When a SMBH eats a neutron star, does a "pseudo pod" emerge?

[halfeye]

According to Wikipedia SGR J1745-2900 is 0.3 light years from Sgr A*, so it's presumably going in sometime, perhaps soon. I thought, being as neutron stars are very dense anyway, when that happens, will the event horizon surge up to eat it? It seems to me that it ought to, but people seem to say that it's perfectly spherical, so I don't know.

https://en.wikipedia.org/wiki/Sagittarius_A*

The relevant text only shows when you expand the image of the magnetar, which is sad.

Does anyone here know definitively what will happen, and is there any clue when?

<edit> Edited the thread title to say SMBH instead of black hole, because apparently some people don't bother reading the first post before replying to the thread title.

[Anymage]

Nothing would emerge. The event horizon is a mathematical construct, not a hungry ooze.

You might get a change from a sphere shape in two ways. The first is just that when you have two large masses in a region of space, each has its own gravity well. If you had a large discrete mass in the black hole somewhere other than the singularity, you might get an oddly shaped event horizon for a few moments until those two masses met up. If you think about two black holes merging, there'd be a moment where the two event horizons just touched and you'd have a shape like two balls touching each other, even if nothing emerged from either to gobble the other one up.

More prosaically, it's rare for two celestial objects to meet head on like that. It's much more likely that they'd spiral around each other for a long while first. Spinning black holes also have imperfectly spherical event horizons. (Also, once the black hole starts to pull on the outer layers of the neutron star, it interrupts the normal gravitational equilibrium that keeps the neutron star stuck together. It's possible that the released matter and energy could kick the neutron star away, but it's just as possible that it starts feeding the black hole and you get something that looks like a black hole eating a more normal gas/plasma neighbor.)

[Yora]

The neutron star itself would deform as it gets closer to the black hole. But since the gravity pulling the neutron star into a sphere is extremely high, that deformation would be much less than you would see in other stars. Also, the tidal forces near a black hole depend on the size of the black hole. With a supermassive black hole the tidal stresses causing tidal stretching can be very low even at the event horizon. The smaller the black hole, the more extreme tidal effects will be as an object reaches the event horizon.

The shape of a black hole depends entirely on what is inside of the event horizon, from what I know. Putting huge masses near the event horizon from the outside should not have any effect on that, I believe.

[halfeye]

Nothing would emerge. The event horizon is a mathematical construct, not a hungry ooze.

You might get a change from a sphere shape in two ways. The first is just that when you have two large masses in a region of space, each has its own gravity well. If you had a large discrete mass in the black hole somewhere other than the singularity, you might get an oddly shaped event horizon for a few moments until those two masses met up.

I really, really don't understand the thinking behind that.


If you think about two black holes merging, there'd be a moment where the two event horizons just touched and you'd have a shape like two balls touching each other, even if nothing emerged from either to gobble the other one up.

That just sounds wrong to me. The gravitational attraction due to the two masses should be a lot more variable than that.


More prosaically, it's rare for two celestial objects to meet head on like that. It's much more likely that they'd spiral around each other for a long while first.

Sure, I'd sort of asume that sort of decaying orbit aproach, but at some point the neutron star actually has to be within a couple of neutron star radii of the event horizon.


Also, once the black hole starts to pull on the outer layers of the neutron star, it interrupts the normal gravitational equilibrium that keeps the neutron star stuck together.

Nope, the surface gravity of a neutron star is ridiculously strong, to do this you need a pretty big black hole, but a supermassive one would be just fine.


The shape of a black hole depends entirely on what is inside of the event horizon, from what I know. Putting huge masses near the event horizon from the outside should not have any effect on that, I believe.

How does that work? all mass produces gravity, gravity at any point should surely be the result of the sum of the nearby masses.

[Knaight]

If anything the event horizon would recede slightly - the overall gravitational field at the former edge of the event horizon is now subject to massive gravitational forces "pulling" from outside that edge, which would make it easier for light to escape. Meanwhile a bit closer to the black hole and further from the neutron star you'd get the precise gravitational force magnitude that defines an event horizon. Then as the star moves in but is near the edge of the black hole contributing a gravitational force in the other direction the event horizon would move outwards, eventually going beyond its original position, before moving back to it (albeit ever so slightly changed by the added mass).

This isn't a clean absorption of a sphere either - the gravity at an event horizon is high, and while the associated tidal forces depend on the size of the black hole they also tend towards being extremely strong. The neutron star would undergo spaghettification in the process (potentially well before the actual event horizon), there's the aforementioned decaying orbit, and overall the entire phenomena just wouldn't look like a pseudopod at all. More like a scratch around a sphere, that first grows slightly then gradually turns into a slight bump instead, all moving from one side of the scratch to the other while the scratch itself moves around the sphere. Though this does move in past the event horizon for sufficiently large black holes.

[factotum]

Just to raise a sideward question, as it were--what makes you think this object is going to fall into the black hole? It's presumably orbiting it, and unless there's some matter in the region dense enough to cause drag that slows its orbit down, it could continue to do so for billions of years. And it would take a *lot* of drag to slow a stellar-mass object down.

[halfeye]

If anything the event horizon would recede slightly - the overall gravitational field at the former edge of the event horizon is now subject to massive gravitational forces "pulling" from outside that edge, which would make it easier for light to escape. Meanwhile a bit closer to the black hole and further from the neutron star you'd get the precise gravitational force magnitude that defines an event horizon. Then as the star moves in but is near the edge of the black hole contributing a gravitational force in the other direction the event horizon would move outwards, eventually going beyond its original position, before moving back to it (albeit ever so slightly changed by the added mass).

I'm not sure about this bit, I suspect you're not correct, but I am by no means sure what's right.


This isn't a clean absorption of a sphere either - the gravity at an event horizon is high, and while the associated tidal forces depend on the size of the black hole they also tend towards being extremely strong. The neutron star would undergo spaghettification in the process (potentially well before the actual event horizon),

This bit I am quite sure is absolutely wrong. We're considering Sgr A*, which is supermassive, the light end of supermassive compared to some, but still supermassive. The gravity at the event horizon is far less than with a smaller black hole.


Supermassive black holes have properties that distinguish them from lower-mass classifications. First, the average density of a SMBH (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some SMBHs.[6] This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. In addition, the tidal forces in the vicinity of the event horizon are significantly weaker for massive black holes. As with density, the tidal force on a body at the event horizon is inversely proportional to the square of the mass: a person on the surface of the Earth and one at the event horizon of a 10 million M☉ black hole experience about the same tidal force between their head and feet. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole.

https://en.wikipedia.org/wiki/Supermassive_black_hole


The gravitational field at the neutron star's surface is about 2×1011 (200 billion) times that of the Earth.

https://en.wikipedia.org/wiki/Neutron_star


there's the aforementioned decaying orbit, and overall the entire phenomena just wouldn't look like a pseudopod at all. More like a scratch around a sphere, that first grows slightly then gradually turns into a slight bump instead, all moving from one side of the scratch to the other while the scratch itself moves around the sphere. Though this does move in past the event horizon for sufficiently large black holes.

Given the above, I'm not at all sure you have this right.


Just to raise a sideward question, as it were--what makes you think this object is going to fall into the black hole? It's presumably orbiting it, and unless there's some matter in the region dense enough to cause drag that slows its orbit down, it could continue to do so for billions of years. And it would take a *lot* of drag to slow a stellar-mass object down.

