SMBH and neutron star subject revisit

[halfeye]

I started a thread about this once before, but it's gone now.

There seemed to be a concensus that if a neutron star is orbiting into a supermassive black hole, on the event horizon at the point next to the approaching neutron star there would be a dimple, due to the gravity of the neutron star acting counter to the attraction of the hole.

The main point I'm interested in this time is that on the line through the centres of both masses once you get past the centre of the neutron star, the gravities add. It seems to me that there ought to be some sort of event horizon or collapse going on inside the neutron star due to this gravity, before the side nearest the hole is absorbed.

[Lord Torath]

I started a thread about this once before, but it's gone now.

There seemed to be a concensus that if a neutron star is orbiting into a supermassive black hole, on the event horizon at the point next to the approaching neutron star there would be a dimple, due to the gravity of the neutron star acting counter to the attraction of the hole.

The main point I'm interested in this time is that on the line through the centres of both masses once you get past the centre of the neutron star, the gravities add. It seems to me that there ought to be some sort of event horizon or collapse going on inside the neutron star due to this gravity, before the side nearest the hole is absorbed.I think the tidal force of the black hole would tend to negate such an event. Also, the neutron star is in free-fall (i.e. orbit) around the black hole, and so is anything on its surface. The only force the star (and its surface) feel from the black hole is its tidal force, which works to pull the star apart, not force it together.

[Anymage]

Halfeye is talking about a supermassive black hole large enough, that the event horizon is far enough away that tidal forces at the event horizon are negligible. He's also talking about a straight-on collision as opposed to the usual getting caught and falling in through an accretion disc.

The specifics lie way beyond my layperson's grasp of the matter. Especially as to what would happen to the neutron star as it got in close. Except to say that:

To an object far enough away from the two objects, they could be treated as one combined mass. This is basic high school physics, and is rather unsurprising on the face of it.
There would briefly be an area of space that would normally be covered by the black hole's event horizon, but that an object could safely pass through because the pull of the neutron star would counteract some of the pull from the black hole.

[halfeye]

Halfeye is talking about a supermassive black hole large enough, that the event horizon is far enough away that tidal forces at the event horizon are negligible. He's also talking about a straight-on collision as opposed to the usual getting caught and falling in through an accretion disc.

Yes, that's a SMBH and what I'm talking about. I am not specifically excluding orbiting in, just more or less ignoring the differences, though I am excluding an accreation disk because as I understand it those exist further out from the hole than the points I'm interested in, in this case.


The specifics lie way beyond my layperson's grasp of the matter. Especially as to what would happen to the neutron star as it got in close. Except to say that:

To an object far enough away from the two objects, they could be treated as one combined mass. This is basic high school physics, and is rather unsurprising on the face of it.

Yes, I'm not talking about any effects on distant objects. I think that this would start to get interesting from a point maybe when the neutron star is something like two neutron star diameters from the event horizon, and then as it went inward. I'm concerned that there appears to be a case that the gravity at the event horizon is matched and exceeded within the neutron star, this seems to me to imply that there is a free floating event horizon without a contained singularity, unless part of the neutron star collapses before crossing the main event horizon.


There would briefly be an area of space that would normally be covered by the black hole's event horizon, but that an object could safely pass through because the pull of the neutron star would counteract some of the pull from the black hole.


That's the dimple.

Possibly, if the black hole is a frozen star with all the matter held at the point it entered the event horizon, moving the event horizon inward might result in an eruption of gamma rays (almost typo-ed grammar rays, those would be interesting). There should also be changed event horizon geometries when black holes collide, it is possible that they are the source of some gamma ray bursts.

[Lord Torath]

Halfeye is talking about a supermassive black hole large enough, that the event horizon is far enough away that tidal forces at the event horizon are negligible. He's also talking about a straight-on collision as opposed to the usual getting caught and falling in through an accretion disc.The point remains that the only forces felt by an object in free-fall (as in the case of a neutron star plunging straight into a black hole) are tidal forces from nearby bodies. In this case, the tidal forces are negligible, and thus there is no contraction of the neutron star as a result of the black hole's gravity.


There would briefly be an area of space that would normally be covered by the black hole's event horizon, but that an object could safely pass through because the pull of the neutron star would counteract some of the pull from the black hole.
For a particular definition of "safely". The tidal forces of the black hole and the neutron star add together, increasing the tendency to "spaghettify" anything passing between them. The neutron star will have some pretty extreme tidal forces, even if the SMBH's are negligible. You could send a beam of light through there safely.

Edit: Halfeye, I think your last post broke the forum formatting. :smallbiggrin:

[halfeye]

The point remains that the only forces felt by an object in free-fall (as in the case of a neutron star plunging straight into a black hole) are tidal forces from nearby bodies. In this case, the tidal forces are negligible, and thus there is no contraction of the neutron star as a result of the black hole's gravity.
For a particular definition of "safely". The tidal forces of the black hole and the neutron star add together, increasing the tendency to "spaghettify" anything passing between them. The neutron star will have some pretty extreme tidal forces, even if the SMBH's are negligible. You could send a beam of light through there safely.

