Do Black Holes Really Exist?

In Black Holes by Brian Koberlein22 Comments

“Black holes don’t exist!” It’s a popular comment made nearly every time I write about black holes. Often such claims come from folks who also don’t believe in things like relativity or the big bang, but another group has a more subtle argument: black holes haven’t yet been proven. In a way, they have a point.

There are basically two lines of evidence to support the existence of black holes. The first is observational. For small (stellar mass) black holes, the best evidence is through micro-quasars, also known as x-ray binaries. These objects emit strong x-rays from an Earth-sized region in space. Since these objects are part of a binary system with another star, we can determine their mass by the way the two stars orbit each other. What we find is these dense objects have masses that range from 1 – 10 times the mass of our Sun.

An example of differential rotation.

An example of differential rotation.

Stellar mass black holes are expected to emit strong x-rays when material near the black hole gets super heated due to all the gravitational compression. So this is exactly what we expect from a black hole. The problem is that neutron stars can also emit strong x-rays because they also have strong gravitational and magnetic fields. But it turns out that neutron stars, like our Sun undergo an effect known as differential rotation. Instead of rotating like a solid object, the equator region of a neutron star makes a complete rotation in less time than their polar regions. As a result their magnetic fields get twisted up until they snap back into alignment. For the Sun differential rotation leads to things like sunspots and solar flares. A similar effect occurs for neutron stars. As a result, some x-ray binaries are known to have differential rotation, and there therefore neutron stars. Other x-ray binaries don’t undergo differential rotation, so they are known as black hole candidates (BHCs).

Animation of stellar orbits near galactic center.

Animation of stellar orbits near galactic center.

We have similar observational evidence for supermassive black holes. For example, we know that quasars can emit more light than 250 billion stars from a region no larger than a light year across. When we observe the motions of stars near the centers of galaxies, they reveal the presence of a dense mass on the order of millions or billions of solar masses. In some cases we can even determine the mass of these central objects with great precision. But some of the strongest evidence comes from our own galaxy. With modern telescopes we’re able to image stars in the center of our galaxy. Over the years we’ve watched these stars as they clearly orbit a large dense mass. We know that this object has a mass of 4.1 million solar masses, and that all this mass can be no larger than our solar system (about 100 astronomical units across to be precise).

The other line of evidence is theoretical, specifically the theory of general relativity. Einstein’s theory of gravity makes very clear predictions about the motions of planets, how light is affected by gravity, gravitational redshift, the timing of GPS, and even the twisting of space and time. Every experimental test we’ve tried so far, general relativity has passed.

General relativity also makes several predictions about black holes. One is that given enough mass, an object will collapse into a black hole. This is true no matter what the mass is made of, since any energy or force trying to prevent the collapse actually starts helping gravity more than it opposes it. As we’ve seen in an earlier post, that upper limit is around 2.5 – 3 solar masses. Interestingly, of all the x-ray binaries we’ve observed, differential rotation has only been observed in ones less than 2 solar masses. The smallest black hole candidate (with so such differential rotation) has a mass of about 3 solar masses.

Another clear prediction is that black holes don’t have visible surfaces. Black holes are so dense that light can’t escape them. Surrounding a black hole is a “distance of no return” known as the event horizon. Anything that crosses that line is forever trapped. While we’ve not yet observed an event horizon, we do have indications that at least one supermassive dense object does not have a surface. A team looked at matter falling into the supermassive object at the center of the M87 galaxy. Unlike the small 4-million solar mass object in our galaxy, M87’s object has a mass of 6 billion suns. If this object had a surface, then matter falling into the object would strike the surface, and the energy released would cause a burst of light. If the object doesn’t have a surface, then there wouldn’t be a secondary brightening as the matter falls into it. The team found no indication of a secondary brightening.

So we know very clearly that neutron stars exist, and that somewhere between 2 – 3 solar masses they switch from being neutron stars to something that looks like a black hole. General relativity predicts that beyond 2.5 – 3 solar masses these objects must be black holes. We know that supermassive objects exist in most galaxies, and there is evidence that such objects do not have a visible surface. Again, according to general relativity, these objects should be black holes. In addition, there’s been a great deal of work modeling the dynamics of black holes within galaxies, and these models are in good agreement with the dynamics we observe. So there is a wealth of evidence to support the existence of black holes, and this is why most astronomers feel that black holes exist.

But there are some who would argue that all of this is still insufficient. All we’ve shown is that either black holes exist, or there are dense objects that closely approximate black holes. Perhaps these objects are all really dense, but don’t have an event horizon. For example the black hole in our galaxy would need to be the size of Earth’s orbit, which is a hundred times smaller than the observational limit thus far. All this evidence implies the existence of black holes, but it doesn’t prove black holes exist.

