Can a blackhole suck in another blackhole?

[magic9mushroo]

This is completely ludicrous. There has never been any such assumption.

Well, IIRC active galactic nuclei are assumed to be due to supermassive black hole accretion.

 

[Sankaku]

Well, IIRC active galactic nuclei are assumed to be due to supermassive black hole accretion.

My objection had nothing to do with the well-established idea that large galaxies have supermassive black holes at their center, nor with the idea that material falls into them. The issue was with the characterization that they are somehow "continuously eating their galaxy."

If that were true, all large galaxies would have active nuclei. Many people have a distorted view of black holes as some sort of vacuum cleaner that goes around looking for things to gobble up. It is just gravity. You either fall in or you don't.

 

[Zentrails]

Wrong. Zentrails - you need to review material, and cite references before you make claims...
Every single post you have made is factually incorrect. Not based on opinion, or perspective, but based on facts.Accreting material may or may not collide with particles outside of the event horizon---the only effect of collisions is to increase the probability that the material is accreted. During most collisions, the the material will not 'transform', but will simply lose energy.Possible. But doubtful.

When do you EVER cite ANY references?
You simply make pontifications and ad hominem attacks.
Your claim seems to be that there are tons of "facts" about black holes, which is patently false.

Massive black holes are certainly seething with energetic materials outside their event horizons which make it impossible for anything to reach the event horizon without colliding with something - why do you demand a reference for such an easy to understand concept?

There is no possible collision where materials DON'T transform.
Have you even heard of Feynman diagrams?
You don't seem to have any grasp of what happens during collisions on the quantum level.
You post like you've taken only a HS physics course, if that.
So, here I find myself copying your ad hominem style, which I don't care for that one whit.
Thanks a lot.

You also seem to like to consider black holes isolated in space.
No "isolated" black holes have been discovered to my knowledge.
I'm considering a far more complex phenomenon than that.
A region of space far larger than just a black hole.
A region of space spanning great distances - sometimes LIGHT YEARS in diameter.

I'm speculating on black holes that have an outer layer, far beyond the event horizon, with an extremely high temperature, high enough
certainly that all matter exists as plasma or maybe a state of matter that exists at even higher temperatures and gravitational extremes.

You want me to cite a reference about that?
It's a little hard to design an experiment that will develop those conditions, don't you think?
So, the only references available consist of astronomical evidence that is very limited, to put it politely.

Or you can cite references that make theoretical predictions using GR or QM, neither of which are particularly apropos for black holes.
In fact, you in particular seem to like to claim that "singularities" prove something which they do not.

 

[Zentrails]

OK here's a reference. Does that make you happy?

"Black holes as a result of their strong gravitational force are able to accelerate particles such as protons and electrons to relativistic speeds (speeds approaching the speed of light) outside their event horizon (Grenier & Laurent 2001)."

http://astronomyonline.org/Stars/BlackHole.asp?Cate=Stars&SubCate=OG04&SubCate2=OG0405

 

[ttmark]

Hey, I don't mean to hijack the topic here but flipping through the first listed link from wikipedia they state that gravitational waves are expected to travel at the speed of light (under the 'Observatories' heading).

I was just wondering why this assumption was made?

There is no basis for that assumption, but since we do not know how gravity works they just invented the assumption that gravity acts at the speed of light. There is no proof that this is or is not the case.

 

[Townes]

Empirically a less than c propagation of gravity would have dramatic effects in e.g. the precession of planetary orbits, which is not observed.

Theoretically, GR predicts speed c propagation of gravity waves. Basically gravity is a massless field (a necessary condition for it to be a long range force) and so must propagate at speed c.

There is basis for that assumption.

 

[ttmark]

There is basis for that assumption.

Do not see any reason that gravity must be limited to the speed of light. For someone to claim that gravity acts as the speed of light they should offer some evidence instead of belief that it must operate according to special relativity, in which the idea of a wave of gravity particles propagating does not reconcile anyway. I tend to think that if the sun were to instantly disappear we would continue to have sunlight for many minutes but the gravitational affect would be experienced before the sunlight disappears. this seems to make the most sense, but we should admit that we simply do not know the speed at which a change in gravity propagates since an experiment has not yet been constructed which has tested this.

 

[Nabeshin]

Do not see any reason that gravity must be limited to the speed of light. For someone to claim that gravity acts as the speed of light they should offer some evidence instead of belief that it must operate according to special relativity, in which the idea of a wave of gravity particles propagating does not reconcile anyway. I tend to think that if the sun were to instantly disappear we would continue to have sunlight for many minutes but the gravitational affect would be experienced before the sunlight disappears. this seems to make the most sense, but we should admit that we simply do not know the speed at which a change in gravity propagates since an experiment has not yet been constructed which has tested this.

