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From a physics perspective, everything that future life may want to create—from habitats and machines to new life forms—is simply elementary particles arranged in some particular way. Just as a blue whale is rearranged krill and krill is rearranged plankton, our entire Solar System is simply hydrogen rearranged during 13.8 billion years of cosmic evolution: gravity rearranged hydrogen into stars which rearranged the hydrogen into heavier atoms, after which gravity rearranged such atoms into our planet where chemical and biological processes rearranged them into life.

Future life that has reached its technological limit can perform such particle rearrangements more rapidly and efficiently, by first using its computing power to figure out the most efficient method and then using its available energy to power the matter rearrangement process. We saw how matter can be converted into both computers and energy, so it’s in a sense the only fundamental resource needed.*7 Once future life has bumped up against the physical limits on what it can do with its matter, there is only one way left for it to do more: by getting more matter. And the only way it can do this is by expanding into our Universe. Spaceward ho!

Gaining Resources Through Cosmic Settlement

Just how great is our cosmic endowment? Specifically, what upper limits do the laws of physics place on the amount of matter that life can ultimately make use of? Our cosmic endowment is mind-bogglingly large, of course, but how large, exactly? Table 6.2 lists some key numbers. Our planet is currently 99.999999% dead in the sense that this fraction of its matter isn’t part of our biosphere and is doing almost nothing useful for life other than providing gravitational pull and a magnetic field. This raises the potential of one day using a hundred million times more matter in active support of life. If we can put all of the matter in our Solar System (including the Sun) to optimal use, we’ll do another million times better. Settling our Galaxy would grow our resources another trillion times..

How Far Can You Go?

You might think that we can acquire unlimited resources by settling as many other galaxies as we want if we’re patient enough, but that’s not what modern cosmology suggests! Yes, space itself might be infinite, containing infinitely many galaxies, stars and planets—indeed, this is what’s predicted by the simplest versions of inflation, the currently most popular scientific paradigm for what created our Big Bang 13.8 billion years ago. However, even if there are infinitely many galaxies, it appears that we can see and reach only a finite number of them: we can see about 200 billion galaxies and settle in at most ten billion.

What limits us is the speed of light: one light-year (about ten trillion kilometers) per year. Figure 6.6 shows the part of space from which light has reached us so far during the 13.8 billion years since our Big Bang, a spherical region known as “our observable Universe” or simply “our Universe.” Even if space is infinite, our Universe is finite, containing “only” about 1078 atoms. Moreover, about 98% of our Universe is “see but not touch,” in the sense that we can see it but never reach it even if we travel at the speed of light forever. Why is this? After all, the limit to how far we can see comes simply from the fact that our Universe isn’t infinitely old, so that distant light hasn’t yet had time to reach us. So shouldn’t we be able to travel to arbitrarily distant galaxies if we have no limit on how much time we can spend en route?

The first challenge is that our Universe is expanding, which means that almost all galaxies are flying away from us, so settling distant galaxies amounts to a game of catch-up. The second challenge is that this cosmic expansion is accelerating, due to the mysterious dark energy that makes up about 70% of our Universe. To understand how this causes trouble, imagine that you enter a train platform and see your train slowly accelerating away from you, but with a door left invitingly open. If you’re fast and foolhardy, can you catch the train? Since it will eventually go faster than you can run, the answer clearly depends on how far away from you the train is initially: if it’s beyond a certain critical distance, you’ll never catch up with it. We face the same situation trying to catch those distant galaxies that are accelerating away from us: even if we could travel at the speed of light, all galaxies beyond about 17 billion light-years remain forever out of reach—and that’s over 98% of the galaxies in our Universe.

