What is a Black Hole? Do Black Holes Even Exist?
What is a black hole?
A black hole is a region of spacetime centred on a point mass called a singularity. A black hole is extremely massive and thus has an immense gravitational pull, which is in fact strong enough to prevent light escaping from it.
A black hole is surrounded by a membrane called an event horizon. This membrane is just a mathematical concept; there is no actual surface. The event horizon is simply a point of no return. Anything that crosses the event horizon is doomed to be sucked towards the singularity – the point mass at the centre of the hole. Nothing  not even a photon of light  can escape a black hole once it has crossed the event horizon because the escape velocity beyond the event horizon is greater than the speed of light in a vacuum. This is what makes a black hole “black” – light cannot be reflected from it.
A black hole is formed when a star above a certain mass reaches the end of its life. During their lifetime, stars "burn" vast quantities of fuel, usually hydrogen and helium at first. The nuclear fusion carried out by the star creates pressure, which pushes outward and stops the star from collapsing. As the star runs out of fuel, it creates less and less outward pressure. Eventually, the force of gravity overcomes the remaining pressure and the star collapses under its own weight. All the mass in the star is crushed into a single point mass – a singularity. This is a rather strange object. All the matter that made up the star is compressed into the singularity, so much so that the volume of the singularity is zero. This means that the singularity must be infinitely dense since the density of an object can be calculated as follows: density = mass/volume. Therefore a finite mass with zero volume must have an infinite density.
Because of its density, the singularity creates a very strong gravitational field that is powerful enough to suck in any surrounding matter it can get its hands on. In this way, the black hole can continue to grow long after the star is dead and gone.
It is thought that at least one supermassive black hole exists at the centre of most galaxies, including our very own Milky Way. It is thought that these black holes played a key role in the formation of the galaxies they inhabit.
It was theorised by Stephen Hawking that black holes emit small amounts of thermal radiation. This theory has been verified, but unfortunately it cannot be directly tested (yet): the thermal radiation – known as Hawking radiation – is thought to be emitted in very small quantities that would be undetectable from Earth.
Has anybody ever seen one?
That’s a slightly misleading question. Remember, the gravitational pull of a black hole is so strong that light cannot escape from it. And the only reason we can see things is light be emitted or reflected from them. So, if you ever saw a black hole, that’s exactly what it would look like: a black hole, a chunk of space devoid of light.
The nature of black holes means that they do not emit any signals – all electromagnetic radiation (light, radio waves etc.) travels at the same speed, c (approximately 300 million meters per second and the fastest speed possible) and is not fast enough to escape the black hole. Thus, we cannot ever directly observe a black hole from Earth. You can’t observe something that won’t give you any information, after all.
Luckily, science has moved on from the old idea of seeing being believing. We can’t directly observe subatomic particles, for example, but we know they’re there and what properties they have because we can observe their effects on their surroundings. The same concept can be applied to black holes. The laws of physics as they stand today will never allow us to observe anything beyond the event horizon without actually crossing it (which would be somewhat fatal).
If we can't see black holes, how do we know they're there?
If electromagnetic radiation can’t escape from a black hole once it’s over the event horizon, how can we possibly observe one? Well, there are a few ways. The first is called “gravitational lensing”. This happens when light from a distant object is made to curve before it reaches the observer, much the same way a light is bent in a contact lens. Gravitational lensing occurs when there is a massive body between the light source and a distant observer. The mass of this body causes spacetime to be “bent” inwards around it. When the light passes through this area, the light travels through the curved spacetime and its path is altered slightly. It’s a strange idea, isn’t it? It is even stranger when you appreciate the fact that the light is still travelling in straight lines, as light must. Hold on, I thought you said the light was bent? It is, sort of. The light travels in straight lines through curved space, and the overall effect is the light’s path is curved. (This is the same concept you observe on a globe; straight, parallel lines of longitude meet at the poles; straight paths on a curved plane.) So, we can observe the distortion of light and deduce that a body of some mass is lensing the light. The amount of lensing can give an indication of the mass of said object.
