Black Holes Are the Terrifying Behemoths of Space. Here's How They Tick

Much like water gushing down a drain, the very fabric of space (and time) also appears to be draining away within some of the most enigmatic things in the universe — black holes. But what exactly are they? 

Are they more common than we think? Should we be concerned about them? What role do they play in the universe? 

These are just some of the "big picture" questions some of the greatest minds of astrophysics have mulled over for many decades. 

Let's see what, if anything, they've managed to learn about the "Great Devourers" of the cosmos. 

What is the definition of a black hole?

According to NASA, black holes can be defined as "a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space."  

As light cannot escape the black 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).

What are black holes made of, and what different kinds of them are there?

How black holes form depends on their type and origin. To date, scientists have managed to define at least four different kinds of black holes: 

• Miniature 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 universe's earliest moments. To date, these tiny black holes are purely theoretical, and it is theorized that most of them may have already evaporated. These tiny black holes are thought to have masses of hundreds of solar masses or less. 

Like miniature black holes, intermediate black holes are only really theoretical. This type of black hole would 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 merging miniature black holes. Others believe that, if they exist, they will form from the collapse of stars with masses equal to hundreds of thousands of solar masses (one solar mass is equal to the mass of our own Sun, or 1.989 × 1030 kg). 

Needless to say, there is little consensus in the field over these types of 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.

As National Geographic explains, "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."

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 (millions or even billions of solar masses in size) 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 “a star”), that may be more than four million times as massive as our sun. Scientists aren't sure how such large black holes come into being, although there are a number of theories.

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 various 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 demonstrate that Newton's law of gravity could be used to show a place where gravity was so intense light could not escape. 

He went even further. Mitchell suggested that although these areas 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 black holes today. 

For decades after this, 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 reinforced in 1971 when two British astronomers, Louise Webster and Paul Murdin, independently announced they had discovered one in space using indirect methods. Murdin worked out of the Royal Greenwich Observatory in London and Webster at the University of Toronto.

They found an intense x-ray source, now 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 is a composite rendering of a petabyte of data collected from a series of radio telescopes 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. 

The black hole in question is thought to have a mass of about 6.5 billion suns. The team attempted to examine and image 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 two years later, they could image the shape of the magnetic fields in the hot gas swirling around the hole. The discovery of this black hole 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. In 2021, astronomers used an ancient gamma-ray burst to detect an intermediate-mass black hole. Information from the Sloan Digital Sky Survey suggests IMBHs may exist in the center of most dwarf galaxies.

What is the definition of a black hole event horizon, and what is it?

A black hole's 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 go 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.

However, the venerable Professor Stephen Hawking 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. That they are, in fact, "apparent horizons" at the edge of black holes, where quantum mechanics goes crazy.

He posited that virtual particles pop in and out of existence here, 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 can 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, as we've previously explained. 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 other 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 you wouldn't. According to one theory, 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, it's 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 they 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 all more than a thousand light-years away from us. At this distance, these black holes will have no discernable effect on our planet or its environment. 

In 2021, astronomers claimed to have found a tiny black hole just 1,500 light-years away, dubbed "the Unicorn".  The black hole is about three times the mass of our sun and appears to be a companion to a red giant star. The miniature black hole was discovered by analyzing the way that light from the red giant appeared to change in intensity and appearance at various points in its orbit. They surmised that the distortion was caused by a very small black hole.

The next nearest black hole, called V616 Monocerotosis, is 3,000 light-years away and has a mass around 9-13 times that of our Sun. After that is Cygnus X-1 which is about 6,000 light-years away, with a mass of around 15-20 suns.

Next up is GRO J0422 + 32, which is another very small black hole, with a mass of around 3 to 5 solar masses, 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 often depicted as. In fact, 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 1067 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 10100 years.

These figures are much longer than the estimated age of our universe, at 13.8 billion 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 they couldn't ever be counted.

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 detecting 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. 

As NASA explains, "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." 

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 waves.

Although once the subject of science fiction and science theory, astrophysicists appear to have actually been able to detect or observe just such an event occurring. In 2019, scientists using gravitational wave detectors at the Caltech's Zwicky Transient Facility at Palomar Observatory spotted what they believe could be a light flare from a pair of colliding black holes.

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 over time 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.

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 a 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 most important contributors to our knowledge of black holes? 

We've already covered a few of the most important scientists who've contributed the most to our understanding of black holes, but there are many other important contributors too. Here are some of the most notable. 

1. John Michell

Year of Main Discovery: 1783

Michell was an English natural philosopher and geologist who was born in 1724. He wrote a letter to Henry Cavendish in which he postulated the idea of a mass so large even light could not escape its pull.

2. Pierre-Simon Laplace

Year of Main Discovery: 1796

Laplace was a French mathematician and astronomer. He promoted the same idea as Michell in his book Exposition du système du Monde.

3. Albert Einstein

Year of Main Discovery: 1915

Needing very little introduction, Einstein, a German-American theoretical physicist who developed his theory of general relativity. This followed his demonstration that light could be influenced by gravity.

4. Karl Schwarzschild

Year of Main Discovery: 1916

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

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

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

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

Kerr, a New Zealand mathematician, derived a solution for a rotating black hole.

9. Ezra Newman

Year of Main Discovery: 1965

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

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

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, are emitted by black holes.

And that, black hole boffins, is your lot for today.

Can we congratulate you on actually making it to the end of this mammoth overview of black holes? 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.

However, this is only the tip of the iceberg of our knowledge of astrophysics' fascinating and ever-developing aspect.