But hold on: didn’t Einstein’s special relativity theory say that nothing can travel faster than light? So how can galaxies outrace something traveling at the speed of light? The answer is that special relativity is superseded by Einstein’s general relativity theory, where the speed limit is more liberal: nothing can travel faster than the speed of light through space, but space is free to expand as fast as it wants. Einstein also gave us a nice way of visualizing these speed limits by viewing time as the fourth dimension in spacetime (see figure 6.7, where I’ve kept things three-dimensional by omitting one of the three space dimensions). If space weren’t expanding, light rays would form slanted 45-degree lines through spacetime, so that the regions we can see and reach from here and now are cones. Whereas our past light cone would be truncated by our Big Bang 13.8 billion years ago, our future light cone would expand forever, giving us access to an unlimited cosmic endowment. In contrast, the middle panel of the figure shows that an expanding universe with dark energy (which appears to be the Universe we inhabit) deforms our light cones into a champagne-glass shape, forever limiting the number of galaxies we can settle to about 10 billion.

If this limit makes you feel cosmic claustrophobia, let me cheer you up with a possible loophole: my calculation assumes that dark energy remains constant over time, consistent with what the latest measurements suggest. However, we still have no clue what dark energy really is, which leaves a glimmer of hope that dark energy will eventually decay away (much like the similar dark-energy-like substance postulated to explain cosmic inflation), and if this happens, the acceleration will give way to deceleration, potentially enabling future life forms to keep settling new galaxies for as long as they last.

How Fast Can You Go?

Above we explored how many galaxies a civilization could settle if it expanded in all directions at the speed of light. General relativity says that it’s impossible to send rockets through space at the speed of light, because this would require infinite energy, so how fast can rockets go in practice?*8 NASA’s New Horizons rocket broke the speed record when it blasted off toward Pluto in 2006 at a speed of about 100,000 miles per hour (45 kilometers per second), and NASA’s 2018 Solar Probe Plus aims to go over four times faster by falling very close to the Sun, but even that’s less than a puny 0.1% of the speed of light. The quest for faster and better rockets has captivated some of the brightest minds of the past century, and there’s a rich and fascinating literature on the topic. Why is it so hard to go faster? The two key problems are that conventional rockets spend most of their fuel simply to accelerate the fuel they carry with them, and that today’s rocket fuel is hopelessly inefficient—the fraction of its mass turned into energy isn’t much better than the 0.00000005% for gasoline that we saw in table 6.1. One obvious improvement is to switch to more efficient fuel. For example, Freeman Dyson and others worked on NASA’s Project Orion, which aimed to explode about 300,000 nuclear bombs during 10 days to reach about 3% of the speed of light with a spaceship large enough to carry humans to another solar system during a century-long journey.5 Others have explored using antimatter as fuel, since combining it with ordinary matter releases energy with nearly 100% efficiency.

Another popular idea is to build a rocket that need not carry its own fuel. For example, interstellar space isn’t a perfect vacuum, but contains the occasional hydrogen ion (a lone proton: a hydrogen atom that’s lost its electron). In 1960, this gave physicist Robert Bussard the idea behind what’s now known as a Bussard ramjet: to scoop up such ions en route and use them as rocket fuel in an onboard fusion reactor. Although recent work has cast doubts on whether this can be made to work in practice, there’s another carry-no-fuel idea that does appear feasible for a high-tech spacefaring civilization: laser sailing.

Figure 6.8 illustrates a clever laser-sail rocket design pioneered in 1984 by Robert Forward, the same physicist who invented the statites we explored for Dyson sphere construction. Just as air molecules bouncing off a sailboat sail will push it forward, light particles (photons) bouncing off a mirror will push it forward. By beaming a huge solar-powered laser at a vast ultralight sail attached to a spacecraft, we can use the energy of our own Sun to accelerate the rocket to great speeds. But how do you stop? This is the question that eluded me until I read Forward’s brilliant paper: as figure 6.8 shows, the outer ring of the laser sail detaches and moves in front of the spacecraft, reflecting our laser beam back to decelerate the craft and its smaller sail.6 Forward calculated that this could let humans make the four-light-year journey to the α Centauri solar system in merely forty years. Once there, you could imagine building a new giant laser system and continuing star-hopping throughout the Milky Way Galaxy.