Well within the confines of general relativity, it is quite clear that gravitational perturbations (i.e. waves) propagate at the speed of light. This is an obvious result and nobody disputes it. It's true that experimentally we have not determined this to a degree of rigor that I'm happy with, but theoretically this is the (strong) prediction.

If you want to talk about whether or not GR is really the whole story, that's another discussion which should not be carried on in this thread.

 

[magic9mushroo]

My objection had nothing to do with the well-established idea that large galaxies have supermassive black holes at their center, nor with the idea that material falls into them. The issue was with the characterization that they are somehow "continuously eating their galaxy."

If that were true, all large galaxies would have active nuclei. Many people have a distorted view of black holes as some sort of vacuum cleaner that goes around looking for things to gobble up. It is just gravity. You either fall in or you don't.

Oh, sure. I just thought it was worth mentioning that the most luminous objects in the universe are black hole accretion, since there were some in this thread doubting the ability of black holes to accrete.

 

[PAllen]

Do not see any reason that gravity must be limited to the speed of light. For someone to claim that gravity acts as the speed of light they should offer some evidence instead of belief that it must operate according to special relativity, in which the idea of a wave of gravity particles propagating does not reconcile anyway. I tend to think that if the sun were to instantly disappear we would continue to have sunlight for many minutes but the gravitational affect would be experienced before the sunlight disappears. this seems to make the most sense, but we should admit that we simply do not know the speed at which a change in gravity propagates since an experiment has not yet been constructed which has tested this.

You should realize that if gravity propagated faster than c, SR would be dead, because you could send faster than light signals, and define absolute simultaneity and time.

How? Imagine an asteroid and some ability to fire large chunks away at rapid (e.g. near c) speeds. Imagine a sensitive torsion balance some significant distance away. Fire chunks in in morse code sequence, torsion balance responds isntantly.

 

[Chronos]

A black hole is no different than any other massive object in the universe. Gravity works the same in all cases. If the sun were to suddenly collapse to form a black hole [not possible under current theory], its gravity would remain exactly the same - the planets would not be 'gobbled up', although sunbathing would be negatively affected.

 

[pawprint]

I haven't been able to find a mention of time dilation in this thread. If one drops a clock into a black hole (from a safe but observable distance) it will be seen to run slower as it approaches the event horizon. From a 'safe' observer's point of view the clock becomes red shifted and never actually reaches the EH.

This raises the possibility that NOTHING can merge with a BH in finite time for any distant observer. Consequently I argue that the answer to this question is 'no' and that GW experiments (LIGO etc.) will never see a collision involving a BH.

These thoughts came to me when I imagined the wave-form of lesser events, e.g. the collision of two closely orbiting neutron stars. One would expect a GW detector to pick up a signal of increasing amplitude and frequency until the stars 'hit'. One would then expect a gravitational square wave. In electromagnetism a square wave is accompanied by a diminishing set of harmonics, and I wondered then if similar harmonics would be associated with Gravity Waves in general, the Big Bang in particular, and if they would still be detectable.

This is my first post on Physics Forums and I would welcome feedback.

 

[jambaugh]

I haven't been able to find a mention of time dilation in this thread. If one drops a clock into a black hole (from a safe but observable distance) it will be seen to run slower as it approaches the event horizon. From a 'safe' observer's point of view the clock becomes red shifted and never actually reaches the EH.

This raises the possibility that NOTHING can merge with a BH in finite time for any distant observer. Consequently I argue that the answer to this question is 'no' and that GW experiments (LIGO etc.) will never see a collision involving a BH.

Careful here! The event horizon of a BH is not a physical object, it is a mathematically defined boundary (corresponding to physical phenomena). Note a coordinate axis can certainly cross an event horizon and even move back and forth across it. Of course such a coordinate wouldn't be very useful.

So your analogy with the clock does not transfer to the infalling second BH.

We can recover the analogy however if we look more closely at what happens when a physical clock (or other real object) falls into a BH rather than just the behavior of the future geodesic path of the object.

Remember that an infalling physical object will have some mass or at least momentum-energy. So the full description of the space-time geometry along the future geodesic will no longer be static. The "infalling object never reaches the event horizon" derivation reflects an idealized limit as the object has zero effect on space-time. This zero effect and infinite time to infall should cancel to a finite outcome.

What should happen (and I'm working heuristically here as I've not seen or done the hard calculations) is that as the infalling object nears the event horizon, the horizon itself should move outward, reflecting the additional curvature of space-time due to the object's mass. Then in a finite time (of the external observer's clock) the event horizon will envelope the object, you'll have a no-longer-spherical BH and its EH will "ring" emitting gravity waves as it settles down to a new ever so slightly larger spherical configuration.

What is ringing here of course is not the EH itself but the gravitational field/space-time geometry. Again the EH is a virtual surface.

I seem to recall discussing this in past elsewhere in the forum. Try searching.

[...]This is my first post on Physics Forums and I would welcome feedback.

