How To Steal Energy from a Black Hole
Making the Greediest Non-Human Object in the Universe Give Up its Mass
Black holes are the most extreme places in the known Universe, sucking everything in with no hope of recovery. Not even light can escape their gravitational pull. It thus seems counterintuitive that any process can take energy from one, yet there are not one but multiple ways by which it can be done. To understand the how and the why of their workings, we first need to understand the very nature of black holes themselves.
Black holes are created by the collapse of supermassive stars, ones dozens of times the mass of our sun. When these giants run out of elements to fuse in their core, the radiation pressure keeping them stable plummets. Gravity takes over, pulling the entire star inwards. The rebounding layers hit the ones coming in, causing a spectacular supernova, something so gigantic our minds cannot comprehend their sheer scale. After the dust settles, what is left behind is the dead core. Depending on the star’s original mass, this could be either a neutron star — in cases where gravity is unable to overcome the neutron degeneracy pressure — or a black hole, where gravity wins and pushes everything together until it’s just one point: the singularity.
Now one thing the Universe likes doing is conserving things, be it mass-energy, charge, or probability. Angular momentum also happens to be on the list, which is why this article is possible in the first place. You see, every star in this Universe spins, and because each is so massive, even a slow rate results in substantial angular momenta. This has to be conserved when the star dies, which means that the black hole also has to spin. And it does, at incredible speeds, hundreds of times a second. Of course, a single point cannot rotate, so the center of a black hole is considered to be a donut with vanishing thickness but non-zero radius, a ‘ringularity’ if you will. It is this ring that rotates, conserving the angular momentum passed down to it by the Universe.
This spin is important because mass affects spacetime, and great mass affects spacetime greatly. The rotation of the ring drags the surrounding space itself as it moves, creating an ergosphere, a place where concepts such as time act strangely but are not completely broken like inside the event horizon. The ergosurface is the boundary at which light traveling opposite to the black hole’s spin appears to stand still to an external observer. This ergosphere also allows us to take energy from the spin of the ringularity by something called the Penrose process.
In a nutshell, the Penrose process involves dropping a mass into the black hole to gain energy. While this may sound counterintuitive, it nevertheless works, provided one has access to a black hole. It’s like being on a spinning stage: running against it won’t make leaving it easy; however, moving along with it will allow you to eventually build up enough speed to escape. Since momentum has to be conserved, ejecting the mass (the gold dot in the diagram) means that the rocket has to move away from the black hole with more energy than before. If carefully done, the rocket ends up having more mass-energy than it went in with, the difference being paid by the black hole. In this manner, all of the rotational energy of the black hole can be taken away until it becomes stationary.
Of course, this plan sounds sketchy at actually providing usable energy and is undoubtedly extremely risky. There is good news, however: there is another way that is much more practical (assuming any process involving black holes can ever be practical at all). This method also inadvertently creates the largest bomb any living thing can ever hope to make, a black hole bomb.
The concept is simple: surround the black hole with mirrors, release light rays inside and let them slosh around a bit. The ergosphere forces some of its rotational energy onto the light, increasing its amplitude. This is known as superradiant scattering. Sure, some light falls past the event horizon and is lost forever, but that is more than made up for by the increased amplitude of what is left. Opening a window allows one to collect the energy in a much safer way than the rocket-ship-dropping-ball method.
Haha, you thought. While it is true that this approach is much more easily manageable than the other one, it also has disastrous consequences if the window isn’t opened to let the light out. The waves would keep increasing in strength, bouncing around and around until the mirrors shatter, releasing tremendous amounts of energy on the scale of supernova explosions. Everything anywhere near the black hole would be destroyed, and the light released visible thousands of lightyears away. It would truly be the weapon to end civilizations.
Assuming it isn’t blown up, though, the good thing about this method is that it is sustainable for what is basically forever: supermassive black holes have lifetimes so obscenely long that human minds cannot distinguish them from infinity. If all goes well, the final humans in the Universe will spend their last days in lights powered by black hole rotation, wondering about their distant ancestors that used to live on a rocky mushy wet planet powered by a hot bright burning star, both relics of the Universe long gone. It is both scary and uplifting at the same time.