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TheIvan
2010-Jun-30, 12:28 PM
So AFAIK a neutron star is as dense as it gets.
So when the neutrons are that densely packed, could this mean the friction coefficient between two layers of a neutron star could grow into infinity?
What happens if a neutron star were to be sucked into a black hole?
Would a black hole have enough gravity to tear a neutron star apart or would it swallow it as one big particle?

Noclevername
2010-Jun-30, 06:54 PM
It would depend on the size of both (a huge Galactic core BH would have a lot less tide than a collapsar), and of what you mean by "swallow". Do you mean passing beyond the event horizon, and if so from what viewpoint?

Nereid
2010-Jun-30, 08:21 PM
So AFAIK a neutron star is as dense as it gets.
So when the neutrons are that densely packed, could this mean the friction coefficient between two layers of a neutron star could grow into infinity?
[...]
The bulk of a neutron star is comprised of a state of matter that is so unlike the ordinary solid-liquid-gas we are used to that 'densely packed' and 'friction coefficient' are likely to need rather radical re-definition to be meaningful.

For example, if a part of a neutron star behaves as some kind of quantum fluid, friction, in the usual sense, would be zero.

Also, a neutron star is not necessarily iso-dense (below any surface layers); there are really strange states of matter that might exist at especially high densities (and temperatures), for example some kind of quarkonium.

Ken G
2010-Jun-30, 08:31 PM
What's more, the internal structure of a neutron star is not actually that well known. Black holes are comparatively easy!

TheIvan
2010-Jun-30, 10:18 PM
@Noclevername: Then please lay out for me how the size combinations would play out.
Actually I'm only thinking of the black hole's tidal force tearing at the neutron star, so a passing of the Schwarzschildradius isn't required (assuming an orbit leading to such isn't required for a great enough tidal force) and the perspective would be the neutron star.

@Nereid: Thanks. I realized I made a false assumption of the neutron star being a very dense solid.
Come to think of it, are the neutrons in a neutron star stable? (afaik neutrons are only stable inside of nuclei).
I lack knowledge in particle physics. Aren't neutrons made of up and down quarks? Wikipedia tells me quarkonium is made of charm xor bottom quarks. Are they intertransformable?
What state of matter would a neutron star resemble?

@Ken G:
Is the "crust" of a neutron star known?
The internal structure of a black hole is known?:surprised Please point me to where I can read about it!

TheIvan
2010-Jun-30, 10:41 PM
I think that friction is an effect of electromagnetism within the intersection of 2 atom's radii.
So if a neutron star is really only neutrons, there shouldn't be any friction at all (assuming there isn't a similar effect if 2 neutron's radii can intersect).

Nereid
2010-Jul-01, 12:32 AM
I think that friction is an effect of electromagnetism within the intersection of 2 atom's radii.
So if a neutron star is really only neutrons, there shouldn't be any friction at all (assuming there isn't a similar effect if 2 neutron's radii can intersect).
The behaviour of matter with the densities typical of neutron stars is not something one can readily grok by thinking of it in terms of classical physics (as you seem to have done). Instead, you need the Standard Model (of particle physics) and quantum theory ... and that's nigh on impossible to think through, intuitively, using only classical physics concepts (with some few exceptions).

Ken G
2010-Jul-01, 01:01 AM
Is the "crust" of a neutron star known?I don't think anything is known about neutron stars, but there are plenty of theories.


The internal structure of a black hole is known?:surprised Please point me to where I can read about it!The internal structure is predicted by GR. That doesn't make it known, but it does at least mean we have a good model.

WayneFrancis
2010-Jul-01, 01:28 AM
As said before the tidal forces of a BH are largely dependant on the size of said black hole. Funny enough a 20 stellar mass black hole is going to do a lot more damage to a neutron star then a super massive black hole. This is because the 20 SM black hole has a larger variation of tidal forces near its EH.

Friction really has nothing do with this scenario. It isn't like the surface of the neutron star is rubbing up against anything even if it isn't classed as some type of frictionless super fluid.