But why stop there? In 1964, the Soviet astronomer Nikolai Kardashev proposed grading civilizations by how much energy they could put to use. Harnessing the energy of a planet, a star (with a Dyson sphere, say) and a galaxy correspond to civilizations of Type I, Type II and Type III on the Kardashev scale, respectively. Subsequent thinkers have suggested that Type IV should correspond to harnessing our entire accessible Universe. Since then, there’s been good news and bad news for ambitious life forms. The bad news is that dark energy exists, which, as we saw, appears to limit our reach. The good news is the dramatic progress of artificial intelligence. Even optimistic visionaries such as Carl Sagan used to view the prospects of humans reaching other galaxies as rather hopeless, given our propensity to die within the first century of a journey that would take millions of years even if traveling at near light speed. Refusing to give up, they considered freezing astronauts to extend their life, slowing their aging by traveling very close to light speed, or sending a community that would travel for tens of thousands of generations—longer than the human race has existed thus far.

The possibility of superintelligence completely transforms this picture, making it much more promising for those with intergalactic wanderlust. Removing the need to transport bulky human life-support systems and adding AI-invented technology, intergalactic settlement suddenly appears rather straightforward. Forward’s laser sailing becomes much cheaper when the spacecraft need merely be large enough to contain a “seed probe”: a robot capable of landing on an asteroid or planet in the target solar system and building up a new civilization from scratch. It doesn’t even have to carry the instructions with it: all it has to do is build a receiving antenna large enough to pick up more detailed blueprints and instructions transmitted from its mother civilization at the speed of light. Once done, it uses its newly constructed lasers to send out new seed probes to continue settling the galaxy one solar system at a time. Even the vast dark expanses of space between galaxies tend to contain a significant number of intergalactic stars (rejects once ejected from their home galaxies) that can be used as way stations, thus enabling an island-hopping strategy for intergalactic laser sailing.

Once another solar system or galaxy has been settled by superintelligent AI, bringing humans there is easy—if humans have succeeded in making the AI have this goal. All the necessary information about humans can be transmitted at the speed of light, after which the AI can assemble quarks and electrons into the desired humans. This could be done either rather low-tech by simply transmitting the two gigabytes of information needed to specify a person’s DNA and then incubating a baby to be raised by the AI, or the AI could nanoassemble quarks and electrons into full-grown people who would have all the memories scanned from their originals back on Earth.

This means that if there’s an intelligence explosion, the key question isn’t if intergalactic settlement is possible, but simply how fast it can proceed. Since all the ideas we’ve explored above come from humans, they should be viewed as merely lower limits on how fast life can expand; ambitious superintelligent life can probably do a lot better, and it will have a strong incentive to push the limits, since in the race against time and dark energy, every 1% increase in average settlement speed translates into 3% more galaxies colonized.

For example, if it takes 20 years to travel 10 light-years to the next star system with a laser-sail system, and then another 10 years to settle it and build new lasers and seed probes there, the settled region of space will be a sphere growing in all directions at a third of the speed of light on average. In a beautiful and thorough analysis of cosmically expanding civilizations in 2014, the American physicist Jay Olson considered a high-tech alternative to the island-hopping approach, involving two separate types of probes: seed probes and expanders.7 The seed probes would slow down, land and seed their destination with life. The expanders, on the other hand, would never stop: they’d scoop up matter in flight, perhaps using some improved variant of the ramjet technology, and use this matter both as fuel and as raw material out of which they’d build expanders and copies of themselves. This self-reproducing fleet of expanders would keep gently accelerating to always maintain a constant speed (say half the speed of light) relative to nearby galaxies, and reproduce often enough that the fleet formed an expanding spherical shell with a constant number of expanders per shell area.

