Can a blackhole suck in another blackhole?

[mdmaaz]

I was wondering if we were to take two black holes, with their event horizons touching each others, what would happen? Both the black holes would be trying to suck the other one up. I'm sorry if this question sounds stupid but please share your ideas about this topic.

 

[Nik_2213]

IIRC, black holes do collide and merge: Solar sized due to gravity-wave energy loss in close binary systems, mega-sized due to galaxy collisions...

I'd anticipate spectacular pyrotechnics as accretion disks are destabilised and gulped...

 

[zhermes]

They will inspiral and merge together to form a single, larger black-hole. I.e. they both succeed in 'sucking' the other one up.

 

[Sankaku]

Black holes are just gravitating masses like any other. I suspect the idea that you have of them 'sucking' things in isn't really accurate. Matter falls into black holes the same way it does into stars or onto planets - by plain old gravity. Two of anything heavy can collide under the right conditions, although it is fairly rare.

We are hoping to see the merger of two black holes by the gravitational waves that the event would send out. This is the basis of the LIGO project and Einstein@home (like SETI@home):

http://en.wikipedia.org/wiki/LIGO
http://einstein.phys.uwm.edu/index.php

 

[FtlIsAwesome]

I was wondering if we were to take two black holes, with their event horizons touching each others, what would happen?

They would collide to make one larger black hole. Imagine the two black holes as two balls of black ink. They merge to form a single larger blob.

And as Sankaku pointed out, such an event would send out gravity waves that we could potentially detect.

Both the black holes would be trying to suck the other one up.

Well, black holes don't "suck". They have gravity like any other mass. If "sucking" meant gravity, then yeah you can say that black holes suck, and so does the Sun, Earth, etc.
The difference between black holes and other objects is that their gravity has compressed them to a smaller size than the event horizon--where escape velocity reaches lightspeed.

I'm sorry if this question sounds stupid but please share your ideas about this topic.

Don't worry, it's not a stupid question.

 

[Lost in Space]

What determines the size of the black hole is its event horizon. I can understand two black holes that merge will form a bigger event horizon but what happens to each of their singularities? Do they merge as well?

 

[espen180]

I suspect an answer to that question requires a quantum theory of gravity. Unfortunately, we don't have one (yet).

 

[jambaugh]

What determines the size of the black hole is its event horizon. I can understand two black holes that merge will form a bigger event horizon but what happens to each of their singularities? Do they merge as well?

Understand that these singularities are not spatial points. They are space-time singularities... time being the operative word.

Here's what it (theoretically) looks like once you've fallen into a black hole. The space around you is like a tube, the direction you're moving in (call it the z-direction) as you fall straight in looks long and drawn out while the other two spatial directions are S^2 periodic (x and y are like coordinates on the surface of a sphere...they are in fact "parallel" to the event horizon's surface), if you (freeze time and) move in anyone of those directions you'll end up back where you started. Now the singularity part is that over time this tube is being drawn out longer and is shrinking width wise. In a finite time you will be pulled/crushed into a straight line. That point in time is the singularity.

Objects who fell into the EH earlier (including the mass which formed the BH) are farther along that line and objects which fell in later are behind you.

Now as to how this would look if two BH's merged. I think it would be somewhat like two zippers zipping up, the two merging into a single tunnel and the contraction to singularity would happen that much sooner. Remember the singularity is not a point in space but a point in the future. You (as an infalling observer) don't see it you reach it as a point in your time-line.

 

[justiny]

Black holes are just gravitating masses like any other. I suspect the idea that you have of them 'sucking' things in isn't really accurate. Matter falls into black holes the same way it does into stars or onto planets - by plain old gravity. Two of anything heavy can collide under the right conditions, although it is fairly rare.

We are hoping to see the merger of two black holes by the gravitational waves that the event would send out. This is the basis of the LIGO project and Einstein@home (like SETI@home):

http://en.wikipedia.org/wiki/LIGO
http://einstein.phys.uwm.edu/index.php

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?