While technically that’s true, it doesn’t gain you much. Any such dense object would violate both the standard model of particle physics and general relativity, which are deeply robust theories. So technically the claim is either these things are black holes, or they are objects that mimic black holes by violating known physics. Of course that could be said about anything. Either electrons exist, or they are objects that mimic electrons through unknown physics. Either neutron stars exist, or they mimic neutron stars through unknown physics. It becomes a game of the “science of the gaps” where one can always demand just one more piece of evidence before they are truly convinced.

So the broad consensus is that black holes definitely exist. There are some astronomers who prefer to withhold judgment until we resolve the region around an event horizon. There are projects in development such as the Event Horizon Telescope which hope to achieve such observations. Even then, such observations will be compared to the predictions of general relativity. If the EHT makes observations in agreement with general relativity, then we can say either there’s an event horizon, or there’s something that mimics an event horizon in a way that defies known physics. Its always a question of how far down that rabbit hole you want to go.

So it’s absolutely clear to me that black holes exist. But as with anything, I could be wrong.

Paper: Avery E. Broderick et al. The Event Horizon of M87. ApJ 805 179 (2015)

Comments

  1. A point I’d like to add is that black holes are the conservative answer to things like quasars. For the immense energy output in a small area would require exotic new physics such as anti-matter for which there is no evidence. Another point to make is – white dwarves and neutron stars definitely exist, we know degeneracy pressure can only withstand about 3 solar masses of gravitational pressure so a person denying the existence of black holes has to answer – what happens then?
    Personally, I think, once pulsars and neutron stars were definitely identified then the case for black holes became overwhelming.

    1. Neutron stars are another so called object that likly doesnot exist. So much of this is only a mathamatical theory with no physical evidence.

  2. Whatever is inside a black hole, I just consider it “degenerate neutron matter” knowing full well that neither general relativity, quantum mechanics, nor the standard model quite work there. I don’t believe that the collapse goes all the way to a singularity, I regard that more an artifact of Schwarzchild’s approach and not a physical reality.

    1. Agreed,JPatrick! I think black holes are simply very dense but they don’t have to be singular. Any supermassive very dense object can exhibit these effects…strong gravitational field, small volume. And what does the black color prove anyway? My shirt is black, no light can escape from it, big deal! As Douglas Adams said: Isn’t it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?

  3. I think that it’s possible to give what seems like a slightly silly answer to this question which is that no, black holes don’t exist, even in completely classical GR (wait before you think I’m some anti-GR person!):

    The reason for this is very simple. Consider, say, the Schwarzschild solution: this solution has an event horizon and obviously describes a black hole. So, now, the interesting question to ask is: for which observers is the event horizon in the past? And, trivially, the answer that it is not in the past of any observer outside the black hole. To put it another way: if it’s in your past you will reach the singularity in finite proper time.

    This is actually obvious: if you’re outside the horizon, then the reason it is black is not because it has some magic property, it’s because it’s in the future, and light does not reach you from the future.

    So black holes don’t exist *yet* for any observer outside them, and never will.

    Well, this seems like a slightly silly point, but it’s not for at least two reasons. First of all I think it gives a clearer idea of what black holes *are* in GR: they’re little bits of spacetime where all of the future directions point inwards.

    Secondly and more importantly I think this view explains why the whole firewall / bounce stuff does not make any difference to distant observers. I have read a number of articles where people (who should know better) will say things like ‘but the whole bounce/firewall stuff must be wrong because we observe things which look like black holes, so they can’t be bouncing just outside the horizon’. But that’s bogus: if they bounce or if there is a firewall then *it makes no difference at all* to anyone far away, because the bounce / firewall is happening (or will happen) in a region of spacetime close to the horizon (or where the horizon would be if there was one) which is arbitrarily redshifted from our perspective: all this stuff happens so slowly that we would need to wait for a huge amount of time to see any difference. (I was going to say ‘astronomical time’, but the timescales involved make astronomical timescales look rather small.)

  4. I don’t understand how black holes can exist. My understanding is that as matter approaches a black holes event horizon for any object observing that matter (e.g. Outside of the event horizon) the object will slow and eventually stop (as according to Einstien at the event horizon matters weight approaches infinity and time stops). Scientists general response is that for the object falling into the black hole time remains the same, but that is not the point. All matter outside of the black hole (e.g. All observers) will see the object freeze at the event horizon. Actually they will not see it and light cannot reflect back, but the object will not continue access the event horizon. If no object can ever cross an event horizon (for outside observers) how can the black hole ever form or ever consume matter?

    1. Author

      The simplistic view of black hole event horizons has to be taken with a huge grain of salt. To really understand them you have to look at the global structure. It is possible for matter to enter a black hole in a finite time even when it might appear to a distant observer that it’s still “outside the horizon.”

    2. Exactly Matt. To look at this another way, imagine a particle approaching the EH. As time is slowing due to the intense gravitational field, the wavelength of the particle extends to infinity at the EH and its energy therefore becomes zero. No energy can therefore enter the BH so how can they exist in the first place.

      In addition, there’s the little matter of the rotation of the star. Does this overcome the gravitational field, spewing material out before it gets neat to the EH? And what about magnetic field effects? The region near to any notional EH must be absolutely chaotic.

