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https://mp.weixin.qq.com/s/CV5U4JSAv1pzy8E8qUZLYw
Blackhole Bomb
来自英文巴士
00:1908:06NB: This may not be a word-for-word transcript.
The Black Hole Bomb and Black Hole Civilizations
Black holes are the largest collections of pure violent energy in the universe. If you come too close, they’ll devour you and add your energy to their collection. And so, the energy is lost to us forever. Or is it? It turns out there’s a universe cheat code – a way of powering civilizations until the very death of everything, or of constructing the largest bomb in the universe. But how? Didn’t we learn that all energy is trapped forever in black holes, even light? This is true. Everything you think you know about the weirdest thing in the universe is about to get weirder, for one simple reason: Black holes are spinning.
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Why black holes spin
When really, really massive stars die, their cores collapse under their own gravity into black holes. This means something very big becomes very, very tiny, like the tiniest anything can be in this universe. But stars are rotating, and a fundamental property of our universe is that things that are spinning don’t want to stop spinning. We call this angular momentum. And this angular momentum can’t go away. A big thing that spins and becomes smaller, spins faster. So, as the core of a star collapses, its momentum makes it spin faster and faster and faster until it collapses into a black hole. And the black hole keeps on spinning, inconceivably fast. Some of them spin millions of times a second.
Why spinning black holes are special
Just like non-spinning black holes, spinning black holes have an event horizon and a singularity at their core, where all of their mass is concentrated. The singularity is usually described as a single infinitely small point with no surface area. But points can’t rotate, so a rotating singularity can’t be a point. Instead, it’s a ringularity. A ringularity is a ring with a thickness of zero and no surface, spinning extremely fast, containing all the mass of the black hole. The black hole is spinning so fast that it morphs space and time itself. It literally drags space with it, such is its power. This creates a new and super weird region of space-time: the ergosphere, which envelops the black hole. If space and time are completely broken inside the event horizon, then they’re only half broken inside the ergosphere.
Inside the ergosphere, nothing makes sense. It’s possible to enter it and then leave it again, but it’s probably not a great experience. You can imagine it like this: Falling into a static black hole is like sliding down a hole. Being inside the ergosphere of a spinning black hole is like spiraling down a deadly drain. The black hole transfers its own kinetic energy in the form of rotation, to everything that enters the ergosphere. The ringularity makes you dance whether you want to or not. You need to move faster than the speed of light just to stand still here, which is impossible. But here’s our cheat code: We can steal this energy, and there’s a lot of energy to steal.
How to steal energy from a monster
Take the supermassive black hole at the center of the Milky Way. We could steal as much energy from it as every single star in the Milky Way emits in a billion years combined. The easiest way to steal this energy is, oddly enough, to drop something into the black hole. We’ve seen that the ringularity forces energy on us when we enter the ergosphere, which is a lot like being in a whirlpool, with space-time rushing around and around. If you’re clever, you can use the water to your advantage, and swim faster than before. In practice, this means sending a rocket into the ergosphere and making a trade with the black hole: We give it some mass-energy, and it gives us some of its rotational energy. But it’s not a fair trade, we get the better deal.
Normally, if you fire a rocket, you exchange chemical energy for kinetic energy. This is like pushing yourself forward in a swimming pool. But if you fire a rocket inside the ergosphere, it’s like pushing yourself forward in a wave pool. The rotational energy of the waves gives you a much stronger boost than you could get just by pushing yourself. The boost from the rotation of the back hole is so big that you leave the ergosphere with much more energy than you entered it. The black hole gives a tiny amount of its rotational energy to you and slows down a little. Obviously, this requires a lot of food. Fortunately, black holes aren’t picky eaters. An advanced future civilization would probably harvest asteroids to drop them into the black hole when they needed an energy boost.
But there’s an even better way to get energy from a black hole, and oddly enough, it builds the biggest bomb any living thing could ever hope to build.
The black hole bomb
We only need two things to build a black hole bomb: a fast-spinning black hole, and a big mirror. The mirror has to completely envelop the black hole, which is similar to a Dyson Sphere, a mega structure that harvests the energy of an entire star. Although our mirror would be easier to build, mirrors are simpler, and black holes are much, much more compact than stars. If we made the mirror 10 centimeters thick, the metal of a big asteroid would probably be enough material for a black hole with the mass of our Sun.
Once our mirror is in place, we only need to open a window, and shoot electromagnetic waves at the black hole. You can imagine what happens next by imagining tossing a ball at a wall and it coming back faster than a bullet. The waves hit the black hole at light speed. A small proportion of the waves falls past the event horizon to disappear forever. But a much larger amount sloshes through the ergosphere, where the black hole forces some of its rotational energy on them and amplifies them. They now begin super radiant scattering, which are fancy science words, meaning: “Bouncing around between mirror and black hole and getting stronger”. Every time they go around, they are getting exponentially stronger. By opening some windows in the mirror, we can extract the energy from the waves as fast as they grow, which we could use, in theory, to create what would be for all practical purposes, an endless source of energy for trillions of years. Or, we could blow it up.
If the waves are not released, they will continue to get stronger and stronger, and take more and more energy from the black hole until the mirror shatters. A supermassive black hole would release as much energy as a supernova, making the bomb the largest explosion any living being could ever create.
The last home in a dying universe
The beauty of the black hole bomb, the Penrose process, and the super radiant scattering is that they are not science-fiction. In the far, far future, this might be the only way to survive in our dying universe. After all the red dwarfs have cooled down and all the white dwarfs transformed into black dwarfs, the universe will turn dark forever. Rotating black holes might be the only sources of energy in the entire universe that life could harvest. If so, the last living being in existence might one day end its life around a black hole, which is equally chilling and uplifting. It turns out that even without any light, there are places we can go.
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