<edit+edit>

I am not talking primarily about the forces on the neutron star, I am thinking about the overall force required to create an event horizon. The graviatational force at points along the line through the centres of the masses should reach a minimum between the neutron star and the event horizon, but there has to be a maximum above that, and it seems to me that makes another event horizon.


Edit: Halfeye, I think your last post broke the forum formatting. :smallbiggrin:

Yeah, but that was the edit before last. I was trying to edit inside a *list* killing the list restored it, but now I don't have a response to your response.

[gomipile]

There was a similar thread over on Physicsforums:

https://www.physicsforums.com/threads/simulation-of-a-neutron-stars-impact-on-a-supermassive-black-hole.952647/

Most over there seemed to think that a neutron star isn't big enough to cause a meaningful effect on the surface of a SMBH.

[halfeye]

There was a similar thread over on Physicsforums:

https://www.physicsforums.com/threads/simulation-of-a-neutron-stars-impact-on-a-supermassive-black-hole.952647/

Most over there seemed to think that a neutron star isn't big enough to cause a meaningful effect on the surface of a SMBH.

Reading some of the first page of that, it seemed to be solely the question poser and one responder. I am not expecting the SMBH to be significantly moved or shaken, I am not expecting the neutron star to get away. I am interested in the theoretical activities that might take place (possibly only for the minutest fraction of a second) when this happens. I am especially interested in whether there is an extra bit of event horizon created in then expanding out of the side of the neutron star furthest from the black hole. If the event horizon is too mystically perfect to be messed with in this way, what about the edge of the photon sphere?

Not particularly relevant to this particular topic, but it seems to me science only progresses when scientific orthodoxy is opposed, the true orthodoxy of science should be scientific heresy.

[Lord Torath]

I am not talking primarily about the forces on the neutron star, I am thinking about the overall force required to create an event horizon. The graviatational force at points along the line through the centres of the masses should reach a minimum between the neutron star and the event horizon, but there has to be a maximum above that, and it seems to me that makes another event horizon. I haven't done the math, but my first guess is "no". That said, these conditions are so far outside of everyday human experience that intuition is not necessarily trustworthy.

Let's assume a super-massive black hole, such that over a distance from the event horizon to a point 10 km away there's a negligible difference in gravitational pull. Let's also assume a neutron star that is just this side of critical density/mass required to initiate collapse into a black hole of its own, having a diameter of 20 km and a mass of 2.56 solar masses, on a course directly toward the singularity of the supermassive black hole.

The Schwarzschild radius for that neutron star is, by my calcs (and with some help from Wikipedia (https://en.wikipedia.org/wiki/Schwarzschild_radius)) 7.566 km, well inside the 10km radius of the neutron star (Maybe? The 10 km figure has one significant digit, meaning the radius could be anywhere from 5 to 14.9 km. I've seen neutron stars listed as having a diameter of 12.4 miles, but that's just the conversion of 20 km. How precisely do we know the diameter of neutron stars? It has to be larger than the Schwarzschild radius (rs), obviously, but how much larger before it collapses? I suspect that as soon as any portion the core's radius equals its own rs, the whole thing collapses into a black hole, and as I understand it, the density of a neutron star increases as you approach its core).

As we've already discussed, the only force the neutron star will feel is the tidal force from the SMBH, so the gravity of the SMBH will not initiate core collapse in the neutron star. Again, I haven’t done the math (and I’m not sure how to do the math), but I suspect that even with the added gravitational pull of the SMBH, the rs of the neutron star will still be within the surface of the neutron star, not resulting in the formation of another event horizon.

[halfeye]

I haven't done the math, but my first guess is "no". That said, these conditions are so far outside of everyday human experience that intuition is not necessarily trustworthy.

Let's assume a super-massive black hole, such that over a distance from the event horizon to a point 10 km away there's a negligible difference in gravitational pull. Let's also assume a neutron star that is just this side of critical density/mass required to initiate collapse into a black hole of its own, having a diameter of 20 km and a mass of 2.56 solar masses, on a course directly toward the singularity of the supermassive black hole.

The Schwarzschild radius for that neutron star is, by my calcs (and with some help from Wikipedia (https://en.wikipedia.org/wiki/Schwarzschild_radius)) 7.566 km, well inside the 10km radius of the neutron star (Maybe? The 10 km figure has one significant digit, meaning the radius could be anywhere from 5 to 14.9 km. I've seen neutron stars listed as having a diameter of 12.4 miles, but that's just the conversion of 20 km. How precisely do we know the diameter of neutron stars? It has to be larger than the Schwarzschild radius (rs), obviously, but how much larger before it collapses? I suspect that as soon as any portion the core's radius equals its own rs, the whole thing collapses into a black hole, and as I understand it, the density of a neutron star increases as you approach its core).

