Most black holes are formed when a massive star at least 30 times the mass of our sun runs out of fuel. It then collapses uncontrolled against all other forces of nature into a theoretically infinitesimally small volume. This region of infinitesimal spacetime is called a singularity. The spherical volume of space around this singularity is what we call a Black Hole. The radius of this region was first described by German physicist Karl Schwarzschild.
The edge of this radius is called the event horizon. It is the point beyond which nothing can escape, not even light. Unlike the singularity at its center, General relativity does describe what happens inside this region beyond the event horizon.
Presuming the mass is concentrated in a tiny volume inside, what we would have between this singularity and the edge of the black hole, is mostly empty space. It may contain things that have fallen through, but if the black hole is not very active, that is, if things are not falling into it, then there would be just empty space inside.
But this empty space would not be anything like the empty space that astronauts see far away from earth. First you wouldn’t see all black on the inside. It would be lit up from all the light that’s falling into it. And everything would be falling towards the singularity. And this singularity would appear to be located in all directions. No matter which way you moved, you would be moving towards it. There would be no other path.
Let’s now follow this astronaut all the way from his spaceship far away from the black hole to the black hole and then to the inside of the black hole, and on to the singularity. Even though time would appear to stop at the event horizon from a perspective far away from the black hole, from the Astronaut’s perspective, time would tick normally and He would simply just go right on through to the inside.
If he were to look back towards you, he wouldn’t see any visible light because most of the light near the event horizon would be so highly blue shifted that it would be in the x-ray part of the light spectrum. Infrared light and the cosmic microwave background light however would now be in the visible spectrum, so the astronaut might see these.
Also the light from all around the black hole may be visible to him, since the severe gravitational well would bend light from all around the black hole. But it would be distorted. What about spaghettification? Ironically, the more massive a black hole is, the less dangerous it is from the perspective of its gravity ripping your body apart.
This is because while gravity grows linearly with mass, it decreases with the square of the distance. Since making a black hole bigger means making its event horizon further away from the singularity where all the mass of the black hole is contained, the net effect is that the gravitational field at the surface goes down. As a consequence, for a sufficiently massive black hole, its surface gravity at the event horizon can be as small as it is on earth.
Now, the astronaut has made it inside the event horizon, what does he see now?
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The singularity, if it exists, would be felt in every direction. No matter which way the astronaut would try to move, he would still be moving towards the singularity. The true nature of the singularity is still a mystery. It is not clear that it would be a physical place inside.
But what would the astronaut actually see after he went inside? Assuming he is not moving at the speed of light,
he would be able to see the light that entered the black hole behind him. He would still see the outside world from inside the black hole. But this light would be a beam of light that would get smaller and smaller as he continued his journey. It would be a beam because only the light directly behind him could reach him. The light from the sides would be headed towards the singularity, so it would not travel sideways for him to be able to view it.
In front he would see total darkness because no light would be able to move backward toward him. So as he approached the singularity, his world would steadily darken until he gets the last glimpse of light from behind, and he smashes into the unknown physics of the singularity.
