When a black hole and a neutron star merge(mpg.de)
mpg.de
When a black hole and a neutron star merge
https://www.mpg.de/21778967/0404-grav-mysterious-object-in-the-gap-152520-x
80 comments
As an FYI, LIGO in Eastern Washington offers full tours of the facility inside and out for free. Was a memorable trip and included a lecture before hand. Highly recommend checking it out!
A lot of publicly funded projects give free tours.
For science experiments and infrastructure this makes a lot of sense: we need to tell tax payers why we're taking their money. I'm a bit sad that it's not more widespread for a lot of critical infrastructure e.g. ferries, power plants, factories, etc: it's a great way to get people interested in engineering and big cool public projects.
It's often worth asking, the reactions can vary a lot. For example taking a ferry, I like to poke around and see how everything works:
- In Long Island Sound, even without going through any doors I got someone asking why I'm interested and essentially telling me there's no way in hell I could e.g. see the engine. He seemed to be 5 seconds away from calling the police.
- In Stockholm the engine mechanic noticed me looking around, invited me to climb down the ladder into the engine room, and showed me how everything worked. It was one of the highlights of that trip.
For science experiments and infrastructure this makes a lot of sense: we need to tell tax payers why we're taking their money. I'm a bit sad that it's not more widespread for a lot of critical infrastructure e.g. ferries, power plants, factories, etc: it's a great way to get people interested in engineering and big cool public projects.
It's often worth asking, the reactions can vary a lot. For example taking a ferry, I like to poke around and see how everything works:
- In Long Island Sound, even without going through any doors I got someone asking why I'm interested and essentially telling me there's no way in hell I could e.g. see the engine. He seemed to be 5 seconds away from calling the police.
- In Stockholm the engine mechanic noticed me looking around, invited me to climb down the ladder into the engine room, and showed me how everything worked. It was one of the highlights of that trip.
My county started offering tours of our new water treatment plant. I HIGHLY recommend doing that in your area if you get the opportunity!
> In Long Island Sound, even without going through any doors I got someone asking why I'm interested and essentially telling me there's no way in hell I could e.g. see the engine. He seemed to be 5 seconds away from calling the police.
Might have had better luck pre-9/11.
> I'm a bit sad that it's not more widespread for a lot of critical infrastructure e.g. ferries, power plants, factories, etc
The rationale is clear (you may debate the necessity or effectiveness): the concern is that terrorists would scout critical infrastructure before an attack.
Might have had better luck pre-9/11.
> I'm a bit sad that it's not more widespread for a lot of critical infrastructure e.g. ferries, power plants, factories, etc
The rationale is clear (you may debate the necessity or effectiveness): the concern is that terrorists would scout critical infrastructure before an attack.
This is the internet: I wouldn't dare debate the necessity or effectiveness of counter-terrorism measures here.
But yes: The guy in Long Island said something about DHS or something and both these things happened after 2001.
But yes: The guy in Long Island said something about DHS or something and both these things happened after 2001.
This is one of the hidden costs of a low-trust society.
So does CERN. Since 2023 they have a new visitor center, which I have yet to visit.
I have visited it in the Easter week, with my 4 year old. It's nice, but I experienced a major disappointment when visiting the immersive experience about the quantum world: they explain quantum entanglement in the wrong way!
The voice-over _clearly_ says: "when two particles are entangled _they can influence each other_ no matter the distance", which is super wrong. If you have two spin entangled particles and you change the spin of one, the spin of the other does not change, there is no influence, only correlation.
Maybe they intended something about the wave function collapse, but they didn't say it. If even explanations coming from CERN get this wrong, I despair for the status of science communication in general...
The voice-over _clearly_ says: "when two particles are entangled _they can influence each other_ no matter the distance", which is super wrong. If you have two spin entangled particles and you change the spin of one, the spin of the other does not change, there is no influence, only correlation.
Maybe they intended something about the wave function collapse, but they didn't say it. If even explanations coming from CERN get this wrong, I despair for the status of science communication in general...
> If you have two spin entangled particles and you change
> the spin of one, the spin of the other does not change
It really irks me we don't use a simple analogy for entanglement: put two balls of different colors in two boxes. Randomize the boxes. Take one of the boxes to the other side of the room then open it.
You now know the colour of the ball in the other box.
Only you can't implement faster-than-light communication with that so we instead mislead people..
It really irks me we don't use a simple analogy for entanglement: put two balls of different colors in two boxes. Randomize the boxes. Take one of the boxes to the other side of the room then open it.
You now know the colour of the ball in the other box.
Only you can't implement faster-than-light communication with that so we instead mislead people..
That's not the correct analogy. In classical case, the color of the ball is fixed even before you open the box (you just don't know it). But in quantum entanglement case, the spin is not fixed until it is measured (because the wave function hasn't collapsed). But as soon as you measure the spin you get either up or down value. If you get up, then the other entangled particle will necessarily have down value when measured, If you get the spin down, the other particle will necessarily have the spin up. Now you may say that, the spin of the first particle was fixed all along and we just didn't know it. This argument is called "hidden variables theory". But it is proven by Bell's inequality that such a theory cannot exist, so the spin of particle 1 is indeed a random outcome. What's "spooky" is that in spite of it being random, it instantaneously fixes the spin of the other particle.
> But it is proven by Bell's inequality that such a theory cannot exist, so the spin of particle 1 is indeed a random outcome.
It's still allowed so long as a) the theory is nonlocal b) the outcome was not actually independent of the observer or c) superdeterminism.
It's still allowed so long as a) the theory is nonlocal b) the outcome was not actually independent of the observer or c) superdeterminism.
Yeah, but I didn't "get" (to the extent that I can without grokking maths) quantum entanglement until I had it explained with this analogy, and then the "but that's not exactly what's happening here" real explanation. Leading with the complicated (albeit correct) wave form collapse explanation spun my head around and got me nowhere.
Good pedagogy (like, you know, science itself) starts simple and adds complexity as you dive deeper into the subject.
Good pedagogy (like, you know, science itself) starts simple and adds complexity as you dive deeper into the subject.
No. There are no “hidden local variables” (color in your example), and it really is “spooky action from a distance”.
See Bell’s Theorem, this has been mathematically and experimentally proved: https://en.m.wikipedia.org/wiki/Bell's_theorem
This thing that might be confusing you is the action you are taking is collapsing the wave function of a pair of entangled particles (by measuring one of them), so they go from a superposition of up|down to each having one definite value. You can’t repeatedly twiddle the bit here.
See Bell’s Theorem, this has been mathematically and experimentally proved: https://en.m.wikipedia.org/wiki/Bell's_theorem
This thing that might be confusing you is the action you are taking is collapsing the wave function of a pair of entangled particles (by measuring one of them), so they go from a superposition of up|down to each having one definite value. You can’t repeatedly twiddle the bit here.
Other people are pointing out how its not actually like that, but I'm gonna buck the trend and say, for the purposes of explaining to the general public the basics of quantum mechanics, its perfectly acceptable.
Quantum mechanics is so counterintuitive that you have to re-calibrate people's intuition before you can really pick at the confusing parts.
So picking the nit re: wave function collapse is the right thing to do, but it needs to be done in the context of "...but its weirder than just we don't know what the colors are until we open the box. It turns out that...", rather than just immediately "correcting" the partially, arguably incorrect information.
As a challenge to the folks correcting the OP over neglecting wave function collapse, can any of you describe what is wrong with the infinite square well very-first-mathematical-example-of-quantum-mechanics? Aside from the "infinite" part, I mean.
Quantum mechanics is so counterintuitive that you have to re-calibrate people's intuition before you can really pick at the confusing parts.
