## Can black holes grow?

### Re: Can black holes grow?

My own theory (I'm NOT scientist) is, there's an event horizon radius for the black hole at its current mass and there is (slightly larger) event horizon radius for the black hole at its mass plus the probe mass. When the probe reaches the distance (from our point of view) equal to difference between these two - and note that for any non-zero distance there is finite amount of our time needed to reach that - the event horizon grows and the probe becomes absorbed by the black hole even from our point of view.

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### Re: Can black holes grow?

I am also not enough expert to look at this problem theoretically, but something else came to my mind:

Does it really matter if the additional mass alredy crossed the eventhorizon in the view of the distant observer? In my eyes the mass of the whol object will grow regardless if the new matter already reached the center or if it appears to be stuck in the eventhorizon. The gravity of a spheere is the same regardless if the matter is concentrated in the center or equally distributed in the whole spheere ore even only in one thin shell like the eventhorizon, so regarding to the gravitational effect to the outside it should not really matter if the mass ever passes the eventhorizon or not, from the view of the infalling mass it will surely do but not in the time visable from the outside.

beye

chriwi

Does it really matter if the additional mass alredy crossed the eventhorizon in the view of the distant observer? In my eyes the mass of the whol object will grow regardless if the new matter already reached the center or if it appears to be stuck in the eventhorizon. The gravity of a spheere is the same regardless if the matter is concentrated in the center or equally distributed in the whole spheere ore even only in one thin shell like the eventhorizon, so regarding to the gravitational effect to the outside it should not really matter if the mass ever passes the eventhorizon or not, from the view of the infalling mass it will surely do but not in the time visable from the outside.

beye

chriwi

bye

chriwi

chriwi

### Re: Can black holes grow?

Kasuha, yes, that's roughly what I was trying to get at. Chriwi, your argument is also a good one, but note that a probe mass falling in will break spherical symmetry. If the probe mass is actually a spherical shell of matter then the argument holds (and in fact this setup is often deployed to simplify matters).

What makes this subtle is that in classical general relativity, the horizon is an imaginary line. There is nothing "there". In fact there are a couple of different, inequivalent ways to define this imaginary line, depending on which physical question you are trying to answer you have to pick the right one. (Here's a review from which you can get an idea: http://arxiv.org/PS_cache/gr-qc/pdf/0508/0508107v2.pdf).

If one does not take quantum mechanics into account, the horizon can only grow - and one can make things precise. In this case, using the notion of "absolute horizon", something like Kasuha's description indeed holds - the black hole horizon begins to expand before the probe mass has actually fallen in, coming out to meet it not halfway, but at the (final) radius corresponding to the mass of the black hole plus the probe mass. The absolute horizon is the point after which no signals can be sent that reach outside observers.

What makes this subtle is that in classical general relativity, the horizon is an imaginary line. There is nothing "there". In fact there are a couple of different, inequivalent ways to define this imaginary line, depending on which physical question you are trying to answer you have to pick the right one. (Here's a review from which you can get an idea: http://arxiv.org/PS_cache/gr-qc/pdf/0508/0508107v2.pdf).

If one does not take quantum mechanics into account, the horizon can only grow - and one can make things precise. In this case, using the notion of "absolute horizon", something like Kasuha's description indeed holds - the black hole horizon begins to expand before the probe mass has actually fallen in, coming out to meet it not halfway, but at the (final) radius corresponding to the mass of the black hole plus the probe mass. The absolute horizon is the point after which no signals can be sent that reach outside observers.

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### Re: Can black holes grow?

Interesting is also the following question:

if no information can come out of a black hole, whats when the center of gravity of a black hole is slightly changed by new mass falling in assymetrically, if this would chaange the direction of the vector of gravity which caqn be measured outside that would be also an information coming out of the black hole what should be impossible.

beye

chriwi

if no information can come out of a black hole, whats when the center of gravity of a black hole is slightly changed by new mass falling in assymetrically, if this would chaange the direction of the vector of gravity which caqn be measured outside that would be also an information coming out of the black hole what should be impossible.

beye

chriwi

bye

chriwi

chriwi

### Re: Can black holes grow?

Hehe yes. The answer is that no information can escape from a black hole when it is in equilibrium, that is when it has settled down into its maximally symmetric state. When you chuck something into a black hole, it will respond dynamically (for instance by emitting gravitational waves) until it is back in equilibrium. Part of this response are the "quasi-normal modes" which are basically ringing modes (think of striking a bell, it then rings with a characteristic frequency).if no information can come out of a black hole, whats when the center of gravity of a black hole is slightly changed by new mass falling in assymetrically, if this would chaange the direction of the vector of gravity which caqn be measured outside that would be also an information coming out of the black hole what should be impossible.

### Re: Can black holes grow?

Let's assume we have a black hole the size (weight) of a sun, and we are orbiting it on a track with diameter of 2 AU (so we are in place where space is not too distorted). Let's assume we have an infinitely strong rope and we'll start to unwind the rope towards our black hole to measure the "real" distance between us and the black hole.

In my opinion, we'd need infinitely long rope to reach the event horizon with it, meaning there's infinite distance between us and the black hole itself.

Now, if something is falling to the black hole, it first needs to travel this infinite distance to get infinitely close to it and at that point there is no change to the common center of weight from our point of view.

In my opinion, we'd need infinitely long rope to reach the event horizon with it, meaning there's infinite distance between us and the black hole itself.

Now, if something is falling to the black hole, it first needs to travel this infinite distance to get infinitely close to it and at that point there is no change to the common center of weight from our point of view.

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### Re: Can black holes grow?

