Black holes and how they work


How do they work, like I’m just confused I don’t understand anything about them

In: Physics

There’s a lot to learn here! Where to start?

Stars have life stages. They spend a lot of time burning fuel and producing light. Way later, after they’ve run out of fuel and maybe gone through some other stages, stars end up condensing down at the end of their lives. They get way smaller and way denser.

The biggest stars condense down so much that they become a black hole. Black holes are basically dead stars that were super massive, so when they condensed down they became super dense. Even denser than the densest elements we know of! Atoms have empty space between the nucleus and the electron shells. But black holes are so dense that they can’t have the empty space there. So the atoms kind of condense down inwards.

Now comes the spooky part. We don’t actually know what black holes look like or what structure they have. This is because they have so much gravity that everything gets pulled in, even light itself if it’s close enough! There is a radius around a black hole that light can’t escape because it’s too close so the gravity is too strong. We call that area the “event horizon.” We call it that because outside of the event horizon we can see what’s going on. Light will get to earth and we can record the “events” that are happening. Inside the event horizon, no light can escape, so we can never really see what happens inside there. We can only make educated guesses.

It’s an admittedly mind-bending concept. But ultimately, it’s just a very dense, very massive object. It’s so dense that the gravity generated from its mass will not let anything escape, not even light.

The most common way for these to happen is a supernova, or a very large star exploding. That leaves behind the super-dense core of the star, which was compressed both by the star’s own weight and the star exploding. If that core got dense enough, it will become a black hole.

For example, let’s imagine earth. It has enough gravitational pull that nearby asteroids can be pulled off their course by the earth, but rockets can still escape earth’s gravity well. If earth had 100x the mass, it would have 100x the gravity. That’s enough to make even the best rockets unable to escape; they just don’t have the speed and would fall back down to the surface. At 100,000x the mass, you could probably measure light’s path being extremely slightly bent as it passes. (IRL earth does theoretically bend light, but it’s such a small effect that we can’t measure it.) At roughly 1,000,000,000x the mass, it could significantly bend light’s path much like earth currently bends a nearby asteroid’s path. At roughly 10,000,000,000,000x earth’s mass (1 with 13 zero’s), an earth-sized object would have so much gravity that not even light could escape; it’d be like a rocket ship at 100x gravity, where it could get off the surface of the object, but it couldn’t actually escape.

(Those Numbers used are probably off, as I didn’t actually calculate the mass of an earth-sized black hole, but they’re good enough for this illustration.)

To understand black holes, you must first understand the concept of gravity. In the 1920s, Albert Einstein formulated his general theory of relativity, which postulates that gravity is not a force that propagates through space, but a warping or curvature of space-time. The greater the mass of an object, the greater the curvature in space time and the greater the gravitational force.

A black hole is the result of the curving of space time by a huge mass. The gravity is so strong because matter has been squeezed into a tiny space. Stellar black holes are formed when center of a very big star falls in upon itself, or collapses.

Around a black hole there is a position of no return, called the event horizon. A black hole it absorbs all the light that hits it, reflecting nothing. A black hole is found by its interaction with matter and gas. Scientists study how strongly gravity affects the stars and gas in a region to work out the presence of a black hole.

One common misconception about them also is that they are giant sucking machines, which is not true. If our sun were to instantly turn into a black hole we would continue to orbit it happily.

Imagine a big, stretchy sheet, pulled out tightly. That’ll be space.

Now imagine pushing a toy car across the sheet. That’ll be an object moving through space.

Now imagine putting a small rock on the sheet. Let’s say that’s a planet out in space. It’ll sink down into a bit of a pocket, right? That’s gravity affecting space.

You push the toy car past the rock.

* Maybe it just flies over the sunken-down rock and keeps going. That’s an object moving quickly enough to overcome the planet’s gravity and continue onwarde.
* Maybe it’s rolls down into sunken-down area that the rock is making, and instead of continuing in its straight line, it starts rolling around the sides of the pocked. That’s an object being pulled in by the planet’s gravity and starting to orbit it.

All good? OK.

Now imagine putting a magical marble on the sheet. This magical marble looks just like a normal marble, the same size as you’d expect…except it magically weighs a hundred tons. That magical marble isn’t just going to “sink down into a bit of a pocket”, right? When you put it on that stretchy sheet, the marble is gonna *slam* straight down to the ground. That’s our black hole.

Now push the toy car past the magical marble.

If that toy car gets anywhere near the marble, it’s heading straight down to it and it’s never coming back up. The pocket around the marble is *so* big and *so* deep that once something goes down there, it ain’t coming back. That’s how black holes work.