So picking the nit re: wave function collapse is the right thing to do, but it needs to be done in the context of "...but its weirder than just we don't know what the colors are until we open the box. It turns out that...", rather than just immediately "correcting" the partially, arguably incorrect information.
As a challenge to the folks correcting the OP over neglecting wave function collapse, can any of you describe what is wrong with the infinite square well very-first-mathematical-example-of-quantum-mechanics? Aside from the "infinite" part, I mean.
What? That's simple deduction. Is it really that trivial? Am I missing something?
No, that’s not quite right. You’re taking two wavefunctions that have not collapsed yet.
This ball box analogy is not the same as quantum entanglement … outcome of the boxes are predetermined but not for two entanglement spins …
> The voice-over _clearly_ says: "when two particles are entangled _they can influence each other_ no matter the distance", which is super wrong. If you have two spin entangled particles and you change the spin of one, the spin of the other does not change, there is no influence, only correlation.
Sounds like you subscribe to the Copenhagen interpretation, whereas they’re using a Bohm interpretation.
Sounds like you subscribe to the Copenhagen interpretation, whereas they’re using a Bohm interpretation.
Granted that I'm not a physicist and I may be wrong about this, but I don't think what you say is correct. This is not a matter of interpretation. In no interpretation changing the spin of an entangled particle will change the spin of the other at a distance.
Imagine if they built fairgrounds next to it and had a ton of science themed rides.
They could call it the CERNival.
They could call it the CERNival.
Only if it has a very fast merry-go-round that transitions into colliding bumper-cars.
The Hanford nuclear site is also nearby and also does day tours. Such a cool piece of history.
The Goldendale Observatory is a couple hours away, as well.
https://www.goldendaleobservatory.com/visit.html
https://www.goldendaleobservatory.com/visit.html
If a neutron star, black hole, and regular star can all exist in the 3 solar mass range, do they have a different curve in spacetime?
I always imagined the example of a ball on cellophane for a gravity well caused by massive objects.
If they have similar masses, how does the black hole bend "deep" enough into space to the point where not even light can escape?
What if it's some other class of object that we have yet to identify? Why black hole if not neutron star?
This is not a criticism of the article, these are curiosities. It's a really good read. Fascinating science.
I always imagined the example of a ball on cellophane for a gravity well caused by massive objects.
If they have similar masses, how does the black hole bend "deep" enough into space to the point where not even light can escape?
What if it's some other class of object that we have yet to identify? Why black hole if not neutron star?
This is not a criticism of the article, these are curiosities. It's a really good read. Fascinating science.
Each mass creates a “dimple” on the cellophane but because they have different density, the shape of that dimple is different. The less dense object has a shallower cone. The infinitely dense object has a steeper one.
If you’re five feet from the surface of the sun you’re hundreds of thousands of miles away from its center of mass. If you’re five feet away from the “surface” of a solar mass black hole you’re just a couple of miles away from the center of mass.
If you’re five feet from the surface of the sun you’re hundreds of thousands of miles away from its center of mass. If you’re five feet away from the “surface” of a solar mass black hole you’re just a couple of miles away from the center of mass.
Except this is not entirely true.
The shape of gravitational field only differs within the body. The shape outside of the perfectly spherical body will be exactly the same regardless of how the mass is distributed (whether it is all on the surface or all in the middle of it).
Or in other words, if you are on the surface of the sun, you will feel exactly the same acceleration as if entire mass of the sun was concentrated in its center.
The only time you feel the difference is if you dip into the sun. As you go deeper and deeper, the pull from all the mass that is further from the center than you are will perfectly nullify each other. As you go deeper and deeper into the sun you will feel less acceleration.
The shape of gravitational field only differs within the body. The shape outside of the perfectly spherical body will be exactly the same regardless of how the mass is distributed (whether it is all on the surface or all in the middle of it).
Or in other words, if you are on the surface of the sun, you will feel exactly the same acceleration as if entire mass of the sun was concentrated in its center.
The only time you feel the difference is if you dip into the sun. As you go deeper and deeper, the pull from all the mass that is further from the center than you are will perfectly nullify each other. As you go deeper and deeper into the sun you will feel less acceleration.
However, when you're 5 miles away from the surface of a black hole, you would be inside the sun, so the shape will be different. We are not comparing equal distances from the center of mass, but equal distances from the surface; which are different distances! Only if you are the same distance from the sun as from a black hole, what you say is correct. But then you will be much farther away from the black hole than in this example.
Distance from the body I have defined as distance from its center.
Black hole does not have a surface. At least not that we know of. Event horizon is a surface, but it is not the surface of the BH in conventional sense. If you were falling into BH nothing special would happen as you crossed event horizon.
And neither does the Sun. Just like Earth's atmosphere, we can only talk about some arbitrary altitudes or pressures, but reality is that the atmosphere of the Sun or Earth does not have any firm beginning or end. And Sun is all atmosphere.
Black hole does not have a surface. At least not that we know of. Event horizon is a surface, but it is not the surface of the BH in conventional sense. If you were falling into BH nothing special would happen as you crossed event horizon.
And neither does the Sun. Just like Earth's atmosphere, we can only talk about some arbitrary altitudes or pressures, but reality is that the atmosphere of the Sun or Earth does not have any firm beginning or end. And Sun is all atmosphere.
So yea, this lets you equate standing on the surface of a given star of X radius, with hovering the same X distance away from a neutron star or black hole of the same mass.
The takeaway is that the cellophane dimple is more warped around the center point, but if, say, our Sun were to suddenly turn into a black hole it wouldn't change anything about our orbit.
The takeaway is that the cellophane dimple is more warped around the center point, but if, say, our Sun were to suddenly turn into a black hole it wouldn't change anything about our orbit.
Nitpick: you will not feel acceleration in a freefall.
You’ll feel the tides, however.
These analogies always fall apart for me because you'll be burnt to nothing long before you reach the surface of the sun.
Your analogy falls apart, too. Sun does not have a surface to be reached. It just sort of gets more and more dense and that's all.
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What is "feeling"? If my senses can register changing spectra of the stars around me then yes, I will "feel" the acceleration.
I didn't think you could accelerate at all in a freefall (which is why you wouldn't feel it), you are just following a geodesic
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Since the speed of light is a constant, mass increases towards infinity as it approaches the speed of light, and time slows for the matter relative to matter with lesser velocity, does that mean that matter accelerating towards a black hole increases the mass in the region and further increases the magnitude of the effect on the spacetime curve?
Is the mass of the black hole perpetually "falling" through spacetime at near the speed of light?
Would the region around an "active" black hole appear more massive when it is consuming matter vs existing in a vacuum?
A big object going really fast would be almost stopped in time, while at the same moment be almost infinitely massive which is increasing the gravitational effect. The combination of these effects is what makes such a big "dent" in spacetime?
Is the mass of the black hole perpetually "falling" through spacetime at near the speed of light?
Would the region around an "active" black hole appear more massive when it is consuming matter vs existing in a vacuum?
A big object going really fast would be almost stopped in time, while at the same moment be almost infinitely massive which is increasing the gravitational effect. The combination of these effects is what makes such a big "dent" in spacetime?
Exactly.
Also, if you are hundreds of thousands of miles from the center of mass of the sun, you'd feel the same force from gravity as if you were the same distance from the center of mass of a black hole (with the same mass).
Far away, the curvature of spacetime is the same, it's just that with black holes you can get waaaay closer to the center of mass whilst still being outside of it.
Also, if you are hundreds of thousands of miles from the center of mass of the sun, you'd feel the same force from gravity as if you were the same distance from the center of mass of a black hole (with the same mass).
