This is kinda big:

The sleeping giant

In short… 300 million lightyears away in the galaxy NGC 4889 is a black hole with the mass of 21 *billion* suns. The event horizon is 130 billion kilometers in diameter. That’s 434.5 AU in *radius.* In comparison, Neptune is a measly 30 AU from the sun.

The black hole seems to be quiet… it appears to have gobbled up all the stars and gas in its immediate vicinity. It has swallowed its own accretion disk. But back when it was active, astronomers estimate that the accretion disk would have been so vast and so hot that it would have emitted a thousand times as much energy as the entire Milky Way galaxy.

Without an accretion disk, the black hole is invisible. However, astronomers have measure the velocity of stars in the region and were able to calculate the mass of the invisible object they are orbiting.

A black hole this vast would have a very gentle entry into the event horizon.  I did some back of the Excel envelope math and came up with an acceleration of gravity at the event horizon of 659.8 m/sec^2, or about 67.27 G’s. Pretty crushing if you were standing there, but if you were simply falling in the acceleration of gravity would not be felt. What you *would* feel is the tidal force. In any documentary about black holes, you will almost inevitably get to the point were they discuss tidal forces, with the presenters inevitably, and gleefully, describing the process of “spaghettification,” where your feet and head are pulled apart from each other. In short, this is because the black hole is so massive, and you are so close to it, that the force of gravity at you feet is vastly higher than the force of gravity at your head, 1.8 (or so) meters away from each other. But for *this* supermassive black hole… assuming my math is right, the delta in acceleration due to gravity at the radius of the event horizon is about 2X10^-11 m/sec^2 per meter. That… is pretty damn weak. You would fall through the event horizon without even noticing.

But when you get closer to the singularity at the center, the tidal forces would increase. At 1 AU, the acceleration is 1.25E8 m/sec^2, and the delta seems to be 124.6 *million* m/sec^2 per meter. That’ll shred ya good. At 0.1 AU, the numbers are 1.87E20 m/sec^2 and 1.87E20 M/sec^2/meter. This math was run without recourse to relativistic effects, largely because I couldn’t be bothered. But you can see that while you could slip past the event horizon without getting squashed, you’ll get nowhere near the center before things turn really quite awful.