# Insights Do Black Holes Really Exist? - Comments

1. Dec 27, 2015

### Staff: Mentor

2. Dec 27, 2015

### 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!

3. Dec 27, 2015

### 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.

4. Dec 28, 2015

### _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.
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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.

Last edited: Dec 28, 2015
5. Dec 28, 2015

### 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).

6. Dec 28, 2015

### 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.

7. Dec 28, 2015

### QuantumQuest

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.

8. Dec 28, 2015

### Greg Bernhardt

Great Insight Peter!

9. Dec 28, 2015

### Bernie G

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 ?

10. Dec 28, 2015

### Staff: Mentor

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

11. Dec 28, 2015

### _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 concensus 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.

12. Dec 28, 2015

### Bernie G

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

13. Dec 28, 2015

### _PJ_

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)

14. Dec 28, 2015

### Bernie G

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?

15. Dec 28, 2015

### _PJ_

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.

16. Dec 28, 2015

### Bernie G

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?

17. Dec 28, 2015

### 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.

18. Dec 29, 2015

### _PJ_

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

19. Dec 29, 2015

### Bernie G

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

20. Dec 29, 2015

### 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?