Similarly, gravity affects the movement of other objects, not just the photons that comprise light. One of the methods used to detect exoplanets (planets outside our solar system) is to examine distant stars for “wobbles”. I’m not even kidding, that's the word. A planet exerts a gravitational pull on the star it orbits, pulling it out of place ever so slightly, "wobbling" the star. Telescopes can detect this wobble and determine that a massive body is causing it. But the body that causes the wobble need not be a planet. Black holes can have the same effect on the star. While the wobble might not mean a black hole is close to the star, it does prove that there is a massive body present, allowing scientists to focus on finding out what the body is.
Spitting Out Xrays  Matter Accretion
Clouds of gas fall into the clutches of black holes all the time. As it falls inwards, this gas tends to form a disc – called an accretion disc. (Don’t ask me why. Take it up with the law of conservation of angular momentum.) Friction within the disc causes the gas to heat up. The further it falls, the hotter it gets. The hottest regions of gas begin to get rid of this energy by releasing enormous amounts of electromagnetic radiation, usually Xrays. Our telescopes might not be able to see the gas initially, but accretion discs are some of the brightest objects in the universe. Even if the light from the disc is blocked by gas and dust, the telescopes can most certainly see Xrays.
Such accretion discs are often accompanied by relativistic jets, which are emitted along the poles and can create vast plumes which are visible in the Xray region of the electromagnetic spectrum. And when I say vast, I mean that these plumes can be bigger than the the galaxy. They’re that big. And they can certainly be seen by our telescopes.
All the black holes
It should come as no surprise that Wikipedia has a list of all known black holes and systems thought to contain black holes. If you'd like to see it (warning: it's a long list) click here.
Do black holes really exist?
“What is real? How do you define real?”
Matrix theories aside, I think we can safely say that anything we can detect is there. If something has a place in the universe, it exists. And a black hole certainly has a “place” in the universe. Indeed, a singularity can only be defined by its location, because that’s all a singularity is. It has no magnitude, only a position. In real space, a point mass like a singularity is pretty much the closest we can get to Euclidian geometry.
Trust me, I wouldn’t have spent all this time telling you about black holes just to say they weren’t actually real. But the point of this hub was to explain why we can prove black holes exist. That is; we can detect them. So, let’s remind ourselves of the evidence that points to their existence.
 They are predicted by theory. The first step in having something recognised as being true is to say why it is true. Karl Schwarzschild created the first modern resolution of relativity that would characterise a black hole in 1916, and later work from many physicists showed black holes are a standard prediction of Einstein’s theory of general relativity
 They can be indirectly observed. As I explained above, there are ways of spotting black holes even when we are millions of light years from them.
 There are no alternatives. Very few physicists would tell you there are no black holes in the universe. Certain interpretations of supersymmetry and some extesnions of the standard model allow for alternatives to black holes. But few physicists support the theories of possible replacements. In any case, no evidence has ever been found to support the weird and wonderful ideas put forward as replacements for black holes. The point is, we observe certain phenomena in the universe (accretion discs, for example). If we don't accept that black holes are causing them, we must have an alternative. But we don't. So, until we find a convincing alternative, science will continue to assert that black holes exist, if only as a "best guess".
I think we can therefore take it as read that black holes do exist. And that they are extremely cool.
Thank you for reading this hub. I really hope you found it interesting. If you have any questions or feedback, please feel free to leave a comment.
Questions & Answers
Comments
Dosent gravity escape
A BLACK HOLE....
You did a superb job on this hub; you really did your homework! You have explained black holes very well. I personally do not believe in infinite density, but I suppose the equations that have taken the scientists to this conclusion.
BTW: In case you forgot my Hubs is entitled:
Do Black Holes Exist?
Look forward to continuing our conversation there.
"[snip] you have claimed that black holes don’t exist."
The positive claim is that "black holes exist." To which I respond..Nope.
You came to my Hub and said this:
"To be clear, black holes do exist. They are predicted by theory and have been directly observed. "
Anyway, the person that makes a claim, hypothesis or theory has to make their case. The onus is on them. Are you telling me to prove a negative?
Now, unless you have counterpoints to the refutation of your claims, then game over.
Anyway, I came over at your invitation (remember?).
I hate to sound rude, but really... you need to try to stay focused.
You may need to focus on law and forget physics for a while. You seem to be having trouble juggling the two.