 

[jambaugh]

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?

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.

 

[FtlIsAwesome]

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

Therefore, if the speed of gravity is less than c, the difference must be very small.

Here is a current thread on the topic: https://www.physicsforums.com/showthread.php?p=3313595

 

[Zentrails]

Black holes are just gravitating masses like any other. I suspect the idea that you have of them 'sucking' things in isn't really accurate. Matter falls into black holes the same way it does into stars or onto planets - by plain old gravity. Two of anything heavy can collide under the right conditions, although it is fairly rare.

We are hoping to see the merger of two black holes by the gravitational waves that the event would send out. This is the basis of the LIGO project and Einstein@home (like SETI@home):

http://en.wikipedia.org/wiki/LIGO
http://einstein.phys.uwm.edu/index.php

That's exactly right. Black holes that approach one another actually orbit their mutual center of mass, traveling faster and faster as they get closer - emitting tremendous amounts of energy. I believe these have been detected. Then they finally merge into one black hole.

It's also quite possible that black holes can orbit their mutual center of gravity for a while, then reach escape velocity.

Black holes cannot collide with another one unless they are headed exactly at each other.

The really odd new finding about black holes is the fact that most physicists now believe that black holes emit EXACTLY the same amount of energy as they absorb, no matter what the source of energy is. If the source of energy contains information, that information is trapped on the event horizon somehow, but the rest of the energy is emitted exactly as you would expect from any black body radiator.

In other words, black holes do not increase in mass (except when they merge with another one) due to absorbed material or photons. They actually lose a little bit more due to Hawking radiation, so theoretically they will eventually evaporate completely.

So, in a fashion, they do sort of suck stuff in, but they spit it right back out! LOL

Good thing, too, because it now appears that every single galaxy out there has a gigantic black hole in their middles.

 

[zhermes]

It's also quite possible that black holes can orbit their mutual center of gravity for a while, then reach escape velocity.

No. Once the system is bound, it will remain bound. Conservation of energy.

Black holes cannot collide with another one unless they are headed exactly at each other.

Incorrect. Gravitational Radiation allows orbiting black-holes to merge from parabolic, or elliptical orbits, close encounters, etc.

In other words, black holes do not increase in mass (except when they merge with another one) due to absorbed material or photons. They actually lose a little bit more due to Hawking radiation, so theoretically they will eventually evaporate completely.

Completely false. Black holes must increase in mass to form and grow in the first place. Hawking radiation is emitted incredibly slowly, for all but the smallest black holes.

 

[Nabeshin]

I believe these have been detected.

It has not.

 

[jambaugh]

They actually lose a little bit more due to Hawking radiation, so theoretically they will eventually evaporate completely.

The current cosmic microwave background gives us a thermal bath of about 2.3K. You can calculate the Hawking Temperature of a BH from its mass or its Schwarzschild radius. A BH with event horizon diameter greater than one millimeter (R=1/2 mm) will be cooler than the cosmic microwave background and thus will not decrease in size.

http://en.wikipedia.org/wiki/Hawking_radiation"

For scale comparison a BH the mass of the Earth has a radius of 9mm so we're talking 1/18th the mass of the Earth.

 

[Sankaku]

It has not.

Inspiralling Binary Pulsars have been detected but Binary Black holes have not been seen directly.

http://en.wikipedia.org/wiki/PSR_B1913+16

Of course, merger has not been seen yet. We are waiting and watching as many binary high-mass objects as possible. For obvious reasons, binary pulsars are easier to find than binary black holes.

http://einstein-dl.aei.uni-hannover.de/EinsteinAtHome/ABP/

 

[Nabeshin]

Inspiralling Binary Pulsars have been detected but Binary Black holes have not been seen directly.

http://en.wikipedia.org/wiki/PSR_B1913+16

Yes, but not the emission of energy part, which is specifically what I was (poorly phrased, I admit) objecting to. I.e. we haven't yet detected gravitational waves from this or any other system.

