Sunday, April 7, 2013

How Would an Astronaut Falling Into a Black Hole Die?

No matter the size of a black hole, gravitational acceleration at the event horizon is c per Planck time. That's not "infinite", but it is maximal. Anything at the event horizon of a black hole will have its velocity increased by c toward the singularity as quickly as theoretically possible. You cannot build a rocket to put out that kind of acceleration in the other direction to keep you in place, because you cannot build a rocket to get you up to c at all, in any amount of time. Your talk about time axes sounds like it's an echo of a description of why you can't build a rocket to get up to c. It has nothing to do with black holes specifically, other than that you would need to get that fast to escape the event horizon of one.

If you were made of light, however, you would be moving at c, and you could orbit at the event horizon. If you're just above the event horizon, you could in theory get moving fast enough to orbit just outside of it. And in orbit, you don't feel any acceleration from gravity; you are free-falling around the black hole, and continually missing it. You could also have some flight path requiring 1g of constant acceleration to keep you from falling in. The size of the black hole doesn't matter for any of that; if you are anywhere outside the event horizon, you can find a flight path that will make you feel any amount of acceleration you want, for as long as you have fuel to maintain that kind of acceleration. (For an orbit, you feel zero acceleration, and so need no fuel and can maintain it indefinitely).

Where the size of a black hole does matter, and what I think you were thinking of in your earlier post about black holes of 100 solar masses or such, is tidal forces. These are the forces which pinch and stretch your body in uneven ways. Imagine you had a tetherball pole in the middle of a schoolyard. You stand far off to the east of it, facing it, with your arms outstretched. The lines from both of your hands, and your elbows, and your nose, toward the tetherball pole, are all roughly westward, so if you were to be pulled toward it, your whole body would be pulled more or less evenly. But if you stand right next to it, with your arms stretched out, your nose is pulled west, but your right hand is now north of it, and your left hand is south of it, so they get pulled south and north respectively, and the pole pulls your hands toward each other. If, like gravity, it also pulls harder the closer to it you are, it will pull your face toward it much harder than it will your hands, and make you smack your nose into it and then hit yourself as your hands fall in behind your head; while from a long ways away, all your body parts are pulled with about the same force.

Likewise with black holes. The closer you are to one, the more the different parts of your body (and spaceship, etc) are pulled in different directions and with different magnitudes. The farther you are from it, the more evenly everything is pulled. A very massive black hole has a very large event horizon, so at the event horizon, you are very far from the center of the black hole, and even though you are still experiencing the same acceleration you would feel at the event horizon of any black hole, it's all pulling you in more or less the same direction, so you could orbit there and suffer no ill effects. Around a small black hole though, even if you were orbiting just above its black hole and feeling no acceleration overall, the parts of you closer to it would need to orbit faster to maintain that effect and so would feel pulled and pinchedcompared to the parts of you further away from it, which would have more speed than they need to orbit and so tend to drift away from it. All in all you would feel pulled in every different direction and your body would be ripped apart. Around a larger black hole, even moving at the same speed to maintain orbit the same distance from the event horizon, all of you would feel roughly the same effects, so you wouldn't even notice them.

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/c_HfZ0hh4mY/story01.htm

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