Are Black Holes Actually Giant Neutron Stars Cloaked in an Event Horizon?

[nikkkom]

I'm a little uncomfortable with that statement. The rules of GR only insist on extreme geometric curvature and pressure in the presence of so much mass/energy, but extreme GR curvature doesn't automatically lead us to infinitely dense objects or "points". Only if our understanding of neutrons and quarks and such is correct, and the Pauli exclusion principle does not apply, can we say with absolute certainty what GR might 'predict' in extreme mass concentration cases.

We do know what GR predicts. Schwarzschild solution is not at all ambiguous.

GR predicts that beyond EH, direction towards the center becomes timelike. This means that there is no way for infalling matter to avoid moving towards it, just like you can't avoid moving into tomorrow.
Moreover, GR predicts that any matter reaches the center in a finite proper time interval. (What happens next, GR can't say, since math breaks down - curvature becomes infinite, etc).

The above is what people mean when they say that "GR predicts singularity".

This, of course, does not imply that this singularity is a mathematically well-behaving object. It is not.
In my opinion, the fact that GR predicts it is a breakdown of GR. Simply put, it does not correctly describe ultra-high-energy and ultra-small-distances behavior of gravitation. In my opinion, there is a better theory which gives more sensible description.

 

[Chronos]

Whatever might be hiding inside an event horizon it almost certainly is not a neutron star for. The most massive known neutron stars to date only run about 2 solar masses and the best existing measurements yield a radius of no less than 10km. That is significantly larger than the Schwarzschild radius for a 2 solar bh - which is about 6km. Assuming 10km is some kind of low limit for the radius of any neutron star. A mass of about 3.5 solar would be necessary to achieve a Schwarzschild radius of 10km. See https://arxiv.org/abs/1505.05155, The Dense Matter Equation of State from Neutron Star Radius and Mass Measurements, for further details. This appears to pretty much rule out the mega neutron star scenario. A quark star is not yet ruled out, but would require an equation of state far beyond nucleon density. As already noted, we have no evidence to suggest any state of matter with a density exceeding that of a neutron star. Most scientists suspect the singularity is nothing more than a mathematical artifact, and nature has demonstrated great resilience in avoiding the singularity paradox when it arises in other situations. I share this suspicion, but, also suspect the way around this disaster will be surprising. The clues are probably staring us in the face - e.g., how does a point particle [electron] avoid having infinite charge density [a charge singularity]?

 

[stefan r]

how does a point particle [electron] avoid having infinite charge density [a charge singularity]?

There is an uncertainty in position and momentum.

The same uncertainty should apply to particles in black holes. Any one electron would be near the singularity give or take a bit. Same/similar to the electron's position in an atom. Black hole is a singularity because there is stellar mass packed into that small space.

 

[Drakkith]

There is an uncertainty in position and momentum.

I'm pretty sure that doesn't resolve the charge singularity.

The same uncertainty should apply to particles in black holes. Any one electron would be near the singularity give or take a bit. Same/similar to the electron's position in an atom. Black hole is a singularity because there is stellar mass packed into that small space.

That's not how a singularity in GR works. It's not simply a point in space. It's also a point in time. You can't have uncertainty when the particle's world-line literally ends at the singularity.

 

[stefan r]

That's not how a singularity in GR works. It's not simply a point in space. It's also a point in time. You can't have uncertainty when the particle's world-line literally ends at the singularity.

I thought there was a time uncertainty too. I may have misunderstood the Eintein-Bohr debate.

Also does it take forever to reach the singularity?

 

[rootone]

No it does not, an infalling something reaches the (theoretical) singularity very shortly after crossing the event horizon.
Why?, because the infalling thing is trying to go faster than light, which as we all know is banned.

 

[nikkkom]

No it does not, an infalling something reaches the (theoretical) singularity very shortly after crossing the event horizon.
Why?, because the infalling thing is trying to go faster than light, which as we all know is banned.

I think the second sentence is wrong. Where did you get it?

 

[Jason R Carrico]

Why wouldn't everything approaching the event horizon already be traveling at or near the speed of light?

 

[rootone]

Yeah. that's what I meant, as far as anyone outside can see

 

[nikkkom]

Why wouldn't everything approaching the event horizon already be traveling at or near the speed of light?

Relative to what? Velocity is relative. Superluminal velocity relative to a distant observer is not prohibited in GR. The speed of light stays constant only locally.

 

[Jason R Carrico]

You're right. So let me clarify my question.

Why wouldn't anything caught in the gravitational field of the black hole be traveling at or near the speed of light, relative to the black hole, as it crossed the event horizon?

My next question would be;

Why would any observer, from any location, measure the speed of matter caught in the gravitational field of a black hole in reference to anything other than the black hole itself?

