Black holes are perhaps one of the most mysterious and intriguing objects in the universe. Misconceptions and misinformation surround black holes. But one thing can be certain, kids are fascinated by them. In my time working outreach for NASA, I have encountered quite a few repeat questions about these strange stellar phenomena. Thus, it is my obligation to clear up some of the burning questions many people have, or else I might break out in a rash.
What are black holes?
The easiest place to start is to describe and define what exactly a black hole is. A black hole is often the remnant of a supergiant star many times the mass of our sun. Our Sun will not become a black hole when it dies in several billion years, so rest assured, you don’t have to worry about that. (1)
Now, pinning down the exact size the star must be to become a black hole has a few moving parts. When a supergiant finally reaches the end of its life cycle, after undergoing a few partial collapses and reignitions, it explodes in a spectacular display known as a supernova. This ejects a huge amount of its material into the space around it, allowing those elements to potentially form other celestial bodies (provided of course they don’t get pulled back into the black hole). (2)
While generally stars of about 15-20 times the mass of our Sun will become a black hole, the cutoff of mass that is required to form a black hole naturally is around 3 times the mass of the Sun. You might have noticed the “naturally” in that previous sentence, and we will get to that later.
How Big are Black Holes?
Now we get to one of the weirdest facts about black holes that doesn’t require a high level physics degree. Size in regards to a black hole is a strange topic. Multiple times above, I specifically used the word “mass” to describe the type of stars that become one. This is because, when a star collapses into a black hole, it collapses hard. The material of the former star crunches down so far that it can’t stop and eventually becomes essentially a singular point in space. This is where all the mass concentrates (this is a very basic explanation of what happens, and we can’t be for sure that’s what happens due to some reasons we’ll get into later).
Because of this, the dimensional size of the black hole, that is how wide, long, and deep the black hole is, winds up being incredibly, unbelievably small, and considering it’s an infinitely small like a singular point in space. This is known as a singularity, though very technically, singularities shouldn’t exist.
To make matters even more complicated, there is a third size related aspect of a black hole: the event horizon. This is the part of the black hole that you can “see.” The event horizon is famously the point of no return around the black hole, where light cannot escape.
How Do We Find Black Holes?
Because light cannot escape a black hole, we cannot see inside a blackhole, as we rely on light to bounce off an object and then enter our eye to be able to see it, rendering the black hole technically invisible. But worry not, we can still observe a blackhole by looking at its effects on the stuff around it. You may have heard that gravity “bends” space around an object, which causes objects to orbit or fall into each other. This is the basis of Einstein’s general relativity theory.
Going much further into this topic would take a whole semester of college, but luckily we don’t need to know much more: essentially, when light from a star or galaxy flies through space and gets close to really massive objects, like our newly formed black hole, the light in bent around it, making the star or galaxy appear in a different spot that it actually is. Imagine looking at a fish in a pond, and how the light bouncing off the fish makes it look like it’s in the wrong place. It’s a very similar effect. When massive objects do this, we call that “gravitational lensing.” (3) Using gravitational lensing, we can find places where we suspect black holes and other huge objects.
The second way we can find black holes is by finding bright clouds of dust and gas that swirl around it, known as an “accretion disk.” Generally, these surround the largest black holes, called supermassive black holes. These are largely relegated to the center of galaxies, where this gas and dust are abundant due to the black hole’s gravity.
What Happens Inside a Black Hole?
This is going to have a very unsatisfying answer to start: we don’t know. Like I said above, we can’t see inside a black hole, which makes trying to figure out what happens in them near impossible. Compounding that with the fact that the closest black hole to us is most likely the one at the center of our galaxy, which is covered in super bright stars and gas and dust, makes observations extremely difficult. In fact we only recently took a picture of the one at the center of the Milky Way, on May 12, 2022. (4) It was a concentrated effort from scientists all over the world pointing their telescopes at the same spot in the sky.
Now for what we speculate happens. Based on the mathematical equations that we figured out that govern gravity and huge stellar objects, if you fell into a black hole, it would not be fun.
