Макс Тегмарк - Life 3.0 - Being Human in the Age of Artificial Intelligence

Figure 6.4: Part of the rotational energy of a spinning black hole can be extracted by throwing a particle A near the black hole and having it split into a part C that gets eaten and a part B that escapes—with more energy than A had initially.

Spinning Black Holes

Fortunately, there are other ways of using black holes as power plants that don’t involve quantum gravity or other poorly understood physics. For example, many existing black holes spin very fast, with their event horizons whirling around near the speed of light, and this rotation energy can be extracted. The event horizon of a black hole is the region from which not even light can escape, because the gravitational pull is too powerful. Figure 6.4 illustrates how outside the event horizon, a spinning black hole has a region called the ergosphere, where the spinning black hole drags space along with it so fast that it’s impossible for a particle to sit still and not get dragged along. If you toss an object into the ergosphere, it will therefore pick up speed rotating around the hole. Unfortunately, it will soon get eaten up by the black hole, forever disappearing through the event horizon, so this does you no good if you’re trying to extract energy. However, Roger Penrose discovered that if you launch the object at a clever angle and make it split into two pieces as figure 6.4 illustrates, then you can arrange for only one piece to get eaten while the other escapes the black hole with more energy than you started with. In other words, you’ve successfully converted some of the rotational energy of the black hole into useful energy that you can put to work. By repeating this process many times, you can milk the black hole of all its rotational energy so that it stops spinning and its ergosphere disappears. If the initial black hole was spinning as fast as nature allows, with its event horizon moving essentially at the speed of light, this strategy allows you to convert 29% of its mass into energy. There is still significant uncertainty about how fast the black holes in our night sky spin, but many of the best-studied ones appear to spin quite fast: between 30% and 100% of the maximum allowed. The monster black hole in the middle of our Galaxy (which weighs four million times as much as our Sun) appears to spin, so even if only 10% of its mass could be converted to useful energy, that would deliver the same as 400,000 suns converted to energy with 100% efficiency, or about as much energy as we’d get from Dyson spheres around 500 million suns over billions of years.

Quasars

Another interesting strategy is to extract energy not from the black hole itself, but from matter falling into it. Nature has already found a way of doing this all on its own: the quasar. As gas swirls even closer to a black hole, forming a pizza-shaped disk whose innermost parts gradually get gobbled up, it gets extremely hot and gives off copious amounts of radiation. As gas falls downward toward the hole, it speeds up, converting its gravitational potential energy into motion energy, just as a skydiver does. The motion gets progressively messier as complicated turbulence converts the coordinated motion of the gas blob into random motion on ever-smaller scales, until individual atoms begin colliding with each other at high speeds—having such random motion is precisely what it means to be hot, and these violent collisions convert motion energy into radiation. By building a Dyson sphere around the entire black hole, at a safe distance, this radiation energy can be captured and put to use. The faster the black hole spins, the more efficient this process gets, with a maximally spinning black hole delivering energy at a whopping 42% efficiency. *4For black holes weighing about as much as a star, most of the energy comes out as X-rays, whereas for the supermassive kind found in the centers of galaxies, much of it emerges somewhere in the range of infrared, visible and ultraviolet light.

Once you’ve run out of fuel to feed your black hole, you can switch to extracting its rotational energy as we discussed above. *5Indeed, nature has already found a way of partially doing that as well, boosting the radiation from accreted gas through a magnetic process known as the Blandford-Znajek mechanism. It may well be possible to use technology to further improve the energy extraction efficiency beyond 42% by clever use of magnetic fields or other ingredients.

Sphalerons

There is another known way to convert matter into energy that doesn’t involve black holes at all: the sphaleron process. It can destroy quarks and turn them into leptons: electrons, their heavier cousins the muon and tau particles, neutrinos or their antiparticles.4 As illustrated in figure 6.5, the standard model of particle physics predicts that nine quarks with appropriate flavor and spin can come together and transform into three leptons through an intermediate state called a sphaleron. Because the input weighs more than the output, the mass difference gets converted into energy according to Einstein’s E = mc 2formula.

Future intelligent life might therefore be able to build what I’ll call a sphalerizer: an energy generator acting like a diesel engine on steroids. A traditional diesel engine compresses a mixture of air and diesel oil until the temperature gets high enough for it to spontaneously ignite and burn, after which the hot mixture re-expands and does useful work in the process, say pushing a piston. The carbon dioxide and other combustion gases weigh about 0.00000005% less than what was in the piston initially, and this mass difference turns into the heat energy driving the engine. A sphalerizer would compress ordinary matter to a couple of quadrillion degrees, and then let it re-expand and cool once the sphalerons had done their thing. *6We already know the result of this experiment, because our early Universe performed it for us about 13.8 billion years ago, when it was that hot: almost 100% of the matter gets converted into energy, with less than a billionth of the particles left over being the stuff that ordinary matter is made of: quarks and electrons. So it’s just like a diesel engine, except over a billion times more efficient! Another advantage is that you don’t need to be finicky about what to fuel it with—it works with anything made of quarks, meaning any normal matter at all.

Figure 6.5: According to the standard model of particle physics, nine quarks with appropriate flavor and spin can come together and transform into three leptons through an intermediate state called a sphaleron. The combined mass of the quarks (together with the energy of the gluon particles that accompanied them) is much greater than the mass of the leptons, so this process will release energy, indicated by flashes.

Because of these high-temperature processes, our baby Universe produced over a trillion times more radiation (photons and neutrinos) than matter (quarks and electrons that later clumped into atoms). During the 13.8 billion years since then, a great segregation took place, where atoms became concentrated into galaxies, stars and planets, while most photons stayed in intergalactic space, forming the cosmic microwave background radiation that has been used to make baby pictures of our Universe. Any advanced life form living in a galaxy or other matter concentration can therefore turn most of its available matter back into energy, rebooting the matter percentage down to the same tiny value that emerged from our early Universe by briefly re-creating those hot dense conditions inside a sphalerizer.