Far away, the curvature of spacetime is the same, it's just that with black holes you can get waaaay closer to the center of mass whilst still being outside of it.
What if it's some other class of object that we have yet to identify? Why black hole if not neutron star?
There's a whole science to this with a fun name:
https://en.wikipedia.org/wiki/Degenerate_matter
Quantum theory predicts and limits the sorts of things you can get once you really put the squeeze on and these predictions align well with observations.
There's a whole science to this with a fun name:
https://en.wikipedia.org/wiki/Degenerate_matter
Quantum theory predicts and limits the sorts of things you can get once you really put the squeeze on and these predictions align well with observations.
A regular star has a significantly lower density. The ongoing fusion causes a pressure that keeps it from collapsing. The atoms here move chaotically through a 3d space in quite a volume. So your ball sits on too large a surface of cellophane to go really deep. That can continue for increasing masses for quite a while. Google tells me the heaviest known star is about 200 solar masses.
Black holes and neutron stars are fundamentally different - there’s no space left between the matter of a neutron star. Now, the physics of that is a bit over my head, but a simplification I got from a PBS Spacetime episode is that the matter in a neutron star pretty much overlaps in 3d and stars filling up some quantum mechanics dimensions. When those cross a threshold, the last forces give way and everything collapses without a limit. So now we’re talking about a ball on your cellophane as small as physically possible for its mass (compact), pushing down significantly deeper… Until it gets replaced with pretty much a puncture, pushing it down to some abstract floor.
Black holes and neutron stars are fundamentally different - there’s no space left between the matter of a neutron star. Now, the physics of that is a bit over my head, but a simplification I got from a PBS Spacetime episode is that the matter in a neutron star pretty much overlaps in 3d and stars filling up some quantum mechanics dimensions. When those cross a threshold, the last forces give way and everything collapses without a limit. So now we’re talking about a ball on your cellophane as small as physically possible for its mass (compact), pushing down significantly deeper… Until it gets replaced with pretty much a puncture, pushing it down to some abstract floor.
My understanding of it is somewhat similar, although I'd argue that's where our mental imagery breaks down: the "weight on fabric" metaphor works to a point then stops conveying what's really happening and becomes misleading.
I think of it like "magnets as rubber bands", and what I call "Feynman lies":
> I can't explain [magnetic] attraction in terms of anything else that's familiar to you. For example, if we’d said the magnets attract like as if they were connected by rubber bands, I would be cheating you. Because they're not connected by rubber bands, I shouldn’t be in trouble, you’d soon ask me about the nature of the bands… and secondly if you were curious enough, you'd ask me why rubber bands tend to pull back together again, and I would end up explaining that in terms of electrical forces, which are the very things that I'm trying to use the rubber bands to explain, so I have cheated very badly, you see.
> [...]
> But I really can't do a good job, any job, of explaining magnetic force in terms of something else that you're more familiar with, because I don’t understand them in terms of anything else that you are more familiar with.
direct link: https://www.youtube.com/watch?v=nYg6jzotiAc&t=1263s
but the whole chapter is worth a watch: https://www.youtube.com/watch?v=nYg6jzotiAc&t=894
I think of it like "magnets as rubber bands", and what I call "Feynman lies":
> I can't explain [magnetic] attraction in terms of anything else that's familiar to you. For example, if we’d said the magnets attract like as if they were connected by rubber bands, I would be cheating you. Because they're not connected by rubber bands, I shouldn’t be in trouble, you’d soon ask me about the nature of the bands… and secondly if you were curious enough, you'd ask me why rubber bands tend to pull back together again, and I would end up explaining that in terms of electrical forces, which are the very things that I'm trying to use the rubber bands to explain, so I have cheated very badly, you see.
> [...]
> But I really can't do a good job, any job, of explaining magnetic force in terms of something else that you're more familiar with, because I don’t understand them in terms of anything else that you are more familiar with.
direct link: https://www.youtube.com/watch?v=nYg6jzotiAc&t=1263s
but the whole chapter is worth a watch: https://www.youtube.com/watch?v=nYg6jzotiAc&t=894
That last sentence is a phenomenal explanation of why it can be harder for experts to teach a subject than non-experts.
https://en.m.wikipedia.org/wiki/Neutron_star#Structure
I think you're misremembering, we don't know what's at the center of a neutron star, or that there aren't other, more dense, types of stars.
I think you're misremembering, we don't know what's at the center of a neutron star, or that there aren't other, more dense, types of stars.
I think you’re right. I misremembered at the very least that the whole star is the quark gluon plasma.
That’s what I get for treating modern physics as a kind of a vaudeville show ;)
That’s what I get for treating modern physics as a kind of a vaudeville show ;)
> I always imagined the example of a ball on cellophane for a gravity well caused by massive objects.
> If they have similar masses, how does the black hole bend "deep" enough into space to the point where not even light can escape?
A back hole is smaller ball
> If they have similar masses, how does the black hole bend "deep" enough into space to the point where not even light can escape?
A back hole is smaller ball
In principle, any object can become a black hole if it is made dense enough. It does not matter for the gravity at long distance: replace the earth with a black hole of the same mass, and it not change the interactions with the other bodies (sun and moon, mostly).
Light does not escape in the black hole case because the black hole is smaller: in case of a black hole earth, light starting at 6370 km (current radius) can still escape. A black hole weighing the same as the earth would have a 9 millimeter radius, no light will escape from there.
Light does not escape in the black hole case because the black hole is smaller: in case of a black hole earth, light starting at 6370 km (current radius) can still escape. A black hole weighing the same as the earth would have a 9 millimeter radius, no light will escape from there.
And, any material (of any density) can be made a black hole if there is enough of it. The "radius" (actually, one should properly talk about the circumference) of a black hole is proportional to its mass, so the "density" is proportional to mass^-2. M87's central black is about the same density as the air you are breathing.
>> any object can become a black hole if it is made dense enough
[big 'if'!]
> any material (of any density) can be made a black hole if there is enough of it
There is an awful lot of intergalactic medium out there. It's of "any density" (albeit very low). How come it's not a black hole? Isn't there "enough of it"? (cf. the "missing baryons problem").
There is an awful lot of atomic and molecular hydrogen gas in the Milky Way and practically every other galaxy. In our galaxy it totally dwarfs all the matter in our central black hole, the stellar black holes, and in all the luminous stars combined. Seems like "enough of it". It's of "any density". How come it's not a black hole?
And so forth.
> (actually, one should properly talk about the circumference)
Why? Also, since we're being properly, what's the circumference of a spinning black hole and how does it evolve with the spin parameter?
Extra credit: assume that you mean the mean density of the interior of M87*, how are you arriving at its volume? (You might cast your eyes over Bengtsson & Jakobsson's extension of work on this question by Christodoulou & Rovelli at respectively https://arxiv.org/abs/1502.01907 and https://arxiv.org/abs/1411.2854 -- when you calculate the volumes in a relativistic spirit, the interiors are very large, far too large to support your (commonly used) throwaway line about dense-as-air/less-dense-than-water). Alternatively, what do you think the BH interior modal densities are?
("In their paper CR [Christodolou & Rovelli] estimate that the black hole at the centre of the Milky Way — whose area radius they assume to be not much larger than the distance to the Moon — now contains enough space to fit a million solar systems. A decent estimate for its spin appears to be a/m ≈ 0.9 [5]. It follows that CR overestimate the volume, but only by a factor of 10 or somewhat less". And from CR: "flat space intuition does not apply to the curved geometry inside the hole [...] and the interior volume keeps growing with time").