Nereid
2010-Jul-01, 02:22 AM
@Noclevername: Then please lay out for me how the size combinations would play out.
Actually I'm only thinking of the black hole's tidal force tearing at the neutron star, so a passing of the Schwarzschildradius isn't required (assuming an orbit leading to such isn't required for a great enough tidal force) and the perspective would be the neutron star.
First, are you familiar with the general idea that the tidal force near a BH depends on the mass of the BH? So that a neutron star, or even a person, would feel only the smallest of tidal forces approaching the EH of a SMBH, but a person would be ripped to shreds (spaghettified) at quite some distance from a sol-mass BH?

If so, then we can calculate some rough numbers.


@Nereid: Thanks. I realized I made a false assumption of the neutron star being a very dense solid.
Come to think of it, are the neutrons in a neutron star stable?
Yes, they are ... neutrons are unstable in isolation because the beta decay is energetically favourable; however, inside a neutron star, beta decay requires energy (this comes with the nuclear degeneracy, which is a quantum effect).


(afaik neutrons are only stable inside of nuclei).
Only in some nuclei; in neutron-rich nuclei they are not, and some will decay, emitting an electron and an anti-neutrino; in neutron-poor nuclei, some protons will capture an electron to become a neutron, and some will decay, emitting a positron and a neutrino.


I lack knowledge in particle physics. Aren't neutrons made of up and down quarks? Wikipedia tells me quarkonium is made of charm xor bottom quarks. Are they intertransformable?
No. These states of matter are describable only within particle physics, but basically it comes down to what states are the lowest energy states.


What state of matter would a neutron star resemble?
Nuclear degenerate matter; the counterpart to electron degenerate matter, which is the state of matter of the bulk of a white dwarf star.



@Ken G:
Is the "crust" of a neutron star known?
The internal structure of a black hole is known?:surprised Please point me to where I can read about it!
If no one has already answered these questions, I'll take a shot later.

TheIvan
2010-Jul-01, 09:06 AM
No I wasn't aware that a smaller BH would produce stronger tidal differences than a larger one and I don't understand why it does so.
@WayneFrancis: initially I thought of the neutron star rubbing against itself while being torn sideways in a spiral.

*sigh* It seems I need to read some textbooks about particle and quantum physics to understand this; thanks for trying me though

Ken G
2010-Jul-01, 03:50 PM
No I wasn't aware that a smaller BH would produce stronger tidal differences than a larger one and I don't understand why it does so.
That's true at the event horizon, and is even somewhat true at the surface of a small planet vs. a large one. The key point is, it's not the strength of the gravity we are talking about, it is the change in gravity over, say, a meter, at the surface of the object. The strength of gravity at the surface (or event horizon) of an object of mass M and radius R scales like M/R^2, but its change over a meter at the surface scales like the derivative of that, or M/R^3. For a rocky planet, M scales like R^3 also, so you get R^3/R^3 and the tidal differences at the surfaces of rocky planets are all about the same. The really big planets are gas giants, which have lower density, so the M scales like R^3 but with a smaller coefficient, so Jupiter actually has a weaker change in its gravity over a meter at its surface than does Earth.

For black holes, M does not scale like R^3, it scales like R of the event horizon, so M/R^3 scales like 1/R^2 or 1/M^2, and smaller mass black holes have stronger tidal differences at the EH.

John Mendenhall
2010-Jul-01, 04:21 PM
Love it!

Nereid
2010-Jul-01, 05:05 PM
Love it!
I'm playing a little fast and loose here ... the usual terms are strange stars, quark stars, and strange matter.

There's also one, or more, hypothesised intermediate stage - pion condensates, lambda hyperons, and more (no doubt) - in which, for example, hyperons (baryons with at least one strange quark, but no charmed, bottom, or top quarks) are stable, a sort of hyperon degenerate state of matter. Here's a good neutron star intro (http://www.astro.umd.edu/~miller/nstar.html) (I don't know how fully accepted everything on it is, but at least what's there is stuff that's under serious consideration).