I'll return to my Hub now, but you are still welcome to come over there and make your case. Take Care.


NOTE: The use of math and authority in this parody is to highlight the fact that WE should not appeal to math and authority. Refer to my Hubs: The REAL Scientific Method and How Theist Can Destroy Big Bang, Existence, A Matter of Science, and There Are No Authorities?
Exhibits for the prosecution:
Exhibit A: Fails conceptually
Exhibit B: Fails at basic math
Exhibit C: Fails at higher math
Now for the prosecution's summary. Not only does the council for the defense fail to prove Black Hole is possible, let alone exists, the prosecution has presented evidence that black hole is founded on failed hypotheses and in opposition to accepted theories.
Black Hole fails on many levels; it fails at the conceptual level (is illogical and irrational). Infinities are impossible and so is zero volume. Black Hole fails at simple math one can not divide by zero. Black Hole fails at higher math, in violation of Einstein's relativity, Schwarchild's solution and classical Newtonian physics, both superposition and escape velocity equations.
All this is confirmed by Einstein
http://www.jstor.org/discover/10.2307/1968902?uid=...
and Schrawzschild and born out by other physicists which have provided expert testimony in their stead.
In conclusion, the defense having not made a case, and the prosecution having shown beyond a reasonable doubt Black Holes are
impossible, the jury must find the Defendant, Black Hole guilty as charged, and must find that Black Hole is nonexistent!
BUT I don't rest my case. There is much, much, more and we must continue the discussion where it began, on my HUb pages.
Your Honor, Ladies and Gentleman of the jury, people of the court. The prosecution alleges that Black Hole is a fraud concocted by priests, promoted by the Rock Stars of physics, and perniciously perpetuated against science and society.
The prosecution has shown that Black Hole fails at every level. Singularities fail conceptually, as zero volume means no length, width or height an irrational proposal with no corollary in reality. Infinite density fails even the most basic math as one can not divide by zero as required by the simple math formula of density, mass and volume, and infinities can not exist in reality regardless of what mathematicians do with their 'higher math.' Black Hole also fails at the higher level of Newtonian and Einsteinian theory and corresponding maths. Black Hole can only be "seen" due to socalled gravitational lensing, which is circumstantial at best, and then only as an ad hoc presentation in lieu of observation of an event horizon or Black Hole itself.
Let's review the case.
Defense Exhibits
Exhibit A: Predicted by Theory
Exhibit B: They can be indirectly observed
Exhibit C: There are no alternatives
The defense alledges that Black Hole is predicted by theory. Their Exhibit A states thusly:
"Karl Schwarzschild created the first modern resolution of relativity that would characterise a black hole in 1916, and later work from many physicists showed black holes are a standard prediction of Einstein’s theory of general relativity"
This is clearly not so as attested to by Karl Schwarzschild himself, in Mr. Schwarzschild's paper entitled On the Gravitational Field of A Mass Point According to Einstein's Theory, and confirmed by Leonard Abrams' paper Black Holes: The Legacy of Error.
Einstein's Special Theory of Relativity clearly prohibits infinite densities. Karl Schwarzschild attests to this fact proving it in his paper on point mass. Not only this, Einstein denied the possibility of black holes multiple times before his death in 1955. Both Einstein Relativity and Schwarzschild's solution theories forbid infinite densities. So does common sense and rationality. Although math can postulate infinite densities in abstraction, reality is having none of it. While Hilbert, can
build Hotels in thought space with an infinite number of rooms, the world of reality does not comply. Though Zeno in his Dichotomy Paradox can halve a distance infinitely (in abstract mathematical equation space) one can not walk halfway to a brick wall indefinitely, and certainly can not halve their distance an infinite number of times. All who have tried end up smacking their foreheads to the wall.
Furthermore, later work in the 40's conveniently ignored relativity and erroneously posited an "infinity of spacetimes differing as to the limiting acceleration of a radially approaching test particle." In other words, Hilbert substituted a variable with A scalar invariant transforming the coordinate location of a point mass. Because of the error the point r=0 becomes a twosphere invalidating Hilbert's assumption.