 

[Chronos]

The reference frame of the observer is irrelevant since c is c in all reference frames. Think of the old intro SR question about 2 rockets approaching from opposite directions at .9 c wrt a ground observer. Do the ground observer and rocket passengers disagree about anything besides whose watch is right?

 

[russ_watters]

Since you guys seem bent on picking apart the question instead of correcting it and answering it, let me ask it this way and see if we can get an answer:

What is the speed a remote observer at rest wrt the black hole measures for a freely falling (from infinity or equivalent) object as it approaches the event horizon?

I would think that since the escape velocity is C at the event horizon we would see the object asymptotically approaching C as it approaches the event horizon...

...unless Relativity effects make it look slower, similar to why we see x-rays generated by infalling matter...perhaps that represents the cutoff speed?

 

[Chronos]

It just like alice watching bob fall into a BH, he never quite reaches the EH from her perspective, he just redshifts into obscurity.

 

[Jason R Carrico]

Why can I not see newer posts on this thread? I keep getting emails saying there have been newer replies but I don't see anything. Why?

 

[phinds]

Why can I not see newer posts on this thread? I keep getting emails saying there have been newer replies but I don't see anything. Why?

Could be a couple of things. First, you get notifications after the posts so if you are active on the site you may see a new post and then a bit later get an email saying that there's a new post but actually it's one you've already seen. Second, sometimes a post will be deleted but it sends out an email when posted so that can cause confusion.

The thing to do is to look at the BODY of the email and see what post you are actually being notified of.

 

[Jason R Carrico]

Why can I not see newer posts on this thread? I keep getting emails saying there have been newer replies but I don'tsee anything. Why?

Could be a couple of things. First, you get notifications after the posts so if you are active on the site you may see a new post and then a bit later get an email saying that there's a new post but actually it's one you've already seen. Second, sometimes a post will be deleted but it sends out an email when posted so that can cause confusion.

The thing to do is to look at the BODY of the email and see what post you are actually being notified of.

The above showed up in my email, but shows up nowhere when I open this thread.

 

[Jason R Carrico]

I see it now. Sorry and thanks

 

[stefan r]

Since you guys seem bent on picking apart the question instead of correcting it and answering it, let me ask it this way and see if we can get an answer:

What is the speed a remote observer at rest wrt the black hole measures for a freely falling (from infinity or equivalent) object as it approaches the event horizon?

I would think that since the escape velocity is C at the event horizon we would see the object asymptotically approaching C as it approaches the event horizon...

...unless Relativity effects make it look slower, similar to why we see x-rays generated by infalling matter...perhaps that represents the cutoff speed?

Suppose the falling object broadcasts a radio message of it's current location. The broadcast starts at one gigahertz but will shift to one megahertz, then kilohertz, hertz etc. The location message was always transmitted at one gigahertz. Now you can make two measurements of velocity. Suppose location A to location B is 2 meter. Light would be able to travel that distance in 6.7 x 10^-9 seconds which is also 6.7 cycles at 1 gigahertz. The distant observer who gets a 1 hertz signal will also observe 6.7 cycles but that will take 6.7 seconds. So the distant observer is seeing the messenger drop into the hole at 0.15 meters per second while the messenger is claiming 150,000,000.
The example is not exactly correct for a drop from infinity. Also the signal and velocity would not be linear from point A to point B.

 

[Drakkith]

Why can I not see newer posts on this thread? I keep getting emails saying there have been newer replies but I don't see anything. Why?

Second, sometimes a post will be deleted but it sends out an email when posted so that can cause confusion.

Phinds is correct. I had to delete multiple posts from this thread.

Suppose the falling object broadcasts a radio message of it's current location. The broadcast starts at one gigahertz but will shift to one megahertz, then kilohertz, hertz etc. The location message was always transmitted at one gigahertz. Now you can make two measurements of velocity. Suppose location A to location B is 2 meter. Light would be able to travel that distance in 6.7 x 10^-9 seconds which is also 6.7 cycles at 1 gigahertz. The distant observer who gets a 1 hertz signal will also observe 6.7 cycles but that will take 6.7 seconds. So the distant observer is seeing the messenger drop into the hole at 0.15 meters per second while the messenger is claiming 150,000,000.
The example is not exactly correct for a drop from infinity. Also the signal and velocity would not be linear from point A to point B.

Hmmm. You sure about this? It doesn't sound right to me, but I confess I'm not sure how to do the math.

 

[stefan r]

Hmmm. You sure about this? It doesn't sound right to me, but I confess I'm not sure how to do the math.

No. Not sure of anything. I have not personally made any observations of black holes. The world looks fairly Newtonian in my house.

Also sure that a linear measurement will be wrong. Just trying to describe time dilation. Most descriptions use the word "clock". I am using a radio broadcast frequency as a "clock".