TheIvan
2010-Jul-01, 07:11 PM
That's true at the event horizon, and is even somewhat true at the surface of a small planet vs. a large one. The key point is, it's not the strength of the gravity we are talking about, it is the change in gravity over, say, a meter, at the surface of the object. The strength of gravity at the surface (or event horizon) of an object of mass M and radius R scales like M/R^2, but its change over a meter at the surface scales like the derivative of that, or M/R^3. For a rocky planet, M scales like R^3 also, so you get R^3/R^3 and the tidal differences at the surfaces of rocky planets are all about the same. The really big planets are gas giants, which have lower density, so the M scales like R^3 but with a smaller coefficient, so Jupiter actually has a weaker change in its gravity over a meter at its surface than does Earth.

For black holes, M does not scale like R^3, it scales like R of the event horizon, so M/R^3 scales like 1/R^2 or 1/M^2, and smaller mass black holes have stronger tidal differences at the EH.
Got that, thanks!

neilzero
2010-Jul-01, 11:49 PM
Infinite rarely (never?) occurs except in theory and hypothesis, but the friction could be extremely high, or not. Black holes of several solar mass are much smaller than neutron stars, so the entire event horizon and much of the accretion disk would be inside the neutron star. Typical neutron stars have their own accretion disk.
Inside the event horizon of several solar mass black holes the temperature, pressure and tide likely continues to rise as you approach the singularity, so the inner most part of the neutron star (near the singularity) would all but surely be shredded and compacted to at least a million times a million, times a million, times a million = 10E24 times the density of water. This would leave a void, into which the outer portion of the neutron star would collapse, perhaps very slowly over billions of years, from the view point of beings, if any, on the surface of the neutron star. The collapse of the surface is slow, because only a tiny portion of the interior of the neutron star can be destroyed per day, not because of time compression. This should be true of both rotating and non rotating black holes, except possibly extremely fast rotation = 0.99999999999999999999999c. I'm guessing so please correct. Neil

WayneFrancis
2010-Jul-02, 01:55 AM
No I wasn't aware that a smaller BH would produce stronger tidal differences than a larger one and I don't understand why it does so.
@WayneFrancis: initially I thought of the neutron star rubbing against itself while being torn sideways in a spiral.

*sigh* It seems I need to read some textbooks about particle and quantum physics to understand this; thanks for trying me though

Tidal forces are due to the difference in angle of gravity. The greater the angle the greater the tidal force.
Here we see a typical neutron star as it approaches the event horizon of a 3 solar mass black hole.
http://www.users.on.net/~waynefrancis/tidal%20forces2.png
You can see the direction of pull of gravity is very different from one side of the black hole to the other.
Now for a super massive black hole the angels are much less.
http://www.users.on.net/~waynefrancis/tidal%20forces.png
because the point of attraction is much further away.

There is always tidal forces that look like this
http://www.users.on.net/~waynefrancis/tidal.png
the difference is how far from the point of gravity you are dealing with and your size.

It is slightly more technical as the angles are in more then just 2 dimensions but the basic idea is that.

astromark
2010-Jul-02, 02:46 AM
Just as the Nutron Star or Super Masive Black Hole are Very big... so is this subject.

To gain a complete understanding of the physics involved could be described as 'complicated'.

However it is not required you work at 'CERN' or a similar facility to gain some understanding of this subject mater...

My brief summary of this is by no means complete and, nothing is a substitute for knowledge. That knowledge is the best tool in the box.

The key for my understanding is locked into the masses involved. As the mass increases so the state of mater changes.

Starting way down the scale and working up. We look at the mass of gas giants like Jupiter and consider.

That at its core a density exists that crushes mater... Moving up through the scale to the super massive Black Hole.

Where that same compression reduces matter to a state we have no knowledge of.

We measure the distortion of space time and from such calculate a mass equivalence...

We have been known to use the word 'gravity force' But, except that is not the whole story...

Noting that a velocity of c is exceeded by that curvature of space. The event horizon.

Assuming a close encounter of two such masses as a Neutron star and BH...

The greater mass will prevail. Consuming and increasing in mass. The Neutron star would fall into the greater mass.

The friction you ask of would not be such as to change the outcome... A plasma state of extreme density. A bigger Black Hole.