Last but not least, there’s the sneaky Hail Mary approach to expanding even faster than any of the above methods will permit: using Hans Moravec’s “cosmic spam” scam from chapter 4. By broadcasting a message that tricks naive freshly evolved civilizations into building a superintelligent machine that hijacks them, a civilization can expand essentially at the speed of light, the speed at which their seductive siren song spreads through the cosmos. Since this may be the only way for advanced civilizations to reach most of the galaxies within their future light cone and they have little incentive not to try it, we should be highly suspicious of any transmissions from extraterrestrials! In Carl Sagan’s book Contact, we Earthlings used blueprints from aliens to build a machine we didn’t understand—I don’t recommend doing this… In summary, most scientists and sci-fi authors considering cosmic settlement have in my opinion been overly pessimistic in ignoring the possibility of superintelligence: by limiting attention to human travelers, they’ve overestimated the difficulty of intergalactic travel, and by limiting attention to technology invented by humans, they’ve overestimated the time needed to approach the physical limits of what’s possible.

Staying Connected via Cosmic Engineering

If dark energy continues to accelerate distant galaxies away from one another, as the latest experimental data suggests, then this will pose a major nuisance to the future of life. It means that even if a future civilization manages to settle a million galaxies, dark energy will over the course of tens of billions of years fragment this cosmic empire into thousands of different regions unable to communicate with one another. If future life does nothing to prevent this fragmentation, then the largest remaining bastions of life will be clusters containing about a thousand galaxies, whose combined gravity is strong enough to overpower the dark energy trying to separate them.

If a superintelligent civilization wants to stay connected, this would give it a strong incentive to do large-scale cosmic engineering. How much matter will it have time to move into its largest supercluster before dark energy puts it forever out of reach? One method for moving a star large distances is to nudge a third star into a binary system where two stars are stably orbiting each other. Just as with romantic relationships, the introduction of a third partner can destabilize things and lead to one of the three being violently ejected—in the stellar case, at great speed. If some of the three partners are black holes, such a volatile threesome can be used to fling mass fast enough to fly far outside the host galaxy. Unfortunately, this three-body technique, applied either to stars, black holes or galaxies, doesn’t appear able to move more than a tiny fraction of a civilization’s mass the large distances required to outsmart dark energy.

But this obviously doesn’t mean that superintelligent life can’t come up with better methods, say converting much of the mass in outlying galaxies into spacecraft that can travel to the home cluster. If a sphalerizer can be built, perhaps it can even be used to convert the matter into energy that can be beamed into the home cluster as light, where it can be reconfigured back into matter or used as a power source.

The ultimate luck will be if it turns out to be possible to build stable traversable wormholes, enabling near-instantaneous communication and travel between the two ends of the wormhole no matter how far apart they are. A wormhole is a shortcut through spacetime that lets you travel from A to B without going through the intervening space. Although stable wormholes are allowed by Einstein’s theory of general relativity and have appeared in movies such as Contact and Interstellar, they require the existence of a strange hypothetical kind of matter with negative density, whose existence may hinge on poorly understood quantum gravity effects. In other words, useful wormholes may well turn out to be impossible, but if not, superintelligent life has huge incentives to build them. Not only would wormholes revolutionize rapid communication within individual galaxies, but by linking outlying galaxies to the central cluster early on, wormholes would allow the entire dominion of future life to remain connected for the long haul, completely thwarting dark energy’s attempts to censor communication. Once two galaxies are connected by a stable wormhole, they’ll remain connected no matter how far apart they drift.

If, despite its best attempts at cosmic engineering, a future civilization concludes that parts of it are doomed to drift out of contact forever, it might simply let them go and wish them well. However, if it has ambitious computing goals that involve seeking the answers to certain very difficult questions, it might instead resort to a slash-and-burn strategy: it could convert the outlying galaxies into massive computers that transform their matter and energy into computation at a frenzied pace, in the hope that before dark energy pushes their burnt-out remnants from view, they could transmit the long-sought answers back to the mother cluster. This slash-and-burn strategy would be particularly appropriate for regions so distant that they can only be reached by the “cosmic spam” method, much to the chagrin of the preexisting inhabitants. Back home in the mother region, the civilization could instead aim for maximum conservation and efficiency to last as long as possible.

How Long Can You Last?

Longevity is something that most ambitious people, organizations and nations aspire to. So if an ambitious future civilization develops superintelligence and wants longevity, how long can it last?