 

[phinds]

Light would be able to travel that distance in 6.7 x 10^-9

Uh ... really? And where exactly is this happening? How close are A and B to the BH?

No. Not sure of anything. I have not personally made any observations of black holes. The world looks fairly Newtonian in my house.

Also sure that a linear measurement will be wrong. Just trying to describe time dilation. Most descriptions use the word "clock". I am using a radio broadcast frequency as a "clock".

It would be helpful if you would draw a diagram of exactly what you are describing. It sounds wrong to me also but I may be misinterpreting what you are describing.

 

[stefan r]

Uh ... really? And where exactly is this happening? How close are A and B to the BH?

speed of light 299,792,458 meters per second in a vacuum.

It would be helpful if you would draw a diagram of exactly what you are describing. It sounds wrong to me also but I may be misinterpreting what you are describing.

Isaac Newton and the royal society did not think of it.
My example is crap because the signal would dilate from both gravity and also doplar effect. For stationary objects:

For a solar mass black hole the Schwarzschild radius is 2950 meters. To get a time dilation of 1 billion, 10^-9 you need √10^-18. So the transmitter is around 3 femtometers from the event horizon. Not a very good example.

Suppose it is broadcasting from 29,500 meters (r_s x 10) outside a solar mass black hole or neutron star. The time dilation is 0.949. So a radio station broadcasting at 105.1 FM will show up as 99.7FM to someone listening from far away.

 

[Drakkith]

Suppose it is broadcasting from 29,500 meters (rs x 10) outside a solar mass black hole or neutron star. The time dilation is 0.949. So a radio station broadcasting at 105.1 FM will show up as 99.7FM to someone listening from far away.

That looks correct to me. My earlier confusion was mostly from your description of the motion of the transmitter.

 

[Ken G]

I really don't see how the presence or absence of a singularity has anything to do with answering any of those questions, I'm sorry. Also, it does not relate to the presence of the "gap" in stable density, as that does not have to do with black holes at all. Whether or not an event horizon forms relates to what happens after the object has already formed, so is just not relevant to those questions. Also, one must be careful not to mistake the infinity in an arbitrary (Schwarzschild-like) time coordinate with the final state of formation of an actual object, as that final state should be in the object's own proper time.

 

[Chronos]

The difference between neuton star & black hole mass is one of the great questions in astrophysics. According to google, the most massive known neutron star [J0348-0432] weighs in at an impressive 2.0 Msun, whereas the least massive black hole [XTE J1650-500] tips the scales at 3.8 Msun. While this is not necessarily spectacular, it certainly is enough to arouse suspicion. Are gap mass degenerates just inexplicably rare within our observational stewardship, or, are we even more naïve than we suspect? Granted, achieving accurate mass measurements at interstellar distances is not always a trivial matter, but, this is akin to randomly turning up a thousand spades of dirt in your backyard only to find all worms revealed run either less than 2 grams or more than 3.8 grams in mass. It certainly appears to suggest either improbable luck, or your yard is inhabited by separate earthworm species.

 

[Ken G]

What doesn't make sense about that is that it usually doesn't matter to the mass if you get a neutron star or a black hole, the mass is set first, and then you get what you get. If there really is a gap from 2 to 3.8, it implies that the mass you end up with is controlled by the object that is created, rather than the other way around. I would find it much easier to believe that either the masses between 2 and 3.8 are hard to pin down, or that they simply aren't classified as either a neutron star or a black hole because it isn't known which to classify them as. If it is true that the mass is pretty well known, and there really is a gap in mass, then I agree that would be of great significance to the formation process of whatever these objects are. Perhaps the neutron star is capable of "bouncing out" any mass that would raise it just a bit above 2 solar masses, but if you really pile on about 4, then it cannot be bounced out. But all that would involve extremely complex physics including rotation, magnetic fields, and equations of state-- not just a treatment of gravity.

 

[Chronos]

The issue is beyond a mere curiousity. In fact,t.one of the leading authorities on stellar mass black holes has already asserted evidence for a preferred mass range of.stellar mass black holes, as discussed in this paper; https://arxiv.org/abs/1006.2834,The Black Hole Mass Distribution in the Galaxy. Ozel also comments on the mass gap between black holes and neutron stars. A variety of methods have been developed to enhance the reliability of black hole mass estimates as discussed here; https://arxiv.org/abs/0902.2852,Determination of Black Hole Masses in Galactic Black Hole Binaries using Scaling of Spectral and Variability Characteristics.and as touched upon in this article https://www.theregister.co.uk/2008/04/01/smallest_black_hole_known_discovered/. For a discussion more specific to the mass gap issue, this may prove interesting; https://arxiv.org/abs/1110.1635,Missing Black Holes Unveil The Supernova Explosion Mechanism. We remain in interesting times.