Owing to the extreme difficulty of the calculus and other maths involved our expert witnesses have reduced the equations to
simpler language for the layperson. There are no known solutions for Einstein's field equations for two or more bodies, and yet proponents of Black HOle alledge multiple masses interacting with each other and matter. The principle of superposition applies to Newtonian masses but not to General Relativity so Newton's escape velocity can not be used in an expression relating to a universe containing only one mass. Einstein's theory, as Schwarzschild shows, pertains to one mass. In other words, Newton's theory contains two masses and superposition. However, r=0 contains no bodies and so therefore can not
accommodate superposition.
The defense exhibit A: Predicted by Theory, fails on multiple counts.
The defense states that Black Hole can be indirectly observed. The socalled evidence sited is that because light can escape Black Hole's event horizon, one must look for gravitational lensing. Of course, as the prosecution has already shown, Newton's escape velocity has to relevence in a universe of only one mass, as required by Einstein and Schwarzschild's theories.
The idea of an escape velocity of light also fails logically as previously stated. If the escape velocity from an event horizon is the speed of light then light can escape an event horizon.
Although eyewitness accounts can be effective in swaying a juror unknowledgable in these matters, it is well known that eyewitnesses are among the least reliable sources.
http://www.sciencedaily.com/releases/2011/01/11012...
http://www.apa.org/monitor/apr06/eyewitness.aspx
This is worse than that because we have witnesses claiming they saw where alledged Black Hole are circumstantially. Furthermore, no one has ever found or photographed an event horizon or a black hole, so instead have provided artist renditions of a black hole, or telescopic images of unidentified objects ad hoc in an attempt to save their failed theories.
Therefore the defense exhibit B:They can be indirectly observed fails on multiple counts.
The defense presents Exhibit C: There are no alternatives.
We are told "Very few physicists would tell you there are no black holes in the universe." The defense admits there are alternative theories to Black Hole when council states, "Certain interpretations of supersymmetry and some extensions of the standard model allow for alternatives to black holes." and yet we are reminded "But few physicists support the theories of possible replacements." Very few scientists would tell you once that the earth was round, but that does not make the earth flat.
The burden of proof falls on the one making a claim, not on another person who forms a different conclusion. Since the prosecution has shown the premises to be false, it follows that the conclusion is false. Furthermore it is a fallacy, a nonsequiter, it does not follow that because we don't have an alternative, that a theory, let alone an invalid theory, is correct. Regardless, in science, when the hypothesis or theory fails, one erases the whiteboard and starts over.
Gravitational lensing is supposedly caused by dark matter itself an unproven and irrational proposition at the hypothesis level. It's not even a theory yet!
Lastly, there ARE multiple theories of gravity. Therefore gravitational lensing is not a settled issue to begin with. Not only that but no satellite including the the gravitational wave observatory LIGO has ever detected a gravitational wave. Additionally, Gravity Probe B did not confirm frame dragging (distortion of space time around a large body) the folks there used a hypothetical model to show why they didn't find anything and then years later altered the data to claim they proved frame dragging.
Therefore the defense Exhibit C fails on multiple levels.


Haha thanks. I think I'm going to have to check out of this debate before we start quoting The Matrix. It's been a fascinating read.
Salvage...I meant salvage a dying theory. Actually it was dead at the hypothesis.
"You want to solve the problems of black holes using rationality. Okay. I like that."
It'll have to wait until I get back from out of town. Maybe you and tphelan can go over the assumptions of Black Hole's existence, by first defining existence.
Right, I meant that all of our understanding of the universe, all the math and physics suggest their existence and we make a rather large amount of assumptions based on their existence. We have yet to concretely "prove" their existence by direct observation, which is usually considered the most simplest and reliable methods of testing. Long story short: they're there, we're smart enough to know they're there.
"Of course there can be disagreement in science."
Yes, but we never agree to disagree. We point out the ambiguity and irrationallity, etc.
"Before you get too hung up on the Schwarzschild solution, it’s worth remembering that it only applies to Schwarzschild black holes (noncharged, nonrotating, boring). Also, does it matter that he was dead? Newton hasn’t been around for quite a while, but Einstein had no problem saying why the guy wasn’t quite right."
