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 X-rays - 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 X-rays. 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 X-rays.
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 X-ray 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.