What are black holes? How are they formed? What are they made of? Where do they go?
These are some of the common questions people have when thinking or talking about the great devourers of the Universe.
We have tried to address some of the more common questions we've found about black holes. As such this should be considered akin to an FAQ.
What is the definition of a black hole?
Black holes are generally defined as "a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because [the] matter has been squeezed into a tiny space." - NASA.
As light is unable to escape the hole's gravity it appears completely black - hence the name. Black holes can, however, be 'seen' with some special analysis of data collected from a wide range of telescopes (more on this later).
How are black holes made and what different kinds of them are there?
- Miniature/small black holes
- Intermediate black holes
- Stellar black holes
- Supermassive black holes
Current theories suggest that small, or miniature, black holes (some as small as an atom) probably formed in the earliest moments of the universe. These tiny black holes are, to date, purely theoretical, and are believed to be tiny vortices of darkness peppered throughout the universe.
These tiny black holes are thought to have masses of hundreds of solar masses.
Like miniature black holes, intermediate black holes are only really known theoretically. These kinds of black holes have several hundred of thousands of solar masses, rather than millions, or even billions of solar masses like their larger cousins.
Some scientists believe that intermediate black holes form from a merging of miniature black holes. Others believe that, if they do indeed exist, that they would form from the collapse of stars with masses equal to hundreds of thousands of our own Sun.
Needless to say, there is little consensus in the field over these enigmatic black holes.
Stellar black holes (about the mass of 20 of our Suns or more) are created when massive stars collapse in on themselves.
"In their final stages, enormous stars go out with a bang in massive explosions known as supernovae. Such a burst flings star matter out into space but leaves behind the stellar core. While the star was alive, nuclear fusion created a constant outward push that balanced the inward pull of gravity from the star's own mass. In the stellar remnants of a supernova, however, there are no longer forces to oppose that gravity, so the star core begins to collapse in on itself." - National Geographic
If this mass collapses into an infinitely small point, a black hole is born—many times the mass of our own sun. There may be thousands of these stellar-mass black holes within our own galaxy.
Supermassive black holes (as much as 1 billion of our Sun's mass or greater) are thought to form at the same time as the galaxy they inhabit is formed and are predicted by Einstein's General Theory of Relativity. The Milky Way has a supermassive black hole at its center, Sagittarius A* (pronounced “ay star”), that is more than four million times as massive as our sun.
Who first discovered black holes?
While everyone has heard of black holes nowadays, have you ever wondered who first discovered them?
Technically speaking, we haven't really "found" a black hole yet, but we can infer their existence through a variety of techniques (more on this later). That being said, scientists have speculated about the existence of something like them for hundreds of years.
In 1783, for example, an English cleric and amateur scientist called John Mitchell managed to show that Newton's law of gravity should be able to show a place where gravity was so intense light cannot escape.
He even went further. Mitchell suggested that although they would be invisible, they should reveal their presence by interfering with things like stars that might orbit them.
His theoretical work would prove to be years ahead of his time with the later groundbreaking work of the great Albert Einstein.
Einstein first predicted that such things should exist way back in 1916, in his General Theory of Relativity. According to him, big enough stars should be able to collapse under their own gravity and create what we call today black holes.
For decades after, black holes remained a purely theoretical concept, and the actual term wasn't coined until 1967 by the American astronomer John Wheeler.
Mitchell and Einstein's work was later reinforced when two British astronomers, Louise Webster and Paul Murdin independently announced they had discovered one in space. Murdin worked out of the Royal Greenwich Observatory in London and Webster at the University of Toronto.
What they had found was an intense x-ray source, called Cygnus X-1, orbiting a blue star around 6,000 light-years away. It would be the first of many.
As amazing as this all is, it wasn't until very recently that scientists managed to "see" one for the first time. Back in 2019, the Event Horizon Telescope (EHT) collaboration managed to release a computerized image of what is believed to be a black hole.
The image itself is actually a composite rendering of a petabyte of data collected from a series of radio telescopes sited around the world.
The EHT focussed the radio telescopes on the center of the Messier 87 Galaxy (Virgo A) where a black hole was thought to lurk. This galaxy is somewhere in the region of 54 million light-years away from Earth.
It is thought that the black hole in question has a mass of about 6.5 billion suns. The team was attempting to examine the black hole's event horizon and accretion disk (a large cloud of hot gas and dust trapped in orbit around the black hole).
This they did, and they were able to map the sudden loss of photons within the black hole's event horizon. This discovery has proved to be groundbreaking, as it is hoped that it will open a whole new area of research into the nature of black holes.