The issue is that Einstein's relativity, and Schwarzschild's solution both forbid black holes, yet this is not what we are being told.
You rephrased wiki reference to Schwarzschild which is entirely incorrect:
"They are predicted by theory. The first step in having something recognised as being true is to say why it is true. Karl Schwarzschild created the first modern resolution of relativity that would characterise a black hole in 1916, and later work from many physicists showed black holes are a standard prediction of Einstein’s theory of general relativity."
Firstly, let's get something out of the way. Science never states that something is true, only that it is possible. Let's just drop that word from the conversation. Theories offer explanations for things that point out the possibility of something, not the certainty or inevitability. In science we use precise terms in our presentation, and never claim truth, because we know better explanations may come along. This is part of the selfcorrecting process of the scientific method.
Not only does relativity forbid black holes because infinite densities are not allowed, Schwarzschild showed that relativity forbids black holes in his own paper that has been conveniently forgotten:
http://www.sjcrothers.plasmaresources.com/schwarzs...
"I suspect you’re right: noone reading this is that interested in mathematical equations. (That said, feel free to elaborate on your point about the nonexistent radius, r)."
See the above paper for the math. If you are really interested I can point you to some scholarly work about that.
Schwarzschild's papers DO NOT predict an event horizon. The notion of a radius of the black hole event horizon is patently false.
"So let’s try it your way. You want to solve the problems of black holes using rationality. Okay. I like that. But it means you have to think about a couple of things. First, physics isn’t very rational when you get right down to it. I’m talking to you, quantum mechanics."
Our senses may be limited, but our intellect is not. We have the ability to critically think and rationally explain anything. Quantum Strangeness is a copout and don't buy into it!
"Second, if we assume for a moment that black holes do not exist, we have a lot of explaining to do. There are certain phenomena observed in theory and  more importantly  in reality that black holes explain. Take them away, and we have to find other explanations for the phenomena."
We never assume a negative in science. 'Observed in theory' is an irrational statement. Theories explain hypotheses. Black Holes are ad hoc explanations for failed theories and for unidentified 'objects' observed by astro physicists through their telescopes. Since no one has ever observed an event horizon, or a black hole, this is an attempt to savage a dying theory.
Yet the theory fails on so many levels, it should have been scrapped a long time ago. We don't look for 'evidence' and then offer ad hoc explanations building our theories around them.
"By the way, don’t worry about the density thing. As you rightly said, density = mass/volume. First we must recall that the mass of a singularity is finite. As the volume decreases to zero, the mass is still constant. Therefore the density must increase to infinity. It’s annoying, I’ll grant you, but that’s maths. It’s worth remembering that physicists tend to have degrees in mathematics. They know the rules better than most."
This is the kind of thing we hear the rock stars of physics tell us all the time. People like Kaku, and Greene, and Hawking would like very much for the rest of us to "not worry" certain violations of math and reason. After all, they "know better than most." The thing is Nes, we can not divide by zero, there is no such thing as a volumeless entity, and infinite anything is impossible in reality.
"PS. You never did say where @tphelan88 said black holes didn’t exist."
What I said was he said, "there was no proof."
Second paragraph: "Black holes were first suggested as far back as the late 1700's but their existence was solidified, but still not proven, by Einstein's Theory of General Relativity."
BTW it was a clergyman that suggested their existence. Hmmm???? Just as a priest invented the Big Bang theory to please the pope. Anything is possible with God.
So, not only does relativity forbid black holes as does the Schwarzschild solution, BH's fail at the most basic level of math:
OK, here is one freebie for you. I'll not return unless you ask me to.
A black hole is supposed to be a gravitational collapse of matter into a single point mass of zero volume. In school we all learned that density is defined as the mass of an object divided by the volume of an object.
Since the volume of a singularity is zero and the mass is not zero we have to divide by zero. Grade school children understand that this is a violation of the rules by mathematics. Yet this is exactly what black hole proponents are doing.
"So, I've gone to some considerable effort to explain why we know black holes exist."
There is no such thing as agreeing to disagree in science. A theory is either possible or not possible. If you wish to hold yourself out as a person who is knowledgeable on Black Holes, you should expect to receive some criticisms.