Welcome!

 

[jambaugh]

Under further consideration, my "reach out and grab" description is not meaningful. Our language of "what happens" and "what can happen" breaks down a bit with the divergence of futures at a BH's event horizon. I'm thinking of a very lengthy exposition (which I will compose somewhere and maybe later post here a reference link) but I will try to summarize as briefly as possible.

Firstly by "a distant observer" we generally mean an observer who will never choose to dive into the black hole (and we'll assume a collapsing universe will never force the issue).

Secondly, it is important to understand what an "event horizon" is and qualify the BH's EH as a "static" null event horizon w.r.t. a certain frame... but it is really a bit more peculiar than that. Any space-like or null-like surface is an event horizon. (I'll henceforth use e.h. for general event horizons and EH for a black hole's EH.) So consider a distant observer (in the above sense) and let him define a "ream" (as in stack of paper) of space-like e.h.'s corresponding to his constant t coordinates which locally to him is proper time. These must deform near the BH forming a nesting of "shells" going back into the past. None of them can ever cross the null EH of the BH. It is in this sense that the BH's EH is in the infinite future of the distant observer and so too is any object's event point as it crosses the black hole.

In this sense then the EH of the black hole itself does not "exist" to the distant observer. This applies also to the OP of this thread in that from a distant observer's p.o.v. neither of two BHs EHs "exist" and so "cannot merge"... but this statement once parsed is less significant than it sounds.

Thirdly, letting our distant observer become rather an initially distant observer we note that he can follow the infalling object into the BH and thence observationally access its future past the point on its time-line where it crossed the EH. This access is necessarily identical to the observer crossing the EH and so the following statements are isomorphic:

* "An external observer never sees an infalling object cross the EH of a BH but rather it asymptotically approaches it as its clock asymptotically approaches a certain value of proper time."
* "An external observer never crosses the EH of a BH."

This reflects the nullness of the EH (it is a future light-cone deformed into a tube) and the fact that with a BH the assumption that all (inertial) observers will cross the future light-cones of all event points in the universe at some future time.

Fourthly, we can consider the extremes of a distant observer's "reem of e.h.'s" in the form of either the series of his future light-cones, or the series of his past light-cones. The past light-cones deform to run into the past asymptotically parallel to the BH's EH. The future light-cones deform but intersect the BH's EH, reflecting the fact that, and the "earliest" points at which the observer could dive into the BH if he took off at a given time at speed c.

And so the real question is: What are we trying to mean when we speak of "a black hole sucking in another black hole". We cannot formulate it in the context of the (rigorously) distant observer as in his context the BH's themselves up to their EH's are not physically existent in his history. But we can formulate it in the context of a once distant observer and his possible future paths. Let us define the merging of two BH's as follows:

If a once distant observer, having sent probes into two distinct BH's can in principle at some future time access both probes in sequence after they have crossed the EH's then we can say that the two BH's will have merged.

I don't know if this is possible... I think I can work it out. It is a question of whether it is possible to get to both before the singularity is acheived.

A more general case is certainly true. Which is that a "new" BH can form around a collection of BH's. Since the collapse of a star into a BH is possible, so too is the collapse of a swarm of smaller BH's. This is I think what I was going for when describing the infalling massive clock. It is not so much that the EH of the BH reaches out to it but that at some point (outside the rigorously distant observer's future since he by definition never "sees" any BH's EH), a "new" BH forms around the clock + original BH.

But really all this is saying is that there is a point BEFORE that final tick of the clock as it crosses the original BH's EH, which the distant observer cannot access and recover his distant position. I do think that, in so far as we can say "Hey Bob! There's a BH over there!" we can at some finite future time say "Hey Bob! There's a BH over there and the clock I threw out is inside it's EH!"

Well I tried to be brief but it just takes too many words to be even close to clear. Our language of absolute tenses is inadequate to clearly and concisely express situations in these extremes of curved space-time.

 

[jambaugh]

Another quick thought on the language of BH's...
We normally speak of possibility in terms of "if we wait long enough will it happen?" but with SR and GR the term "wait" has more depth. The qualifier becomes "if we wait long enough in the right direction...". For the infalling clock, "if we wait long enough in the right direction i.e. into the BH ourselves, then we will 'see' the clock pass the EH".

It's something to keep in mind as one ponders these questions.

 

[PAllen]

Jambaugh,

The following:

https://www.physicsforums.com/showpost.php?p=3348591&postcount=2

posted by George Jones seems relevant here. I think the key concept is that the actual causal event horizon 'now' is affected by the total future of a black hole. Thus the actual boundary of which events are causally disconnected from distant observers is affected by any future growth of the black hole. It seems to me, then that the causal event horizon does grow in anticipation, if you will, as the merger approaches.

Also, there are lots of numeric solutions of black hole mergers showing GW seen by distant observers. So, in this sense, the merger must be observable.