As we've already discussed, the only force the neutron star will feel is the tidal force from the SMBH, so the gravity of the SMBH will not initiate core collapse in the neutron star. Again, I haven’t done the math (and I’m not sure how to do the math), but I suspect that even with the added gravitational pull of the SMBH, the rs of the neutron star will still be within the surface of the neutron star, not resulting in the formation of another event horizon.

I think we're talking past each other.

I'm not primarily interested in whether the neutron star collapses before it crosses the event horizon (though that would be interesting, if it was going to happen), what I'm thinking is that there's a region on, in and above (taking the centre of the black hole as being down) the neutron star where the combined gravity of the neutron star and the black hole is so strong that light entering that region is pulled (in so far as gravity pulls) so hard that it inevitably ends up in the black hole. This is a counterpart to the dimple in the event horizon, in that both are due to the interaction of the gravity of the black hole with the gravity of the neutron star. I suppose I'm hypothesising an event horizon without an enclosed singularity, I do not suggest that the existence of the one would in any way suggest the existence of the converse.

[georgie_leech]

I strongly suspect that the mass of the Neutron Star is sufficiently dwarfed by the SMBH that it won't have any meaningful "extension" of the Event Horizon on the side opposite the SMBH, if that's what you're asking. Not so long as the NS is outside of the Event Horizon, anyway. The math for what happens when the NS begins crossing the EH is beyond me though.

[halfeye]

I strongly suspect that the mass of the Neutron Star is sufficiently dwarfed by the SMBH that it won't have any meaningful "extension" of the Event Horizon on the side opposite the SMBH, if that's what you're asking. Not so long as the NS is outside of the Event Horizon, anyway. The math for what happens when the NS begins crossing the EH is beyond me though.

I am not saying this would be significant relative to the SMBH. I am ignoring reflections of light off the neutron star. Locally to the neutron star, the gravitational field of the neutron star would be much stronger than that of the black hole because the singularity is so much further away. I don't know when this would start to happen, I don't think it'll have started when the neutron star is 20 neutron star radii from the event horizon, I suspect it's well and truly begun by the time the near point of the surface of the neutron star is 2 neutron star radii from the normal position of the event horizon. My current thought is that the mini-event horizon will appear exactly at the surface of the neutron star (supposing the neutron star is a perfect sphere, or a perfect elipsoid if it's stretched), on the opposite side from the singularity of the SMBH.

[Lord Torath]

I think we're talking past each other.

I'm not primarily interested in whether the neutron star collapses before it crosses the event horizon (though that would be interesting, if it was going to happen), what I'm thinking is that there's a region on, in and above (taking the centre of the black hole as being down) the neutron star where the combined gravity of the neutron star and the black hole is so strong that light entering that region is pulled (in so far as gravity pulls) so hard that it inevitably ends up in the black hole. This is a counterpart to the dimple in the event horizon, in that both are due to the interaction of the gravity of the black hole with the gravity of the neutron star. I suppose I'm hypothesising an event horizon without an enclosed singularity, I do not suggest that the existence of the one would in any way suggest the existence of the converse.Right. Yes, I understand what you're talking about. But I don't think you could have an event horizon on the non-black-hole side of the neutron star that wouldn't also initiate the collapse of the neutron star. But as I said in the last paragraph of my previous post, I don't know how to do the math on that. I mean, I could do the standard "what's the force of gravity from the SMBC, what's the force from the neutron star, and add them together", but that's using Newtonian physics, and I'm quite sure they don't apply here.

Also, the gravitational force of the SMBH is a good bit stronger than that of the Neutron Star. We are right on the edge of the black hole, and it is massive enough that there is no meaningful difference between the force just before the event horizon and the force 20 km away (negligible tidal force on the neutron star from the SMBH is one of our assumptions).

[Chronos]

As an aside, a couple of relevant points about event horizons:

1: An event horizon is defined globally, not locally. If, from a given point at a given time, there exists any path from that point to destinations infinitely far away, then that point at that time is not inside of a horizon. If, from any given point at any given time, there does not exist any such path, then that point at that time is inside of a horizon. There will not typically be anything special going on locally right at an event horizon, especially not for dynamic situations like this, and it is in fact for an observer to cross an event horizon without having any indication whatsoever that an event horizon even exists. It is possible, albeit ludicrously unlikely, that we crossed an event horizon ten minutes ago, and just don't know it yet.

2: The total area of event horizons never decreases (except extremely slowly, via Hawking radiation, but that can be neglected). When two holes merge, the final hole has at least as much surface area as the sum of the two originals, and usually a fair bit more. And any perturbation to the event horizon must be such as to increase its area.

That said, to the original question: Because the Schwarzschild radius is proportional to mass, the moment when two black holes merge into a single hole is precisely the moment when their event horizons touch. Since a neutron star is always larger than a black hole of the same mass (because everything is always larger than a black hole of the same mass), the effect you're describing wouldn't occur until part of the neutron star was already within the original event horizon of the hole. More precisely, it would happen when the center of the neutron star was a distance from the original horizon equal to the star's Schwarzschild radius, which is smaller than its physical radius.