فصل ششم : وقف کیهانی ما یک میلیارد سال بعد و فراتر از آن

Chapter 6

Our Cosmic Endowment: The Next Billion Years and Beyond

Our speculation ends in a supercivilization, the synthesis of all solar-system life, constantly improving and extending itself, spreading outward from the sun, converting nonlife into mind.

Hans Moravec, Mind Children

To me, the most inspiring scientific discovery ever is that we’ve dramatically underestimated life’s future potential. Our dreams and aspirations need not be limited to century-long life spans marred by disease, poverty and confusion. Rather, aided by technology, life has the potential to flourish for billions of years, not merely here in our Solar System, but also throughout a cosmos far more grand and inspiring than our ancestors imagined. Not even the sky is the limit.

This is exciting news for a species that has been inspired by pushing limits throughout the ages. Olympic games celebrate pushing the limits of strength, speed, agility and endurance. Science celebrates pushing the limits of knowledge and understanding. Literature and art celebrate pushing the limits of creating beautiful or life-enriching experiences. Many people, organizations and nations celebrate increasing resources, territory and longevity. Given our human obsession with limits, it’s fitting that the best-selling copyrighted book of all time is The Guinness Book of World Records.

So if our old perceived limits of life can be shattered by technology, what are the ultimate limits? How much of our cosmos can come alive? How far can life reach and how long can it last? How much matter can life make use of, and how much energy, information and computation can it extract? These ultimate limits are set not by our understanding, but by the laws of physics. This, ironically, makes it in some ways easier to analyze the long-term future of life than the short-term future.

If our 13.8-billion-year cosmic history were compressed into a week, then the 10,000-year drama of the last two chapters would be over in less than half a second. This means that although we cannot predict if and how an intelligence explosion will unfold and what its immediate aftermath will be like, all this turmoil is merely a brief flash in cosmic history whose details don’t affect life’s ultimate limits. If the post-explosion life is as obsessed as today’s humans are with pushing limits, then it will develop technology to actually reach these limits—because it can. In this chapter, we’ll explore what these limits are, thus getting a glimpse of what the long-term future of life may be like. Since these limits are based on our current understanding of physics, they should be viewed as a lower bound on the possibilities: future scientific discoveries may present opportunities to do even better.

But do we really know that future life will be so ambitious? No, we don’t: perhaps it will become as complacent as a heroin addict or a couch potato merely watching endless reruns of Keeping Up with the Kardashians. However, there is reason to suspect that ambition is a rather generic trait of advanced life. Almost regardless of what it’s trying to maximize, be it intelligence, longevity, knowledge or interesting experiences, it will need resources. It therefore has an incentive to push its technology to the ultimate limits, to make the most of the resources it has. After this, the only way to further improve is to acquire more resources, by expanding into ever-larger regions of the cosmos.

Also, life may independently originate in multiple places in our cosmos. In that case, unambitious civilizations simply become cosmically irrelevant, with ever-larger parts of the cosmic endowment ultimately being taken over by the most ambitious life forms. Natural selection therefore plays out on a cosmic scale and, after a while, almost all life that exists will be ambitious life. In summary, if we’re interested in the extent to which our cosmos can ultimately come alive, we should study the limits of ambition that are imposed by the laws of physics. Let’s do this! Let’s first explore the limits of what can be done with the resources (matter, energy, etc.) that we have in our Solar System, then turn to how to get more resources through cosmic exploration and settlement.

Making the Most of Your Resources

Whereas today’s supermarkets and commodity exchanges sell tens of thousands of items we might call “resources,” future life that’s reached the technological limit needs mainly one fundamental resource: so-called baryonic matter, meaning anything made up of atoms or their constituents (quarks and electrons). Whatever form this matter is in, advanced technology can rearrange it into any desired substances or objects, including power plants, computers and advanced life forms. Let’s therefore begin by examining the limits on the energy that powers advanced life and the information processing that enables it to think.