I of course expect this comment to be thrown into a deep (gravitational) well, actually.
[big 'if'!]
> any material (of any density) can be made a black hole if there is enough of it
There is an awful lot of intergalactic medium out there. It's of "any density" (albeit very low). How come it's not a black hole? Isn't there "enough of it"? (cf. the "missing baryons problem").
There is an awful lot of atomic and molecular hydrogen gas in the Milky Way and practically every other galaxy. In our galaxy it totally dwarfs all the matter in our central black hole, the stellar black holes, and in all the luminous stars combined. Seems like "enough of it". It's of "any density". How come it's not a black hole?
And so forth.
> (actually, one should properly talk about the circumference)
Why? Also, since we're being properly, what's the circumference of a spinning black hole and how does it evolve with the spin parameter?
Extra credit: assume that you mean the mean density of the interior of M87*, how are you arriving at its volume? (You might cast your eyes over Bengtsson & Jakobsson's extension of work on this question by Christodoulou & Rovelli at respectively https://arxiv.org/abs/1502.01907 and https://arxiv.org/abs/1411.2854 -- when you calculate the volumes in a relativistic spirit, the interiors are very large, far too large to support your (commonly used) throwaway line about dense-as-air/less-dense-than-water). Alternatively, what do you think the BH interior modal densities are?
("In their paper CR [Christodolou & Rovelli] estimate that the black hole at the centre of the Milky Way — whose area radius they assume to be not much larger than the distance to the Moon — now contains enough space to fit a million solar systems. A decent estimate for its spin appears to be a/m ≈ 0.9 [5]. It follows that CR overestimate the volume, but only by a factor of 10 or somewhat less". And from CR: "flat space intuition does not apply to the curved geometry inside the hole [...] and the interior volume keeps growing with time").
I of course expect this comment to be thrown into a deep (gravitational) well, actually.
> There is an awful lot of intergalactic medium out there. It's of "any density" (albeit very low). How come it's not a black hole? Isn't there "enough of it"? (cf. the "missing baryons problem").
Nothing to do with the missing baryons problem. If you had a large enough cloud of intergalactic matter it would form an event horizon. Obviously “large enough” is larger than the size of our galaxy and logically any existing galaxy .
One theory of how very early SMBHs may have formed is from direct collapse of large but diffuse clouds.
Nothing to do with the missing baryons problem. If you had a large enough cloud of intergalactic matter it would form an event horizon. Obviously “large enough” is larger than the size of our galaxy and logically any existing galaxy .
One theory of how very early SMBHs may have formed is from direct collapse of large but diffuse clouds.
> Nothing to do with the missing baryons problem.
WHIM. Nicastro et al (2018) https://www.nature.com/articles/s41586-018-0204-1 (you can get a PDF at </https://arts.units.it/bitstream/11368/2935866/4/2935866_s415...> or the preprint at <https://arxiv.org/abs/1806.08395> and there is a nice summary at <https://www.esa.int/Science_Exploration/Space_Science/XMM-Ne...> ).
> If you had a large enough cloud of intergalactic matter it would form an event horizon
I assume you mean that you get gravitational collapse and some form of ~ 2GM black hole horizon (pfdietz's "any material (of any density) can be made a black hole if there is enough of it"). Trivially you can get an event horizon by making the cloud a Rindler observer, or even more trivially by making it an Eulerian observer in de Sitter space. So let's think about what makes a diffuse cloud collapse in a universe like ours at a comparable scale factor.
At the GR level (e.g. matter dominated era OS or LTB like versions of Einstein-de Sitter or similar dark energy dominated era approaches) the problem you encounter is that you end up having to exchange matter between the collapsing "large enough" cloud and the expanding radiation (and other FLRW fluid) filled cosmological region in which it is immersed, and you have to keep a check on the false intuition that leaking stress-energy from the nonvacuum cosmological region into the vacuum in the collapsing cloud -> black hole region is OK to stabilize pressure, because that's not what we see.
We could alternatively look at it through the lens of Jeans instability. To get collapse you need undamped perturbations with oscillation length longer than the Jeans length, which increases in proportion to temperature and decreases in proportion to mass density and mean molecular mass. And then you have to avoid fragmentation. Which takes us to...
> direct collapse [SMBHs]
Sure, in z > 15, the metal-poor radiation dominated era or even earlier, it's plausible (but see below). You need to avoid the formation of molecules or the gas will radiatively cool much faster than it will contract, fragment your cloud and drive cluster star formation. Greedy metals (carbon, oxygen) will kill direct collapse dead. More relevantly to a hypothetical cold intergalactic medium, you need a lot of UV (Lyman, Werner) photons to bust up molecular hydrogen. (But as far as we can tell from e.g. <https://savechandra.org/> the actual intergalactic medium is not cold: <https://astronomy.swin.edu.au/cosmos/I/Intergalactic+Medium> and so the Jeans length is lonnnnnnng).
However, if we have a sufficiently large and cold gas cloud in the dark energy dominated era, with quasars dim and distant, how do we keep it from diluting away with expansion or alternatively fragmenting into overdense and underdense regions? I think the inevitable answer is you need higher mass density and not "any density", which was my point.
Now I'll stray from my island of comfort. I don't do galaxy dynamics or formation and evolution, and I think black holes are mostly pretty boring, so the next two paragraphs and the final one are out of my areas of focus, and there are likely to be gaps and errors. For me, this stuff is all the wrong age, the wrong spatial volume, and/or the wrong energy.
> SMBHs ... from direct collapse [again]
DCBHs (if they exist and if they are the missing step in a hierarchical building of SMBHs by z ~ 7; here supermassive means 10^5 M_sun rather than some of the monsters in the local universe) are due to a direct GR instability in supermassive pop III stars, and have masses around 10^5 M_sun. DCBHs are metallicity limited (thermonuclear kablams), with critical metallicity increasing enormously near and above 10^6 M_sun or so.
If we take the lower end of SMBHs at ~ 10^5 M_sun, then you're right that there are theories of how they may have formed from direct collapse of large but diffuse clouds. A number centre on Priyamvada Natarajan. The introduction of Goulding, Natarajan et al. 2023 <https://arxiv.org/abs/2308.02750> on the weirdo UHZ1 (very dust obscured luminosities from <https://savechandra.org/> X-ray and NIRSpec has a very early z ~ 10 SMBH that seems very unusually massive compared to the host galaxy) lists a few: "Theorists have therefore explored alternate seed formation models, with heavier seed BH mass functions (∼ 10^4 M⊙) that could form from the direct collapse of gas in the high redshift Universe (Lodato & Natarajan 2006, 2007; Volonteri et al. 2008; Inayoshi et al. 2022, see for instance)." Natarajan in a nearly-simultaneous paper on UHZ1 <https://arxiv.org/abs/2308.02654> accepts that a 10^4 M_sun heavy seed is sufficient to explain UHZ1's SMBH:host galaxy mass ratio, and of course 10^4 M_sun is within the range for pop III DCBHs.
Context for next paragraph: "larger than the size of our galaxy ... direct collapse of large ... clouds". I apologize if you did not mean to link your second and third paragraphs that way.
I don't know of many ideas that propose direct collapse into SMBHs much above that lower limit. Directly collapsing a dying supergiant star is hard enough, and much of the work is done in the earlier life of the star. Directly collapsing a 10^12 M_sun or heavier cloud without going through star formation (which would fragment and blow apart the cloud through radiative cooling and reheating) is a pretty wild idea!
