Insights Do Black Holes Really Exist? - Comments

[PeterDonis]

PeterDonis submitted a new PF Insights post

Do Black Holes Really Exist?

Continue reading the Original PF Insights Post.

 

[jambaugh]

Yes. They really do... They are observed in galactic centers.

I find people get most confused by the characterization of event horizons, as if the proverbial event horizon of a black hole is some unique new physical entity. We pass through event horizons constantly. space-like hyper-surface is an event horizon, the future, and past light cones of any space-time event are examples of an event horizon, i.e. a boundary across which causal signals and matter can only travel one way. Event horizons don't need some extreme circumstance to be formed. The issue is whether gravitation can curve space-time so that we can draw an event horizon into a shape we describe as a black hole. GR says yes. Astronomical observations show something in the center of most galaxies that seems to confirm this theoretical prediction so... Yea, you betcha!

 

[Jonathan Scott]

What is observed in galactic centers is dense supermassive objects, which can be described as "black hole candidates".
If General Relativity is still accurate in such extreme situations, such objects are theoretically predicted to be black holes. GR has been confirmed to give very accurate predictions for the solar system and for example for loss of energy of the Hulse-Taylor binary pulsar system through gravitational waves. However, the most sensitive test these observations have checked so far only confirms GR to one "Post-Newtonian" correction term - the $\beta$ parameter in the Parameterized Post-Newtonian (PPN) model, which can be measured through the perihelion precession of Mercury and Lunar Laser Ranging. For black holes to occur, GR has to be accurate to further terms which have not yet been confirmed.
Clearly, GR is a neat and self-consistent theory and it is generally expected that black holes will eventually be confirmed, which is why there is no problem with calling these objects "black hole candidates". However, in the mean time, there are various observations which do not fit so well with GR, such as an apparent strong magnetic field in the vicinity of the core of a quasar (where a black hole was not expected to be able to sustain such a field) and the way in which GR apparently needs to be supplemented by mysterious dark matter to explain galactic rotation curves. It is also quite tricky to tell the difference between a hypothetical extremely red-shifted object which is not a black hole (if GR has some sort of limit that prevents black holes) and an actual black hole.
For the moment, GR is the best theory of gravity that we have and it predicts black holes, but at present that is a theoretical prediction, not an experimentally confirmed one.

 

[_PJ_]

I disagree. I am still unaware of any DIRECT OBSERVATION of Black Holes - not saying anyone above is wrong just that I'm not aware of such.

Indirectly, though, the evidence for Black Holes is overwhelming. The output from what used to be called 'Quasars' and 'Active Galactic Nuclei' is readily explained by current models of the energies from electromagnetic fields and the friction of the accretion material due to the incredible power of the BH frame dragging and its radial speed.
Even more recent measurements of lower frequencies to penetrate the amassed dust and obscuring clouds at the heart of the Milky Way, and the measured orbital paths (size, parabolicity and speeds) of the stellar objects around the "Great Attractor" Sag A* not only fit with the model with a supermassive Black Hole at the gravitational centre, but also, there is no known, nor generally accepted reasonable alternative possibility for something so massive, yet so spatially compact to produce such results.
It's a logical deduction that the most obvious, reasonable and plausible cause is that there MUST be a Black Hole.

I, too, would find it extremely unlikely that this is not the case, yet as a matter of direct, irrefutable proof and direct measurements confirming an actual Black Hole, there are none.
_____________

I also would consider Cauchy surface horizons and the effective surfaces of light cones in spacetime as being Absolute Horizons, which INCLUDE Event Horizons, but the nature of a Black Hole EVENT HORIZON is more than simply a 'one way street', the reason for the name "Event" Horizon refers to the extreme nature of the Black Hole in warping spacetime so that no more events are applied to a causal timeline that crosses the boundary.

Yes. They really do... They are observed in galactic centers.

I find people get most confused by the characterization of event horizons, as if the proverbial event horizon of a black hole is some unique new physical entity. We pass through event horizons constantly. space-like hyper-surface is an event horizon,

 

[CalcNerd]

I am on the side that is convinced by the available information that Black Holes exist. However, everyone has a different level of proof needed to be convinced. And I am NOT a professional physicist or astronomer. Using Occam's razor, I believe that Black Holes in the Galactic cores is the simplest explanation for what we observe. However, dark matter and other anomalies may lead to a more complex model that may not rely upon or allow something else to explain away the Black hole formation. A Black Hole seems to be the simplest and most reasonable explanation at this time, so I am convinced.

We can all see Jupiter through a modest telescope, but have ANY of us actually been there? Similar level of proof. We see activity and effects of a super massive object at the center of our Galaxy. We don't see any object, just lots of starts racing around a darkened core. It agrees with our mathematical model of a billion solar mass object. It doesn't radiate any perceivable light (of course we are tens of thousands of light years away, so we can't see low levels of radiation if they were there). Hence we now assume we have a black hole (candidate for the more severe doubting Thomas's).