Happy reading... Mark.

neilzero
2010-Jul-02, 11:41 AM
Hi WayneFrancis: That seems correct except a 3 solar mass neutron star has about the same gravity field as a 3 solar mass black hole until the black hole is partially or completely inside the neutron star. Now the gravity tide effects are much stronger near the black hole's singularity, so likely that disrupts a small portion of the neutron star which was likely distorted only slightly as the neutron star just touched the event horizon. You illustration is likely to scale for a 1 solar mass neutron star verses a 10 solar mass black hole. I'm guessing. The event horizon is a mathematical boundary with no substance, so it can easily enter the neutron star or even another event horizon. The accretion disks are puny compared to the gravity binding of a neutron star. Is it probable that the the collision will be brief = one second and both the neutron star and the black hole will continue on their original path with only moderate changes, other than perhaps loss of half the mass of the neutron star?
There are, perhaps some neutron stars out there with only 1/10 th solar mass as the rest was lost to a black hole? Would a 1/10 th solar mass neutron star emit lots neutrons (at escape velocity) and thus shrink to even less mass? Neil

WayneFrancis
2010-Jul-03, 04:44 PM
Hi WayneFrancis: That seems correct except a 3 solar mass neutron star has about the same gravity field as a 3 solar mass black hole until the black hole is partially or completely inside the neutron star.


yes a 3 solar mass black hole and a typical neutron star would have similar gravitational fields up to the point of the surface of the neutron star.
The gravity at the surface of a neutron star is pretty extreme but no nearly as extreme as the gravity near the EH of a black hole.
The big issue is that the matter of a neutron star is very much susceptible to external gravitational forces while the mass of a black hole is VERY safe from being ripped apart.


Now the gravity tide effects are much stronger near the black hole's singularity, so likely that disrupts a small portion of the neutron star which was likely distorted only slightly as the neutron star just touched the event horizon.


Even if the Neutron star could hold together as it approached the black hole's event horizon it would definitely be ripped apart before it even got close to the EH.
The hole neutron star doesn't need to enter into the Roche limit before it will start getting ripped apart. Once any part of the neutron star is closer to the centre of a black hole of equal mass then its own radius then the BH's gravity is stronger for that point then it is from the neutron star itself. Before this happens the neutron star will be warped because of the interaction of gravitational fields. This means for the neutrons at those points the gravitational effect of the neutron star will start to become cancelled out by the black hole. This in turn will cause a shift in the make up of the neutron star as the neutrons will themselves become unstable and will start to beta decay.



You illustration is likely to scale for a 1 solar mass neutron star verses a 10 solar mass black hole.

No actually my illustration is very much to scale for a 3 solar mass black hole and 3 solar mass neutron star. A 10 solar mass black hole would be ~30km in diameter (its Event Horizon that is). A 3 solar mass neutron star is about 10km in diameter.



I'm guessing. The event horizon is a mathematical boundary with no substance, so it can easily enter the neutron star or even another event horizon.


Doesn't matter if it was a hard shell. The maths works out to be the same.



The accretion disks are puny compared to the gravity binding of a neutron star. Is it probable that the the collision will be brief = one second and both the neutron star and the black hole will continue on their original path with only moderate changes, other than perhaps loss of half the mass of the neutron star?


I'm not even thinking about the accretion disk ... It isn't matter in the accretion disk of a black hole that is the problem for the neutron star....it is the fact that the neutron star would not survive a close encounter with a black hole. There would be no glancing blow and the neutron star escaping with just a loss of some of its mass like some cosmic salamander that dropped its tail while being chased by a mouse.



There are, perhaps some neutron stars out there with only 1/10 th solar mass as the rest was lost to a black hole?

No...a neutron star can not be only 1/10th of a solar mass. It takes a certain amount of mass to maintain the hold over electron degeneracy pressure. There needs to be about 1.5 solar masses to have the gravity to become a neutron star. Take a spoon full of neutron star off the star and it will explode very violently.



Would a 1/10 th solar mass neutron star emit lots neutrons (at escape velocity) and thus shrink to even less mass? Neil

No it wouldn't because it can't exist.