Of course, merger has not been seen yet. We are waiting and watching as many binary high-mass objects as possible. For obvious reasons, binary pulsars are easier to find than binary black holes.

http://einstein-dl.aei.uni-hannover.de/EinsteinAtHome/ABP/

I imagine what will happen first is we will detect a burst from an inspiraling source (hopefully with advanced LIGO/VIRGO), and then we will try to identify it with an optical counterpart, rather than the other way around. The distance out to which these detectors are sensitive might rule out an optical confirmation of any kind of system, though.

 

[Zentrails]

No. Once the system is bound, it will remain bound. Conservation of energy.Incorrect. Gravitational Radiation allows orbiting black-holes to merge from parabolic, or elliptical orbits, close encounters, etc.Completely false. Black holes must increase in mass to form and grow in the first place. Hawking radiation is emitted incredibly slowly, for all but the smallest black holes.

Sorry, I meant collide without first orbiting. I suspect that black holes could temporarily orbit another one without being permanently bound. In fact, if something is orbiting around a black hole that collided with another object, surely one of them could reach escape velocity. Not likely of course, but possible.

Black holes are created (as far as I know) by imploding stars. They do not increase in mass, they increase in DENSITY. The only way they can increase in mass is by merging with another black hole. It's a new finding that surprises me, too, and maybe I read it wrong. It's just that the idea that black holes "suck" stuff in is apparently not quite right.

Once the black hole is formed, they no longer increase in mass by absorbing any mass outside the event horizon, if I understand current findings correctly. They supposedly emit just as much mass in the form of EM radiation as they absorb. I remember that from an article from Hawking himself, IIRC. INFORMATION from the absorbed mass is not lost though, like Hawking once thought. Instead it is incorporated somehow into the surface of the event horizon itself something like a hologram.

Hawking radiation is certainly incredibly slow, but I believe it has been detect by experiments - that is not the mechanism obviously for most of the emitted radiation.

I believe most black holes that have been positively identified usually have a powerful EM emitter associated with it in one form or another. Black holes also usually have powerful magnetic fields associated with them. The area around black holes must be incredibly complex, so it makes it tough to make blanket statements like I did above.

I probably have it wrong.

I've also read that black holes spin and have charge, but how could anyone possibly know that for sure?
Gravitational radiation is widely believed to exist, but has yet to be detected.

I'll look for the refs and post them when I find them.

 

[Nabeshin]

They do not increase in mass, they increase in DENSITY. The only way they can increase in mass is by merging with another black hole.

No, this is very mistaken. In the absence of hawking radiation (which is completely dwarfed by even the CMB radiation), a black hole's mass will increase only, which results in a DECREASE of its density. Now, I never do like talking about the density of a black hole. As a singularity, the object obviously has infinite density. So the only sensible definition is the total mass divided by the volume within the event horizon. But at any rate, as the black hole's mass increases, the density decreases.

Once the black hole is formed, they no longer increase in mass by absorbing any mass outside the event horizon, if I understand current findings correctly. They supposedly emit just as much mass in the form of EM radiation as they absorb.

Black holes certainly do absorb mass and certainly do increase in mass. This is very well accepted within GR. They do not emit EM radiation at all, save for Hawking radiation.

Hawking radiation is certainly incredibly slow, but I believe it has been detect by experiments - that is not the mechanism obviously for most of the emitted radiation.

It has not been detected, and likely never will be detected. The smallest black hole we know of would emit radiation much weaker than even the CMB.

I believe most black holes that have been positively identified usually have a powerful EM emitter associated with it in one form or another. Black holes also usually have powerful magnetic fields associated with them. The area around black holes must be incredibly complex, so it makes it tough to make blanket statements like I did above.