There is "frame dragging" from the SMBH, gravity dragging in my words which is probably the wrong way to put it.

https://en.wikipedia.org/wiki/Frame-dragging

[Knaight]

This bit I am quite sure is absolutely wrong. We're considering Sgr A*, which is supermassive, the light end of supermassive compared to some, but still supermassive. The gravity at the event horizon is far less than with a smaller black hole.

I was working with the generic black hole case - hence the last line about that not applying to sufficiently large black holes.

[shawnhcorey]

Black holes do not exist. Gravity travels at the speed of light. Light cannot escape a black hole. Therefore gravity cannot. So, black holes have no gravity (or they don't exist).

If space is curved, the all measurements of distance are greater than the Cartesian distance.

r > √( Δx² + Δy² +Δz² )

That makes black holes an infinite distance away. Which means they have zero gravity.

Hypermasses do not form mythical black holes. They form quark stars (https://en.wikipedia.org/wiki/Quark_star).

[Rockphed]

Black holes do not exist. Gravity travels at the speed of light. Light cannot escape a black hole. Therefore gravity cannot. So, black holes have no gravity (or they don't exist).

If space is curved, the all measurements of distance are greater than the Cartesian distance.

r > √( Δx² + Δy² +Δz² )

That makes black holes an infinite distance away. Which means they have zero gravity.

Hypermasses do not form mythical black holes. They form quark stars (https://en.wikipedia.org/wiki/Quark_star).

I'm fairly certain that you are very confused about a great many things. I'm not even going to try to say that any particular point you are making is wrong, simply that the net argument is wrong.

[Excession]

This bit I am quite sure is absolutely wrong. We're considering Sgr A*, which is supermassive, the light end of supermassive compared to some, but still supermassive. The gravity at the event horizon is far less than with a smaller black hole.

Just to correct this point, the gravity at the event horizon is not lower for larger block holes, but the gravity gradient is shallower. Spaghettification occurs when one end of an object is closer to the black hole than the other end, and the difference in gravitational attraction across the object pulls it apart. While I haven't done the math, I suspect the difference in gravity across a 20 km neutron star next to Sagittarius A* won't be all that big when compared to the gravity of the neutron star itself. From reading something where someone had done the math, the tidal forces at the event horizon of a 100 million solar mass black hole are apparently less than the tidal forces on the surface of Earth.


Black holes do not exist. Gravity travels at the speed of light. Light cannot escape a black hole. Therefore gravity cannot. So, black holes have no gravity (or they don't exist).

Changes in gravity travel at the speed of light. Gravity itself is the deformation of space and time caused by mass. The deformation of time means that any change of gravity also can't escape the event horizon, even after the event horizon has expanded to destroy (from one way of looking at it) the mass creating that deformation.

It's all more complicated than that really, and pretty much any statement about black holes needs to specify the reference frame of the observer, because relatively is very important for this. What the neutron star experiences is quite different from what a distant observer sees.

[Knaight]

I'm fairly certain that you are very confused about a great many things. I'm not even going to try to say that any particular point you are making is wrong, simply that the net argument is wrong.

Shawnhcorey has a... unique perspective on a lot of physics. I'd recommend taking that into account for any physics question, including classical physics.

[Rockphed]

Shawnhcorey has a... unique perspective on a lot of physics. I'd recommend taking that into account for any physics question, including classical physics.

I did a search on the internet, and I am fairly certain his stance about black holes is almost unique. As in I haven't found any other proponents of his ideas, but I do not conclusively rule out that they exist.

[shawnhcorey]

Yeah, everyone says I'm wrong but they don't point out where in the math I am. Their arguments are fluff.

If black holes exist, then they have no gravity and are infinitely far away. That's the math of curved space.

[enderlord99]

{scrubbed}

[shawnhcorey]

{scrubbed}

[hamishspence]

Sagittarius A* being a black hole is "beyond any reasonable doubt":

https://en.wikipedia.org/wiki/Sagittarius_A*

After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimate the object's mass at 4.31 ± 0.38 million solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009.[2] Reinhard Genzel, team leader of the research, said the study has delivered "what is now considered to be the best empirical evidence that super-massive black holes do really exist. The stellar orbits in the Galactic Center show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt."[24]


Quark star theories might explain some "oversized neutron stars" but I don't think astrophysicists have ever tried to argue that supermassive black holes are quark stars.

[shawnhcorey]

Quark star theories might explain some "oversized neutron stars" but I don't think astrophysicists have ever tried to argue that supermassive black holes are quark stars.

You mean none of the astrophysicists you have read. There are a lot of things in astronomy that aren't mentioned in the popular press.

[Rockphed]

Then point us to a scholarly article that agrees with your argument. (scholar.google.com)

[shawnhcorey]

Then point us to a scholarly article that agrees with your argument. (scholar.google.com)

Science is not a religion. Anyone can do science. Why don't you do the simple algebra needed to prove it?

[enderlord99]

Science is not a religion. Anyone can do science. Why don't you do the simple algebra needed to prove it?

Plenty of people have done the math, and you're the only one who got the result that black holes can't exist.

Also, space could just as easily be curved the other way, such that all measurements are less than that stuff you said they have to be greater than. Spherical geometry isn't the only alternative form (https://en.wikipedia.org/wiki/Hyperbolic_geometry).

[Rockphed]

Science is not a religion. Anyone can do science. Why don't you do the simple algebra needed to prove it?

Because, as the person who is denouncing the orthodoxy, the burden of proof lies on your shoulders. If you can prove that black holes cannot exist with simple algebra, then write it up and submit it to Science or The Astrophysical Journal. As you said, anyone can do science; what is stopping you?

As for why I don't write up a proof that black holes work the way I think they do: I have never been good at proofs. When I finish my thesis, however, I will take a crack at it.

[shawnhcorey]

Plenty of people have done the math, and you're the only one who got the result that black holes can't exist.

Nope, there's something even weirder. (https://en.wikipedia.org/wiki/Magnetospheric_eternally_collapsing_object)

[shawnhcorey]

Because, as the person who is denouncing the orthodoxy, the burden of proof lies on your shoulders.

I did prove it. But I can do it in geometry too, if you like.

Here is an image of the suppose curved space around a black hole. http://ichef.bbci.co.uk/wwfeatures/wm/live/624_351/images/live/p0/2r/z7/p02rz7lq.jpg

The mass is suppose to be in the middle of the funnel. So, what's the distance from any point to the mass? Simple: draw a path from the point along the surface to mass. The path goes down the funnel and never reaches the mass. The distance to the mass is infinite.

[enderlord99]

I did prove it. But I can do it in geometry too, if you like.

Here is an image of the suppose curved space around a black hole. (image snipped)

The mass is suppose to be in the middle of the funnel. So, what's the distance from any point to the mass? Simple: draw a path from the point along the surface to mass. The path goes down the funnel and never reaches the mass. The distance to the mass is infinite.

That is absolutely correct: the singularity is an infinite distance away. The event horizon, however, starts as soon as it goes vertical. The term "black hole" can refer to either concept, which is fine since they always occur together (by which I mean, the singularity (which, from a practical standpoint, doesn't exist) is an infinite distance into the event horizon (which does exist.))

