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From here: http://www.reddit.com/r/askscience/comments/f1lgu/what_would_happen_if_the_event_horizons_of_two/
(About a page down from the top. You can't miss it.)
Imagine, just for a moment, that you are aboard a spaceship equipped with a magical engine capable of accelerating you to any arbitrarily high velocity. This is absolutely and utterly impossible, but it turns out it'll be okay, for reasons you'll see in a second.
Because you know your engine can push you faster than the speed of light, you have no fear of black holes. In the interest of scientific curiosity, you allow yourself to fall through the event horizon of one. And not just any black hole, but rather a carefully chosen one, one sufficiently massive that its event horizon lies quite far from its center. This is so you'll have plenty of time between crossing the event horizon and approaching the region of insane gravitational gradient near the center to make your observations and escape again.
As you fall toward the black hole, you notice some things which strike you as highly unusual, but because you know your general relativity they do not shock or frighten you. First, the stars behind you — that is, in the direction that points away from the black hole — grow much brighter. The light from those stars, falling in toward the black hole, is being blue-shifted by the gravitation; light that was formerly too dim to see, in the deep infrared, is boosted to the point of visibility.
Simultaneously, the black patch of sky that is the event horizon seems to grow strangely. You know from basic geometry that, at this distance, the black hole should subtend about a half a degree of your view — it should, in other words, be about the same size as the full moon as seen from the surface of the Earth. Except it isn't. In fact, it fills half your view. Half of the sky, from notional horizon to notional horizon, is pure, empty blackness. And all the other stars, nearly the whole sky full of stars, are crowded into the hemisphere that lies behind you.
As you continue to fall, the event horizon opens up beneath you, so you feel as if you're descending into a featureless black bowl. Meanwhile, the stars become more and more crowded into a circular region of sky centered on the point immediately aft. The event horizon does not obscure the stars; you can watch a star just at the edge of the event horizon for as long as you like and you'll never see it slip behind the black hole. Rather, the field of view through which you see the rest of the universe gets smaller and smaller, as if you're experiencing tunnel-vision.
Finally, just before you're about to cross the event horizon, you see the entire rest of the observable universe contract to a single, brilliant point immediately behind you. If you train your telescope on that point, you'll see not only the light from all the stars and galaxies, but also a curious dim red glow. This is the cosmic microwave background, boosted to visibility by the intense gravitation of the black hole.
And then the point goes out. All at once, as if God turned off the switch.
You have crossed the event horizon of the black hole.
Focusing on the task at hand, knowing that you have limited time before you must fire up your magical spaceship engine and escape the black hole, you turn to your observations. Except you don't see anything. No light is falling on any of your telescopes. The view out your windows is blacker than mere black; you are looking at non-existence. There is nothing to see, nothing to observe.
You know that somewhere ahead of you lies the singularity … or at least, whatever the universe deems fit to exist at the point where our mathematics fails. But you have no way of observing it. Your mission is a failure.
Disappointed, you decide to end your adventure. You attempt to turn your ship around, such that your magical engine is pointing toward the singularity and so you can thrust yourself away at whatever arbitrarily high velocity is necessary to escape the black hole's hellish gravitation. But you are thwarted.
Your spaceship has sensitive instruments that are designed to detect the gradient of gravitation, so you can orient yourself. These instruments should point straight toward the singularity, allowing you to point your ship in the right direction to escape. Except the instruments are going haywire. They seem to indicate that the singularity lies all around you. In every direction, the gradient of gravitation increases. If you are to believe your instruments, you are at the point of lowest gravitation inside the event horizon, and every direction points "downhill" toward the center of the black hole. So any direction you thrust your spaceship will push you closer to the singularity and your death.
This is clearly nonsense. You cannot believe what your instruments are telling you. It must be a malfunction.
But it isn't. It's the absolute, literal truth. Inside the event horizon of a black hole, there is no way out. There are no directions of space that point away from the singularity. Due to the Lovecraftian curvature of spacetime within the event horizon, all the trajectories that would carry you away from the black hole now point into the past.
In fact, this is the definition of the event horizon. It's the boundary separating points in space where there are trajectories that point away from the black hole from points in space where there are none.
Your magical infinitely-accelerating engine is of no use to you … because you cannot find a direction in which to point it. The singularity is all around you, in every direction you look.