It's true that the accretion disks of black holes can emit a lot of EM radiation, particularly in the x-ray region. It's also true that this process can be incredibly efficient in transforming energy into radiation, but after all is said and done the black hole certainly still gains mass. (Plus, it is wrong to say that the BH is radiating anything at this point, since everything is happening well outside the EH, the only sensible boundary for the object we are referring to as the black hole).

I've also read that black holes spin and have charge, but how could anyone possibly know that for sure?

Well, there are two reasons:
1) Solutions to Einstein's equations exist for black holes with three properties, which can be identified with mass, spin, and charge. So on the basis of their existence as solutions, we might expect them to be physical objects, but also

2) We see astrophysical objects with spin and (not really) charge, so we expect them to carry their properties over when they become black holes. [Note: Charge is by and large ignored, since any large object will quickly become electrically neutral. Sure, these objects could possibly exist, but realistically we do not expect a significant charge on either a star or a black hole] The fact is, it would be downright odd to find a black hole without spin, and would be rather like finding an orbit which was perfectly circular.

Gravitational radiation is widely believed to exist, but has yet to be detected.

This one is correct :)

 

[zhermes]

Nabeshin has well covered most of it.

Gravitational radiation is widely believed to exist, but has yet to be detected.

This one is correct :)

As a technicality, I'd like to clarify that grav radiation hasn't been directly detected. It has been indirectly detected: http://en.wikipedia.org/wiki/Hulse-Taylor_binary"

 

[Zentrails]

No, this is very mistaken. In the absence of hawking radiation (which is completely dwarfed by even the CMB radiation), a black hole's mass will increase only, which results in a DECREASE of its density. Now, I never do like talking about the density of a black hole. As a singularity, the object obviously has infinite density. So the only sensible definition is the total mass divided by the volume within the event horizon. But at any rate, as the black hole's mass increases, the density decreases.
Black holes certainly do absorb mass and certainly do increase in mass. This is very well accepted within GR. They do not emit EM radiation at all, save for Hawking radiation.
It has not been detected, and likely never will be detected. The smallest black hole we know of would emit radiation much weaker than even the CMB. It's true that the accretion disks of black holes can emit a lot of EM radiation, particularly in the x-ray region. It's also true that this process can be incredibly efficient in transforming energy into radiation, but after all is said and done the black hole certainly still gains mass. (Plus, it is wrong to say that the BH is radiating anything at this point, since everything is happening well outside the EH, the only sensible boundary for the object we are referring to as the black hole).

Well, there are two reasons:
1) Solutions to Einstein's equations exist for black holes with three properties, which can be identified with mass, spin, and charge. So on the basis of their existence as solutions, we might expect them to be physical objects, but also

2) We see astrophysical objects with spin and (not really) charge, so we expect them to carry their properties over when they become black holes. [Note: Charge is by and large ignored, since any large object will quickly become electrically neutral. Sure, these objects could possibly exist, but realistically we do not expect a significant charge on either a star or a black hole] The fact is, it would be downright odd to find a black hole without spin, and would be rather like finding an orbit which was perfectly circular.
This one is correct :)

I'll find the refs and post them. What you are posting are old notions that have been somewhat disproved. It's hard enough to detect a black hole much less measure an increase in it's mass over time, so everything you are saying relies too much on GR.

Astronomers have assumed for quite some time now that massive black holes at the center of galaxies are continuously eating their galaxy. If that were true, the universe would have nothing but black holes in it by now. Instead, the universe is dominated by "dark matter."

I don't understand that first one, though. Stars are not especially dense, they collapse into neutron stars until they are about 10x the size of our sun, IIRC. A neutron star is certainly more dense than the original star but will contain significantly less mass than that original star had before collapsing since much of the gas envelope will blow off.

So, are you suggesting that a star that collapses into a black hole is less dense than a neutron star? I don't get your logic. I'm not talking about a black hole that has already formed.

