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.

 

[PAllen]

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."

If you actually look at the reference [3], given below, you will see it is a purported claim of an optical simulation of Hawking radiation, not actual Hawking radiation. The disputes center not so much on reproducibility as on relevance. I remember the announcement of this result, and my reaction was 'cool experiment, doesn't count as anything more than plausibility demonstration'.

There is, as yet, not a single claim of detection of actual Hawking radiation.

http://arxiv.org/abs/1009.4634

 

[Zentrails]

If you actually look at the reference [3], given below, you will see it is a purported claim of an optical simulation of Hawking radiation, not actual Hawking radiation. The disputes center not so much on reproducibility as on relevance. I remember the announcement of this result, and my reaction was 'cool experiment, doesn't count as anything more than plausibility demonstration'.

There is, as yet, not a single claim of detection of actual Hawking radiation.

http://arxiv.org/abs/1009.4634

Yep, that experiment proves nothing so far, nevertheless, there's plenty of interesting info coming from that satellite:

"In March 2010 it was announced that active galactic nuclei are not responsible for most gamma-ray background radiation.[32] Though active galactic nuclei do produce some of the gamma-ray radiation detected here on Earth, less than 30% originates from these sources. The search now is to locate the sources for the remaining 70% or so of all gamma-rays detected. Possibilities include star forming galaxies, galactic mergers, and yet-to-be explained dark matter interactions."

https://secure.wikimedia.org/wikipedia/en/wiki/Fermi_Gamma-ray_Space_Telescope

 

[cph]

A galactic black hole can suck up anything, including compact objects.

 

[Zentrails]

A galactic black hole can suck up anything, including compact objects.

I guess my objection with that "suck" claim is not that they can theoretically suck stuff in, but rather that they can't because an object or photon cannot possibly get close to the event horizon.

Because it will be blocked way outside the event horizon by plenty of rapidly moving particles and photons orbiting the black hole. The only way in is to collide many times with the circling stuff, transforming in the process to many new things.

It is the combined effect of high gravity and high energy collisions that muddies the water.
I think it is possible that what goes in might be almost entirely photons.

A black hole sitting way out in space somewhere, far from much matter would be a different story.

 

[zhermes]

I guess my objection with that "suck" claim is not that they can theoretically suck stuff in, but rather that they can't because an object or photon cannot possibly get close to the event horizon.

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.

Because it will be blocked way outside the event horizon by plenty of rapidly moving particles and photons orbiting the black hole. The only way in is to collide many times with the circling stuff, transforming in the process to many new things.

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.

It is the combined effect of high gravity and high energy collisions that muddies the water.
I think it is possible that what goes in might be almost entirely photons.

Possible. But doubtful.

 

[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.