Building Dyson Spheres

When it comes to the future of life, one of the most hopeful visionaries is Freeman Dyson. I’ve had the honor and pleasure of knowing him for the past two decades, but when I first met him, I felt nervous. I was a junior postdoc chowing away with my friends in the lunchroom of the Institute for Advanced Study in Princeton, and out of the blue, this world-famous physicist who used to hang out with Einstein and Gödel came up and introduced himself, asking if he could join us! He quickly put me at ease, however, by explaining that he preferred eating lunch with young folks over stuffy old professors. Even though he’s ninety-three as I type these words, Freeman is still younger in spirit than most people I know, and the mischievous boyish glint in his eyes reveals that he couldn’t care less about formalities, academic hierarchies or conventional wisdom. The bolder the idea, the more excited he gets.

When we talked about energy use, he scoffed at how unambitious we humans were, pointing out that we could meet all our current global energy needs by harvesting the sunlight striking an area smaller than 0.5% of the Sahara desert. But why stop there? Why even stop at capturing all the sunlight striking Earth, letting most of it get wastefully beamed into empty space? Why not simply put all the Sun’s energy output to use for life?

Inspired by Olaf Stapledon’s 1937 sci-fi classic Star Maker, with rings of artificial worlds orbiting their parent star, Freeman Dyson published a description in 1960 of what became known as a Dyson sphere.1 Freeman’s idea was to rearrange Jupiter into a biosphere in the form of a spherical shell surrounding the Sun, where our descendants could flourish, enjoying 100 billion times more biomass and a trillion times more energy than humanity uses today.2 He argued that this was the natural next step: “One should expect that, within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star.” If you lived on the inside of a Dyson sphere, there would be no nights: you’d always see the Sun straight overhead, and all across the sky, you’d see sunlight reflecting off the rest of the biosphere, just as you can nowadays see sunlight reflecting off the Moon during the day. If you wanted to see stars, you’d simply go “upstairs” and peer out at the cosmos from the outside of the Dyson sphere.

A low-tech way to build a partial Dyson sphere is to place a ring of habitats in circular orbit around the Sun. To completely surround the Sun, you could add rings orbiting it around different axes at slightly different distances, to avoid collisions. To avoid the nuisance that these fast-moving rings couldn’t be connected to one another, complicating transportation and communication, one could instead build a monolithic stationary Dyson sphere where the Sun’s inward gravitational pull is balanced by the outward pressure from the Sun’s radiation—an idea pioneered by Robert L. Forward and by Colin McInnes. The sphere can be built by gradually adding more “statites”: stationary satellites that counteract the Sun’s gravity with radiation pressure rather than centrifugal forces. Both of these forces drop off with the square of the distance to the Sun, which means that if they can be balanced at one distance from the Sun, they’ll conveniently be balanced at any other distance as well, allowing freedom to park anywhere in our Solar System. Statites need to be extremely lightweight sheets, weighing only 0.77 grams per square meter, which is about 100 times less than paper, but this is unlikely to be a showstopper. For example, a sheet of graphene (a single layer of carbon atoms in a hexagonal pattern resembling chicken wire) weighs a thousand times less than that limit. If the Dyson sphere is built to reflect rather than absorb most of the sunlight, then the total intensity of light bouncing around within it will be dramatically increased, further boosting the radiation pressure and the amount of mass that can be supported in the sphere. Many other stars have a thousandfold and even a millionfold greater luminosity than our Sun, and are therefore able to support correspondingly heavier stationary Dyson spheres.

If a much heavier rigid Dyson sphere is desired here in our Solar System, then resisting the Sun’s gravity will require ultra-strong materials that can withstand pressures tens of thousands of times greater than those at the base of the world’s tallest skyscrapers, without liquefying or buckling. To be long-lived, a Dyson sphere would need to be dynamic and intelligent, constantly fine-tuning its position and shape in response to disturbances and occasionally opening up large holes to let annoying asteroids and comets pass through without incident. Alternatively, a detect-and-deflect system could be used to handle such system intruders, optionally disassembling them and putting their matter to better use.

