Where the boundary lies
The horizon is the distance from the center where escape velocity equals the speed of light. Closer than that, nothing can get out - not because anything holds a signal back: all trajectories there lead inward. There is no matter at the boundary itself; it is pure geometry of empty space.
The horizon's radius is set by mass and grows in proportion to it. Compress Earth into a black hole, and the boundary would be a cherry-sized sphere. For the Sun, it would be a sphere with a radius of about 3 km. For Sagittarius A* at the center of our Galaxy, with roughly 4 million solar masses, the horizon lies 12 million km out.
There is no local sign of this line: instruments on a falling spacecraft would show nothing. And you cannot calculate the horizon's position from one nearby region - it depends on the black hole's entire future. If a gas cloud falls into it a year from now, the horizon will start expanding in advance.
Why the falling person won't feel the crossing
The boundary itself is not dangerous - the difference in gravity is: your feet are pulled more strongly than your head, stretching your body. At the horizon, this difference is weaker the more massive the black hole. Near a stellar-mass black hole, a person would be torn apart hundreds of kilometers before reaching the horizon. At a supermassive one, the stretching at the boundary is so slight that you would not feel it - the tearing would happen later, deep inside.
How noticeable crossing the boundary is for someone falling into a supermassive black hole.
And yet everything has changed. Beyond the horizon,
What you see from the outside
From the outside, the picture is reversed. The closer an object gets to the horizon, the more slowly its clock runs for a distant observer, and the redder its light becomes. Formally, its image never crosses the boundary: it freezes forever at the very edge.
But
Brightness of a falling object for an outside observer: each step is 3 times dimmer.
The orange ring in black hole images is not the horizon either. The dark spot in the center is about 2.6 times larger in radius: light passing nearby curves around the black hole, and this optics sets the shadow's edge. All of this is predicted by general relativity; physicists still debate what happens at the horizon when quantum effects are taken into account.
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Sources
- Horizon sizes - the Schwarzschild solution (1916). Shadow images - Event Horizon Telescope: M87* (2019), Sagittarius A* (2022)