WHIM. Nicastro et al (2018) https://www.nature.com/articles/s41586-018-0204-1 (you can get a PDF at </https://arts.units.it/bitstream/11368/2935866/4/2935866_s415...> or the preprint at <https://arxiv.org/abs/1806.08395> and there is a nice summary at <https://www.esa.int/Science_Exploration/Space_Science/XMM-Ne...> ).
> If you had a large enough cloud of intergalactic matter it would form an event horizon
I assume you mean that you get gravitational collapse and some form of ~ 2GM black hole horizon (pfdietz's "any material (of any density) can be made a black hole if there is enough of it"). Trivially you can get an event horizon by making the cloud a Rindler observer, or even more trivially by making it an Eulerian observer in de Sitter space. So let's think about what makes a diffuse cloud collapse in a universe like ours at a comparable scale factor.
At the GR level (e.g. matter dominated era OS or LTB like versions of Einstein-de Sitter or similar dark energy dominated era approaches) the problem you encounter is that you end up having to exchange matter between the collapsing "large enough" cloud and the expanding radiation (and other FLRW fluid) filled cosmological region in which it is immersed, and you have to keep a check on the false intuition that leaking stress-energy from the nonvacuum cosmological region into the vacuum in the collapsing cloud -> black hole region is OK to stabilize pressure, because that's not what we see.
We could alternatively look at it through the lens of Jeans instability. To get collapse you need undamped perturbations with oscillation length longer than the Jeans length, which increases in proportion to temperature and decreases in proportion to mass density and mean molecular mass. And then you have to avoid fragmentation. Which takes us to...
> direct collapse [SMBHs]
Sure, in z > 15, the metal-poor radiation dominated era or even earlier, it's plausible (but see below). You need to avoid the formation of molecules or the gas will radiatively cool much faster than it will contract, fragment your cloud and drive cluster star formation. Greedy metals (carbon, oxygen) will kill direct collapse dead. More relevantly to a hypothetical cold intergalactic medium, you need a lot of UV (Lyman, Werner) photons to bust up molecular hydrogen. (But as far as we can tell from e.g. <https://savechandra.org/> the actual intergalactic medium is not cold: <https://astronomy.swin.edu.au/cosmos/I/Intergalactic+Medium> and so the Jeans length is lonnnnnnng).
However, if we have a sufficiently large and cold gas cloud in the dark energy dominated era, with quasars dim and distant, how do we keep it from diluting away with expansion or alternatively fragmenting into overdense and underdense regions? I think the inevitable answer is you need higher mass density and not "any density", which was my point.
Now I'll stray from my island of comfort. I don't do galaxy dynamics or formation and evolution, and I think black holes are mostly pretty boring, so the next two paragraphs and the final one are out of my areas of focus, and there are likely to be gaps and errors. For me, this stuff is all the wrong age, the wrong spatial volume, and/or the wrong energy.
> SMBHs ... from direct collapse [again]
DCBHs (if they exist and if they are the missing step in a hierarchical building of SMBHs by z ~ 7; here supermassive means 10^5 M_sun rather than some of the monsters in the local universe) are due to a direct GR instability in supermassive pop III stars, and have masses around 10^5 M_sun. DCBHs are metallicity limited (thermonuclear kablams), with critical metallicity increasing enormously near and above 10^6 M_sun or so.
If we take the lower end of SMBHs at ~ 10^5 M_sun, then you're right that there are theories of how they may have formed from direct collapse of large but diffuse clouds. A number centre on Priyamvada Natarajan. The introduction of Goulding, Natarajan et al. 2023 <https://arxiv.org/abs/2308.02750> on the weirdo UHZ1 (very dust obscured luminosities from <https://savechandra.org/> X-ray and NIRSpec has a very early z ~ 10 SMBH that seems very unusually massive compared to the host galaxy) lists a few: "Theorists have therefore explored alternate seed formation models, with heavier seed BH mass functions (∼ 10^4 M⊙) that could form from the direct collapse of gas in the high redshift Universe (Lodato & Natarajan 2006, 2007; Volonteri et al. 2008; Inayoshi et al. 2022, see for instance)." Natarajan in a nearly-simultaneous paper on UHZ1 <https://arxiv.org/abs/2308.02654> accepts that a 10^4 M_sun heavy seed is sufficient to explain UHZ1's SMBH:host galaxy mass ratio, and of course 10^4 M_sun is within the range for pop III DCBHs.
Context for next paragraph: "larger than the size of our galaxy ... direct collapse of large ... clouds". I apologize if you did not mean to link your second and third paragraphs that way.
I don't know of many ideas that propose direct collapse into SMBHs much above that lower limit. Directly collapsing a dying supergiant star is hard enough, and much of the work is done in the earlier life of the star. Directly collapsing a 10^12 M_sun or heavier cloud without going through star formation (which would fragment and blow apart the cloud through radiative cooling and reheating) is a pretty wild idea!
If you imagine balls of different sizes but with the same mass on cellophane, and look at the shape of cellophane, then you'll see that smaller balls will make cellophane more pointed. If instead of a ball you take a point mass, then the depression will converge into a sharp point. Light can climb out from some depressions but there is a limit on a steepness it can overcome.
This analogy probably is not very good for this kind of things, but it shows the idea.
This analogy probably is not very good for this kind of things, but it shows the idea.
[deleted]
https://www.youtube.com/watch?v=eLWNqkQfjTE&t=384s
Les Grossman in Tropic of Thunder: "Speedman is a dying star... a white dwarf heading for a black hole. That's physics... it's inevitable..."
(Sorry, I couldn't resist! <g> :-) <g>)
Les Grossman in Tropic of Thunder: "Speedman is a dying star... a white dwarf heading for a black hole. That's physics... it's inevitable..."
(Sorry, I couldn't resist! <g> :-) <g>)
a comedic triumph. what a film
This was a refreshingly good press release about the event
Should the intricacies of the universe fill you with awe, consider the gold in your possession. Touch it. You have on your hands, the product from the fusion at the core of two colliding neutron stars.
Wouldn’t a neutron star orbiting a black hole be an aberration though?
Isn’t it more likely that a normal neutron star trapped in orbit of a black hole would sweep up the substantial accretion disk, than that a heretofore theoretical massed neutron star fell into the orbit of a black hole?
Isn’t it more likely that a normal neutron star trapped in orbit of a black hole would sweep up the substantial accretion disk, than that a heretofore theoretical massed neutron star fell into the orbit of a black hole?
No, the size of the accretion disk depends on the mass of the black hole but it not that large compared to stellar orbits.
The disk well defined inner radius of 3rg (Schwarzschild radius), the outer radius is blurred, but usually even in super massive black-holes does not extend all that much beyond 0.1ly. A star rarely orbits another star (black-holes) that close in a stable orbit.
For normal blackholes the disk it is going to very small.
The disk well defined inner radius of 3rg (Schwarzschild radius), the outer radius is blurred, but usually even in super massive black-holes does not extend all that much beyond 0.1ly. A star rarely orbits another star (black-holes) that close in a stable orbit.
For normal blackholes the disk it is going to very small.
"GW230529 was formed by the merger of a compact object with 1.3 to 2.1 times the mass of our Sun with another compact object with 2.6 to 4.7 times the solar mass. Whether these compact objects are neutron stars or black holes cannot be determined with certainty from gravitational-wave analysis alone. However, based on all the known properties of the binary, astronomers believe that the lighter object is a neutron star and the heavier is a black hole."
I believe what is interesting about this is that the supposed black hole is lighter than is expected - it's in the gap they mention, not the neutron star!
I believe what is interesting about this is that the supposed black hole is lighter than is expected - it's in the gap they mention, not the neutron star!
Are there any astronomers willing to answer this:
How likely is it that a neutron star "crashes" with a black hole? Why do not they just fly past each other or just rotate like Earth and the sun?