If you calculate the density of a black hole by dividing it's mass (which can be calculated by it's gravity) by the volume of the sphere made by the event horizon (using Euclidian geometry which is perfectly valid in this case) surely the density you come up with will be greater than that of a neutron star.

If you are suggesting that the density is lower because of space-time distortion, you are literally going down a slippery slope, where GR or any other physics is no longer valid. A singularity suggests infinite volume in Rieman space-time not infinite density.

As long as you stay a decent distance away from the event horizon, you would not be able to detect any volume change due to space-time distortion anyway. The gravitational effects would appear to be coming from the center of mass of that sphere and they will not be strong enough to have any relativistic effects.

So, singularities aside. there is no reason to think that you can't describe the density of a black hole. You simply measure the mass by the gravitational effects and divide by a reasonable estimate of an effective estimated volume.

 

[Nabeshin]

everything you are saying relies too much on GR.

I'm sorry, please point me to the most standard, accepted gravitational theory. Oh wait.

But in all seriousness, black holes are an artifact of GR so it makes sense to discuss them in the context of GR. Sure, there are notions of the holographic principle and whatnot that we've acquired over the last 10-15 years, but I'm not sure these ideas are well developed enough such that we should be stating them as fact over GR. Certainly, I don't feel comfortable doing that. If anyone here has studied quantum gravity or string theory in enough detail to describe these things, then fine (or, I suppose you could post references to peer reviewed papers, but I'm not sure this thread is the best place for that).

I don't understand that first one, though. Stars are not especially dense, they collapse into neutron stars until they are about 10x the size of our sun, IIRC. A neutron star is certainly more dense than the original star but will contain significantly less mass than that original star had before collapsing since much of the gas envelope will blow off.

So, are you suggesting that a star that collapses into a black hole is less dense than a neutron star? I don't get your logic. I'm not talking about a black hole that has already formed.

If you calculate the density of a black hole by dividing it's mass (which can be calculated by it's gravity) by the volume of the sphere made by the event horizon (using Euclidian geometry which is perfectly valid in this case) surely the density you come up with will be greater than that of a neutron star.

If you are suggesting that the density is lower because of space-time distortion, you are literally going down a slippery slope, where GR or any other physics is no longer valid. A singularity suggests infinite volume in Rieman space-time not infinite density.

As long as you stay a decent distance away from the event horizon, you would not be able to detect any volume change due to space-time distortion anyway. The gravitational effects would appear to be coming from the center of mass of that sphere and they will not be strong enough to have any relativistic effects.

So, singularities aside. there is no reason to think that you can't describe the density of a black hole. You simply measure the mass by the gravitational effects and divide by a reasonable estimate of an effective estimated volume.

I was referring to a black hole which has already formed and is then consuming more mass.

 

[zhermes]

I'll find the refs and post them. What you are posting are old notions that have been somewhat disproved. It's hard enough to detect a black hole much less measure an increase in it's mass over time, so everything you are saying relies too much on GR.

Please find those references and post them, or refrain from further confabulations.
You seem to have virtually zero understanding of physics; PhysicsForums is a place to learn and discuss science, not to make it up. Please consult thehttps://www.physicsforums.com/showthread.php?t=414380"for posting.

 

[Sankaku]

Astronomers have assumed for quite some time now that massive black holes at the center of galaxies are continuously eating their galaxy.

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

 

[chronon]

Groups at Caltech and Cornell have recently been doing simulations of black hole collisions. See http://www.black-holes.org/

 

[Zentrails]

Please find those references and post them, or refrain from further confabulations.
You seem to have virtually zero understanding of physics; PhysicsForums is a place to learn and discuss science, not to make it up. Please consult thehttps://www.physicsforums.com/showthread.php?t=414380"for posting.

Are you a mod?
Have you read Wheeler's "Gravitation" book cover to cover like I have?
Have you taken UI/Chamgaign-Urbana core engineering physics courses like I have?