 

[Bernie G]

"For the moment, GR is the best theory of gravity that we have and it predicts black holes"

Do we need complex ideas like GR or curved space to predict black holes? A body that falls under the force of gravitational attraction of mass M from infinity, starting with zero velocity, will strike the mass with a velocity equal to its escape velocity. Is the escape velocity or impact velocity v of a falling body on any object (even a neutron star) given exactly by the Newtonian formula v^2 = 2GM/r ? Note this equation gives the Schwarzschild radius when v = c.

 

[QuantumQuest]

Do we need complex ideas like GR or curved space to predict black holes?

In my opinion, this sounds like some people beginning with Albert Einstein, just wanted to make things look complex, which can in no way be true. Real Nature and Universe are indeed complex, regarding their mathematical description. Of course Newton's theories were great achievements for their time, but it's like describing an object as you see its surface, having no idea what hides inside or where it comes from. Again, this in no way relegates the great work of Newton, who after all, had nothing more than a few of previous theories and very few observations - or what did that mean back then. But Einstein went a great way further with GR and finally found very innovative ways to express his ideas. I don't think that any mathematically rigorous prediction, can exist outside some rigorous treatment and I definitely agree that what we have so far in this regard, is GR. I think that black holes exist, but I also think that quantum world has a lot to reveal in the future.

 

[Greg Bernhardt]

Great Insight Peter!

 

[Bernie G]

In my opinion, this sounds like some people beginning with Albert Einstein, just wanted to make things look complex, which can in no way be true.

Of course Einstein didn't want to just make things look complex. In a Newtonian sense the acceleration of a small falling object shouldn't be affected even by a relativistic increase in its effective mass-energy. Absent other forces, can the impact velocity on any far object (even a neutron star) be given exactly by the formula v^2 = 2GM/r ?

 

[russ_watters]

I disagree. I am still unaware of any DIRECT OBSERVATION of Black Holes...

My suspicion is that you are using a definition of "direct observation" here that it far more limited than you would use in other situations (just seeing with your eyes?). Because there are several direct observations of properties of black holes. Gravitational field strength is measured by timing orbits. Size is measured by observing radiation from infalling matter.
https://www.cfa.harvard.edu/seuforum/bh_reallyexist.htm

 

[_PJ_]

No. By Direct Obsevration of a Black Hole, I mean, any measurement that detects the actual properties of a Black Hole directly, rather than an indirect inference from a measurement of some other property which (ALTHOUGH HIGHLY UNLIKELY) may still be yet shown to be due to some other process.

Infalling Matter tells us the gravitational power accelerating objects, there is no observation of to-what this matter is falling into.*

Now the consensus is overwhelmingly in favour of, and, again as I mentioned, seems to largely reject any reasonable alternative possibilities given the mass/energy densities involved, that it can only really be a Black Hole. HOWEVER, and I am making an extreme exaggeration for the sake of the point, consider that some alien civilisation created super powerful energy rays which, when focussed onto a single concentrated point result in an extreme gravitational event.
This event would also exist in a small space, with a high gravitational force, for all intents and purposes of the 'indirect mweasurements', would still accrete infalling matter and accelerate it to relativistic speeds. The nature of the energy rays may still exhibit a powerful magnetic field and emit jets of high energy charge. This phenomena would still pull nearby stars into extremely tight, fast orbits around a space in which no stable stellar object could exist and none are visible. In effect, you have an entity which ticks all the boxes for a Black Hole, but is not one.

I personally am absolutely in agreement that Black Holes exist, and do not doubt that the measurements made as described are indirectly evidencing this - however, I maintain that it's simply not enough to warrant any claim of confirming the definite, undeniable such a phenomena as a Black Hole.

 

[Bernie G]

Is the concept of curved space required to predict black holes?

 

[_PJ_]

Is the concept of curved space required to predict black holes?

Light always travels in straight lines. In curved spacetime, this straight path is seen to be curved.
So although the basic idea of a highly dense, collapsed star (such as Laplace's Dark Star) were put forth even in 18th century, they were based on inaccurate understanding of light.

Part of the definition for a Black Hole is that the gravitational strength is such that the escape velocity at a particular altitude up the gravitational potential well is faster than the speed of light. This causes light to follow a trajectory that appears as curving towards and ultimately into the Black Hole.

So in some ways, yes, curved space is necessarily part of the actual definition of what a Black Hole is, but the idea of Black Holes in essence existed in a classical form (although not entirely accurate)

 

[Bernie G]

Light always travels in straight lines... in some ways, yes, curved space is necessarily part of the actual definition of what a Black Hole is ...

Is the escape velocity from any large object (even a neutron star or black hole) described exactly by the formula v^2 = 2GM/r ? If the Schwarzschild radius (called SR) is defined as the radius where the escape velocity equals the speed of light, can we then simply say that SR = 2GM/(c^2)? The concept of an object with a mass/radius ratio large enough to contain light doesn't require curved space along with the concept of light always traveling in straight lines. Why can't we simply say that light bends around an object? If a neutral object from far away drops straight into a basic non-spinning and non-magnetic black hole, is its relative velocity c when it reaches the event horizon?

 

[_PJ_]

Why can't we simply say that light bends around an object??

Because Light must travel in straight lines. If light was "bent" or curved, it would necessitate a change in velocity which necessarily entails a temporal metric which implies that light is not relativistic and violates both of Einstein's theories in one go.