@Orion:

he assumes contraction of the rope due to gravity (general relativity) as far as I understand it I agree with him.

beye

Christian

he assumes contraction of the rope due to gravity (general relativity) as far as I understand it I agree with him.

beye

Christian

bye

chriwi

chriwi

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### Re: Can black holes grow?

Sorry for moving off topic a little, but I have a question...

"Distance to the event horizon is infinite" ...

I suppose that is another way of saying that "for an outside observer it takes some object forever to fall into a black hole" (whereas for an observer who travels with the falling object it only takes a short time before tidal forces rip the object apart)?

"Distance to the event horizon is infinite" ...

I suppose that is another way of saying that "for an outside observer it takes some object forever to fall into a black hole" (whereas for an observer who travels with the falling object it only takes a short time before tidal forces rip the object apart)?

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- chriwi
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### Re: Can black holes grow?

Thats what scinetist say nad more than that also for the rope each meter added to come closer to the evevthorizon brings you less and less closer to it the closer you are and in the end you have to add an infinite length to ever reach it by lowering slowly using a rope.

bye

chriwi

chriwi

- TwoPointOh
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### Re: Can black holes grow?

So, black holes. Can they grow? Here is the definitive answer from an ex-physicist.

A black hole is what's called a singularity. A singularity is essentially a point; it has no measurable length in the 3 normal spatial dimensions. What it does have is an event horizon. This is the edge that is essentially the "point of no reutrn" (beyond this point, the escape velocity required to escape the gravitational pull of the singularity is greater than the speed of light). If you think of such an object in 3-dimensional space, the event horizon looks like a black sphere with massive gravitational lensing around it. This is, essentially, the physical manifestation of the singularity, or black hole.

Now, say you throw a small mass (like a planet) into the black hole. This mass, when it reaches the event horizon, will not be able to escape, and so will be pulled into the singularity and added to its mass. In this way, a black hole can grow. Not in the sense of the actual singularity growing in physical size, but rather the

I hope this helps.

~2.0

A black hole is what's called a singularity. A singularity is essentially a point; it has no measurable length in the 3 normal spatial dimensions. What it does have is an event horizon. This is the edge that is essentially the "point of no reutrn" (beyond this point, the escape velocity required to escape the gravitational pull of the singularity is greater than the speed of light). If you think of such an object in 3-dimensional space, the event horizon looks like a black sphere with massive gravitational lensing around it. This is, essentially, the physical manifestation of the singularity, or black hole.

Now, say you throw a small mass (like a planet) into the black hole. This mass, when it reaches the event horizon, will not be able to escape, and so will be pulled into the singularity and added to its mass. In this way, a black hole can grow. Not in the sense of the actual singularity growing in physical size, but rather the

*area of effect*of the singularity increases, due to the mass increase in the singularity.I hope this helps.

~2.0

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### Re: Can black holes grow?

@2.0:

that is a good and true explaination, but I think we already agreed from the beginning that groth not mean spacial groth, but groth in mass.

that is a good and true explaination, but I think we already agreed from the beginning that groth not mean spacial groth, but groth in mass.

bye

chriwi

chriwi

- TwoPointOh
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### Re: Can black holes grow?

Ah, fair enough. I'm too lazy to read the scrollback

### Re: Can black holes grow?

@2.0:

The original question was, if due to relativistic effects any object falling to the black hole takes infinite time to reach the event horizon for a stationary observer in safe distance, how can the black hole ever "swallow" anything from the point of view of the same observer - and so if what we see today are really black holes or just lots of matter frozen in time infinitely close to event horizon boundary.

Even from the point of view of the planet thrown at the black hole, it will pass the point of space where it detected event horizon before, but I believe it will never actually pass the event horizon which will keep shrinking and running from it due to its changing frame of reference.

Actually, singularity is just outcome of mathematic equations. I don't believe universe's black holes are really singularities, there may be forces and effects yet unknown to us taking effect when matter is compressed enough which will stop it falling to a point. Rather than some new repulsive force I think it will be some kind of lack of compression force. But for studying effects of black holes it's irrelevant, whatever is under event horizon does not affect the result in any way.

The original question was, if due to relativistic effects any object falling to the black hole takes infinite time to reach the event horizon for a stationary observer in safe distance, how can the black hole ever "swallow" anything from the point of view of the same observer - and so if what we see today are really black holes or just lots of matter frozen in time infinitely close to event horizon boundary.

Even from the point of view of the planet thrown at the black hole, it will pass the point of space where it detected event horizon before, but I believe it will never actually pass the event horizon which will keep shrinking and running from it due to its changing frame of reference.

Actually, singularity is just outcome of mathematic equations. I don't believe universe's black holes are really singularities, there may be forces and effects yet unknown to us taking effect when matter is compressed enough which will stop it falling to a point. Rather than some new repulsive force I think it will be some kind of lack of compression force. But for studying effects of black holes it's irrelevant, whatever is under event horizon does not affect the result in any way.

- TwoPointOh
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### Re: Can black holes grow?

Ah, fair enough. Maybe next time I'll read the question.

- March_Hare
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### Re: Can black holes grow?

1. Not according to what I have read about it (don't remember where): for an outside observer (at a safe distance etc.) the object never reaches the horizon, but for an observer falling into a black hole the ride down is swift and very short.Kasuha wrote: Even from the point of view of the planet thrown at the black hole, it will pass the point of space where it detected event horizon before, but I believe it will never actually pass the event horizon which will keep shrinking and running from it due to its changing frame of reference.

2. Isn't that just Zeno's Achilles-Turtle paradox in a new setting?

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