What is the definition of a black hole event horizon and what is it?
A black holes' event horizon is its outermost boundary. This is the point at which the gravitational force overcomes light's ability to escape the pull of gravity from the black hole. To escape from the event horizon, you would have to be going faster than the speed of light.
It is the literal point of no return - you cannot escape once you pass it. At least that was the traditional view.
The venerable Professor Stephen Hawking, during his life, was adamant that the definition of a black hole should be changed.
He believed that event horizons, as they are traditionally understood, don't actually exist at all. They are, in fact, "apparent horizons" at the edge of black holes, where quantum mechanics goes crazy.
He posited that here, virtual particles pop in and out of existence, causing the horizon to fluctuate, rather than act as a specific point in space..
Theoretically, these "apparent horizons" are also a point where quantum effects create streams of hot particles that radiate back out into the Universe - the so-called Hawking radiation. It is theorized that this will eventually cause the black hole to radiate away all its mass and disappear.
What is at the center of a black hole?
A black hole singularity or gravitational singularity is a point at the very center of a black hole. It is a one-dimensional point that contains enormous amounts of mass in an infinitely small space.
Here gravity and density become infinite, space-time curves infinitely and the laws of known physics are thought to no longer apply.
Kip Thorne, the eminent American physicist, describes it as "the point where all laws of physics break down".
What does a black hole look like?
As light cannot escape once past the black holes' event horizon they can't actually be 'seen' in a traditional sense. We can, however, infer their existence from their effects on other bodies in space (like Suns and gas clouds) that we can see.
It might soon be possible to detect the boundary of the event horizon around the black hole - or rather, detect the Hawking radiation emanating from it.
Hawking radiation is theorized to consist of photons, neutrinos, and to a lesser extent all sorts of massive particles.
What would happen to you if you fell into a black hole?
In theory, so long as it's a supermassive black hole you wouldn't feel anything -- you'd actually be in freefall (what Einstein once called his "happiest thought"). You'd exist and then inevitably you wouldn't. The tidal forces would become too strong too fast for you to survive to the event horizon, resulting in your spaghettification (the actual technical term).
For an observer, however, its a very different story.
As you approach the event horizon you will appear to immediately accelerate, stretch and distort obscenely. Interestingly you will appear to move in slow motion the closer you get to the horizon, until you freeze (as if on pause).
Now for the fun bit.
As you approached the event horizon, a faraway observer would watch your image slow down and redden. Although your image would appear to freeze at the event horizon, in practice you would disappear: it becomes harder for photons to climb out of the black hole’s gravitational well, and their wavelength would increase until it could no longer be detected. The image would then become effectively invisible. So, the observer would see your image redden and dim with time, and then fade entirely.
For smaller black holes you undergo a process commonly termed "spaghettification". This is a very different, and somewhat more disturbing, story.
Here's an interesting video on just this subject.
What is at the center of a black hole?
At the center of a black hole, it is often postulated there is something called a gravitational singularity, or singularity. This is where gravity and density are infinite and space-time extends into infinity.
Just what the physics is like at this point in the black hole no-one can say for sure.
What is the closest black hole to Earth?
The closest black holes yet discovered to Earth are several thousand light-years away from us. At this distance, these black holes will have no discernable effect on our planet or its environment.
To date, the nearest black hole, called V616 Monocerotosis, is 3,000 light-years away and has a mass around 9-13 times that of our Sun. The next closest is Cygnus X-1 (about 6,000 light-years away with a mass of 15 suns).
Next up is GRO J0422 + 32, which is actually one of the smallest yet 'discovered' and is roughly 7,800 light-years away.
As far as we know the nearest supermassive black hole, Sagittarius A*, to us sits in the middle of our home galaxy - The Milky Way. This monster is roughly 27,000 light-years away from us.
You can 'find' it in the approximate direction of the Sagittarius constellation.
Our galaxy's supermassive black hole is estimated to be several million times (approx 4.1 million times to be precise) the mass of our sun. But don't worry, its enormous distance from us doesn't directly affect our solar system - at least not yet.
It is thought that in about 4 billion years our galaxy will collide with our neighbor galaxy, Andromeda. When this happens stars, and their respective black holes could be mixed together into a new blended galaxy.
However, black holes aren't exactly the “cosmic vacuum cleaners,” they are depicted as. Objects must be fairly close to one to be "sucked in".
How long does it take for a black hole to die?