In your given equations, when dealing with relativistic speeds, one must factor in the Lorenz transformations, which you seem to be missing.

 

[Bernie G]

In your given equations, when dealing with relativistic speeds, one must factor in the Lorenz transformations, which you seem to be missing.

Sure, a particle falling straight down towards a black hole will have Lorenz transformations, but do the Lorenz transformations at any point affect the velocity it will have?

 

[Bernie G]

"Do Black Holes Really Exist?"

Probably, but could what we think are black holes be compact stars (larger than their Schwarzschild radius) if they had the following characteristics?: (1) They were a mixture of normal matter and ultra-relativistic matter. (2) They had a crust that was mostly a light absorber.

 

[_PJ_]

Sure, a particle falling straight down towards a black hole will have Lorenz transformations, but do the Lorenz transformations at any point affect the velocity it will have?

It will have a constantly changing velocity anyway, if it's "falling".

 

[Bernie G]

It will have a constantly changing velocity anyway, if it's "falling".

Yes, but would its acceleration be affected by the Lorenz transformations?

 

[Bernie G]

"Do Black Holes Really Exist?"

The suggestion of a stable compact star consisting of normal matter and ultra-relativistic matter is probably a bad idea.

Suggestion (2): Could a compact star of 5 solar masses exist if its radius was 20 or 25 km? Would it have to collapse? Could we distinguish it from a black hole?

 

[russ_watters]

No. By Direct Obsevration of a Black Hole, I mean, any measurement that detects the actual properties of a Black Hole directly, rather than an indirect inference from a measurement of some other property...

So do you accept that the gravitational field strength and size measurements are "direct" measurements? I can't tell from what you are saying. What is the difference between/definition of "direct/indirect" measurements"? Rather than direct/indirect measurements, you now seem to be talking about some sort of indirect properties, and I've never heard of such a thing either.

...which (ALTHOUGH HIGHLY UNLIKELY) may still be yet shown to be due to some other process.

That's a different issue than whether the measurements are "direct". In science, theories predict properties and if properties are detected that match the theory and no other viable theories exist, then the theory is validated. Your line of logic sounds more like wishing another explanation will be found than accepting the scientific process that already found a viable explanation.

Infalling Matter tells us the gravitational power accelerating objects, there is no observation of to-what this matter is falling into.*

Gravitational acceleration is caused by mass. Mass is a property of objects. So that's an observation of the property of mass of the object it is falling into.

I maintain that it's simply not enough to warrant any claim of confirming the definite, undeniable such a phenomena as a Black Hole.

That's just the vanilla "you can't prove anything absolutely" fundamental reality of science. It's true of anything in science and nobody would ever claim black holes or anything else were 100% proven.

 

[jambaugh]

"Direct" vs "Indirect" observation may be problematic to define. "I don't see the moon, I see photons that, if they behave as my theory predicts, imply the existence of the moon." We extend our "selves" to our instruments, and our definition of what actually is "our instruments" to some pretty broad categories of phenomena. Always there is the possibility of an update to our models not just of the systems in question but of how our instruments behave and we may find that the "observed BH's" upon update cease to exist. Likewise we might find upon updating theory that photons or say electrons, should no longer be considered to be "observed". But until then I go with the current orthodox model.

I assert that the current inference of Black holes in galactic centers is stronger than say the inference of the existence of quarks or of the recently "observed" Higgs boson.

 

[Martin0001]

Unquestioned belief in existence of BH displayed by many here is breath taking.

What we can say is that some ultradense objects which do not appear to possesses a surface are detected.
Of course there might be a very red shifted surface indeed and such an object would not be a BH.
There are few credible alternatives indeed.

Ever heard about BH Firewall Paradox?
https://www.quantamagazine.org/20121221-alice-and-bob-meet-the-wall-of-fire/
This paradox is indicating distinct possibility of existence some other peculiar objects, compacted stars of diameter of Schwartzchild radius but entirely different from BH in their workings.

What about so called Magnetospheric Eternally Collapsing Objects.
https://en.m.wikipedia.org/wiki/Magnetospheric_eternally_collapsing_object

What about objects which do form event horizon and yet hold large, surface possessing body instead singularity under said horizon?

If proven, existence of any of these (and there are also many other possibilities) makes existence of BH very unlikely, if not impossible.

The only argument which I hear for is:
Very dense solution of GR with no detectable surface.
Well, that is rather wishful thinking and unwarranted jumping to conclusion.

 

[rootone]

A black hole is a prediction of GR, but nobody has made any claims of knowing exactly what happens inside of the event horizon of a BH,
and obviously whatever does go on cannot be observed directly.
The fact that the simplest models end up with a mathematical singularity is a strong indication that some kind of presently unknown physics comes into play, as a mathematical singularity implying infinite density cannot be describing any conceivable physical object.
However which ever way one chooses to interpret it, 'black hole candidates' do exist, and in particular the evidence for the SMBH in our galaxy's centre is overwhelming.
There are beyond any doubt star systems which are rapidly orbiting an extremely massive yet small invisible object.
Whatever object exists there it fulfils the GR description of a black hole, so until such time as there is contrary evidence we may as well call it a black hole.

