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When you think of a black hole, what comes to mind? If it's an image of a colossal void floating through space, sucking in planets and stars and evaporating them to dust, you aren't alone. But black holes are actually thought to be full of matter. In simple terms, a black hole is an area with such strong gravity that nothing, including light, has the energy to escape.
The nature of a black hole makes them hard to observe and, as a result, study. This means that while we do know a lot about black holes, there's potentially even more that we don't know about them — and there are a lot of assumptions, too; scientists can infer things about black holes from the impact they have on matter around them, but can't study the black hole directly.
If we know so little about black holes, what does that mean? How do they form? Could Earth get sucked into one? Would we even know if we were? In this article, we will discuss what black holes are, how we can study them, and what it would be like inside of one.
What is a black hole?
Black holes are regions of space where a large amount of matter occupying a very small area exerts a huge gravitational pull. There are different kinds of black holes, assigned a category according to mass. The two main types of black hole are stellar and supermassive black holes, which vary massively in size: a stellar black hole is categorised as between 5 to 10 times the mass of the Sun, whereas a supermassive black hole contains millions of solar masses.
How do black holes form?
Each kind of black hole forms in a different way. Stellar black holes form, as you may have guessed, from stars. When a large star collapses in on itself, it causes a supernova, and what's left of the star's core becomes a stellar black hole. This only occurs if the star is over a certain mass, otherwise, the star becomes a neutron star rather than a black hole.
Supermassive black holes are at the centre of almost every galaxy, but scientists don't know how they were formed. It's thought that they may have begun to form with the collapse of supermassive stars in the early universe — but we don't know whether this is the case.
The boundary of a black hole is called its event horizon. This is the point of no return for all matter which crosses its path; nothing can escape it as the velocity needed to do so is higher than the speed of light, which is as fast as it's possible to travel. As such, light emitted on the other side of a black hole's event horizon would not be visible to someone looking at it from outside — but if you were inside the event horizon, you'd be able to see light as normal. This all sounds scary — and in a way, it is — but as we will come to explain, you are incredibly unlikely ever to know what it's like to cross a black hole's event horizon.
Intermediate-mass black holes
As well as stellar and supermassive black holes, there is some evidence for the existence of another, mid-sized kind of black hole, known as an intermediate-mass black hole. The material around black holes means that they emit large amounts of X-rays, so some believe that observing a strong source of X-rays points to the existence of a black hole.
In an article published in May 2023, NASA reported that astronomers using the Hubble Space Telescope claimed to have found "some of their best evidence yet" for intermediate-mass black holes — in the star cluster Messier 4 (M4), to be specific. Astronomers looked at 12 years worth of Hubble's M4 observations to study the potential black hole. Since black holes themselves cannot be seen, scientists instead calculated their mass "by studying the motion of stars caught in its gravitational field, like bees swarming around a hive". They found that M4's black hole could be up to 800 times the mass of the Sun, which would make it an intermediate-mass black hole.
Primordial black holes
There is actually another type of black hole which scientists also believe may exist, called primordial black holes. No primordial black hole has ever been observed or recorded. They are thought to have originated, as the name suggests, at the beginning of time; in fact, if primordial black holes exist, they would probably have formed in the first second following the big bang. The mass of a primordial black hole could be incredibly small or incredibly large, depending on how close to the big bang it formed.
Primordial black holes would explain at least a portion of dark matter — maybe even all of it — and they are essentially believed to exist (or have previously existed) because of the unique universal conditions in the seconds following the birth of the universe. As NASA explains it: "in that moment, pockets of hot material may have been dense enough to form black holes ... then as the universe quickly expanded and cooled, the conditions for forming black holes this way ended". These black holes, over time, could have evaporated, or alternatively, they could still be out there — or a mix of the two. Unfortunately, we don't know for sure if they ever existed, let alone what might have happened to them in the billions of years since their formation.
Unravelling the mysteries of black holes
Black holes are difficult to see, study, and even to comprehend at all. As such, they are among the biggest scientific mysteries that scientists face today. But considering how elusive black holes are, we actually know quite a bit about them. How have we achieved this level of understanding?
How do we study black holes?
The nature of black holes means they cannot be observed with telescopes which detect electromagnetic radiation such as visible light, x-rays, or infrared radiation. Instead, scientists study black holes by observing their effect on nearby matter, and inferring a black hole's presence based on the occurrence of these effects in a given area.