How likely is it that a neutron star "crashes" with a black hole? Why do not they just fly past each other or just rotate like Earth and the sun?
If you look at the animation linked in the article, you can see that they did orbit each other, not only that but if you look at the time scale in the lower part of the screen, you can see that they were doing that at a crazy frequency (about 8 full revolutions in the last 30 milliseconds) https://www.youtube.com/watch?v=3PKsBwH_bJE
Not an astronomer, but…
They usually do fly past each other or (at first) rotate like the earth and sun.
For similar reasons to if you drop a marble into a bathtub or sink, it will almost never go straight into the drain immediately.
For it to immediately drop into the drain requires a bunch of velocity vectors to have very specific values (in a just right kind of way) that are statistically improbable in such a chaotic environment.
It’ll occasionally, very rarely, happen of course.
But more likely is that if they don’t just fly past each other (marble flies out of the sink!), is that eventually tidal losses/gravitational wave losses/mass transfers will slow the bodies down relative to each other until they merge.
A much slower version of the marble rolling around, bouncing off things, and losing speed until it drops into the drain.
Depending on the starting states of the system, it could take billions of years, or mere months.
If there are other bodies involved like multiple stars or black holes, one of them might even just get ejected out of the system.
Like if you drop a bunch of marbles into the sink at once, occasionally one of them will get knocked out even if none of them individually would have had the energy to do so, and it will be that much harder for any one of them to fall in since just when it is getting close another marble will come in to knock it away.
That’s essentially what is happening with accretion disks, and why they get so hot.
They usually do fly past each other or (at first) rotate like the earth and sun.
For similar reasons to if you drop a marble into a bathtub or sink, it will almost never go straight into the drain immediately.
For it to immediately drop into the drain requires a bunch of velocity vectors to have very specific values (in a just right kind of way) that are statistically improbable in such a chaotic environment.
It’ll occasionally, very rarely, happen of course.
But more likely is that if they don’t just fly past each other (marble flies out of the sink!), is that eventually tidal losses/gravitational wave losses/mass transfers will slow the bodies down relative to each other until they merge.
A much slower version of the marble rolling around, bouncing off things, and losing speed until it drops into the drain.
Depending on the starting states of the system, it could take billions of years, or mere months.
If there are other bodies involved like multiple stars or black holes, one of them might even just get ejected out of the system.
Like if you drop a bunch of marbles into the sink at once, occasionally one of them will get knocked out even if none of them individually would have had the energy to do so, and it will be that much harder for any one of them to fall in since just when it is getting close another marble will come in to knock it away.
That’s essentially what is happening with accretion disks, and why they get so hot.
Well, according to this: https://www.space.com/dancing-black-holes-merge
"Based on all the observations of gravitational waves, astronomers estimate that there are somewhere between 15 and 38 black hole mergers every year within every cubic gigaparsec of volume in the universe (about 1/12000th of the total volume of the observable universe)"
According to this: https://bigthink.com/starts-with-a-bang/40-quintillion-black...
"By combining information about stars, black holes, and stellar and cosmic evolution all together, astronomers have the first robust estimate for black holes in the Universe: 40 quintillion"
So, just for black hole mergers, it would be really, really rare. There are probably orders of magnitude more black holes that are just orbiting each other.
Neutron star and black hole statistics I leave as an exercise for the reader ;)
"Based on all the observations of gravitational waves, astronomers estimate that there are somewhere between 15 and 38 black hole mergers every year within every cubic gigaparsec of volume in the universe (about 1/12000th of the total volume of the observable universe)"
According to this: https://bigthink.com/starts-with-a-bang/40-quintillion-black...
"By combining information about stars, black holes, and stellar and cosmic evolution all together, astronomers have the first robust estimate for black holes in the Universe: 40 quintillion"
So, just for black hole mergers, it would be really, really rare. There are probably orders of magnitude more black holes that are just orbiting each other.
Neutron star and black hole statistics I leave as an exercise for the reader ;)
IANAA, but black holes often have an accretion disc orbiting them. Anything that would cross this plane would get impacted by the orbiting particles, which would slowly cause the object to begin to orbit.
Neutron stars also have enormous gravitational and magnetic fields which could steal mass from the accretion disc. If you (the neutron star) gain mass near the gravitational field, it pulls you in closer.
Neutron stars also have enormous gravitational and magnetic fields which could steal mass from the accretion disc. If you (the neutron star) gain mass near the gravitational field, it pulls you in closer.
> If you (the neutron star) gain mass near the gravitational field, it pulls you in closer.
This conflicts with the Strong Equivalence Principle (SEP) in its universality of free fall form, or more colloquially a rule that the trajectory of a self-gravitating object depends only on its initial position and velocity and not on its internal composition (including the evolution of its mass).
There is a good test of the SEP in a system involving the millisecond pulsar https://en.wikipedia.org/wiki/PSR_J0337%2B1715 which is in a hard binary with a white dwarf; a second white dwarf orbits the inner binary. Here is a trace of the orbits https://www.youtube.com/watch?v=oDgfqq_W_uM -- I recommend reading the video description.
Reference 2 (Archibald et al.) at the wikipedia page is an excellent paper.
There are a couple of good astrobites aimed at (advanced, or possibly final-year) astrophysics undergrads at https://astrobites.org/2014/01/14/timing-is-everything-a-pul... (2014) and https://astrobites.org/2019/03/25/testing-einsteins-equivale... (2019)
You're right that the flow of mass in a complex system might be relevant to the merging binary, but in general a stellar black hole's circumstellar accretion structure isn't going to do much to a close-in neutron star except create a lot of (relatively small) nuclear explosions on the latter's surface. There just isn't going to enough material in the accretion structure in comparison to the bigger bodies, which will have masses greater than our sun.
Accretion structures are more relevant for softer (that is more widely separated) binaries, which are a long time before final merger. Circumstellar accretion discs can be in association with a circumbinary disc (CBD; outside the binary), with a flow of matter between them (one would expect the CBD to be the origin of the circumstellar discs, but jet outflows make that expectation complicated). CBDs tend to circularize wide binary orbits, for example. They also provide mechanisms to harden binaries that might seem so soft that a merger could be avoided.
You're right that some neutron stars may have very strong magnetic fields that may affect its accretion disc or the (randomly aligned) accretion disc of its black hole binary partner, but again the flows of mass near the binary must be much less than the masses of the relativistic stars (the NS and the BH), as the each of the stars are very very strongly bound self-gravitation.
Your idea bout mass flow is interesting -- it takes my mind to how much of the accretion disc might be blown into ~polar jets, and how complicated the alignment of spin axes are to the orbital plane can be late in the merger. The accretion structures are I guess likely to be blown into a CBD (or out of the system entirely) well before final merger. Either way, the masses involved likely won't change the inspiral significantly.
If the NS shreds (as opposed to being swallowed whole, which is the more likely outcome, and the apparent outcome in this detection) the additional matter in an accretion structure (including any accretion material blown out of the system via jetting) could be relevant in the r-process and s-process heavy element formation processes. Realistically though that's the sort of question more relevant to binary neutron star mergers.
(As a sort of self-bookmarking and because it's probably interesting to other readers here, Maria Massi's 2020 "Accretion" teaching slides at https://www3.mpifr-bonn.mpg.de/staff/mmassi/lezione2WEdd.pdf are awesome, and takes one to some work on x-ray binary circumstellar discs and even circumbinary accretion. More recent review on the latter <https://arxiv.org/abs/2211.00028>, and figure 1 is probably helpful for a couple of my paragraphs above).