 

[Martin0001]

A black hole is a prediction of GR, but nobody has made any claims of knowing exactly what happens inside of the event horizon of a BH,
and obviously whatever does go on cannot be observed directly.
The fact that the simplest models end up with a mathematical singularity is a strong indication that some kind of presently unknown physics comes into play.
However which ever way one chooses to interpret it, 'black hole candidates' do exist, and in particular the evidence for the SMBH in our galaxy's centre is overwhelming.
There are beyond any doubt star systems which are rapidly orbiting an extremely massive yet small invisible object.
Whatever object exists there it fulfils the GR description of a black hole, so until such time as there is contrary evidence we may as well call it a black hole.

Actually you may read more about BH Firewall Paradox.
It is hot topic in physics today.

It implies that one of these 2 notions must be true:

1. Firewall at the place where should be event horizon, that imply that certain assumptions of GR are rubbish.

2. Information in the hole is lost, that imply that cornerstone of QM (unitarity) is rubbish

This is a really substantial trouble, one which should ask us to consider possibility of existence other than BH, very red shifted and extremely compact objects there, but not necessarily one with no surface, event horizon like features.

 

[|Glitch|]

"Do Black Holes Really Exist?"

Probably, but could what we think are black holes be compact stars (larger than their Schwarzschild radius) if they had the following characteristics?: (1) They were a mixture of normal matter and ultra-relativistic matter. (2) They had a crust that was mostly a light absorber.

I tend to agree. Mass appears to be the determining factor. A star with ≤ 4.8 M_☉ will eventually form a degenerate white dwarf capable of resisting gravity's effects. Whereas a star > 4.8 M_☉ ≤ 10 M_☉ will eventually form a neutron star capable of resisting gravity's effects. It certainly seems plausible that ultra-relativistic matter, such as quarks or dileptons, in stars with a certain mass range (> 10 M_☉) could be capable of resisting gravity's effects. The apparent effect, to an outside observer, would be identical to a black hole with a Schwarzschild radius event horizon and an apparent horizon. While being incredibly dense (possibly an order of magnitude more dense than a neutron star) it still would not be "infinitely" dense, as in a "singularity."

 

[Bernie G]

Questioning conventional theory is a good thing but as was said above its pretty much a fact that there are small-size large-mass objects that are not visible. The GR description of a black hole is a logical explanation although we don’t have to accept it as gospel. An object that gravitationally contains light could be predicted without GR or curved space. A compact star with a light absorbing surface could also appear as a black hole. And there are other theories mentioned above.

There might be a clue from the probable fact that neutron star mass is limited by some process to 2 solar masses. Logically this process must occur within the star. Jets forming outside the star that limit the stars mass by preventing all material from falling to the star seem illogical. In older neutron stars large amounts of mass-energy could be shed by super extreme surface temperature, but this is not observed. Nuclear surface explosions are not an explanation. The alternative is the direct ejection of ultra-relativistic matter from the star. (Matter with a velocity of only 0.1c can’t escape the star). Ultra-relativistic matter is a logical result of the collapse of neutrons in the core, and this would explain the jets from neutron stars, not so interesting except this would probably have implications for the 5 – 10 solar mass “black holes” with jets. It seems that many people have a mental block to considering the possibility of neutron collapse at the core of a neutron star. It would be good to hear any suggestions for a process that limits the mass of neutron stars.

 

[Martin0001]

Questioning conventional theory is a good thing but as was said above its pretty much a fact that there are small-size large-mass objects that are not visible. The GR description of a black hole is a logical explanation although we don’t have to accept it as gospel. An object that gravitationally contains light could be predicted without GR or curved space. A compact star with a light absorbing surface could also appear as a black hole. And there are other theories mentioned above.

There might be a clue from the probable fact that neutron star mass is limited by some process to 2 solar masses. Logically this process must occur within the star. Jets forming outside the star that limit the stars mass by preventing all material from falling to the star seem illogical. In older neutron stars large amounts of mass-energy could be shed by super extreme surface temperature, but this is not observed. Nuclear surface explosions are not an explanation. The alternative is the direct ejection of ultra-relativistic matter from the star. (Matter with a velocity of only 0.1c can’t escape the star). Ultra-relativistic matter is a logical result of the collapse of neutrons in the core, and this would explain the jets from neutron stars, not so interesting except this would probably have implications for the 5 – 10 solar mass “black holes” with jets. It seems that many people have a mental block to considering the possibility of neutron collapse at the core of a neutron star. It would be good to hear any suggestions for a process that limits the mass of neutron stars.

Something called "electroweak star" is proposed where neutrons/quarks would be burned to leptons in small core of otherwise "normal" neutron star.
http://arxiv.org/abs/0912.0520

 

[Jonathan Scott]

There might be a clue from the probable fact that neutron star mass is limited by some process to 2 solar masses.