Can life escape a black hole?
To start with, once passing the event horizon, that’s it, you’re gone forever. When we say light cannot escape a black hole, what we specifically mean is that not even light is fast enough to escape from a black hole. It’s not that there’s some strange property of black holes that makes it snack on light exclusively, it’s that light, you, your dog, the most powerful rocket ship you can imagine, are not fast enough to outrun the powerful gravity of a black hole once it crosses the event horizon.
Now here is where it gets strange, what happens to you inside the black hole, as far as we can tell, depends on the mass of the black hole. For one with many times the mass of our sun, you could fall in for quite some time without experiencing any ill effects. That is until you got close enough to the center. For smaller black holes, with only a few times the mass of the Sun, as well as when you do get close to the center of larger ones, the gravity becomes incredibly extreme and exhibits a strange effect. If you were to fall feet first into the black hole, the gravity at your feet would actually be many times stronger than the gravity on your head. The technical term for this is called “spaghettification” because you get stretched like a spaghetti noodle. As you can imagine, you wouldn’t survive this process. (5)
Questions from Kids I Get Frequently
This section is dedicated to the questions I frequently get from kids in particular, along with some general questions that get asked often.
What happens when two black holes fall into each other or collide?
It’s pretty anticlimactic, but they just form a single, larger black hole. There’s no big explosion, as nothing can escape a black. Now, what is fascinating about two black holes colliding is their effects on space around. If you recall above, gravity bends space around objects, and the heavier the object the more space bends. You can imagine this as a huge sheet, pulled tight, and a big ball being placed on it. The ball bends the fabric. Now if you had two large balls falling into each other and then orbiting each other, they produce microscopic ripples in the fabric as they fall closer and closer. The same thing happens to space. These waves are called gravitational waves, and we can actually detect them. (6) They aren’t detrimental to us, and you probably go your whole life not knowing the hundreds of gravitational waves propagating through you every second.
What happens if Earth/Moon/Sun/etc fell into a black hole?
Short answer, it gets ripped apart and destroyed. It would not be good for anyone involved
What if we could survive falling into a black hole?
This is one kids love to ask, and I’m glad they are curious enough to ask. That’s why it makes me sad that I can’t give them a satisfying answer. We don’t know. This falls into the realm of speculation currently, and kids love talking about wormholes and other universes. Luckily there are a few simulation videos on what it would look like to fall into a black hole, but currently we can’t confirm them to be true.
What is a white hole?
I want you to understand that there have been quite a few kids that have asked about white holes. As in a weird amount. Some of my colleagues hadn’t heard of white holes before being asked by the kiddos. Luckily, I know enough to at least placate their hungry minds. Short answer, white holes are the inverse of a black hole. Essentially they constantly emit energy, material etc and nothing can enter them. (7) Right now, these are purely theoretical objects as they would have some weird properties. Do black holes lead to a white hole somewhere in our universe or another universe? Well, read on.
Are black holes wormholes?
One theoretical version of a wormhole is called an Einstein-Rosen bridge which basically consists of a blackhole that leads to a white hole. The physics surrounding how this works is complicated but oddly enough the math works out that they could exist. The only caveat is that if Einstein-Rosen Bridges exist, we have yet to find evidence of white holes in our universe. In addition, the Bridge would necessarily have to lead to another Universe. This means that if Einstein-Rosen Bridges exist, then other universes would also exist. But that’s a big if. (8)
Can the Earth/Sun/anything become a Black Hole?
Yes, anything can become a black hole, if it is squeezed down into a small enough volume. How much it needs to be crushed depends solely on the mass of the object, giving us something called the “Schwarzschild Radius,” the radius of the sphere for how big that volume is. It is also the radius of the event horizon as viewed from outside the blackhole. Now collapsing an object this small is extremely difficult for small objects. In fact, the Schwarzschild radius of the Earth is a little less than a centimeter. Imagine taking the entirety of the Earth’s mass and crushing it down into the size of a peanut. Not easy to do. (9)