OK, you are just repeating some incorrect assumptions that came about based on the fact that most people can not understand the mathematical descriptions offered up for the magical mysterious black hole. This is not a settled issue for science as you would have us believe by throwing around the words true and proof.
I have explained to you rationally how black holes are not possible on one of my Hubs. However, If you would like I am happy to go into some greater detail here for your benefit and for the benefit of your readers.
But for now let's look at your number one reason why you think BH's are 'true.'
You state:
"Karl Schwarzschild created the first modern resolution of relativity that would characterise a black hole in 1916, and later work from many physicists showed black holes are a standard prediction of Einstein’s theory of general relativity."
Yes, much later in the 60's (and Einstein wasn't there to defend his theories. BTW, he did not think that BH's were possible and Schwarzschild didn't even think of the possibility of a collapsed star).
I could tell you that the proponents of the Black Hole are making much to do about the 'r' in the "Schwarzschild solution." The claim is that r is the radius. This is not so because it is not even a distance in Schwarchild spacetime. I could tell you "r plays the role of the inverse square root of the Gaussian curvature of the spherically symmetric geodesic surface in the spatial section of Schwarzchild spacetime and so does not in itself denote any distance whatsoever in Schwarzschild spacetime." That makes it unnecessary to go over the complicated mathematics and renders the black hole invalid. I could continue, but neither you or your readers are likely to understand it.
The black hole came out of a solution of field equation Ric=0 which is a spacetime with no matter. This BH is called the Schwarzschild black hole even though it is not his solution nor does Schwazschild's actual solution contain a black hole.The imaginary universe is constructed without any matter and therefore there is nothing for it to interact with.
Schwarzschild is rolling over in his grave at the use of his name and misappropriation of his theory.
So now you are forced to research what it is I am talking about, check the math for yourself, or accept my authority on it. None of this is practical or advisable, so let me offer up another solution. We can think of this rationally. We can solve the issues of singularities, point mass, and infinities on a conceptual level with no need for complicated math or language. Just say the word, and I'll come back and do this for you.
"Escape velocity is the speed you must *exceed* to escape. "
Sorry, but no.
Here is the definition (my appeal to authority and popularity) just so you don't think I made it up:
"The minimum speed that an object at a given distance from a gravitating body must have so that it will continue to move away from the body instead of orbiting about it. "
http://dictionary.reference.com/browse/escape+velo...
(Anyways nothing can go faster than light according to relativity, so an escape velocity of greater than sol doesn't even make sense).
As tphelan says in his article on BHs, there is no proof of black holes.
If we are going to depend on observation for our 'proof' since no one has ever found an infinitely dense point mass singularity, or an event horizon, no one has ever found a black hole. No event horizon, no singularity, no proof.
NO you didn't explain how escape velocity works. Escape velocity is the velocity needed to escape. If the escape velocity of a black hole is the speed of light, then light would escape. If light can not escape the black hole, then the escape velocity is not the speed of light. Can't have it both ways.
The laws of physics break down inside of a black hole. All we are able to do with the singularity is theorize. Einstein's theories predict the existence of the very thing that could unravel them. Kind of poetic of the universe I think.
Black hole theory claims an infinitely dense point mass singularity, yet Special Relativity forbids infinite mass.
But so does rationality. There is no actual point mass. It is an abstract mathematical concept, as is infinity. These do not exist in reality.
"No, because you're being pushed downriver at the same rate you move upriver. "
Well, then the black hole has no escape velocity.
"Nothing  not even a photon of light  can escape a black hole once it has crossed the event horizon because the escape velocity is equal to the speed of light in a vacuum. This is what makes a black hole “black” – light cannot be reflected from it."
If the escape velocity is the speed of light, then pray tell, why doesn't light escape?
There's a really exciting opportunity for astronomers to study the effects of black holes coming up. A giant gas cloud is approaching the supermassive black hole at the center of our galaxy called Sagittarius A*. The gas cloud will pass about 40 billion km away from Sgr A* (about 36 light hours) sometime mid2013 and we'll be able to directly observe what happens to it!
Awesome Nesbyte. I love your idea of a photograph of a black hole.
Fascinating! I had heard of black holes before but this hub explains the concept in an accessible way. Thanks for the primer!
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