You're saying this in a way which suggests that you think some mechanism prevents an existing neutron star from becoming any larger. I'm not aware of any evidence for this. What is thought to happen is that there is a threshold mass at which a neutron star will collapse to become a black hole, and there may be intermediate phases at which a neutron star might be transformed to a more dense hypothetical object such as a "quark star".
Also, neutron stars are currently primarily distinguished from any more compact form by X-ray bursts which are thought to be from fusion of accumulated helium (produced by hydrogen immediately fusing to helium when it falls to the surface). I've just started another thread to ask about whether a neutron star might be able to become sufficiently massive that falling hydrogen might have enough energy for much of it to fuse beyond helium immediately, in which case there will be no accumulation of helium to cause an X-ray flash, making it more difficult to distinguish it from a black hole. Here's the thread: https://www.physicsforums.com/threads/x-ray-bursts-might-not-happen-for-larger-neutron-stars.850627/

 

[Jonathan Scott]

Something called "electroweak star" is proposed where neutrons/quarks would be burned to leptons in small core of otherwise "normal" neutron star.
http://arxiv.org/abs/0912.0520

I note that this suggestion violates baryon and lepton number, which contradicts all experimental evidence to date. (But so do black holes).

 

[Bernie G]

You're saying this in a way which suggests that you think some mechanism prevents an existing neutron star from becoming any larger. I'm not aware of any evidence for this. What is thought to happen is that there is a threshold mass at which a neutron star will collapse to become a black hole, and there may be intermediate phases at which a neutron star might be transformed to a more dense hypothetical object such as a "quark star".
Also, neutron stars are currently primarily distinguished from any more compact form by X-ray bursts which are thought to be from fusion of accumulated helium (produced by hydrogen immediately fusing to helium when it falls to the surface). I've just started another thread to ask about whether a neutron star might be able to become sufficiently massive that falling hydrogen might have enough energy for much of it to fuse beyond helium immediately, in which case there will be no accumulation of helium to cause an X-ray flash, making it more difficult to distinguish it from a black hole. Here's the thread: https://www.physicsforums.com/threads/x-ray-bursts-might-not-happen-for-larger-neutron-stars.850627/

"You're saying this in a way which suggests that you think some mechanism prevents an existing neutron star from becoming any larger.": YES!

"I'm not aware of any evidence for this.": The maximum observed mass of neutron stars is thought to be about 2 M☉.

"What is thought to happen is that there is a threshold mass at which a neutron star will collapse to become a black hole": If this is so where does the collapse start? At the core (neutron collapse!)? Or at the surface?

"A neutron star might be able to become sufficiently massive that falling hydrogen might have enough energy for much of it to fuse beyond helium immediately.": I'm digesting your new thread and prefer to wait for other comments. Its an interesting idea. If hydrogen can do it maybe helium could do it. You are suggesting the smaller mass "black holes" might be compact stars? Expect a lot of flak!

 

[Jonathan Scott]

You're saying this in a way which suggests that you think some mechanism prevents an existing neutron star from becoming any larger.": YES!
"I'm not aware of any evidence for this.": The maximum observed mass of neutron stars is thought to be about 2 M☉.

You seem to have missed the point. It is theoretically expected that there will be a maximum possible mass for a neutron star between 1.4 and 3 solar masses, after which the neutron star will collapse to some other state (starting from the core with what could indeed be described loosely as "neutron collapse"). If the initial state is not a black hole, it is expected that only a relatively small further increase in mass would be enough to create a black hole. Regardless of whether the new state is a something like a quark star or a black hole, it could appear very similar to the original neutron star, as the appearance is normally dominated by radiation from the accretion disk.
One difference is that if an object is clearly a pulsar, this is thought to distinguish it from a black hole, as it is not generally thought that a black hole can emit similar pulsed radiation. However, not all neutron stars show pulsar characteristics (although that is normally the easiest way of identifying them), possibly due to viewing them at the wrong angle. (It is of course also theoretically possible that there might be some unknown physics which causes pulsars to switch off above a certain mass, but that doesn't seem relevant here).
Another difference is that neutron stars can produce sudden X-ray bursts which are thought to be from helium fusion in chain reaction. However, the absence of such bursts does not prove that an object is not still a neutron star, especially if it is possible, as I've queried in the other thread, that a sufficiently massive neutron star could cause much of the infalling hydrogen to undergo immediate fusion beyond helium and hence suppress that particular type of X-ray bursts. If this were the case, it would be possible that objects somewhat greater than 2 solar masses could still be neutron stars even though they did not show X-ray bursts, making it more difficult to establish a threshold for black hole formation.

 

[Bernie G]

It is theoretically expected that there will be a maximum possible mass for a neutron star between 1.4 and 3 solar masses, after which the neutron star will collapse to some other state (starting from the core with what could indeed be described loosely as "neutron collapse".

Then when the neutrons at the core collapse, what happens to them? What would be the "other state"? Recent high energy collider experiments indicate that when a nucleus collapses the mass is converted roughly 10+% quark type matter and roughly 90% energy. I think the same would probably happen when some core neutrons collapse. If the resulting quark matter and energy quickly exit the star by some process, pressure is then relieved and core collapse should stop.