And it is getting closer.
(About a page down from the top. You can't miss it.)
Imagine, just for a moment, that you are aboard a spaceship equipped with a magical engine capable of accelerating you to any arbitrarily high velocity. This is absolutely and utterly impossible, but it turns out it'll be okay, for reasons you'll see in a second.
Because you know your engine can push you faster than the speed of light, you have no fear of black holes. In the interest of scientific curiosity, you allow yourself to fall through the event horizon of one. And not just any black hole, but rather a carefully chosen one, one sufficiently massive that its event horizon lies quite far from its center. This is so you'll have plenty of time between crossing the event horizon and approaching the region of insane gravitational gradient near the center to make your observations and escape again.
As you fall toward the black hole, you notice some things which strike you as highly unusual, but because you know your general relativity they do not shock or frighten you. First, the stars behind you — that is, in the direction that points away from the black hole — grow much brighter. The light from those stars, falling in toward the black hole, is being blue-shifted by the gravitation; light that was formerly too dim to see, in the deep infrared, is boosted to the point of visibility.
Simultaneously, the black patch of sky that is the event horizon seems to grow strangely. You know from basic geometry that, at this distance, the black hole should subtend about a half a degree of your view — it should, in other words, be about the same size as the full moon as seen from the surface of the Earth. Except it isn't. In fact, it fills half your view. Half of the sky, from notional horizon to notional horizon, is pure, empty blackness. And all the other stars, nearly the whole sky full of stars, are crowded into the hemisphere that lies behind you.
As you continue to fall, the event horizon opens up beneath you, so you feel as if you're descending into a featureless black bowl. Meanwhile, the stars become more and more crowded into a circular region of sky centered on the point immediately aft. The event horizon does not obscure the stars; you can watch a star just at the edge of the event horizon for as long as you like and you'll never see it slip behind the black hole. Rather, the field of view through which you see the rest of the universe gets smaller and smaller, as if you're experiencing tunnel-vision.
Finally, just before you're about to cross the event horizon, you see the entire rest of the observable universe contract to a single, brilliant point immediately behind you. If you train your telescope on that point, you'll see not only the light from all the stars and galaxies, but also a curious dim red glow. This is the cosmic microwave background, boosted to visibility by the intense gravitation of the black hole.
And then the point goes out. All at once, as if God turned off the switch.
You have crossed the event horizon of the black hole.
Focusing on the task at hand, knowing that you have limited time before you must fire up your magical spaceship engine and escape the black hole, you turn to your observations. Except you don't see anything. No light is falling on any of your telescopes. The view out your windows is blacker than mere black; you are looking at non-existence. There is nothing to see, nothing to observe.
You know that somewhere ahead of you lies the singularity … or at least, whatever the universe deems fit to exist at the point where our mathematics fails. But you have no way of observing it. Your mission is a failure.
Disappointed, you decide to end your adventure. You attempt to turn your ship around, such that your magical engine is pointing toward the singularity and so you can thrust yourself away at whatever arbitrarily high velocity is necessary to escape the black hole's hellish gravitation. But you are thwarted.
Your spaceship has sensitive instruments that are designed to detect the gradient of gravitation, so you can orient yourself. These instruments should point straight toward the singularity, allowing you to point your ship in the right direction to escape. Except the instruments are going haywire. They seem to indicate that the singularity lies all around you. In every direction, the gradient of gravitation increases. If you are to believe your instruments, you are at the point of lowest gravitation inside the event horizon, and every direction points "downhill" toward the center of the black hole. So any direction you thrust your spaceship will push you closer to the singularity and your death.
This is clearly nonsense. You cannot believe what your instruments are telling you. It must be a malfunction.
But it isn't. It's the absolute, literal truth. Inside the event horizon of a black hole, there is no way out. There are no directions of space that point away from the singularity. Due to the Lovecraftian curvature of spacetime within the event horizon, all the trajectories that would carry you away from the black hole now point into the past.
In fact, this is the definition of the event horizon. It's the boundary separating points in space where there are trajectories that point away from the black hole from points in space where there are none.
Your magical infinitely-accelerating engine is of no use to you … because you cannot find a direction in which to point it. The singularity is all around you, in every direction you look.
And it is getting closer.