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Of all the places in the universe you could go, a black hole is probably one of the last places you'd really want to be. Even if you know very little about black holes, you might think it's probably not one of those places you absolutely must visit. In fact, there are many reasons why you'd want to avoid black holes, but if you want to know exactly what would happen if you encountered one, why don't we take a look and see?
What is a black hole?
Simply put, a black hole is a place in space where gravity has become so strong that nothing can escape, including light itself. That's why we call it black: it's physically impossible to see because light can't escape. Without light, there's nothing to see.
Two types of black holes may exist in our Universe. The first are primordial black holes. They are thought to have formed shortly after the Big Bang. They would be less dense than another type of black hole, called stellar black holes.
The primordial black hole may have formed even before stars emerged. It's theorized that dense clumps of matter may have existed in the early universe, just when everything was very hot and just beginning to evolve. These clumps may have collapsed under their own gravity, forming a smaller version of what we now call a black hole.
A more common type of black hole is a stellar black hole, which forms inside a star. When a star becomes too massive at the end of its lifespan—if it's large enough, by which we mean about 25 times larger than our Sun—it collapses in on itself, becoming so dense that a black hole forms.
The Universe also contains supermassive black holes, which, as their name suggests, are quite impressive. Supermassive black holes are sometimes found at the centers of galaxies. It is theorized that they were formed by the collapse of massive stars billions of years ago, and since then, the black hole, like other stars, has been able to attract matter, growing ever larger. They may even have been able to absorb other black holes.
How many are there in total?
Basically, nothing can escape from a black hole; you don't have enough torque to escape something that even light can't escape. That means they're pretty dangerous. Surely they're not that common, right? They're like sharks in the ocean, you just have to watch out for them. That's harder to assess, though.
For a long time, the most reasonable answer to the question of how many black holes there are in the universe was: too many. Too many to count. Impossible to estimate. This is because detecting cosmic phenomena is already difficult, given the size of the universe. Detecting something you can't see is even more difficult. You need to measure gravitational waves and observable phenomena typically associated with stars. In a sense, it's like counting stones in a pond by the splashes.
Of course, any numbers we come up with will only be estimates; we're not providing exact figures. And please follow us the same way we estimate the Earth's population, without counting literally every single person. But we've made progress over the past few years, and we have a rough estimate of the number of black holes we think may exist. Are you ready? You might want to sit down, look over your shoulder, and make sure nothing's about to suck you into oblivion.
Working with large volumes of complex data, astrophysicists have calculated that there are approximately 40 quintillion black holes in the universe. In case you weren't aware, there are 18 zeros in a quintillion.
How big do they grow?
At the center of our galaxy, the Milky Way, lies a black hole known as Sagittarius A. It's not the largest black hole we've ever discovered. However, to give you an idea of its size, it's 4 million times larger than our Sun.
A typical black hole can be several times larger than our Sun, or even several hundred times. It's important to remember that this is only during its formation. Like any newborn object, it will grow. As the black hole begins to "feed," drawing in gases and particles from the surrounding system, it may begin to devour other stars, growing larger and larger. The older it gets, the larger its size will be.
As we mentioned, Sagittarius A is 4 million times larger than our Sun. You'll be pleased to know that it's a small galaxy. There's also a galaxy called Holmberg 15A, which has a black hole at its center. It's 40 billion times larger than our Sun.
Supermassive black holes like the one in the galaxy Holmberg 15A may have formed in the first billion years of our Universe. They likely devoured several stars during their lifetimes, and they may also be the result of a collision between two galaxies, one black hole consuming the other. They may also accumulate dark matter.
An interesting contrast here is the primordial black holes we discussed earlier. These black holes are much less dense than stellar black holes and supermassive black holes. While it's only theoretical, it's thought that some of them could be 100,000 times smaller than a paper clip. Thus, we're talking about rather tiny, unimpressive cosmic phenomena in terms of their size. On the other hand, they can be 100,000 times denser than our Sun, so an encounter with them might still not be worth it.
What happens inside a black hole?