 

[Ken G]

That's quite interesting, thank you!

 

[Ken G]

General relativity is normally thought to imply that anything that creates an event horizon around itself will also collapse into a singularity. I personally don't know what theorems are needed for that conclusion, but I still don't see in the above any evidence that the mass of the remnant is determined by the equation of state of the remnant in the range 2 - 5 solar masses, that feedback is missing from the argument. Intermediate mass black holes are much more massive than that.

 

[Ken G]

The interesting gap is between 2 and 5 solar masses, not 15 and 50,000, as the latter is expected from stellar mass issues. Also, there is not a direct connection between a mass gap and a density gap, as the latter is expected and the former is not.

 

[Chronos]

There is plenty of evidence to challenge a 15 - 50,000 Msolar slot limit. The first LIGO GW detections involved a pair of ~30 Msolar black holes. Other candidates include the 5000 Msolar specimen reported here https://arxiv.org/abs/1601.02628 and 7600 Msolar reported here https://arxiv.org/abs/1705.01612.

 

[Chronos]

There is always uncertainty in data - especially astrophysical data. But strong outliers [like 5000-7500] must be taken seriously when uncertainty of the data is more tightly constrained than those of any assumptions underlying predictive models. The lack of prolific exceptions is less noteworthy than the existence of any confirmed exception..

 

[PAllen]

The difference between neuton star & black hole mass is one of the great questions in astrophysics. According to google, the most massive known neutron star [J0348-0432] weighs in at an impressive 2.0 Msun, whereas the least massive black hole [XTE J1650-500] tips the scales at 3.8 Msun. While this is not necessarily spectacular, it certainly is enough to arouse suspicion. Are gap mass degenerates just inexplicably rare within our observational stewardship, or, are we even more naïve than we suspect? Granted, achieving accurate mass measurements at interstellar distances is not always a trivial matter, but, this is akin to randomly turning up a thousand spades of dirt in your backyard only to find all worms revealed run either less than 2 grams or more than 3.8 grams in mass. It certainly appears to suggest either improbable luck, or your yard is inhabited by separate earthworm species.

This gap seems to have been bridged by the LIGO neutron star merger, which is believed by most subject matter experts to have produced a 2.7 solar mass BH within about 10 to 100 milliseconds after initial merger.

 

[PeterDonis]

A few misconceptions/open questions that have appeared in this thread need to be cleared up:

if a neutron star is just on the cusp of having enough mass to be a black hole, and then gains that mass, what's to say it doesn't just gain an event horizon at that point?

This is not possible; there is not a continuous series of stable (i.e., non-collapsing) states between any neutron star and any black hole. The reason is something called Buchdahl's Theorem, which says that no stable configuration of matter can have a radius smaller than 2.25M, where 2M would be the Schwarzschild radius of a black hole with the same mass. So there's no way for a stable object like a neutron star to be "just short" of being a black hole, because that would correspond to a stable configuration of matter having a radius of, say, 2.0001M--i.e., just a bit larger than a black hole of the same mass--and that is ruled out by Buchdahl's Theorem.

an infalling something reaches the (theoretical) singularity very shortly after crossing the event horizon.
Why?, because the infalling thing is trying to go faster than light

This is not correct. No locally measured speed will be faster than light, even inside the horizon. A coordinate speed in particular coordinates might be greater than $c$, but this has no physical meaning. And none of this has anything to do with whether a singularity is present or how long it takes an infalling observer to reach it.

Why wouldn't everything approaching the event horizon already be traveling at or near the speed of light?

It is, relative to an observer "hovering" at a constant altitude just above the horizon. Only local relative speeds are physically meaningful in a curved spacetime.

how do we know it's not a super compact body?

Because no compact body can exist with a radius smaller than 2.25M. See above.

I thought there was a time uncertainty too. I may have misunderstood the Eintein-Bohr debate.

The Einstein-Bohr debate is irrelevant, as is the uncertainty principle; we are talking about classical GR here, not QM. If you want to talk about how quantum gravity might affect possible black hole states, please start a new thread (and it should be either in the QM forum or, more likely, the Beyond the Standard Model forum, since there is no established theory of quantum gravity at present).

General relativity is normally thought to imply that anything that creates an event horizon around itself will also collapse into a singularity. I personally don't know what theorems are needed for that conclusion

The Hawking-Penrose singularity theorems are the ones that establish this conclusion: a good brief statement of the conclusion is that the presence of a trapped surface implies geodesic incompleteness. The assumptions required are an energy condition (which one depends on what kind of geodesic incompleteness is being addressed--timelike or null) and a condition on the global structure of the spacetime (typically that there is a Cauchy surface with certain properties). The Wikipedia page gives a decent brief overview:

https://en.wikipedia.org/wiki/Penrose–Hawking_singularity_theorems