The universe destroys something in different ways. If you try to hold your breath in space, your lungs will explode; if you breathe in every molecule of air instead, you will lose consciousness. In some places, you will freeze, having lost the last of your body heat; others will be so hot that the atoms in your body will turn into plasma. But of all the ways the universe gets rid of objects, the most fun is to send it into a black hole.
What's Beyond the Event Horizon?
According to our theory of gravity - Einstein's general theory of relativity - the properties of a black hole are determined by three things. Namely:
- Mass, or the total amount of matter and the equivalent amount of energy (according to the formula E = mc2), which go to the formation and growth of a black hole to its current state.
- Charge, or the total electrical charge that exists in a black hole from all positively and negatively charged objects that fell into the black hole in the entire history of its life.
- Angular momentum (moment), or spin, which is a measure of the total amount of rotational motion that a black hole has by nature.
In reality, all black holes that physically exist in our universe must have large masses, significant amounts of angular momentum, and negligible charges. This makes the situation extremely difficult.
When we usually imagine a black hole, we imagine a simple version of it, which is described only by its mass. It has an event horizon surrounding one point, and an area surrounding that point, beyond which light cannot go. This area is completely spherical and has a boundary separating areas from which light can escape and from which it cannot: the event horizon. The event horizon is at a certain distance (Schwarzschild radius) from the singularity in all directions simultaneously.
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This is a simplified version of a realistic black hole, but a great place to start thinking about physics taking place in two different places: beyond the event horizon and inside the event horizon.
Outside the event horizon, gravity behaves as you'd normally expect. Space bends in the presence of mass, which causes every object in the universe to accelerate towards the central singularity. If you were at a great distance from a black hole at rest and let an object fall into it, what would you see?
Assuming you've managed to keep still, you will see the falling object slowly accelerate from you towards this black hole. It will accelerate towards the event horizon, after which something strange will happen. It will seem to you that it slows down, fades out and becomes redder. But it will not disappear completely. It will only get closer to it: it will become dull, red and more difficult to detect. You can always see it if you look closely enough.
Now let's imagine the same scenario, but this time let's imagine that you are the same object falling into a black hole. The experience will be completely different.
The event horizon will get bigger much faster than you expected as the curvature of space gets stronger. Space is so curved around the event horizon that you will see many images of the universe, which is from the outside, as if it were reflected and turned over.
And once you cross the event horizon, you will not only still be able to see the outer universe, but part of the universe within the event horizon. In the final moments, the space will look completely flat.
What's in a black hole?
The physics of all of this is complex, but the calculations are fairly simple and most elegantly done by Andrew Hamilton of the University of Colorado in a series of papers from the late 2000s and early 2010s. Hamilton also created a series of impressive renderings of what you will see falling into a black hole based on these calculations.
After examining these results, we can draw a number of conclusions, many of which are illogical. To try to make sense of them, you need to change the way you represent space. We usually think of it as an immobile fabric and think that the observer is "descending" somewhere. But within the event horizon, you are always on the move. Space moves - like a treadmill - continuously, moving everything in itself towards the singularity.
And it moves everything so fast that even if you accelerate straight from the singularity with infinite force, you will still fall towards the center. Objects beyond the event horizon will still send you light from all directions, but you will only be able to see a fraction of the objects from beyond the event horizon.
The line that defines the boundary between what any observer can see is mathematically described by the cardioid, where the component with the largest radius touches the event horizon and the component with the smallest radius is at the singularity. This means that a singularity, even as a point, does not necessarily connect everything that falls into it with everything else. If you and I fall into the event horizon from different directions at the same time, we will never see each other's light after the event horizon crosses.
The reason for this is the constantly moving fabric of the Universe itself. Inside the event horizon, space travels faster than light, so nothing can escape from the black hole. That's why when you hit a black hole, you start to see strange things like multiple images of the same object.
You can understand this by asking the question: where is the singularity?
From within the black hole event horizon, whichever direction you move, you end up encountering the singularity itself. Therefore, oddly enough, the singularity appears in all directions. If your legs are pointing in the direction of acceleration, you will see them in front of you, but also above you. All this is easy to calculate, albeit extremely illogical. And that's just for a simplified case: a non-rotating black hole.
Now let's move on to the physically interesting case: when the black hole rotates. Black holes owe their origin to systems of matter - like stars - that always rotate at some level. In our universe (and in general relativity), angular momentum is the absolute enclosed quantity for any closed system; there is no way to get rid of it. When the aggregate of matter collapses to a radius that is less than the radius of the event horizon, the angular momentum is trapped inside it, just like mass.
The solution we have here will be much more complicated. Einstein presented general relativity in 1915, and Karl Schwarzschild obtained a solution for a non-rotating black hole a couple of months later, in early 1916. But the next step in modeling this problem in a more realistic way - where the black hole has angular momentum, not just mass - was only taken in 1963, when Roy Kerr found the exact solution in 1963.
There are several fundamental and important differences between Schwarzschild's more naive and simpler solution and Kerr's more realistic and complex solution. Among them:
- Instead of a single decision about where the event horizon is, a rotating black hole has two mathematical solutions: an inner and outer event horizon.
- Beyond even the outer event horizon, there is a place known as the ergosphere, in which space itself moves at a rotational speed equal to the speed of light, and the particles in it experience tremendous accelerations.
- There is a maximum allowable ratio of angular momentum to mass; if the momentum is too strong, the black hole will radiate this energy (through gravitational radiation) until it drops to the limit.
- And the most interesting thing: the singularity at the center of the black hole is no longer a point, but a one-dimensional ring, the radius of which is determined by the mass and angular momentum of the black hole.
With all this in mind, what happens when you hit a black hole? Yes, it's the same as what happens if you fall into a non-rotating black hole, except that all space does not behave as if it is falling towards the central singularity. Instead, space also behaves as if it moves along the direction of rotation, like a swirling funnel. The greater the ratio of angular momentum to mass, the faster it rotates.
This means, if you see something falling into a black hole, you will see that it becomes dimmer and redder, but also smeared into a ring or disk in the direction of rotation. If you fall into a black hole, you will be spun like a carousel that pulls you towards the center. And when you reach the singularity, it will be a ring; different parts of your body will meet a singularity - on the inner ergosurface of the Kerr black hole - in different spatial coordinates. You will gradually stop seeing other parts of your own body.
The most important thing you need to understand from all this is that the fabric of space itself is in motion, and the event horizon is defined as a place in which even if you move at the speed of light, whichever direction you choose, you will inevitably collide with a singularity.
Andrew Hamilton's visualizations are the best and most accurate models of what happens when you fall into a black hole, and so illogical that they have to be viewed over and over again until you begin to understand something (you don't really start). It's creepy and beautiful, and if you're adventurous enough to ever fly into a black hole and cross the event horizon, this will be the last thing you've ever seen.
Ilya Khel