Seems pretty simple, to me.

[shawnhcorey]

Seems pretty simple, to me.

Yup, black holes are too far away for their gravity to effect anything. Glad we agree.

[hamishspence]

Gravity beyond the event horizon follows an inverse square law, regardless of what exactly's going on inside the event horizon.

[Rockphed]

I did prove it. But I can do it in geometry too, if you like.

No, you haven't. You have not produced a rigorous proof that black holes cannot exist any more than I have produced a rigorous proof that they must.

We don't know the distribution of mass inside a black-hole, we only know that at a certain radius nothing, not even light, can escape. For all that it matters, it can be a shell of mass or an undifferentiated sphere of mass occupying some set of space inside the event horizon. I'm fairly certain that I could work the integrals such that either one produces the same net gravity outside the event horizon with the same mass.

[shawnhcorey]

We don't know the distribution of mass inside a black-hole, we only know that at a certain radius nothing, not even light, can escape.

You assume that black holes exist, unproven.

I shall repeat myself.

1. Gravity travels at the speed of light. Einstein.

2. Light cannot escape black holes because it does not travel faster enough. Schwarzschild

3. Black holes have no gravity because gravity does not travel fast enough. QED.

[Rockphed]

2. Light cannot escape black holes because it does not travel faster enough. Schwarzschild

Schwarzschild developed 2 things as corollaries to special relativity. He developed an exterior metric that is valid outside a spherically symmetric object, and an interior metric that is valid inside a spherically symmetric object of constant composition. These metrics are used to compute the distance between points. So, yes, if we have a static point mass it will take infinite time for its gravity to propagate outwards because the metric has a singularity at r = 0. However, we can replace that point mass with any other spherical mass smaller than the Schwarzschild radius and the metric is the same outside the Schwarzschild radius. We can even, by change of coordinate systems, change to a system where there is not a singularity at the Schwarzschild radius, allowing a single solution to describe everything outside of the mass. The interior solution is what predicts the pressure and density becoming infinite in the center of a black hole. However, they are time-harmonic solutions, meaning that these solutions do not hold as soon as the situation starts to change.

And congratulations! You discovered the problem with point mass black holes: they take an infinitely long time to form. This also means that gravity would have an infinite time to propagate back up their gravity well.

[Bohandas]

Wouldn't the event horizon actually recede slightly due to the counter pull of the neutron star's own gravity

[Rockphed]

Wouldn't the event horizon actually recede slightly due to the counter pull of the neutron star's own gravity

Possibly. I don't feel like figuring out how the math works.

[SpoonR]

From a sufficient distance, black hole meeting neutron star will have the same equations of motion as two black holes merging. Which... hey! We found some of those!

https://en.wikipedia.org/wiki/LIGO

I am not Kip Thorne, so I know better than to try the calculus, but someone already did a simulation and posted the results...
https://en.wikipedia.org/wiki/Binary_black_hole

- D

"If you think you understand infinity, then you don't understand infinity."

[Rockphed]

"If you think you understand infinity, then you don't understand infinity."

Perhaps, but I still think that any time you hit multiple infinities at the same time you need to try L'hopital's rule.

[enderlord99]

Yup, black holes are too far away for their gravity to effect anything. Glad we agree.

We don't, and you know it.

[Knaight]

I did prove it. But I can do it in geometry too, if you like.

Here is an image of the suppose curved space around a black hole. http://ichef.bbci.co.uk/wwfeatures/wm/live/624_351/images/live/p0/2r/z7/p02rz7lq.jpg

The mass is suppose to be in the middle of the funnel. So, what's the distance from any point to the mass? Simple: draw a path from the point along the surface to mass. The path goes down the funnel and never reaches the mass. The distance to the mass is infinite.

That's not an image of the supposed curved space around a black hole. That's an analogy used for teaching concepts underlying the mathematics. Last I checked, space wasn't 2D, and switching to space-time isn't going to remove a dimension. Poking holes in the simplified teaching analogy means approximately jack-all.

[Rockphed]

That's not an image of the supposed curved space around a black hole. That's an analogy used for teaching concepts underlying the mathematics. Last I checked, space wasn't 2D, and switching to space-time isn't going to remove a dimension. Poking holes in the simplified teaching analogy means approximately jack-all.

The problem, of course, is that he is half right. If I understand this article (https://en.wikipedia.org/wiki/Schwarzschild_metric) properly, the distance between adjacent points approaches infinity as the distance from the center of the black hole approaches zero. So, it would, indeed, take infinite time for changes in a black hole to propagate out of the black hole. However, it will also take an infinite amount of time for a black hole that is a shell of mass at the Schwartzschild radius to transition into a point mass. Due to the speed of light barrier, that second infinity is at least as long as the first infinity, so gravity will have propagated back to the event horizon. And, unless I am completely misunderstanding this other article (https://en.wikipedia.org/wiki/Interior_Schwarzschild_metric), for a system that starts out as an undifferentiated, spherical mass, there isn't a singularity in the distance metric.

None of this is particularly exciting, much less ground-breaking: pretty much every treatment of black holes I have ever seen has made the caveat that black holes take infinite time to transition to point mass. And while some physicist theorize that high mass neutron stars transition into quark stars, we literally know nothing about quarks that aren't contained in mesons or nucleons. We have some guesses, but no data.

As for black holes being too far away to affect us, tell that to Ligo. I'm pretty sure they detected a merger of black holes, a merger of neutron stars, and a neutron star falling into a black hole. The second one apparently sent a stream of platinum more massive than Earth into the sky.

[shawnhcorey]

As for black holes being too far away to affect us, tell that to Ligo. I'm pretty sure they detected a merger of black holes, a merger of neutron stars, and a neutron star falling into a black hole. The second one apparently sent a stream of platinum more massive than Earth into the sky.

If black holes are infinitely far away, then what LIGO detected wasn't a black hole.

BTW,space is curved. What you are calling the "insides" of a black hole is infinitely far away. That would place it outside of space.

[Rockphed]

If black holes are infinitely far away, then what LIGO detected wasn't a black hole.

A true point mass would, if I follow the math right, be infinitely far away. It would also take infinite time to form. A black hole is simply any object whose mass is concentrated within its Schwartzschild radius. The Schwartzschild radius of the Earth is a few millimeters, the sun has a Schwartzschild radius of a few kilometers. You only get the infinite distance thing from true points sources. Distributed sources inside an event horizon will still have regular space gradients all the way down to their center.




BTW,space is curved. What you are calling the "insides" of a black hole is infinitely far away. That would place it outside of space.

The Earth curves space. Are you going to claim that the curvature of space from the Earth causes the center of the Earth to be infinitely far away?

Furthermore, just because something is infinite does not necessarily mean that it is intractable. We cannot integrate 1/x from 1 to infinity, but we can integrate 1/x^2 from 1 to infinity. I suspect, though I do not feel like doing the math, that even though a point mass is infinitely far away, the volume of space inside its event horizon will still be finite and constant no matter the distribution of mass inside the event horizon. It is like the paradox of Gabriel's Horn, a surface with infinite surface area and finite volume.

[Brother Oni]

Shawnhcorey has a... unique perspective on a lot of physics. I'd recommend taking that into account for any physics question, including classical physics.

From my arguments with him, I'd expand that to include the field of biology (at the very least).