While black holes may feel like a fairly recent scientific disovery, the first known theory regarding black holes actually dates back to the 18th century. Natural philosopher John Mitchell proposed the existence of 'dark stars', which we have come to know as black holes, in 1783. He theorised that the gravitational pull of these stars would mean that nothing can escape them, including light. He even went as far as to suggest that we could identify a 'dark star' by looking for stars whose gravitational behaviour would suggest that two stars are present, but only one could be seen. Although his theory was largely ignored at the time, it isn't far off from how we understand black holes today.
Scientists identified Cygnus X-1, the first known black hole, in 1964 — almost 200 years later. We have since made many more discoveries and have a much deeper understanding of black holes than ever before. Despite our progress, however, there's still a lot for us to learn.
Who studies black holes?
Astronomers and astrophysicists study black holes, examining the way they affect matter around them. In doing so, they can test theories on how the universe works — for example, black holes helped scientists to test Einstein's theory of general relativity.
Dark matter, one of science's biggest mysteries, could also link to black holes: it has been theorised that primordial black holes could account for all the dark matter in the universe. If scientists found more evidence to support this theory, then we could be closer than ever to what many would consider the biggest breakthrough in modern science.
Inside a black hole
While we understand how black holes affect matter around them, what still remains something of a mystery is what's inside them. Given that we can't just jump inside of one to see what's on the other side, a lot of theories as to what's inside a black hole remain just that — theories. However, no concrete evidence doesn't mean no evidence at all, and we do have a somewhat substantial idea of what lies past a black hole's impenetrable event horizon.
Singularity: the heart of a black hole
At the centre of a black hole is thought to be a singularity. Described by NASA as where "matter is crushed to infinite density", it is estimated that here, the laws of physics break down and spacetime doesn't exist how it does everywhere else. In other words, gravity is so strong here that the singularity cannot be defined in terms of where or when — it's immense gravity means it is not part of the same spacetime as we are.
If you went through a black hole's singularity, you would most likely undergo what has been called 'spaghettification' — you would essentially be stretched out like a piece of spaghetti until you were pulled apart.
What is on the other side of a black hole?
It has actually been theorised that black holes could be wormholes — that on the other side of a black hole is just another part of the universe. The existence of such wormholes was theorised by scientists Albert Einstein and Nathan Rosen, and as such, they are known as Einstein-Rosen bridges. The idea is that black holes contain 'bridges' through spacetime which are essentially shortcuts to another place in the universe. Theoretically, if you went through an Einstein-Rosen bridge, you could come out at not only another location within the universe, but also a different time.
Unfortunately, there's not much point trying just yet, as even if this theory is correct, you still couldn't pass through the black hole without being crushed. But if the theory does hold some truth, black holes could revolutionise space travel. Even if we can't yet make the journey without becoming a piece of cosmic spaghetti, we'd be one huge step closer to interstellar space travel — and maybe even time travel.
Would we know if we were inside a black hole?
Now comes the real question: could we be inside a black hole, and would we know it if we were? The evidence suggests that we would certainly know if we passed through a black hole, as we'd all likely be crushed to an infinitely tiny point. But how would we know if we had always been inside one? Theoretically, we could have at some point passed through an Einstein-Rosen bridge, but it would be hard to know if we had done so, and even if we had, we wouldn't technically be inside a black hole, we'd just have passed through one. We couldn't live inside a black hole, so we wouldn't know if we were inside one — we would no longer exist, at which point we wouldn't know much about anything.
In terms of getting sucked into a black hole, it's looking pretty unlikely. The Sun doesn't have enough mass to become a black hole, and even if it did, a black hole would have to be closer for its gravitational pull to suck up the Earth. The Earth going into a black hole isn't something that will happen in the near future, if ever — so it's not worth worrying about, really.
Mystery or certainty?
As we have established, most of the things we 'know' about black holes are actually just what we've decided is the most likely theory. However, the accepted theories have a strong basis in science, and are likely to be at least largely true. Similarly, increasingly advanced technology will enable us to continue learning more and more about black holes. For example, the James Webb Space Telescope has already produced data and images which have advanced our knowledge of black holes.
We have learnt more about black holes in the last 60 years than ever before; in another 60, we could have an understanding that we never thought possible. It's even possible that a greater understanding of black holes could help us to uncover mysteries such as the origins of dark matter and the mechanism of time travel. If this is the case, expanding our knowledge of black holes could revolutionise science as we know it.