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).