The lifespan of a black hole varies depending on its mass. You can only really know by running quantum field theory calculations to find out - which is complex, to say the least.
As a general rule, the loss of mass from Hawking radiation is thought to occur at different rates relative to the 'size' of the black hole. Interestingly lower-mass black holes are theorized to lose their mass quicker than larger ones.
This is because the curvature they create in space is more intense around their event horizon. But even so, it takes a very, very long time indeed.
By way of example, it is estimated it would take 10^67 years for a black hole with the Sun's mass to completely dissipate. For the larger black holes in the Universe, it could take an unbelievable 10^100 years.
These figures are much longer than the estimated age of our Universe, at 13.8 times 10^9 years, but it's not forever. That means that when all stars and planets have long since perished, black holes will dominate before eventually disappearing themselves.
How many black holes are there in the universe?
How long is a piece of string? How many grains of sand are there on a beach? How many stars are there in the Galaxy? These questions are nigh on impossible to answer.
The same is true for the number of black holes in the Universe, as it has been postulated that there are so many you couldn't ever hope to count them.
Even if we tried, we would never get the right answer as a large part of the Universe will be obscured from our view, forever. If such an attempt was made we would first need to limit our count to what is more correctly called the "Observable Universe".
We can, however, make some educated guesses.
Stellar-mass black holes form from the supernovae of massive stars. Our Milky Way alone likely contains thousands of stellar-mass black holes.
This should mean that there might be as many as 100 million stellar-scale black holes in our galaxy. But this number is theoretically increasing with every second that passes.
New, stellar-mass type black holes are thought to form once every second or so.
If we are talking about supermassive black holes, these tend to lurk at the center of galaxies. In our local region of space, there could be 100 Billion supermassive black holes or thereabouts.
How is it possible to detect a black hole?
Given the nature of these celestial phenomena, it's not actually possible to directly observe them with telescopes that rely on x-rays, light, or any other form of EM radiation.
Rather, finding or detect them requires a bit of lateral thinking. They can be inferred by their gravitational impact on other nearby matter and objects.
A classic example would be if the black hole passes through an interstellar cloud. This event will draw matter inward towards the black hole in a process known as accretion.
Stars can also be deflected from their 'normal' motion if they pass near a black hole or, of course, can be torn apart.
In the latter scenario, the star's matter is accelerated as it moves towards the black hole and this emits x-rays into space.
"Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma-ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others." - NASA.
You can also 'see' the perimeter of space that is close to the black holes' event horizon through something called the "lensing effect' or gravitational lensing.
You can also attempt to observe the black hole's Hawking radiation. Other than these methods, the recent work of the EHT collaboration may open up new avenues to not only detect them but also make tentative observations of them.
Can you destroy a black hole?
As we've seen above you don't need to (if you could possibly live long enough), just wait for them to destroy themselves. But it might be theoretically possible to destroy a black hole artificially.
It turns out that black holes might actually have an Achilles heel - their event horizons. Some physicists have theorized that if we could increase the black hole's angular momentum and/or charge of the event horizon, we might be able to reverse its inherent inequality.
This would, in turn, cause the black hole to dissipate and might just reveal its central singularity. However, just how you would do this is anyone's guess.
One of the main issues is that anything with angular momentum tends to also have mass. If we feed a black hole in an attempt to destroy it, that would put it into a dynamic state and there is no guarantee it would settle back in a steady state without shedding any excess artificially added.
But physicists admit they have no idea what the actual consequences of doing this would be.
What would happen if two black holes collided?
If two (of equal mass) were to collide the result would be one new double-sized black hole. But the event would be incredibly violent.
Such an event would release enormous amounts of energy and could cause long-ranging ripples in the very fabric of space-time, so-called gravitational ripples.
Although once the subject of science fiction and science theory, astrophysics appear to have actually been able to detect or 'observe' just such an event occurring.
Do black holes eventually collapse?
The answer to this depends on your meaning behind the use of the term 'collapse'.
If by collapse the questioner means an end to the black hole then yes they do. Black holes can exist for a very long time but they are not 'immortal'.
Although they do die out over time it's not because they 'collapse' in the traditional sense of the word.
Black holes, namely their event horizons, become their very downfall. It is hypothesized that, after they have consumed all matter around them that is possible they eventually evaporate as the energy and mass are sapped overtime via Hawking's radiation.
If, however, we consider the meaning of collapse literally then the answer is very different indeed. Black holes are, in effect, the very definition of collapse.
In this sense, black holes can do nothing other than collapse.