For today’s humans, life on or in a Dyson sphere would at best be disorienting and at worst impossible, but that need not stop future biological or non-biological life forms from thriving there. The orbiting variant would offer essentially no gravity at all, and if you walked around on the stationary kind, you could walk only on the outside (facing away from the Sun) without falling off, with gravity about ten thousand times weaker than you’re used to. You’d have no magnetic field (unless you built one) shielding you from dangerous particles from the Sun. The silver lining is that a Dyson sphere the size of Earth’s current orbit would give us about 500 million times more surface area to live on.

If more Earth-like human habitats are desired, the good news is that they’re much easier to build than a Dyson sphere. For example, figures 6.1 and 6.2 show a cylindrical habitat design pioneered by the American physicist Gerard K. O’Neill, which supports artificial gravity, cosmic ray shielding, a twenty-four-hour day-night cycle, and Earth-like atmosphere and ecosystems. Such habitats could orbit freely inside a Dyson sphere, or modified variants could be attached outside it.

Building Better Power Plants

Although Dyson spheres are energy efficient by today’s engineering standards, they come nowhere near pushing the limits set by the laws of physics. Einstein taught us that if we could convert mass to energy with 100% efficiency,*1 then an amount of mass m would give us an amount of energy E given by his famous formula E = mc2, where c is the speed of light. This means that since c is huge, a small amount of mass can produce a humongous amount of energy. If we had an abundant supply of antimatter (which we don’t), then a 100% efficient power plant would be easy to make: simply pouring a teaspoonful of anti-water into regular water would unleash the energy equivalent to 200,000 tons of TNT, the yield of a typical hydrogen bomb—enough to power the world’s entire energy needs for about seven minutes.

In contrast, our most common ways of generating energy today are woefully inefficient, as summarized in table 6.1 and figure 6.3. Digesting a candy bar is merely 0.00000001% efficient, in the sense that it releases a mere ten-trillionth of the energy mc2 that it contains. If your stomach were even 0.001% efficient, then you’d only need to eat a single meal for the rest of your life. Compared to eating, the burning of coal and gasoline are merely 3 and 5 times more efficient, respectively. Today’s nuclear reactors do dramatically better by splitting uranium atoms through fission, but still fail to extract more than 0.08% of their energy. The nuclear reactor in the core of the Sun is an order of magnitude more efficient than those we’ve built, extracting 0.7% of the energy from hydrogen by fusing it into helium. However, even if we enclose the Sun in a perfect Dyson sphere, we’ll never convert more than about 0.08% of the Sun’s mass to energy we can use, because once the Sun has consumed about about a tenth of its hydrogen fuel, it will end its lifetime as a normal star, expand into a red giant, and begin to die. Things don’t get much better for other stars either: the fraction of their hydrogen consumed during the main lifetime ranges from about 4% for very small stars to about 12% for the largest ones. If we perfect an artificial fusion reactor that would let us fuse 100% of all hydrogen at our disposal, we’d still be stuck at that embarrassingly low 0.7% efficiency of the fusion process. How can we do better?

Method Efficiency

Digesting candy bar 0.00000001%

Burning coal 0.00000003%

Burning gasoline 0.00000005%

Fission of uranium-235 0.08%

Using Dyson sphere until Sun dies 0.08%

Fusion of hydrogen to helium 0.7%

Spinning black hole engine 29%

Dyson sphere around quasar 42%

Sphalerizer 50%?