      Whilst it’s certainly presumptuous to question Hawking’s theories, he has many questions to answer.

  5. Thanks for a very interesting discussion. However, I still find Matt’s post above worth more pondering…

    In your reply, Brian, you invoked the fact that matter can enter a black hole “in finite time”. The question is: in what coordinate systems?
    I know this is true in coordinate systems which is attached to the falling observer (with “time” being defined as the time coordinate at the observer’s location).
    But what about us static distant observers? Do black holes exist for us? Not sure I understand what this would really mean… but intuitively, if we use the Schwarzschild coordinate system, and we are a static observer located at some r (> 2m), it seems the horizon doesn’t yet exist. For example, imagine that the collapse occurs now. What we would see is matter collapsing but slowing down as it redshifts and never really form a horizon.
    Then, how can we say things like: “there’s a black hole in the center of the galaxy?” if that blackhole’s even horizon hasn’t been created yet and will be created only at t=infinity, for us?
    In short: are there really event horizons which exist in the universe as we observe it? If there are, they would have to have pre-existed, which precludes astrophysical blackholes and leaves us with some exotic primordial stuff?

    Will be happy if anyone can solve my confusion. Thanks!

  6. I would appear that the creation of black hole (based on the theory of General Relativity) also requires the theory of Gravity.

    “General relativity also makes several predictions about black holes. One is that given enough mass, an object will collapse into a black hole. This is true no matter what the mass is made of, since any energy or force trying to prevent the collapse actually starts helping gravity more than it opposes it.”

    This would therefore mean that a theory of a black hole is reliant upon the theory of General Relativity which relies upon the theory of Gravity… hmmm.. that’s three theories, none of which are proven fact.

    Articles discussing any of these three theories could be considerably shortened, made more accurate, and prevent the misleading of people with the following words. “We don’t know what is really happening, or how it works”. Certainly such articles should not be discussing such issues as though they are proven or factual.

    That’s my opinion

    1. Author

      Actually, the theory of general relativity is a theory of gravity, and a very well tested one. Since this article was written we’ve also observed the gravitational waves of a black hole merger, so we have plenty of evidence to say that black holes are very real. Comments such as yours could be considerably shortened and made more accurate by simply stating “I don’t understand this stuff, and I don’t care to learn, but let this comment note my disdain for your work.”

  7. ‘dense objects that closely approximate black holes’ how can you know what could approximate a black hole if we don’t know how a black hole behaves since we havent found one yet?
    ‘Stellar mass black holes are expected to emit strong x-rays’ you say expected as if you know how a black hole behaves which makes no sense since we havent found a black hole yet?

  8. What about the work of Laura Mersini-Houghton? She was able to merge GR and quantum mechanics and show that as a star collapses under its own gravity, it produces Hawking radiation, as we all know, but that by giving off this radiation, the star also sheds mass to the point that it no longer has the density to become a black hole. http://arxiv.org/abs/arXiv:1409.1837

  9. This theory to me is jest a loads of assumptions …. I recently watched a movie named “Inception” it also says loads of theories and says a person can enter into another persons dream and really can do something in the dream and can change to his favor…. if I wanted to believe in black hole I also wanted to believe in this theory too…!!!!

  10. Hawking in 2014 suggested that the intense radiation zone around black holes doesn’t actually exist. If this were true it would not disprove black holes but rather mean that black holes cannot explain quasars.

  11. Science is a matter of observing and fomenting an hypothesis that, the next time you observe that thing, you will have a pre-setermined notion as to the cause, which you then fine-tune and make certain.

    Black holes don’t exist.

    They’ve never been observed, and there’s no indirect evidence, either. Therefore we don’t even get to the first bullet point of science – “did you observer something?”, as the answer is no.

    This ends the debate.

    As for the math, this then becomes, no matter how much you want me to buy into your 40 Million elvis fans and appeal to authority falacies, moot. There’s no evidence these things exist, either directly observed or indirectly.

    The model used by these tenured physicists is faulty, and can be debunked by grade-school kids, same as Michelson-Morley and special relativity ( if the observer to the rocket and earth is equi-distant from both, how does the observer observe either being in a state of motion?)

  12. Black Holes doesn’t exist, relativity still to be proven, although with big chances (I’m still reading and try to see if the allegedly recent prove of the General Relativity makes sense) and the Big Bang theory…hahahaha a Big LOL, just a big inexplicable fairy tale.
    It is simple, the wasted trying to prove Black Holes, should be used to better matters, that could help our daily lives.

  13. A lot of evidence for something that has not actually been observed. Theoretical mathematics is not proof. Observed data can be translated in different ways to imply different things. Again, that is not proof.

    1. Author

      Since this post was written, there’s even further evidence of black holes, such as the direct detection of gravitational waves produced by merging black holes. Theoretical mathematics is not proof, but evidence is. Your inability to understand that evidence not withstanding.

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