Do black holes die?
Yes, they do, eventually. But, it takes a very long time indeed.
The process is a very slow one and requires the black hole to be starved of fresh matter from other celestial bodies nearby. The process of black hole decay is the emission of Hawking radiation, as we have previously mentioned.
In most cases, this process will likely take longer than the current age of the universe. By way of example, if you took a black hole with the mass of our Sun, it would take somewhere in the region of 2×1067 years to evaporate.
To put that into perspective, the age of the universe is only 13.8×109 years. Such a black hole would take more than 1057 times the current age of the universe for that black hole to evaporate. An amazing thought.
What is a black hole made of?
Put simply we cannot really be sure. Black holes are by definition regions of spacetime where extreme gravitational forces prevent anything, including light, from escaping.
Once past the event horizon, as matter "goes down the rabbit hole", the more and more our understanding of what's going on in there completely falls apart.
"Thanks to General Relativity, we think we understand what happens in this extreme gravity and, with the help of Quantum Mechanics, we can make an intelligent estimate as to what happens at smaller, microscopic scales. But if the two theories are combined – like they would be at the center of a black hole – they break down, leaving us with no idea as to what’s going on!" - spaceanswers.
What is on the other side of a black hole?
Are they gateways to other universes? Perhaps they form wormholes we can use to quickly circumnavigate the vastness of space? These and many other theories exist for what could possibly be on the other side of a black hole, but the reality is actually thought to be somewhat disappointing.
These massive black holes are more of a final stop than a route to somewhere else.
Although we can't be entirely sure what's going on beyond the event horizon, most physicists agree that you'd go absolutely nowhere. Crossing the point of no return would simply mean anything consumed by the black hole simply becomes part of it.
They are a literal road to nowhere. Objects that fall into the black hole are torn apart and incorporated into the greater mass of the entity until they end up within the singularity.
Their sacrifice will lead to the black hole becoming that little bit bigger and stronger. All that and rather than finding a nirvana of some sort, all that awaits you is disassembly and death.
Who are the top scientists whose work was about black holes and what did they discover/claim?
The following 11 are some of the most important scientists whose labors helped forge our modern understanding of black holes.
1. John Michell
Year of Main Discovery: 1783
Description: Michell was an English natural philosopher and geologist who was born in 1724. He wrote a letter to Henry Cavendish in which he postulates the idea of mass so large even light could not escape its pull.
2. Pierre-Simon Laplace
Year of Main Discovery: 1796
3. Albert Einstein
Year of Main Discovery: 1915
Description: Einstein, a German-American theoretical physicist and all-around 'badass' developed his theory of general relativity. This followed his demonstration that light can be influenced by gravity.
4. Karl Schwarzschild
Year of Main Discovery: 1916
Description: Schwarzchild, a German physicist, was the first to provide an application of general relativity that could be used to characterize a black hole.
5. Arthur Eddington
Year of Main Discovery: 1924
Description: Eddington, a British Astrophysicist, noted that the singularity in Einstein's work could disappear after the coordinates were altered.
6. Robert Oppenheimer
Year of Main Discovery: 1939
Description: One of the pre-eminent physicists of all time, Oppenheimer predicted that neutron stars in excess of 3 solar masses would likely collapse to form black holes.
7. David Finkelstein
Year of Main Discovery: 1958
Description: Finkelstein, an American physicist, recognized that the Schwarzschild surface was actually an event horizon. He was also able to extend the Schwarzschild solution for the future of observers falling into a black hole.
8. Roy Kerr
Year of Main Discovery: 1963
Description: Kerr, a New Zealand mathematician, derived a solution for a rotating black hole.
9. Ezra Newman
Year of Main Discovery: 1965
Description: Newman, an American physicist, postulated the axisymmetric solution for a black hole that is both rotating and electrically charged.
10. James Bardeen
Year of Main Discovery: 1970's
Description: Bardeen, an American physicist, along with Jacob Bekenstein, Brandon Carter, and Stephen Hawking, worked on the formulation of black hole thermodynamics.
11. Stephen Hawking
Year of Main Discovery: 1974
Description: Hawking, the British theoretical physicist, and cosmologist, showed that black holes are not actually entirely 'black'. He postulated that small amounts of thermal radiation, called Hawking radiation, is emitted by black holes.
Congratulations you've made it to the end of the article. By now, we hope you have gathered a good understanding of what black holes are, how they form, and how they can die over time.
But, should you still have some questions about these strange cosmic phenomena, feel free to share your thoughts in the comments.