[shawnhcorey]

Shawnhcorey has a... unique perspective on a lot of physics. I'd recommend taking that into account for any physics question, including classical physics.

It's not unique. A simple web search will prove that.

[shawnhcorey]

You only get the infinite distance thing from true points sources. Distributed sources inside an event horizon will still have regular space gradients all the way down to their center.

Doesn't matter. An event horizon prevents anything from getting out.


Furthermore, just because something is infinite does not necessarily mean that it is intractable. We cannot integrate 1/x from 1 to infinity, but we can integrate 1/x^2 from 1 to infinity.

Space is not the integral under the curve. Space is the curve.


I suspect, though I do not feel like doing the math,...

I know. It's much easier to argue for anything if you don't do the math. Especially when the math proves you're wrong.

[shawnhcorey]

It's not unique. A simple web search will prove that.

Speaking of simple searches, here are two interesting articles.

Now Hawking is suggesting a resolution to the paradox: Black holes do not possess event horizons after all, so they do not destroy information. (https://news.nationalgeographic.com/news/2014/01/140127-black-hole-stephen-hawking-firewall-space-astronomy/)

...Laura Mersini-Houghton, a physics professor at UNC-Chapel Hill in the College of Arts and Sciences, has proven, mathematically, that black holes can never come into being in the first place. (https://phys.org/news/2014-09-black-holes.html)

[halfeye]

Wouldn't the event horizon actually recede slightly due to the counter pull of the neutron star's own gravity

Thinking about the situation further, it seems to me that this is probably correct, for the space between the neutron star and the main event horizon, but I think there ought to be an extra bit of event horizon around the side of the neutron star furthest from the black hole, which would emerge from the interior of the neutron star as it approached the black hole. Since a neutron star is not a singularity, there would be no naked singularities here.

[Anymage]

Speaking of simple searches, here are two interesting articles.[/URL]

I don't doubt that theoretical physicists have some disputes as to what exactly happens inside an event horizon. Our understanding of physics is far from complete, there will be competing ideas.

We've observed things that act an awfully lot like black holes, though. And the theories that predicted them have held up pretty well. Dissent around the particulars does not mean that the main thing does not exist.


Thinking about the situation further, it seems to me that this is probably correct, for the space between the neutron star and the main event horizon, but I think there ought to be an extra bit of event horizon around the side of the neutron star furthest from the black hole, which would emerge from the interior of the neutron star as it approached the black hole. Since a neutron star is not a singularity, there would be no naked singularities here.

Assuming the simple case where the neutron star was shot straight at the black hole, there would be a brief period where you could model it as two point masses instead of one. (One at the central singularity, one wherever the bulk of the neutron star happened to be.) This would create a slightly more elliptical event horizon. Ellipse, though, not pseudopod. In order to have a noticeable bulge in an event horizon, you'd probably need the second mass in your equation to be a sizable black hole all on its own. And this bulge would only last as long as the second mass was close to the edge of the event horizon.

Remember that you can already have nonspherical event horizons. Spinning black holes being the classic example. It's just that any mass inside the event horizon will be zooming towards the central singularity, making twin mass black holes very short lived. And in most cases (neutron star vs. hypermassive black hole certainly counting), not sufficient to be noticeable.

(Yes, I am aware that matter inside the event horizon cannot send any signal to the outside universe. Since mass and momentum are among the properties that are conserved by black holes, I'd guess that the expected trajectory of any infalling matter can still be predicted, and might show in the shape of the event horizon until it aggregates with everything else in the center.)

To simplify the issue, imagine that instead of a hypermassive black hole vs. a neutron star, you had two similarly massed black holes on straight collision course with each other.

Before they came into contact, you'd basically have two spherical event horizons. There may be slight deformations due to the nearness of another highly massive object, but those should be easy to math out.

At the moment that they came into contact, you'd basically have two spheres just touching each other.

After that, treating it as two point masses should work. You'd see two bubbles merging into a more elliptical shape, before finally rounding out to a larger sphere as the two singularities meet each other and merge. The same logic should apply with two significantly different masses, except that obviously a negligible mass compared to the main singularity should have similarly negligible effects on the event horizon's shape.)

[hamishspence]

...Laura Mersini-Houghton, a physics professor at UNC-Chapel Hill in the College of Arts and Sciences, has proven, mathematically, that black holes can never come into being in the first place. (https://phys.org/news/2014-09-black-holes.html)

When I googled this - I found other scientists (like William Unruh) dismissing it as pseudoscientific nonsense.

[Bohandas]

The Earth curves space. Are you going to claim that the curvature of space from the Earth causes the center of the Earth to be infinitely far away?


No, because the earth has finite density

[hamishspence]

There's also (according to various commentators) an element of media sensationalism in headlines like that - saying "singularities don't exist" is not the same thing as saying "objects with event horizons don't exist".

[Bohandas]

When I googled this - I found other scientists (like William Unruh) dismissing it as pseudoscientific nonsense.

Furthermore other articles on that site seem to be about work based on theories that were discredited long before the article was written (there;s one from 2014 that mentions the Big Crunch (https://phys.org/news/2014-02-astrophysicists-duo-planck-star-core.html#nRlv) for instance)

[Bohandas]

This wikipedia article (https://en.wikipedia.org/wiki/Binary_black_hole) has a simulation from Caltech of two black holes colliding. They seem to distort almost into hemispheres immediately before merging

https://en.wikipedia.org/wiki/Binary_black_hole

[Rockphed]

Doesn't matter. An event horizon prevents anything from getting out.

The event horizon prevents some things from leaving, but not everything. While the Schwartzschild black hole is the simplest case it is a solution for a simplified case with neither charge nor momentum. Real objects, having one or both, will result in different behavior.


Space is not the integral under the curve. Space is the curve.

Spacetime is a 4 dimensional thing. Gravity does not distort it into a 5th dimension. Those curve pictures you keep posting are, in general, a depiction of things with 2 dimensions held constant (normally zenith and time) and a third dimension showing the Schwartzschild metric.


I know. It's much easier to argue for anything if you don't do the math. Especially when the math proves you're wrong.

This sort of statement would be more persuasive if you included a link to someone actually doing the math with it or if you displayed any ability to actually comprehend and comment on the math involved. Part of why I'm not doing the math is because I have built an intuition about how math like this works over the last decade. The other part is because this is a forum on the internet without good functions for displaying equations. Also, you haven't offered any mathematical rebuttals to my math, leading me to believe that you either cannot understand the math involved or simply refuse to do so.


No, because the earth has finite density

See, this sort of reply evidences an understanding of the physics and math involved that shawnhcorey has heretofore refused to produce.

[halfeye]

Thinking about the dimple in the event horizon, if there were matter directly underneath where it used to be, that might be exposed, it might do nothing, or if it's a firewall, it might erupt, so that's interesting.

[SpoonR]

Perhaps, but I still think that any time you hit multiple infinities at the same time you need to try L'hopital's rule.

Hey, you got real math into my gradually accreting sig! :smalltongue:

True though. And then there's renormalization which is somehow related, but I haven't taken enough grad classes to try and explain that. But in my experience, the weirdest theories come out when there is a single infinity involved.

Of course, any theory of "black holes don't exist" would have to start by explaining how to handle already being disproven. I mean, someone came up with a theory of what two black holes colliding would look like, ran the test, and found events with that signature. So, it's something that looks like a black hole, acts like a black hole, but is actually a duck?