This conflicts with the Strong Equivalence Principle (SEP) in its universality of free fall form, or more colloquially a rule that the trajectory of a self-gravitating object depends only on its initial position and velocity and not on its internal composition (including the evolution of its mass).
There is a good test of the SEP in a system involving the millisecond pulsar https://en.wikipedia.org/wiki/PSR_J0337%2B1715 which is in a hard binary with a white dwarf; a second white dwarf orbits the inner binary. Here is a trace of the orbits https://www.youtube.com/watch?v=oDgfqq_W_uM -- I recommend reading the video description.
Reference 2 (Archibald et al.) at the wikipedia page is an excellent paper.
There are a couple of good astrobites aimed at (advanced, or possibly final-year) astrophysics undergrads at https://astrobites.org/2014/01/14/timing-is-everything-a-pul... (2014) and https://astrobites.org/2019/03/25/testing-einsteins-equivale... (2019)
You're right that the flow of mass in a complex system might be relevant to the merging binary, but in general a stellar black hole's circumstellar accretion structure isn't going to do much to a close-in neutron star except create a lot of (relatively small) nuclear explosions on the latter's surface. There just isn't going to enough material in the accretion structure in comparison to the bigger bodies, which will have masses greater than our sun.
Accretion structures are more relevant for softer (that is more widely separated) binaries, which are a long time before final merger. Circumstellar accretion discs can be in association with a circumbinary disc (CBD; outside the binary), with a flow of matter between them (one would expect the CBD to be the origin of the circumstellar discs, but jet outflows make that expectation complicated). CBDs tend to circularize wide binary orbits, for example. They also provide mechanisms to harden binaries that might seem so soft that a merger could be avoided.
You're right that some neutron stars may have very strong magnetic fields that may affect its accretion disc or the (randomly aligned) accretion disc of its black hole binary partner, but again the flows of mass near the binary must be much less than the masses of the relativistic stars (the NS and the BH), as the each of the stars are very very strongly bound self-gravitation.
Your idea bout mass flow is interesting -- it takes my mind to how much of the accretion disc might be blown into ~polar jets, and how complicated the alignment of spin axes are to the orbital plane can be late in the merger. The accretion structures are I guess likely to be blown into a CBD (or out of the system entirely) well before final merger. Either way, the masses involved likely won't change the inspiral significantly.
If the NS shreds (as opposed to being swallowed whole, which is the more likely outcome, and the apparent outcome in this detection) the additional matter in an accretion structure (including any accretion material blown out of the system via jetting) could be relevant in the r-process and s-process heavy element formation processes. Realistically though that's the sort of question more relevant to binary neutron star mergers.
(As a sort of self-bookmarking and because it's probably interesting to other readers here, Maria Massi's 2020 "Accretion" teaching slides at https://www3.mpifr-bonn.mpg.de/staff/mmassi/lezione2WEdd.pdf are awesome, and takes one to some work on x-ray binary circumstellar discs and even circumbinary accretion. More recent review on the latter <https://arxiv.org/abs/2211.00028>, and figure 1 is probably helpful for a couple of my paragraphs above).
I appreciate the correction. It's too late to edit my post. I have been watching 8 years of astrophysics on YouTube and thought I might be able to pitch in on layman's questions but it seems as though I still have much to learn. Thanks
Not an astronomer, but I can imagine a crashing window as a thin straight pipe that is bended by spacetime's gravitational waves, magnetic waves and BH ejection material.
Good question! I'll try an answer and hope it's a good balance between accuracy and what I guess is your (and other readers') knowledge of astrophysics.
[tl;dr: a neutron star (NS) and black hole (BH) in a close binary are practically guaranteed to "crash" into each other eventually. There seem to be lots of those binaries. Neutron star - black hole mergers (NSBH) from other possible arrangements (wider binaries, isolated individual NSes and BHs) will be vanishingly rare in "the local universe".]
There are two principal branches in an answer to your question.
First branch: there are probably lots of individual isolated neutron stars and black holes within galaxies, and those are unlikely to form binaries with each other, or get anywhere close to each other (except maybe possibly in the very centre of galaxies, or in the very far future). I won't spend time on this branch in this comment, even though this is the branch which interests me personally (at a theoretical rather than astrophysical level). In reality, isolated stars and star systems within galaxies just do not seem to collide with one another and isolated neutron stars and (stellar mass) black holes are no different.
Independent isolated stars may form binaries under rare circumstances such as galaxy-galaxy mergers or in radiating globular clusters within galaxies. The same goes for isolated neutron stars or stellar mass black holes. In those cases they can fall into the next paragraph.
Second branch: there are probably lots of neutron stars and black holes already in close orbit with one another, and those orbits will over long times decay (because such binaries radiate nontrivial gravitational waves) leading to a "crash". Tight orbits involving two relativistic bodies (white dwarf (WD), neutron stars, black holes) are unstable and practically always lead to a merger; there are a number of ways a looser/softer binary of such bodies can become a tighter/harder one, including during the transition of a dying star into a WD, NS, or BH.
Multi-star systems are commonplace[1] and likely mostly born as multi-star systems (e.g. within a nebula originating in the explosive demise of earlier more massive stars), rather than starting as individual stars that just get close enough to start orbiting each other.
One could compare this to how Jupiter and the sun most likely formed from the same part of a giant molecular cloud (GMC), rather than a fully-formed Jupiter being captured later by the sun. Likewise, the Alpha Centauri system probably formed as triple-star system within a different part of the same GMC, rather than the inner AB binary capturing Proxima Centauri (C) or each other. (Triple star systems with an outer member orbiting an inner binary are pretty common, even very close to us in the galaxy).
We'd expect therefore for a proportion of aging multi-star systems to evolve into binaries of "dead stars" including inner neutron star - black hole binaries. Those inner binaries will practically inevitably decay resulting in a neutron star - black hole merger (NSBH) producing a signal in-principle detectable by gravitational wave detectors like KAGRA, Virgo, and LIGO. (The outer member of a triple or surrounding gas, dust, asteroids, and comets can help tighten the inner binary.)
Gravitational wave detectors like LIGO are range-limited, with the current range about 200 megaparsecs (about 600 million light years), which is on cosmological scales quite local. We've counted about a quarter of a million galaxies[2] in that range. By comparison there are hundreds of billions of galaxies in our sky.
A fraction of neutron stars are pulsars. Because of the practicalities of radiotelescopes, at present we can only really find pulsars nearby within our galaxy (dust obscures a lot of the galaxy at the relevant radio frequencies, and there is only one extragalactic pulsar known, and it's in the neighbouring Large Magellanic Cloud). However, we've found several thousand pulsars (see final paragraph below). A sizable fraction are in multi-star systems, which we know from tracking radial acceleration in several ways, and because among these are a handful of eclipsing binaries. There are known pulsar-pulsar pairs (famously, <https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_pulsar>) as well as more complicated systems which include white dwarfs and maybe black holes.
From gravitational wave observations, we can spot a NSBH even if we can't see either individual body (in radio or other parts of the electromagnetic spectrum) because of obscuration by dust and other foreground matter, or because the NS is not a pulsar and thus both objects are very faint in radio. We don't have a good census of dim neutron stars even in our part of the galaxy. However, there will be more dim neutron stars than radio-bright pulsars, and it is reasonable to guess that there will be more dim neutron stars in binary- and multi star systems with a black hole than there are radio-bright pulsars in such systems.
