Black holes are perhaps the most mysterious objects in the universe. They are so dense that the force of gravity does not allow anything, not even light, to leave the black hole. Physicists have discovered many black holes, ranging from small to supermassive, weighing in the millions or billions of the sun. An important property of the event horizon - that light cannot cross it - creates a boundary in space: once you cross it, you are doomed to be in a singularity. But what will you see falling into a black hole? Will the lights go out or stay? Physicists know the answer, and you'll love it.
At the center of our own galaxy, we saw stars move around a central point of mass 4 million solar masses, emitting no light. This object, Sagittarius A *, is a clear black hole candidate that we can determine directly by measuring stars in its orbit.
But there are some very strange things that happen when you get closer to the horizon of a black hole, and they get even weirder when you cross it. There is a reason why you, having overcome this invisible barrier, will never be able to leave it. And it doesn't matter what class of black hole sucked you in, what spaceship is trying to take you out of there, or something else. General relativity is a big deal, especially when it comes to black holes. The reason has to do with Einstein's greatest achievement: it has to do with HOW a black hole bends spacetime.
When you are very far from the black hole, the fabric of space is curved less. In fact, when you are very far from a black hole, its gravity is indistinguishable from any other mass, be it a neutron star, an ordinary star, or just a diffuse cloud of gas. Spacetime can be curved, but all you can tell from afar is the presence of mass, with no data on the distribution of that mass. But if you look with your own eyes, then instead of a cloud of gas, a star or a neutron star, there will be an absolutely black sphere in the center that does not emit any light.
This spherical region, known as the event horizon, is not something physical, but rather a region of space of a certain size from which light cannot escape. One would assume that from a distance the size of the black hole seems to be what it really is. In other words, if you get close to a black hole, it will look like a completely black hole against the background of space, along the edges of which light is distorted.
For a black hole with the mass of the Earth, this sphere will be tiny: about 1 centimeter in radius; and for a black hole with a mass of the sun, this sphere will be about 3 kilometers in radius. If you scale the mass (and size) to a supermassive black hole - like the one in the center of our galaxy - you get the size of a planetary orbit or a giant red star like Betelgeuse.
What happens when you get close and eventually fall into a black hole?
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From a distance, the geometry of what you see will meet your expectations and calculations. But as you progress in your perfectly constructed and indestructible spacecraft, you will begin to notice something strange as you approach the black hole. If you divide the distance between you and the star in half, the angular size of the star appears to be twice as large. If you shorten the distance to a quarter, it will be four times larger. But black holes are different.
Unlike all other objects that you are used to, which the closer they seem, the larger they seem, the black hole grows in size much faster, thanks to the incredible curvature of space.
From our point of view on Earth, a black hole at the galactic center will appear tiny, its radius measured in micro-arc seconds. But compared to the naive radius that you calculate in general relativity, it will seem 150% larger due to the curvature of space. If you get close to it, by the time the event horizon is the size of the full moon in the sky, it will be four times larger than that. The reason, of course, is that spacetime bends more and more as you get closer to the black hole.
Conversely, the observed area of the black hole grows more and more; by the time you are within several Schwarzschild radii from it, the black hole will grow to such a size that it will obscure almost the entire forward view of the ship. Regular geometric objects do not behave this way.
As you approach the innermost stable circular orbit - which is 150% of the radius of the event horizon - you will notice that the forward view on your ship will turn completely black. Once you cross this one exactly, even behind you everything will start to plunge into darkness. Again, this has to do with how the paths of light from different points move in this highly curved spacetime.
At this point, if you have not crossed the event horizon, you can still exit. If you apply sufficient acceleration away from the event horizon, you can leave its gravity and return to safe space-time away from the black hole. Your gravity sensors will tell you where the downward gradient towards the center gives way to a plane where starlight can be seen.
But if you keep falling towards the event horizon, you will eventually see the starlight shrink to a tiny point behind you, changing color to blue due to gravitational blue shift. At the last moment, when you cross the event horizon, this point will turn red, white, and then blue as the cosmic microwave and radio wave backgrounds shift into the visible spectrum.
And then … there will be darkness. Nothing. From within the event horizon, no light from the outer universe can reach your ship. Now you will think about the powerful engines of your ship and think about how you could escape from this trap with their help. You will remember in which direction the singularity lay, and try to determine the gravitational gradient towards it. This is provided that there is no other matter or light behind or in front of you.
Surprisingly, even if a lot of light falls outside the event horizon with you - you will see "half" of the visible Universe - you will also have gravitational sensors on board. And as soon as you cross the event horizon, with or without light, something strange will happen.
Your sensors will tell you that the gravitational gradient that goes towards the singularity will be everywhere, in all directions. Even in the opposite direction to the singularity.
How is this possible?
And like that, because you are beyond the event horizon, right there. Any ray of light that you radiate now will go towards the singularity; you're too deep inside the black hole for it to go anywhere else.
How long does it take after crossing the horizon in a supermassive black hole to be in its center? Believe it or not, even though the event horizon can be a light hour in diameter in our frame of reference, it only takes about 20 seconds to reach the singularity. Severely curved space is a scary thing.
The worst part is that any acceleration will bring you closer to the singularity even faster. It is impossible to increase the survival time at this stage. The Singularity exists in all directions, wherever you look. Resistance is futile.
Ilya Khel