Then there's the fact that infinity is irrelevant, but would be really cool if it mattered. Outside the event horizon, Gauss' law would apply - you know there is a certain mass inside, but the spatial distribution doesn't matter. If "infinite gravity" somehow did turn a black hole inside out I guess that would be a kind of wormhole, and there would probably be some beautiful explosions as that "infinite tube" intersects earth (and everywhere else).

Argumentum ad Nobelum Comitteum - To know what goes on inside the event horizon, and even moreso near the singularity, step 1 is unify quantum mechanics and gravity. I haven't heard any shouting from Stockholm (or my local college campus). Officially, we don't know what happens inside the event horizon, which is why we can have arguments about the details of evaporation or black hole information/entropy. Now it's possible that paper said something to the effect of "black holes do not exist under these very specific conditions", which would be interesting but a much weaker statement

As far as what two colliding black holes, or a black hole and neutron star, actually look like, I'll have to leave that to the astrophysics black hole specialists. Probably if you do journal searches starting with LIGO and working backwards, you'll find a paper describing it.

[Rockphed]

Then there's the fact that infinity is irrelevant, but would be really cool if it mattered. Outside the event horizon, Gauss' law would apply - you know there is a certain mass inside, but the spatial distribution doesn't matter. If "infinite gravity" somehow did turn a black hole inside out I guess that would be a kind of wormhole, and there would probably be some beautiful explosions as that "infinite tube" intersects earth (and everywhere else).

Isn't Gauss's law about magnetic flux and the electric currents going through a loop? We can apply it to mass and gravity? How? I mean I know that a spherically symmetric mass behaves identically to an equal point mass, so long as you are outside the body. But surely the distribution of mass matters to some extent (e.g. 2 point masses versus a line mass).

[SpoonR]

Gauss - constants are different, but the equation is basically the same since e&m and gravity are both square-law with distance.

Your standard black hole is spherically symmetrical. I'd have to check "History of Time" for the equations, but the result was that because of the event horizon, the hole has total mass, radius, and temperature, no mass distribution. IIRC part of the problem of information content is that from the outside, all you can see is the mass. "position inside the horizon" is one of those pieces of information that is apparently destroyed. For a black hole with diameter of a few km, if you want to know what is happening at further than 1 light year I think it it is a point source to first order

Now trying to calculate the effect at a couple km from two black holes a couple km apart, things could get messy.

[Bohandas]

Gauss - constants are different, but the equation is basically the same since e&m and gravity are both square-law with distance.

At conventional scales at any rate. IIRC gravity drops off much more quickly on the subatomic scale.

[factotum]

At conventional scales at any rate. IIRC gravity drops off much more quickly on the subatomic scale.

I don't think it does drop off quicker? However, at the subatomic scale gravity is an incredibly weak force to start with, so it gets totally dominated by electromagnetic and nuclear forces.

[Rockphed]

I don't think it does drop off quicker? However, at the subatomic scale gravity is an incredibly weak force to start with, so it gets totally dominated by electromagnetic and nuclear forces.

Electricity is insanely much stronger than gravity. A pair of electrons 1 meter apart will have a force between them in the 10^-28 newtons. They will have a gravitational force between them in the 10^-71 newtons. The only reason gravity gets to act at all is because most of the universe is comprised of neutral matter. This is, of course, also why it takes electricity almost no space at all to stop something that has been falling for miles.

Edit to avoid doublepost:


Thinking about the dimple in the event horizon, if there were matter directly underneath where it used to be, that might be exposed, it might do nothing, or if it's a firewall, it might erupt, so that's interesting.

I only just noticed this. When LIGO detected 2 neutron stars merging they saw a stream of platinum ejected from the collision (https://www.ligo.caltech.edu/page/press-release-gw170817). I imagine that if there is material near the surface of the black hole that some of it might get dislodged. Of course, telling that material from the chunks of former neutron star that got dislodged is hard.

[Caesar]

In so far as known physics can describe it, there is nothing inside of a blackhole, no matter to interact with, only the imprint on spacetime caused by the previous presence of matter. This imprint consists of 11 measureables, resolving as mass, position, angular momentum, and perhaps most curiously, electrical charge.

An approaching massive object will alter spacetime geometry just as the blackhole does. Vector addition means that it flattens spacetime as it approaches, which reduces the schwarzchild radius. So no, you dont get an emerging pod, you get a flattening of one side of the blackhole. Given the gravitational force between such massive objects as a neutron star and a comparable black hole, however, this flattening would only occur for a brief moment in time relative to an outside observer. The two "bodies" would be drawn towards eachother with astonishing speed once they got close enough.

However with enough net angular momentum in the two-body system, which is the most likely scenario, the neutron star would instead spiral around the black hole for some more time, being torn to shreds in the process. Regardless, once the star was close enough, the process would occur with astonishing speed.

[halfeye]

However with enough net angular momentum in the two-body system, which is the most likely scenario, the neutron star would instead spiral around the black hole for some more time, being torn to shreds in the process. Regardless, once the star was close enough, the process would occur with astonishing speed.

Another person who replied to the title without even reading the first post, let alone any other posts.

We've already had the discussion about the neutron star being torn to shreds, if the black hole is stellar mass, it probably does when it gets very close, but since it's own surface gravity is 200 billion Earth surface gravities, not until it is very close indeed. If the black hole is small, it will be swallowed by the neutron star and eat it from the inside, which might take hundreds or thousands of years, depending on just how small the black hole was. For intermediate or supermassive black holes, which this thread is actually about (I think I'm going to edit the thread title), the neutron star is just fine until it is taken into the event horizon.

[Bohandas]

The edited title is even harder to understand

[halfeye]

The edited title is even harder to understand

Is it? :smallredface: :smallsigh:

There is a character limit on thread title length, I don't know how close the previous was to the limit. I could try again, but constantly editing the title would be bad.

[Bohandas]

People aren;t necessarily familiar with that abbreviation, and some may be familiar enough to know it's a black hole but forget the exact acronym and incorrectly assume the sm means "solar mass"

[Anymage]

For intermediate or supermassive black holes, which this thread is actually about (I think I'm going to edit the thread title), the neutron star is just fine until it is taken into the event horizon.

Neutron stars have very strong surface gravity, but the material also has strong outwards pressure. Lesser the inwards pressure keeping it all contained (by, say, having a strong outwards force due to a strong gravitational pull from outside), there's a good chance that some of the neutronium on that side will break free.

We can talk about what would happen if two similarly massed black holes merged. We can talk about what would happen if two black holes of different sizes merged. (The latter, not surprisingly, would be "similar to the first case, but less so".) Bohandas gave a good answer to that a bit back (https://en.wikipedia.org/wiki/Binary_black_hole). We can talk about hypothetical objects that never break down (probably best modeled by treating it as a similarly massed black hole), or at what point a real neutron star would break down. If you're really curious, we can even talk about what would happen in the region of space that used to be covered by an event horizon, but currently isn't*.

And count me as another person who thinks that the new title is more confusing than the last one. SMBH vs. neutron star is mostly answered, with the main remaining question being how well the neutron star would actually structurally hold up before breaking down and becoming a swirling cloud of particles. Other related questions can either help illustrate the underlying ideas, or be interesting in their own right.