 

[Jonathan Scott]

Then when the neutrons at the core collapse, what happens to them? What would be the "other state"? Recent high energy collider experiments indicate that when a nucleus collapses the mass is converted roughly 10+% quark type matter and roughly 90% energy. I think the same would probably happen when some core neutrons collapse. If the resulting quark matter and energy quickly exit the star by some process, pressure is then relieved and core collapse should stop.

Please do not continue to promote this extremely speculative idea which has already been the subject of another thread. I have already pointed out that if you wish to discuss it further, you first need find appropriate references then start a new thread. (In that thread I already pointed out that no additional kinetic energy can be found without violating baryon number conservation, and your hand-waving assertion that such relativistic material could find its way from the core to the surface at such a speed as to escape the gravitational field seems totally fanciful).

 

[Bernie G]

It is theoretically expected that there will be a maximum possible mass for a neutron star between 1.4 and 3 solar masses, after which the neutron star will collapse to some other state (starting from the core with what could indeed be described loosely as "neutron collapse".

What happens to the neutrons which collapse? Do they disappear? Just saying its loosely described as neutron collapse says little. What happens to the mass-energy of the neutrons which collapse?

 

[rootone]

A neutron consists of three quarks bound by gluons, so in principle that is what a disintegrated neutron should turn into.
Whether or not that condition of free quarks and gluons can remain stable is (I think) unknown.
Matter in this state has been hypothesised though for two cases.
Firstly there *may be* a stable condition of this sort when a neutron star collapses instead of collapse directly to a black hole, (hypothetical quark star)
Quark-gluon plasma is also hypothesised as being a possible state of the Universe very shortly after the big bang.

 

[Jonathan Scott]

What happens to the neutrons which collapse? Do they disappear? Just saying its loosely described as neutron collapse says little. What happens to the mass-energy of the neutrons which collapse?

As far as I know, there is very little evidence as to what might happen in this case, but I think the general opinion is that neutrons might collapse into their components, i.e. quarks and gluons, in which case the total mass-energy would remain unchanged, but the density could be somewhat higher. If the pressure was removed, the quarks and gluons would form the original number of neutrons (or protons and electrons) again. If the density is able to increase significantly relative to a neutron star, this would directly trigger collapse to a black hole anyway, otherwise it may be possible to add a bit more mass before that happens.

 

[rjbeery]

Excellent article PeterDonis! Personally, my objection doesn't lay in the math of GR, but in our definition of "existence". In the past when I've pressed on this point the typical response is that the discussion has turned semantic and philosophical. I disagree. I believe the entire subject rests on what we mean, precisely, when we say that something exists*. The definition of this word is where the discussion should be taking place if we want to discuss the subject at all.

*such as a definition of "exists" which applies to a region of spacetime that is not in the causal past of future null infinity

 

[Bernie G]

A neutron consists of three quarks bound by gluons, so in principle that is what a disintegrated neutron should turn into.

From the Wiki article on protons: "it is now known to be composed of three valence quarks: two up quarks and one down quark. The rest masses of the quarks contribute only about 1% of the proton's mass, however.[2] The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together."

It says the quark mass contribution is only 1%(! ) and the rest is energy. Sure sounds like ultra-relativistic stuff to me. Question: Suppose you disintegrated 10 neutrons, resulting in quark matter and energy, then half the energy was lost, and you recombined what was left. Would you have about 5 neutrons?

 

[Martin0001]

From the Wiki article on protons: "it is now known to be composed of three valence quarks: two up quarks and one down quark. The rest masses of the quarks contribute only about 1% of the proton's mass, however.[2] The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together."

It says the quark mass contribution is only 1%(! ) and the rest is energy. Sure sounds like ultra-relativistic stuff to me. Question: Suppose you disintegrated 10 neutrons, resulting in quark matter and energy, then half the energy was lost, and you recombined what was left. Would you have about 5 neutrons?

Quarks formed this way will also form a sort of *degenerate matter*, very much like initial neutrons did but a denser one
Gross of energy will be retained in the system and pressure of degeneracy of degenerate quark matter will take care of it.

 

[Jonathan Scott]

Question: Suppose you disintegrated 10 neutrons, resulting in quark matter and energy, then half the energy was lost, and you recombined what was left. Would you have about 5 neutrons?

The energy is what is keeping the quarks apart (and they cannot be completely isolated). There could also be extra quark / anti-quark pairs produced from interactions involving the excess energy. If you removed enough energy for the quarks to recombine, they would combine back into 10 neutrons or the equivalent in protons (plus leptons). They could in theory initially combine back into heavier particles, for example including strange or charm quarks, but the final result could not be less than 10 protons in rest mass because of baryon number conservation.

(From Special Relativity, if you have a system containing lots of internal kinetic energy but the overall momentum is small, the kinetic energy effectively counts towards the rest mass of the overall system).

 

[rjbeery]

When discussing whether or not black holes exist there are three options that I see:

1) Define "exists" and "black holes" to exclude them from existence
2) Reject the word "exists" completely as an unscientific term
3) Define "exists" and "black holes" to include them in existence

The definition for black holes in the Insight article is given as a region of spacetime that is not in the causal past of future null infinity, and that's the one I'll use here.