The first thorough scientific analysis of our far future was performed by none less than Freeman Dyson, and table 6.3 summarizes some of his key findings. The conclusion is that unless intelligence intervenes, solar systems and galaxies gradually get destroyed, eventually followed by everything else, leaving nothing but cold, dead, empty space with an eternally fading glow of radiation. But Freeman ends his analysis on an optimistic note: “There are good scientific reasons for taking seriously the possibility that life and intelligence can succeed in molding this universe of ours to their own purposes.”8 I think that superintelligence could easily solve many of the problems listed in table 6.3, since it can rearrange matter into something better than solar systems and galaxies. Oft-discussed challenges such as the death of our Sun in a few billion years won’t be showstoppers, since even a relatively low-tech civilization can easily move to low-mass stars that last for over 200 billion years. Assuming that superintelligent civilizations build their own power plants that are more efficient than stars, they may in fact want to prevent star formation to conserve energy: even if they use a Dyson sphere to harvest all the energy output during a star’s main lifetime (recouping about 0.1% of the total energy), they may be unable to keep much of the remaining 99.9% of the energy from going to waste when very hefty stars die. A heavy star dies in a supernova explosion from which most of the energy escapes as elusive neutrinos, and for very heavy stars, a large amount of mass gets wasted by forming a black hole from which the energy takes 1067 years to seep out.

What When

Current age of our Universe 1010 years

Dark energy pushes most galaxies out of reach 1011 years

Last stars burn out 1014 years

Planets detached from stars 1015 years

Stars detached from galaxies 1019 years

Decay of orbits by gravitational radiation 1020 years

Protons decay (at the earliest) > 1034 years

Stellar-mass black holes evaporate 1067 years

Supermassive black holes evaporate 1091 years

All matter decays to iron 101500 years

All matter forms black holes, which then evaporate 101026 years

As long as superintelligent life hasn’t run out of matter/energy, it can keep maintaining its habitat in the state it desires. Perhaps it can even discover a way to prevent protons from decaying using the so-called watched-pot effect of quantum mechanics, whereby the decay process is slowed by making regular observations. There is, however, a potential showstopper: a cosmocalypse destroying our entire Universe, perhaps as soon as 10–100 billion years from now. The discovery of dark energy and progress in string theory has raised new cosmocalypse scenarios that Freeman Dyson wasn’t aware of when he wrote his seminal paper.

So how’s our Universe going to end, billions of years from now? I have five main suspects for our upcoming cosmic apocalypse, or cosmocalypse, illustrated in figure 6.9: the Big Chill, the Big Crunch, the Big Rip, the Big Snap and Death Bubbles. Our Universe has now been expanding for about 14 billion years. The Big Chill is when our Universe keeps expanding forever, diluting our cosmos into a cold, dark and ultimately dead place; this was viewed as the most likely outcome back when Freeman wrote that paper. I think of it as the T. S. Eliot option: “This is the way the world ends / Not with a bang but a whimper.” If you, like Robert Frost, prefer the world to end in fire rather than ice, then cross your fingers for the Big Crunch, where the cosmic expansion is eventually reversed and everything comes crashing back together in a cataclysmic collapse akin to a backward Big Bang. Finally, the Big Rip is like the Big Chill for the impatient, where our galaxies, planets and even atoms get torn apart in a grand finale a finite time from now. Which of these three should you bet on? That depends on what the dark energy, which makes up about 70% of the mass of our Universe, will do as space continues to expand. It can be any one of the Chill, Crunch or Rip scenarios, depending on whether the dark energy sticks around unchanged, dilutes to negative density or anti-dilutes to higher density, respectively. Since we still have no clue what dark energy is, I’ll just tell you how I’d bet: 40% on the Big Chill, 9% on the Big Crunch and 1% on the Big Rip.