So looking at one galaxy (ours), we can guess that if we waited a few million years we would see a decaying hard neutron star/black hole binary eventually merge. So naively, we might think that if we waited a few years we'd see an NSBH merger from among about a quarter of a million galaxies. And at gravitational wave detectors we're seeing more NSBHs than that naive estimate. So perhaps pulsarNS-BH binaries are rare compared to nonpulsarNS-BH ones. Or perhaps there are lots of pulsarNS-BH binaries in the Milky Way hidden behind dust and sources of radio noise.
Finally, this interactive map of known pulsars https://ishivvers.github.io/maps/pulsars.html is so cool I've submitted it separately at https://news.ycombinator.com/item?id=39970373
[1] "Stellar Multiplicity" (arxiv version of published ARA&A paper), Duchene et al. 2013 https://arxiv.org/abs/1303.3028
"Multiple Star Systems" (multimedia outreach) https://science.nasa.gov/universe/stars/multiple-star-system...
(examples) https://en.wikipedia.org/wiki/Category:Triple_star_systems
(more examples, quadruples and beyond) https://en.wikipedia.org/wiki/Star_system#Quadruple
[2] section 5.3 of "Census of the Local Universe [..]", Cook et al. 2019 <https://ui.adsabs.harvard.edu/abs/2019ApJ...880....7C/abstra...>
[tl;dr: a neutron star (NS) and black hole (BH) in a close binary are practically guaranteed to "crash" into each other eventually. There seem to be lots of those binaries. Neutron star - black hole mergers (NSBH) from other possible arrangements (wider binaries, isolated individual NSes and BHs) will be vanishingly rare in "the local universe".]
There are two principal branches in an answer to your question.
First branch: there are probably lots of individual isolated neutron stars and black holes within galaxies, and those are unlikely to form binaries with each other, or get anywhere close to each other (except maybe possibly in the very centre of galaxies, or in the very far future). I won't spend time on this branch in this comment, even though this is the branch which interests me personally (at a theoretical rather than astrophysical level). In reality, isolated stars and star systems within galaxies just do not seem to collide with one another and isolated neutron stars and (stellar mass) black holes are no different.
Independent isolated stars may form binaries under rare circumstances such as galaxy-galaxy mergers or in radiating globular clusters within galaxies. The same goes for isolated neutron stars or stellar mass black holes. In those cases they can fall into the next paragraph.
Second branch: there are probably lots of neutron stars and black holes already in close orbit with one another, and those orbits will over long times decay (because such binaries radiate nontrivial gravitational waves) leading to a "crash". Tight orbits involving two relativistic bodies (white dwarf (WD), neutron stars, black holes) are unstable and practically always lead to a merger; there are a number of ways a looser/softer binary of such bodies can become a tighter/harder one, including during the transition of a dying star into a WD, NS, or BH.
Multi-star systems are commonplace[1] and likely mostly born as multi-star systems (e.g. within a nebula originating in the explosive demise of earlier more massive stars), rather than starting as individual stars that just get close enough to start orbiting each other.
One could compare this to how Jupiter and the sun most likely formed from the same part of a giant molecular cloud (GMC), rather than a fully-formed Jupiter being captured later by the sun. Likewise, the Alpha Centauri system probably formed as triple-star system within a different part of the same GMC, rather than the inner AB binary capturing Proxima Centauri (C) or each other. (Triple star systems with an outer member orbiting an inner binary are pretty common, even very close to us in the galaxy).
We'd expect therefore for a proportion of aging multi-star systems to evolve into binaries of "dead stars" including inner neutron star - black hole binaries. Those inner binaries will practically inevitably decay resulting in a neutron star - black hole merger (NSBH) producing a signal in-principle detectable by gravitational wave detectors like KAGRA, Virgo, and LIGO. (The outer member of a triple or surrounding gas, dust, asteroids, and comets can help tighten the inner binary.)
Gravitational wave detectors like LIGO are range-limited, with the current range about 200 megaparsecs (about 600 million light years), which is on cosmological scales quite local. We've counted about a quarter of a million galaxies[2] in that range. By comparison there are hundreds of billions of galaxies in our sky.
A fraction of neutron stars are pulsars. Because of the practicalities of radiotelescopes, at present we can only really find pulsars nearby within our galaxy (dust obscures a lot of the galaxy at the relevant radio frequencies, and there is only one extragalactic pulsar known, and it's in the neighbouring Large Magellanic Cloud). However, we've found several thousand pulsars (see final paragraph below). A sizable fraction are in multi-star systems, which we know from tracking radial acceleration in several ways, and because among these are a handful of eclipsing binaries. There are known pulsar-pulsar pairs (famously, <https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_pulsar>) as well as more complicated systems which include white dwarfs and maybe black holes.
From gravitational wave observations, we can spot a NSBH even if we can't see either individual body (in radio or other parts of the electromagnetic spectrum) because of obscuration by dust and other foreground matter, or because the NS is not a pulsar and thus both objects are very faint in radio. We don't have a good census of dim neutron stars even in our part of the galaxy. However, there will be more dim neutron stars than radio-bright pulsars, and it is reasonable to guess that there will be more dim neutron stars in binary- and multi star systems with a black hole than there are radio-bright pulsars in such systems.
So looking at one galaxy (ours), we can guess that if we waited a few million years we would see a decaying hard neutron star/black hole binary eventually merge. So naively, we might think that if we waited a few years we'd see an NSBH merger from among about a quarter of a million galaxies. And at gravitational wave detectors we're seeing more NSBHs than that naive estimate. So perhaps pulsarNS-BH binaries are rare compared to nonpulsarNS-BH ones. Or perhaps there are lots of pulsarNS-BH binaries in the Milky Way hidden behind dust and sources of radio noise.
Finally, this interactive map of known pulsars https://ishivvers.github.io/maps/pulsars.html is so cool I've submitted it separately at https://news.ycombinator.com/item?id=39970373
[1] "Stellar Multiplicity" (arxiv version of published ARA&A paper), Duchene et al. 2013 https://arxiv.org/abs/1303.3028
"Multiple Star Systems" (multimedia outreach) https://science.nasa.gov/universe/stars/multiple-star-system...
(examples) https://en.wikipedia.org/wiki/Category:Triple_star_systems
(more examples, quadruples and beyond) https://en.wikipedia.org/wiki/Star_system#Quadruple
[2] section 5.3 of "Census of the Local Universe [..]", Cook et al. 2019 <https://ui.adsabs.harvard.edu/abs/2019ApJ...880....7C/abstra...>
I'm assuming there was no gamma ray burst detected at that time, or else the press release would have mentioned it.
I bet it’s much harder to detect the gamma ray burst if you don’t know what direction to look since only one of the gravitational wave detectors was running
Short GRBs are detected by omnidirectional detectors; they're so short there's no time to alter detector orientation.
What happens if a black hole gobbles a strangelet?
A strange star is still matter so that matter would just fall into the black hole as any other matter would. Tidal forces prior to the merger could result in some of the stranglet material being spewed out into space. AFAIK strange stars are an unconfirmed theory at present and the 'super stable' nature of strange matter, I'm unsure of what would happen. It'd be interesting to know this for sure.
For anyone interested in what this 'Strange matter' is see https://en.wikipedia.org/wiki/Strange_matter
For anyone interested in what this 'Strange matter' is see https://en.wikipedia.org/wiki/Strange_matter
...the "Generally Safe Distance" is measured in kiloparsecs.
For anyone else curious: that's 3,260 light years.
True...but all sentient beings should be aware that counterfeit, cut-rate, or old "Singularity Merger Glasses" do NOT suddenly become "good enough" at 1.0 kpc. Doesn't matter if you've previously looked at mere Typo O stars with your unprotected photoreceptors, or your spaceship's viewports have a Gamma Protection Factor that's over 50, or ...