Regarding #1, we could try to define "exists" in terms of simultaneity. There are obvious ambiguities in doing this, but it's a start. Events A and B could be said to co-exist if observer C claimed they were simultaneous; we could then say that A exists with B for C...but that's all. Co-existence is relative to an observer; however, this does give us a chance to put bounds on co-existence. If two events A and B are lightlike or timelike separated then there is no observer C who could make the claim that they are in co-existence. Using these definitions, no events A at or within a hypothetical event horizon co-exist with events B outside of the event horizon because there is no observer C who can make the claim that they are simultaneous.

Regarding #2, ambiguity suggests that perhaps "exists" should not enter a scientific discussion. I disagree with this. By the same logic we should not discuss velocity in a scientific context due to its relative nature. In any event, if we choose to reject "existence" completely then we certainly can't also make the claim that black holes exist.

That leaves us with #3. As external observers to any theoretical black hole, we could make the claim that the black hole exists due to the thought experiment of simply imagining observer B traveling to the region of a suspected black hole A and "falling in". The black hole exists for B because he can cross the event horizon A in finite proper time, right? The math is clear on this, but there is baggage with the view that this constitutes existence; namely, it requires a block universe in which timelike separated events are considered to co-exist. There is simply no external observer C who can make the claim that A and B co-exist until B reaches A...and that never happens for C, D, E or any other external observers. If we simply declare that A and B co-exist by fiat, or by definition, then we must also accept that Julius Caesar and Christmas Day of 2100 co-exist. This is a consequence that I doubt most people would accept.

 

[Jonathan Scott]

I think that philosophical discussion about the meaning of "exists" is outside the scope of these forums.

The scientific position as I understand it is that standard GR accurately predicts observations relating to weak gravitational fields and also predicts that extremely dense objects will collapse gravitationally into what we describe as a black hole, which has some rather bewildering and awkward properties. There are modified versions of GR and other gravitational theories which do not predict such collapse, although they are typically far more complex and contrived than GR and hence are less satisfactory, by Occam's razor. We therefore eagerly await experimental confirmation or refutation of whether objects predicted by GR to be black holes actually have the predicted properties.

(Although I accept GR as the best theory we have so far, I personally have a strong suspicion that Einstein's Field Equations might effectively be only a weak field approximation. It would certain stir things up a bit if someone could spot an object soon that GR says should be a black hole but which clearly isn't, such as a pulsar around 30 solar masses!).

 

[Martin0001]

When discussing whether or not black holes exist there are three options that I see:

1) Define "exists" and "black holes" to exclude them from existence
2) Reject the word "exists" completely as an unscientific term
3) Define "exists" and "black holes" to include them in existence

The definition for black holes in the Insight article is given as a region of spacetime that is not in the causal past of future null infinity, and that's the one I'll use here.

Regarding #1, we could try to define "exists" in terms of simultaneity. There are obvious ambiguities in doing this, but it's a start. Events A and B could be said to co-exist if observer C claimed they were simultaneous; we could then say that A exists with B for C...but that's all. Co-existence is relative to an observer; however, this does give us a chance to put bounds on co-existence. If two events A and B are lightlike or timelike separated then there is no observer C who could make the claim that they are in co-existence. Using these definitions, no events A at or within a hypothetical event horizon co-exist with events B outside of the event horizon because there is no observer C who can make the claim that they are simultaneous.

Regarding #2, ambiguity suggests that perhaps "exists" should not enter a scientific discussion. I disagree with this. By the same logic we should not discuss velocity in a scientific context due to its relative nature. In any event, if we choose to reject "existence" completely then we certainly can't also make the claim that black holes exist.

That leaves us with #3. As external observers to any theoretical black hole, we could make the claim that the black hole exists due to the thought experiment of simply imagining observer B traveling to the region of a suspected black hole A and "falling in". The black hole exists for B because he can cross the event horizon A in finite proper time, right? The math is clear on this, but there is baggage with the view that this constitutes existence; namely, it requires a block universe in which timelike separated events are considered to co-exist. There is simply no external observer C who can make the claim that A and B co-exist until B reaches A...and that never happens for C, D, E or any other external observers. If we simply declare that A and B co-exist by fiat, or by definition, then we must also accept that Julius Caesar and Christmas Day of 2100 co-exist. This is a consequence that I doubt most people would accept.

All fine, but why to make issue complicated, if one can keep it simple?
Let's define a BH as a compact object, detectable only by its immense gravity which does not have a hard surface up to the area where EH should be present.
Such object should only give away to our investigation few details like mass, angular momentum and electric charge but only information about mass is easy to access.
Forget what is inside, so for example object with mathematical singularity as well as one with hard surface of compact star just below EH are both BH.

Now let's get more observations and find out if existing candidates meet these objections.
So for example if we are finding that candidates are showing magnetic fields which cannot be explained by accretion discs we must consider them not to be BH.
There are several classes of such objects, which might pretend to be BH but they are not.
So let's exclude some objects like compact stars made of degenerate matter of any sort, then exclude MECO, then exclude "fuzzballs", gravastars etc.
Once these are excluded let's assume that we have "proven" BH to exist, but *if* we have proven that object is MECO, fuzzball, gravastar or something alike, let's assume that BH do not exist.