*(In this case, the answer is not that you can retrieve an object that might have fallen past the horizon. Even from oversimplified models, it would have fallen further in than you could get the horizon to recede. If you want to get something out of a black hole, either look into the physics on their decay or look into how, from the perspective of the outside universe, all infalling matter is seen as being squished infinitely thin and frozen in time at the event horizon. The specifics of this are outside my area of focus. And without any way to reasonably gather experimental evidence, there are likely multiple competing models amongst theoretical physicists.)

[halfeye]

Neutron stars have very strong surface gravity, but the material also has strong outwards pressure. Lesser the inwards pressure keeping it all contained (by, say, having a strong outwards force due to a strong gravitational pull from outside), there's a good chance that some of the neutronium on that side will break free.


Supermassive black holes have properties that distinguish them from lower-mass classifications. First, the average density of a SMBH (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some SMBHs.[6] This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. In addition, the tidal forces in the vicinity of the event horizon are significantly weaker for massive black holes. As with density, the tidal force on a body at the event horizon is inversely proportional to the square of the mass: a person on the surface of the Earth and one at the event horizon of a 10 million M☉ black hole experience about the same tidal force between their head and feet. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole.

https://en.wikipedia.org/wiki/Supermassive_black_hole#Description


We can talk about what would happen if two similarly massed black holes merged. We can talk about what would happen if two black holes of different sizes merged. (The latter, not surprisingly, would be "similar to the first case, but less so".) Bohandas gave a good answer to that a bit back (https://en.wikipedia.org/wiki/Binary_black_hole). We can talk about hypothetical objects that never break down (probably best modeled by treating it as a similarly massed black hole), or at what point a real neutron star would break down. If you're really curious, we can even talk about what would happen in the region of space that used to be covered by an event horizon, but currently isn't*.

The last one interests me, the others not so much (particularly the neutron star in the light of the above quote from Wikipedia, which to me reads as it goes in entirely intact).


And count me as another person who thinks that the new title is more confusing than the last one. SMBH vs. neutron star is mostly answered, with the main remaining question being how well the neutron star would actually structurally hold up before breaking down and becoming a swirling cloud of particles. Other related questions can either help illustrate the underlying ideas, or be interesting in their own right.

I am sorry if the new title is confusing, but the first post clearly mentions Sgr A*, and we kept getting people replying to the thread title as if that first post didn't exist.

I don't think we have reached a consensus about the event horizon, it seems to me that there is probably a dent in the event horizon between the neutron star and the black hole, and another event horizon outside the neutron star, that probably widens as the neutron star approaches the black hole, and presumably some sort of gap between the two, the shape of which might be interesting.


*(In this case, the answer is not that you can retrieve an object that might have fallen past the horizon. Even from oversimplified models, it would have fallen further in than you could get the horizon to recede. If you want to get something out of a black hole, either look into the physics on their decay or look into how, from the perspective of the outside universe, all infalling matter is seen as being squished infinitely thin and frozen in time at the event horizon. The specifics of this are outside my area of focus. And without any way to reasonably gather experimental evidence, there are likely multiple competing models amongst theoretical physicists.)

It's the frozen in time at the event horizon scenario that seems interesting to me, that might just reveal some ordinary matter, which is probably the reasonable result, or that stuff might go bang in exciting ways, but that's probably not what will really happen.

[Anymage]

Supermassive black holes wouldn't produce significant tidal forces if you happened to be near the surface. They still create strong gravity. It's just that there isn't a sharp distinction between the pull felt at the top and at the bottom.

Just because the top half of a neutron star is pulled inwards with roughly the same force as the bottom does not mean that the bottom feels no force. It's still an accelerated frame of reference.

And while it'd be nice if someone with better astrophysics background than just taking a bunch of online courses hopped in and cleared things up, it's not that the bottom gets pulled off by the gravitational differential. It's that the whole thing really wants to expand, and only fails to due to the gravitational pressure holding it all together. Counter that with a strong external force, it'll start to let off material all on its own.

[halfeye]

Supermassive black holes wouldn't produce significant tidal forces if you happened to be near the surface. They still create strong gravity. It's just that there isn't a sharp distinction between the pull felt at the top and at the bottom.

The tidal force is the gravity. The event horizon of a SMBH is huge.


Just because the top half of a neutron star is pulled inwards with roughly the same force as the bottom does not mean that the bottom feels no force. It's still an accelerated frame of reference.

No, it's not, it's a warped spacetime frame of reference. There is no noticable acceleration in freefall, just a change in velocity.


And while it'd be nice if someone with better astrophysics background than just taking a bunch of online courses hopped in and cleared things up, it's not that the bottom gets pulled off by the gravitational differential. It's that the whole thing really wants to expand, and only fails to due to the gravitational pressure holding it all together. Counter that with a strong external force, it'll start to let off material all on its own.

Yes, but a difference of one Earth gravity compared to approximately 200 billion Earth gravities is going to do nothing at all. It would be really nice to hear from a specialist, but I doubt Kip Thorne frequents these fora.

[Rockphed]

No, it's not, it's a warped spacetime frame of reference. There is no noticable acceleration in freefall, just a change in velocity.

Um, isn't that what acceleration is? Now, with limited tidal forces and in free fall you wouldn't notice acceleration because our brains detect acceleration by an unevenly applied external force. A field force we have a much harder time detecting.

[halfeye]

Um, isn't that what acceleration is?

I don't rightly know any more, I thought I did once, but then I heard about relativity. The thing is anymage seems to be saying that because there is a lot of acceleration, there will be an equal and opposite reaction to that, and in that sense, gravity doesn't accelerate a body, there is no reaction to acceleration by gravity.


Now, with limited tidal forces and in free fall you wouldn't notice acceleration because our brains detect acceleration by an unevenly applied external force. A field force we have a much harder time detecting.

What our inner ears detect, is the change in inertia, that's not what happens with gravity, with gravity you stand on the Earth, and you are accelerated upwards by the surface of the Earth beneath you, that generates a change in inertia that our inner ears detect, and any changes in that rate of change are also detected. I'm probably explaining this bit really badly.

The main thing is, when a neutron star approaches the event horizon of a SMBH, it is not going to be spaghettified, SMBHs probably do do that, but only when objects are a very long way inside the event horizon.

[Anymage]

The main thing is, when a neutron star approaches the event horizon of a SMBH, it is not going to be spaghettified, SMBHs probably do do that, but only when objects are a very long way inside the event horizon.

We're agreed on this.

Just to make sure everybody is on the same page, neutronium is inherently unstable and doesn't want to stay so compressed. Only being in the center of a massive gravity well keeps it that way, and if removed from that level of severe pressure it would escape as a large stream of particles.

Two black holes will deform each others event horizons as they merge. So we know that being in freefall towards an object does not mean that you don't experience any sense of force from it.

So again, and I'll shut up if a real astrophysicist says I'm talking out my rear, neutron stars already exist in a state between outwards degenerate pressure vs. inwards gravitational pressure. If a third outwards force is added to that equation, it can tilt the balance towards what the degenerate pressure would want to do. No spaghettification or tidal forces required.

[Lord Torath]

The tidal force is the gravity. The event horizon of a SMBH is huge.Tidal force is not gravity! Tidal force is the change in gravity over distance.

