> {A_{occupied} \over A_{mass} } &lt; 4<br />

 

[stever19]

how about the jets on black holes?

Also, is there a limit to how small/big a black hole can get? i.e. is there a breaking point at which the black hole functions in reverse and gives off matter?

 

[DaleSwanson]

how about the jets on black holes?

Also, is there a limit to how small/big a black hole can get? i.e. is there a breaking point at which the black hole functions in reverse and gives off matter?

In theory, there's no upper or lower bounds on a black hole's size. There are some practical lower limits, though. First, there's the minimum amount of mass needed to naturally cause a http://en.wikipedia.org/wiki/Stellar_black_hole" . A neutron star of about 3 solar masses or more will collapse further into a black hole.

The second practical lower limit is created by http://en.wikipedia.org/wiki/Hawking_radiation" . Hawking radiation becomes more significant as the mass decreases. Thus, there reaches a point where the radiation emitted is greater than the CMB radiation absorbed. At that point the black hole will tend to lose mass over time. That assumes there's no mass or energy falling into the black hole other than the CMB radiation. According to Wikipedia, the tipping point is about the mass of the moon. Which would put it well below naturally occurring stellar black holes.

As a side note, this is why fears about artificially created black holes destroying the Earth are unfounded. They would be so small that they would evaporate away in a small fraction of a second.

At the upper limit, as far as I know there is none, practical or otherwise. You can read about http://en.wikipedia.org/wiki/Supermassive_black_hole" for more info.

The http://en.wikipedia.org/wiki/Relativistic_jet" , which is the normal way for matter to fall to a massive body.

 

[stever19]

Thanks for that information.

It seems like once there is a black hole there will always be that black hole and it will never lose mass-- is that correct?

If this is correct would that not imply that the universe will eventually have nothing but black holes or just one giant black hole if they all combined together?

 

[71STARS]

Until you understand exactly what a "black hole" really is, you cannot ask questions regarding functions to and from such a misnamed object. The Russians in the 1930s originally called this a "frozen star" (to which I agree). A black hole is badly names as it is merely dead star; remnant of exhausted dying material which will stay in space until finally dissipated over universal Time. With dying energy, there can be no "jets coming off" such a totally energy depleted object. Whoever has made this assumption then has to admit they have "seen" a black hole, which to date has not happened to my knowledge.

 

[DaleSwanson]

It seems like once there is a black hole there will always be that black hole and it will never lose mass-- is that correct?

No.

Hawking radiation becomes more significant as the mass decreases. Thus, there reaches a point where the radiation emitted is greater than the CMB radiation absorbed. At that point the black hole will tend to lose mass over time. That assumes there's no mass or energy falling into the black hole other than the CMB radiation. According to Wikipedia, the tipping point is about the mass of the moon. Which would put it well below naturally occurring stellar black holes.

All black holes are continually emitting Hawking radiation. It is just that for any stellar black hole there is a greater influx of energy than outflux.

As for the universe ending up as nothing but black holes, as long as the universe continues expanding enough to overtake gravity (and it seems it will) then the only matter that will end up in black holes is the matter which is in groups greater than about three solar masses. In fact, as the CMB radiation would decrease with large passage of time, it would seem that Hawking radiation would eventually be able to evaporate larger and larger black holes. This last part is pure speculation on my part, but I see no reason it wouldn't hold.

 

[DaveC426913]

If this is correct would that not imply that the universe will eventually have nothing but black holes or just one giant black hole if they all combined together?

It is important to be aware, in case you were under the misapprehension, that black holes are nothing special gravitationally. They do not "suck in everything around them, growing infinitely" as is often thought.

If our sun were magically whisked away and replaced with a BH of 1 solar mass, nothing at all would change in Earth's orbit; it would happily continue to follow its one year orbit.

A sun-sized BH will suck in no more material than our Sun currently does.

Likewise, satellites in orbit around Earth are orbiting 3900 miles from the centre of the Earth (Earth is 3800 miles in radius, + 100 mile orbital altitude). If Earth were whisked away and replaced with a BH of one Earth mass, the satellites would never know the difference. An Earth-sized BH will not eat any more satellites than the Earth currently does.

What makes a BH special is that because of its small size, you can get a lot closer to it before you reach its "surface". And closer means high Gs.

If one of those satellies were deorbited, it wouldn't stop at 3800 miles (like it would if Earth were still there); it could fall another 3799 miles before it reached the tiny BH, and that means many, many Gs.

But, generally in the universe, there's not much reason for objects to get that close. So, it's not like the universe is going to be swallowed up by BHs.

 

[tiny-tim]

Yes, black holes have no sucky-power!

Jets do not come out of black holes, they are not really understood, but they come from material falling towards the black hole being somehow flung off along the axis of rotation.

Hawking radiation from a "normal-sized" black hole is extremely small (the larger the black hole, the smaller the radiation), and is almost certainly far less than the increase in mass from matter randomly falling in.

 

[alphachapmtl]

How big can a black hole get?

QUOTE]

There is no size limit. The bigger a black hole, the less dense it is.
Our universe is a black hole.
Nothing can escape it (cause the far away parts of the universe are receding at the speed of light).
Matters keep on falling into it (cause the horizon is receding from us, uncovering new matter as part of our universe).

 

[Chronos]

The theoretical lower limit on a black hole is the Planck mass. It would survive for about a Planck time before going poof. There is no theoretical upper limit on black hole mass.

 

[stevebd1]

While it's not an upper limit, the largest black hole found to date is OJ 287 at 18 billion sol which has a 'smaller' black hole in orbit at 100 million sol.

http://en.wikipedia.org/wiki/OJ_287

 

[DaveC426913]

Our universe is a black hole.
Nothing can escape it (cause the far away parts of the universe are receding at the speed of light).
Matters keep on falling into it (cause the horizon is receding from us, uncovering new matter as part of our universe).

The universe is not a black hole.
Though it is an amusing analogy.

 

[tiny-tim]

The universe is not an analogy …

what would it be an analogy to? ​

 

[DaveC426913]

The universe is not an analogy …

what would it be an analogy to? ​

A black hole is an analogy.