Let's hope that current efforts to produce images of Sag. A by arrays of radiotelescopes will deliver some more information in this respect.

 

[rjbeery]

I think that philosophical discussion about the meaning of "exists" is outside the scope of these forums.

Outside the scope of a thread entitled "Do Black Holes Really Exist"?

 

[maline]

Events A and B could be said to co-exist if observer C claimed they were simultaneous

The idea that an observer "claims" that two events are simultaneous- meaning that there is a well-defined inertial frame centered on that observer- is true in SR & locally in GR but not in situations involving large distances & gravity. Any coordinate map you define on spacetime has some objects with coordinate acceleration that does not come from any force. So to ask whether a spacelike separated object "exists now" is really and truly meaningless- if you like, and work at it long enough, you can draw up a map that will label your point of interest with the same time coordinate as your present. It will be just as valid as any other map, so long as you work out all the Christoffel symbols etcetera whenever you want to predict anything. As far as I know you can do this for the interior of a BH as well, as long as it's not in your future light cone.

 

[jambaugh]

Unquestioned belief in existence of BH displayed by many here is breath taking.

What we can say is that some ultradense objects which do not appear to possesses a surface are detected.
Of course there might be a very red shifted surface indeed and such an object would not be a BH.
There are few credible alternatives indeed...

Your remaining post not withstanding, here's my position. There are 2 questions:
i. Is GR right or mostly right? (e.g. to the extent that its prediction of BH's requires they exist in galactic centers given the amount of mass there.)
and
ii. Do Black Holes Exist? (as a more general question than above.)

The first question we can consider by doing other experiments and so in so far as GR has been confirmed by evidence, there you go.
Since the original concept of a BH was as a prediction of GR to answer the second question which I take to be the OP's question, I would argue that "some ultradense objects which do not appear to possesses a surface" comes mighty close to an "if it quacks like a duck" definition of a BH. However as this doesn't quite address this begged question I've pointed out then we really should answer that one, first...

If we wish to generalize the idea to theories distinct from Einstein's GR, what do we mean by a "Black Hole"?

Keep in mind also that when we speak of curved space-time this is a MODEL for GR and other such theories of gravity, not the theory itself. So do not try to describe a BH in terms of the geometry of invisible space-time but rather in terms of what is or can be observed. (e.g. one cannot communicate observations across an event horizon.)

But Martin, my belief in BH's is not "unquestioned" any more than is my belief in quarks or the Higgs particle or even the photon itself. All my beliefs are tentative subject to change with new data. Now if you want to speak of unquestioned beliefs we could open a discussion on Anthropogenic Global Warming! ;)

 

[rjbeery]

The idea that an observer "claims" that two events are simultaneous- meaning that there is a well-defined inertial frame centered on that observer- is true in SR & locally in GR but not in situations involving large distances & gravity. Any coordinate map you define on spacetime has some objects with coordinate acceleration that does not come from any force. So to ask whether a spacelike separated object "exists now" is really and truly meaningless- if you like, and work at it long enough, you can draw up a map that will label your point of interest with the same time coordinate as your present. It will be just as valid as any other map, so long as you work out all the Christoffel symbols etcetera whenever you want to predict anything. As far as I know you can do this for the interior of a BH as well, as long as it's not in your future light cone.

C's choice of coordinates and simultaneity calculation is irrelevant because the only thing that matters in my argument is causality. No valid choice of coordinates can change causal ordering.

 

[rjbeery]

Your remaining post not withstanding, here's my position. There are 2 questions:
i. Is GR right or mostly right? (e.g. to the extent that its prediction of BH's requires they exist in galactic centers given the amount of mass there.)
and
ii. Do Black Holes Exist? (as a more general question than above.)

My position is that GR does not predict that BH's exist in galactic centers given the amount of mass there. I'm claiming that GR could be perfectly correct but that we are misinterpreting (or simply not clarifying precisely) what we mean by "exists".

 

[jambaugh]

My position is that GR does not predict that BH's exist in galactic centers given the amount of mass there. I'm claiming that GR could be perfectly correct but that we are misinterpreting (or simply not clarifying precisely) what we mean by "exists".

At the classical level (where this discussion currently resides) a (categorical) objects existence is quite meaningful. An object of category X exists if a physical object has been observed with properties similar to one of category X so that current orthodox theory forbids it being other than of category X. By orthodox theory I mean theory which has as yet not been overturned by virtue of empirical evidence and which has already supplanted a prior theory the same way.

Note the word "exists" here is rather more like "is actualizable" i.e. "can exist". For example it is not that the Tooth Fairy doesn't exist because she has been killed last week, but rather her existence is not possible.

(Now if you want to get into whether the singularity of a BH exists we got issues since we are getting into the quantum domain where ontology breaks down and we have to get phenomenological.)

Now I am not above playing semantics games and we should be clear that we agree to the meaning of the words we use, but... Stars exist. Roc's do not exist. Black holes? Well that depends on the precision of the latest observations and your degree of skepticism. The M87 observations: http://arxiv.org/abs/1503.03873 point to a mass sufficiently large and compressed that infalling matter is not impacting a surface outside the calculated S. Rad.

Quack Quack Quack!