What about the other 50% of my money? I’m saving it for the “none of the above” option, because I think we humans need to be humble and acknowledge that there are basic things we still don’t understand. The nature of space, for example. The Chill, Crunch and Rip endings all assume that space itself is stable and infinitely stretchable. We used to think of space as just the boring static stage upon which the cosmic drama unfolds. Then Einstein taught us that space is really one of the key actors: it can curve into black holes, it can ripple as gravitational waves and it can stretch as an expanding universe. Perhaps it can even freeze into a different phase much like water can, with fast-expanding death bubbles of the new phase offering another wild-card cosmocalypse candidate. If death bubbles are possible, they would probably expand at the speed of light, just like the growing sphere of cosmic spam from a maximally aggressive civilization.

Moreover, Einstein’s theory says that space stretching can always continue, allowing our Universe to approach infinite volume as in the Big Chill and Big Rip scenarios. This sounds a bit too good to be true, and I suspect that it is. A rubber band looks nice and continuous, just like space, but if you stretch it too much, it snaps. Why? Because it’s made of atoms, and with enough stretching, this granular atomic nature of the rubber becomes important. Could it be that space too has some sort of granularity on a scale that’s simply too small for us to have noticed? Quantum gravity research suggests that it doesn’t make sense to talk about traditional three-dimensional space on scales smaller than about 10-34 meters. If it’s really true that space can’t be stretched indefinitely without undergoing a cataclysmic “Big Snap,” then future civilizations may wish to relocate to the largest non-expanding region of space (a huge galaxy cluster) that they can reach.

How Much Can You Compute?

After exploring how long future life can last, let’s explore how long it might want to last. Although you might find it natural to want to live as long as possible, Freeman Dyson also gave a more quantitative argument for this desire: the cost of computation drops when you compute slowly, so you’ll ultimately get more done if you slow things down as much as possible. Freeman even calculated that if our Universe keeps expanding and cooling forever, an infinite amount of computation might be possible.

Slow doesn’t necessarily mean boring: if future life lives in a simulated world, its subjectively experienced flow of time need not have anything to do with the glacial pace at which the simulation is being run in the outside world, so the prospects of infinite computation could translate into subjective immortality for simulated life forms. Cosmologist Frank Tipler has built on this idea to speculate that you could also achieve subjective immortality in the final moments before a Big Crunch by speeding up the computations toward infinity as the temperature and density skyrocketed.

Since dark energy appears to spoil both Freeman’s and Frank’s dreams of infinite computation, future superintelligence may prefer to burn through its energy supplies relatively quickly, to turn them into computations before running into problems such as cosmic horizons and proton decay. If maximizing total computation is the ultimate goal, the best strategy will be a trade-off between too slow (to avoid the aforementioned problems) and too fast (spending more energy than needed per computation).

Putting together everything we’ve explored in this chapter tells us that maximally efficient power plants and computers would enable superintelligent life to perform a mind-boggling amount of computation. Powering your thirteen-watt brain for a hundred years requires the energy in about half a milligram of matter—less than in a typical grain of sugar. Seth Lloyd’s work suggests that the brain could be made a quadrillion times more energy efficient, enabling that sugar grain to power a simulation of all human lives ever lived as well as thousands of times more people. If all the matter in our available Universe could be used to simulate people, that would enable over 1069 lives—or whatever else superintelligent AI preferred to do with its computational power. Even more lives would be possible if their simulations were run more slowly.9 Conversely, in his book Superintelligence, Nick Bostrom estimates that 1058 human lives could be simulated with more conservative assumptions about energy efficiency. However we slice and dice these numbers, they’re huge, as is our responsibility for ensuring that this future potential of life to flourish isn’t squandered. As Bostrom puts it: “If we represent all the happiness experienced during one entire such life by a single teardrop of joy, then the happiness of these souls could fill and refill the Earth’s oceans every second, and keep doing so for a hundred billion billion millennia. It is really important that we make sure these truly are tears of joy.” Cosmic Hierarchies

The speed of light limits not only the spread of life, but also the nature of life, placing strong constraints on communication, consciousness and control. So if much of our cosmos eventually comes alive, what will this life be like?