If you have 1 G pulling on your feet, but half a G pulling on your head, the tidal force acting on your body is half a G.

With SMBHs, the event horizon is very far from the singularity, so the difference in gravity felt by a human "standing" on the event horizon between their head and their feet is very, very small. The over all force of gravity acting on that human though, is still 299,792,458m/s2, or roughly 30.6 million Gs.

[halfeye]

Tidal force is not gravity! Tidal force is the change in gravity over distance.

If you have 1 G pulling on your feet, but half a G pulling on your head, the tidal force acting on your body is half a G.

With SMBHs, the event horizon is very far from the singularity, so the difference in gravity felt by a human "standing" on the event horizon between their head and their feet is very, very small. The over all force of gravity acting on that human though, is still 299,792,458m/s2, or roughly 30.6 million Gs.

Hm, where did you get that figure please?

I'm still saying that 200 billion gs trumps 30 million by quite enough to keep a neutron star stable.

[Lord Torath]

Hm, where did you get that figure please?

I'm still saying that 200 billion gs trumps 30 million by quite enough to keep a neutron star stable.I arrived at the figure of 299,792,458 m/s2 through the entirely-logical-but-probably-completely-wrong method of assuming that the acceleration of gravity at the event horizon should be exactly enough to overcome the speed of light. As light travels through vacuum at a speed of 299,792,458 m/s, the gravitational acceleration to counteract that should be 299,792,458 m/s2. And since 1 G is 9.807 m/s, then the force of gravity at the event horizon is 30.57 million Gs.*

Numbers aside, the basic concept is still valid. Calculate the pull due to the SMBH on the near end of the neutron star, the pull on the far side, and find the difference to get the tidal force. If that tidal force is greater than the force holding the neutron star together, the star will be ripped apart. If it is less, the star will intact through the event horizon. There will be a particular mass for the black hole at which the two values are equal, which means the neutron star will be ripped apart right at the event horizon. Less massive black holes will rip it part farther from the event horizon, more massive black holes will not.

* I should go google to see how it really gets calculated! Back in a while! <scurries off to Google>

[halfeye]

I arrived at the figure of 299,792,458 m/s2 through the entirely-logical-but-probably-completely-wrong method of assuming that the acceleration of gravity at the event horizon should be exactly enough to overcome the speed of light. As light travels through vacuum at a speed of 299,792,458 m/s, the gravitational acceleration to counteract that should be 299,792,458 m/s2. And since 1 G is 9.807 m/s, then the force of gravity at the event horizon is 30.57 million Gs.*

Oh, right, yeah, I don't know how to get the right answer, but that is totally wrong, for one thing it would be the same for all black holes, and that is not true.

The thing is, light doesn't get stopped at the event horizon, it keeps on keeping on travelling at c, but it gets pulled around so it heads back in. That's why the gravity at the surface of a small hole is huge, the event horizon forms where light can be pulled around, and to make light take a tight corner requires a huge amount of gravity, to make light turn a less wide corner requires less gravity. This hypothetical 10 million solar mass super massive black hole that Wikipedia likes with the event horizon about as wide as the orbit of Mercury has minutes to turn a beam of light (from the event horizon being that wide that it takes minutes at c to go around it).

You can presumably derive the gravity at the event horizon from the circumference of the EH and c, though that's probably not the order in which real scientists make calculations.

[Rockphed]

Oh, right, yeah, I don't know how to get the right answer, but that is totally wrong, for one thing it would be the same for all black holes, and that is not true.

The thing is, light doesn't get stopped at the event horizon, it keeps on keeping on travelling at c, but it gets pulled around so it heads back in. That's why the gravity at the surface of a small hole is huge, the event horizon forms where light can be pulled around, and to make light take a tight corner requires a huge amount of gravity, to make light turn a less wide corner requires less gravity. This hypothetical 10 million solar mass super massive black hole that Wikipedia likes with the event horizon about as wide as the orbit of Mercury has minutes to turn a beam of light (from the event horizon being that wide that it takes minutes at c to go around it).

You can presumably derive the gravity at the event horizon from the circumference of the EH and c, though that's probably not the order in which real scientists make calculations.

I thought the escape velocity of a black hole at the event horizon was the speed of light. Then again, once you get in to relativity concepts like velocity and acceleration get intrinsically tied up with frame of reference and the math gets sufficiently dense as to make my head hurt.

[halfeye]

I thought the escape velocity of a black hole at the event horizon was the speed of light.

Yes that is correct. However, that means that anything going at that speed or less is unable to get away, it does not mean that it is stopped dead in no time at all. The path of light is bent by gravity, it curves around. An Earth mass black hole would have the mass of the Earth, at the radius of the Earth from the singularity, the gravity would be that experienced on the surface of the Earth. Which is nowhere near enough to bend light into a circle, so it turns out the the event horizon has a diameter of a cm or so? I can't refind that reference, here's one for a lunar mass black hole:


To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole would need a mass less than the Moon. Such a black hole would have a diameter of less than a tenth of a millimeter.[120]

https://en.wikipedia.org/wiki/Black_hole#Evaporation

[Excession]

Yes that is correct. However, that means that anything going at that speed or less is unable to get away, it does not mean that it is stopped dead in no time at all.

Time stopping at the event horizon is what an outside observer sees though.

Something I'm not so sure about is the effect of different time dilation across a body near a black hole. That might be more important than the more Newtonian tidal forces. If it does manage to pull the neutron star apart, you'll end up with the universe's largest fission explosion, though some of that energy will be spent climbing back out of the gravity well before we get to see it.

[halfeye]

I was thinking about this while trying to sleep, and it occurred to me that when I was talking about an event horizon appearing in the far side of the neutron star from the SMBH, I was talking about the formation of a new baby black hole. Those go bang! and there would be an increasing sequence of those as the neutron star approached the black hole, up until the point where the black hole was instantaneously big enough to be stable. So this would be a somewhat energetic event, particularly if neutronium was ejected.

[Douglas]

Yes that is correct. However, that means that anything going at that speed or less is unable to get away, it does not mean that it is stopped dead in no time at all. The path of light is bent by gravity, it curves around.
Suppose you have an object heading exactly directly out from a strong source of gravity, at a speed less than escape velocity. Its path will not curve. It will not turn aside in any direction. It will instead slow down and reverse course, all while moving on the same never-changing straight line.

Light can't do that. Light always moves at the same speed. Always. Light loses energy when it climbs out of a gravity well, but this loss takes the form of reduced frequency rather than reduced speed. For light to be unable to escape, it must be stopped dead in an instant, because that's the only way light can be stopped.


An Earth mass black hole would have the mass of the Earth, at the radius of the Earth from the singularity, the gravity would be that experienced on the surface of the Earth. Which is nowhere near enough to bend light into a circle, so it turns out the the event horizon has a diameter of a cm or so? I can't refind that reference, here's one for a lunar mass black hole:



https://en.wikipedia.org/wiki/Black_hole#Evaporation
The effect and strength of a black hole's gravity at millions or billions of times the distance of its event horizon has very little to do with the effect and strength at the event horizon.

[halfeye]

Suppose you have an object heading exactly directly out from a strong source of gravity, at a speed less than escape velocity. Its path will not curve. It will not turn aside in any direction. It will instead slow down and reverse course, all while moving on the same never-changing straight line.