The title of this thread is "can a black hole SUCK IN another black hole?"
and you want to lecture me about lack of physics knowledge?

I'm refuting the SUCK IN notion of black holes and the notion that you can make blanket statements about black holes pretending they exist in isolated areas of space without being integral parts of the universe.

Here's two refs from NASA to start with.
http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html
http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html

 

[Zentrails]

I looked at my older posts and I'm being too strident, arrogant, and irritating.
Sorry, please allow me to start over.

Here's one reference that has some interesting points.

https://secure.wikimedia.org/wikipedia/en/wiki/Hawking_radiation

"Hawking's analysis became the first convincing insight into a possible theory of quantum gravity. In September 2010, Hawking radiation was claimed to have been observed in a laboratory experiment, however the results remain unrepeated and debated.[3] Other projects have been launched to seek this radiation. In June 2008, NASA launched the GLAST satellite, which will search for the terminal gamma-ray flashes expected from evaporating primordial black holes. In speculative large extra dimension theories, CERN's Large Hadron Collider may be able to create micro black holes and observe their evaporation."

So what I mis-remembered was one experiment that has not been duplicated, but if that experiment was interpreted correctly, this new satellite should confirm the results of that experiment.

So, I admit that I remembered that incorrectly, sorry.

Black holes below a certain mass cannot increase in mass, (according to the above article, as predicted using QM) and even smaller black holes actually lose mass over time.

Granted, that mass has to be pretty small. But there is no reason to believe that most black holes are bigger than that mass. They may be out there, but they are much harder to detect than the bigger ones - and it's only the bigger ones that have been found so far, except for that ONE experiment.

I'm on shaky ground, but my intuition leads me to believe that one experiment. I wouldn't call that mere speculation, though - and it shouldn't take too long for the experiment to be confirmed if that experiment was correct.

Some people suspect that the Large Hadron Collider will create micro-black holes, but you won't find anyone associated with that project enthusiastic about discussing that possibility, because of the perception that "black holes suck things in."

If black holes are indeed detected by the LHC, then that would suggest that tiny black holes are being formed ALL THE TIME, then quickly evaporating - and were formed in gigantic numbers when the universe was younger and much hotter.

That's all I'm trying to say about black hole "sucking."
Sometimes they do, sometimes they don't.

 

[Zentrails]

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

OK, you're right, I overstated that point.

https://secure.wikimedia.org/wikipedia/en/wiki/Black_hole

"There is growing consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our Milky Way."

That is what I should have posted, sorry. I don't think I was being "completely ludicrous," though, either.

 

[Lost in Space]

What I'd like to know is how we can witness jets of radiation emanating from the results of objects crossing the event horizon of black holes. Surely relativistic time differences should make this impossible? If we on the outside are witnessing such events, shouldn't we see all of the objects responsible for these jets sitting on the edge of the EH as they would (to our observation) never cross it?

 

[Zentrails]

What I'd like to know is how we can witness jets of radiation emanating from the results of objects crossing the event horizon of black holes. Surely relativistic time differences should make this impossible? If we on the outside are witnessing such events, shouldn't we see all of the objects responsible for these jets sitting on the edge of the EH as they would (to our observation) never cross it?

Here's a good newspaper article from 2007 that discusses these amazing jets.

http://www.washingtonpost.com/wp-dyn/content/article/2007/12/17/AR2007121701266.html

I would assume that the area around the EH to be a seething caldron of sorts with exceedingly high temperatures and perhaps some collisions on the order of magnitude of the collisions with the LHC that's just now coming on line.

There is also relativistic effects due to being so close to a powerful gravitational source that must be added to the relativistic effects due to high particle velocities.

Which could mean that there might be particles in that caldron which would normally have very short life times, but in that high g environment, could exist far longer than we would see in particle acceleration-collision experiments at similar velocities.