Black hole evaporation 90%

Evaporating Black Holes

In his book A Brief History of Time, Stephen Hawking proposed a black hole power plant.2 This may sound paradoxical given that black holes were long believed to be traps that nothing, not even light, could ever escape from. However, Hawking famously calculated that quantum gravity effects make a black hole act like a hot object—the smaller, the hotter—that gives off heat radiation now known as Hawking radiation. This means that the black hole gradually loses energy and evaporates away. In other words, whatever matter you dump into the black hole will eventually come back out again as heat radiation, so by the time the black hole has completely evaporated, you’ve converted your matter to radiation with nearly 100% efficiency.3 A problem with using black hole evaporation as a power source is that, unless the black hole is much smaller than an atom in size, it’s an excruciatingly slow process that takes longer than the present age of our Universe and radiates less energy than a candle. The power produced decreases with the square of the size of the hole, and the physicists Louis Crane and Shawn Westmoreland have therefore proposed using a black hole about a thousand times smaller than a proton, weighing about as much as the largest-ever seagoing ship.3 Their main motivation was to use the black hole engine to power a starship (a topic to which we return below), so they were more concerned with portability than efficiency and proposed feeding the black hole with laser light, causing no energy-to-matter conversion at all. Even if you could feed it with matter instead of radiation, guaranteeing high efficiency appears difficult: to make protons enter such a black hole a thousandth their size, they would have to be fired at the hole with a machine as powerful as the Large Hadron Collider, augmenting their energy mc2 with at least a thousand times more kinetic (motion) energy. Since at least 10% of that kinetic energy would be lost to gravitons when the black hole evaporates, we’d therefore be putting more energy into the black hole than we’d be able to extract and put to work, ending up with negative efficiency. Further confounding the prospects of a black hole power plant is that we still lack a rigorous theory of quantum gravity upon which to base our calculations—but this uncertainty could of course also mean that there are new useful quantum gravity effects yet to be discovered.

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.*4 For 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.*5 Indeed, 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 = mc2 formula.

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.*6 We 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.

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.

To figure out how efficient an actual sphalerizer would be, one needs to work out key practical details: for example, how large does it need to be to prevent a significant fraction of the photons and neutrinos from leaking out during the compression stage? What we can say for sure, however, is that the energy prospects for the future of life are dramatically better than our current technology allows. We haven’t even managed to build a fusion reactor, yet future technology should be able to do ten and perhaps even a hundred times better.

Building Better Computers

If eating dinner is 10 billion times worse than the physical limit on energy efficiency, then how efficient are today’s computers? Even worse than that dinner, as we’ll now see.

I often introduce my friend and colleague Seth Lloyd as the only person at MIT who’s arguably as crazy as I am. After doing pioneering work on quantum computers, he went on to write a book arguing that our entire Universe is a quantum computer. We often grab beer after work, and I’ve yet to discover a topic that he doesn’t have something interesting to say about. For example, as I mentioned in chapter 2, he has lots to say about the ultimate limits of computing. In a famous 2000 paper, he showed that computing speed is limited by energy: performing an elementary logical operation in time T requires an average energy of E = h∕4T, where h is the fundamental physics quantity known as Planck’s constant. This means that a 1 kg computer can perform at most 5 × 1050 operations per second—that’s a whopping 36 orders of magnitude more than the computer on which I’m typing these words. We’ll get there in a couple of centuries if computational power keeps doubling every couple of years, as we explored in chapter 2. He also showed that a 1 kg computer can store at most 1031 bits, which is about a billion billion times better than my laptop.

Seth is the first to admit that actually attaining these limits may be challenging even for superintelligent life, since the memory of that 1 kg ultimate “computer” would resemble a thermonuclear explosion or a little piece of our Big Bang. However, he’s optimistic that the practical limits aren’t that far from the ultimate ones. Indeed, existing quantum computer prototypes have already miniaturized their memory by storing one bit per atom, and scaling that up would allow storing about 1025 bits/kg—a trillion times better than my laptop. Moreover, using electromagnetic radiation to communicate between these atoms would permit about 5 × 1040 operations per second—31 orders of magnitude better than my CPU.

In summary, the potential for future life to compute and figure things out is truly mind-boggling: in terms of orders of magnitude, today’s best supercomputers are much further from the ultimate 1 kg computer than they are from the blinking turn signal on a car, a device that stores merely one bit of information, flipping it between on and off about once per second.