Light can't do that. Light always moves at the same speed. Always. Light loses energy when it climbs out of a gravity well, but this loss takes the form of reduced frequency rather than reduced speed. For light to be unable to escape, it must be stopped dead in an instant, because that's the only way light can be stopped.

Exactly and directly outward is, depending the degree of exactness, very unlikely. What happens to that photon is I suspect unknown, but it really doesn't matter, because in the lifetime of a SMBH there is likely to be one or less that are that exactly aligned.


The effect and strength of a black hole's gravity at millions or billions of times the distance of its event horizon has very little to do with the effect and strength at the event horizon.

If the inverse square law applies, then the ratio between them is known. I think I got distracted or something, maybe it didn't seem relevant any more when I got to it, but I think I was intending to compare the rate of fall off of the Earth's gravitational field with that of the SMBH, since the inverse square law implies that at two Earth radii the gravity of the Earth will have dropped off to 1/4 of the surface gravity, whereas if the SMBH had one Earth gravity at the event horizon, one extra Earth radius was going to make very little difference to that, due to the radius of the EH being of the order of Mercury's orbit. When people started throwing up their hands at the idea that there was any difference between the gravity at the EH of a SMBH and an Earth or Lunar mass black hole with an EH radius on the scale of centimetres or millimetres, discussing the effective rate of drop off didn't seem so relevant, why I left it in I don't remember, I think it was because it seems obvious to me that if the radius of the EH is very small, the gravity at the EH must be tremendous, so I thought that would be convincing.

Anyhow, we've apparently got a magnetar at 0.3 of a light-year from Sgr A*, does anyone know when it's exspected to go in?

[Rockphed]

Exactly and directly outward is, depending the degree of exactness, very unlikely. What happens to that photon is I suspect unknown, but it really doesn't matter, because in the lifetime of a SMBH there is likely to be one or less that are that exactly aligned.

Photons are both particles and waves. Specifically, their motion is always a solution to maxwell's equations. How maxwell's equations interact with relativity I have never figured out, but essentially, photons go in all possible directions from the point they are emitted from.

And now I am wondering if you can build a laser using a black hole instead of mirrors.

[Douglas]

Exactly and directly outward is, depending the degree of exactness, very unlikely. What happens to that photon is I suspect unknown, but it really doesn't matter, because in the lifetime of a SMBH there is likely to be one or less that are that exactly aligned.
It does matter, because the event horizon is defined as the point where it is impossible for light to escape, not merely the point where it is improbable.


If the inverse square law applies, then the ratio between them is known.
It doesn't, actually. If you take the general relativity equations for gravity and replace the speed of light with infinity, they simplify to inverse square, but near a black hole the difference is big enough to matter.

[halfeye]

It does matter, because the event horizon is defined as the point where it is impossible for light to escape, not merely the point where it is improbable.

Definitions are human things, and the universe doesn't care. If an event is rare enough, impossible and improbable are very nearly synonyms.


It doesn't, actually. If you take the general relativity equations for gravity and replace the speed of light with infinity, they simplify to inverse square, but near a black hole the difference is big enough to matter.

Well maybe, though that's news since the origination of this thread to me, however, from the photon sphere out, it presumably does apply, and that's all the way from 1.5 cm to the crust.

[Douglas]

Definitions are human things, and the universe doesn't care. If an event is rare enough, impossible and improbable are very nearly synonyms.
The General Theory of Relativity, which is the original basis for predicting black holes and their properties* in the first place, has math that works out to it being literally impossible for light to escape the event horizon. Not "very nearly" impossible, but "actually, truly, we really mean it" impossible. Said math is complicated, and I'm not going to pretend I fully understand it, but there is quite an overwhelming consensus among the serious scientists that do understand it about what it means.

* Including the photon sphere and its location


Well maybe, though that's news since the origination of this thread to me, however, from the photon sphere out, it presumably does apply, and that's all the way from 1.5 cm to the crust.
The gravitic field around a simple spherical mass follows the Schwarzschild metric (https://en.wikipedia.org/wiki/Schwarzschild_metric), and it does so at ALL distances. As distance increases it gets closer and closer to inverse square, but no matter how far you get inverse square remains an approximation. There is no cutoff point where gravity suddenly switches equations.

[gomipile]

Definitions are human things, and the universe doesn't care. If an event is rare enough, impossible and improbable are very nearly synonyms.



Well maybe, though that's news since the origination of this thread to me, however, from the photon sphere out, it presumably does apply, and that's all the way from 1.5 cm to the crust.

The inverse square law doesn't even apply precisely in our inner solar system, at the orbit of Mercury. We were able to measure the discrepancy from Newtonian prediction as early as 1859:

https://en.m.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_ of_Mercury

[halfeye]

Do people think that a baby black hole will arise inside the magnetar before it crosses the event horizon? Neutron stars are pretty dense objects, and it's own gravity combined with that of the SMBH might it seems to me be enough.

[Lord Torath]

Do people think that a baby black hole will arise inside the magnetar before it crosses the event horizon? Neutron stars are pretty dense objects, and it's own gravity combined with that of the SMBH might it seems to me be enough.I would tend to think not. The neutron star is in freefall*, so the only forces it feels from the black hole are the tidal forces trying to pull it apart. I would think it would never form a black hole, just merge into the one pulling it in.

* When you're in freefall, you can't feel the Earth's gravity pulling on you. You can feel the resistance of the air if you're in the atmosphere, but you can't actually feel the Earth's pull. It's certainly pulling you, but there's nothing resisting that pull, so you can't feel it.

[halfeye]

I would tend to think not. The neutron star is in freefall*, so the only forces it feels from the black hole are the tidal forces trying to pull it apart. I would think it would never form a black hole, just merge into the one pulling it in.

* When you're in freefall, you can't feel the Earth's gravity pulling on you. You can feel the resistance of the air if you're in the atmosphere, but you can't actually feel the Earth's pull. It's certainly pulling you, but there's nothing resisting that pull, so you can't feel it.

I'm not sure any more. I''m pretty sure there's an event horizon, because of the combined gravity of the SMBH and the neutron star, but the stuff inside that isn't under any greater gravity than the rest of the neutron star, so maybe it stays neutronium and doesn't form a singularity? If it's a micro black hole then it emits Hawking radiation at a furious rate, but I think the horizon hangs around (emitting like mad iff that constitutes a black hole) getting bigger until the neutron star is entirely enclosed by the event horizon of the SMBH. The starting position of the new event horizon would be somewhere between the midpoint of the radius of the neutron star and the surface of the neutron star, on the far side from the SMBH, if there were no spins or frame dragging it would be exactly on the extension of the line through the singularity and the centre of the neutron star, but that's almost certainly not the case, something will be spinning, possibly everything will be, and there will be some frame dragging too. I think that event horizon will form inside the neutron star, but I really can't see any way to derive it's position more closely than above the half way mark from the centre to the surface, but below the surface.

<edit> The reason for this is in part an equal and opposite reaction of the dimple in the event horizon of the SMBH, the dimple is caused by the combined gravity of the SMBH and the neutron star being opposed, but on the other side of the neutron star they are conjoined, so there the gravitational field must increase, and an event horizon will form IMHO.

I hope that LIGO (or if it's a long time from now, its replacement) is running when that magnetar is engulfed by Sgr A*