Do you like spaghetti? You'll agree, it's quite tasty. And of all the pasta options, it's one of the best. It's easy to eat, simply twirling it around your fork or slurping it, not exactly fancy. It's fun! Unless, of course, spaghetti is your thing.
Spaghettification is a ridiculously funny name for the process that occurs when an object falls into a black hole of the right mass. The hole's gravity will essentially pull you toward it, like spaghetti. If you've ever tried to form yourself into a spaghetti shape, you know it's impossible to do comfortably. It will kill you, badly.
Even in an object as small as the space shuttle, the gravitational forces at the front will be significantly stronger than at the rear. This is a result of the steep gravitational gradient you experience as you approach the black hole. Imagine a bowl of Jell-O, where you use a straw to suck it from the middle, upper part.
The bottom of the bowl won't be damaged for now, but you'll destroy everything the straw touches. The same thing will happen to your ship. Everything will accumulate in that straw, whether it was intended for that purpose or not. The tidal forces of the black hole will stretch you out and then tear you apart. This is spaghettification.
It's important to remember that spaghettification isn't possible in every black hole. It depends largely on the mass of the black hole itself. Supermassive black holes have such a strong gravitational pull that this gradient won't affect you at all. It wouldn't be a pleasant experience; you'd simply fall in and experience all the other terrible things that come with it.
On the other hand, while this isn't necessarily a positive, you'll likely die long before that happens. Around a black hole exists what's called an accretion disk, where all the particles of matter that haven't yet been absorbed accumulate. It's like a cosmic ring around a bathtub, all this goo surrounding the center.
The problem with the accretion ring is that it's subject to the gravitational force of the black hole itself. It gets so hot that it begins to emit radiation. Gravity and friction combine to produce X-rays and gamma rays. So, before the black hole turns you into cosmic marshmallows, the accretion disk will incinerate you.
But that's not all! Because of the time dilation effect inherent to a black hole, as you approach the event horizon, to most of the rest of the universe you'll appear to be slowing down. In fact, you could come to a complete stop and become frozen at this point relative to the rest of the universe, potentially forever. To achieve this, we have to ignore spaghettification and radiation. But, as already noted, once you enter the event horizon, you'll never leave.
So what is an event horizon?
The term "Event Horizon" can be a bit difficult to understand. It sounds grandiose, doesn't it? It has the word "event" in it. And "horizon" is what you see in the distance. So it sounds like something huge, monumental, and far away. In reality, it's simply the boundary of the black hole. Think of it as a wall that isn't physical. It's the part that separates the black hole from the space around it.
The key feature of a black hole, what makes its event horizon so crucial, is that it's the point of no return. Technically, it's what allows us to see a black hole. Since light can't escape beyond the event horizon, we have to look at its edge to even know the black hole exists. This is where all the gases, cosmic dust, unfortunate UFOs, and other objects accumulate, causing them to heat up dramatically before crossing this boundary. It's like a cosmic whirlpool, called a halo because it's visible in an incredibly dark region that no Earth-based instrument can see.
Can anything escape from a black hole?
Given the gravitational force so strong that light can't escape it, it seems a reasonable and safe assumption that nothing in the universe could escape a black hole unless it were capable of traveling faster than the speed of light. Physics tells us that this is actually impossible, except in theoretical cases. But there are some things that can resist the gravitational force of a black hole: subatomic particles.
Black holes emit jets of plasma filled with positrons and electrons. These jets fly out from the black hole itself, back toward the event horizon, at speeds close to the speed of light. There, they bounce violently and are ejected beyond the event horizon again, back into space. But how the hell does this happen?
The working theory is that the black hole attracts all these particles, some of which are negative-energy particles. It's likened to negative-calorie food—a food that requires more calories to digest than you get from digesting it. Thus, the black hole fills with these negative-energy particles, which effectively drain energy from the black hole, reducing its overall force and accelerating the particles themselves. While inside the black hole, they gain energy, while the black hole loses energy.
So, while most substances will never be able to escape, there are limited circumstances in which particles can. And as for us? We're out of luck.
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