This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
It might not seem like it, but the inhabitants of Earth are on the threshold of leaving the solar system. On cosmic timescales—measured in millions or even billions of years—it will be no time at all before our descendants begin to spread outward and across the Milky Way Galaxy. Unless technological progress is brought to a halt by some kind of major catastrophe, the era of our Earthly confinement is about to come to a close.
Imagine that over the course of the next hundred million years or so a hyperadvanced civilization emerges from Earth—much as our early ancestors once migrated out of Africa—and expands across the entirety of the Milky Way. Futurists have long argued such beings will build structures called Dyson spheres around stars, capturing and putting to use as much of the energy released in starlight as possible. It seems entirely conceivable that 100 million years from now every star in the Milky Way will be surrounded by a Dyson sphere, providing roughly a trillion trillion, or 1024 times more power than human beings currently produce and consume.
I don’t claim to have any great insights into what this kind of civilization will chose to use their vast quantities of energy to accomplish. Maybe they’ll use it to tackle unsolved math problems or to create and maintain vast simulations of virtual worlds. Perhaps they will direct it to the expansion of their civilization or to the creation of art or other such pursuits. Regardless of their goals, however, one thing is clear. Every act, including thinking, requires energy. This means no matter what an advanced civilization may want to accomplish, harnessing the power of as many stars as possible will help them to do it.
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To such a civilization, the greatest long-term threat will be the expansion of space. The stars, planets and other forms of matter that make up our galaxy are bound together by the force of gravity, and thus we don’t easily notice the effects of this expansion. But across much larger distances the expansion of space is steadily pulling galaxies apart from one another, riding on the very fabric of spacetime. Exacerbating this further is the fact our universe is not only expanding but is doing so at an accelerating rate, being driven by what cosmologists call “dark energy.”
One of the most interesting implications of this expansion is that it creates an absolute limit to how far away we can travel, or even observe. At our current moment in cosmic history, the most distant objects from which we can see light, or anything else, are about 46.5 billion light-years away. All of space beyond this distance is forever lost from our world. In this sense we are surrounded by an impenetrable horizon that for all practical purposes is the very boundary of our universe. Everything beyond this distance is not merely obscured from our vision, but is utterly and permanently disconnected from us.
As the universe’s expansion continues to accelerate, stars and galaxies that were once within our horizon fall beyond our reach. Every star in our universe represents a potential source of energy, which could be directed toward any number of valuable uses. But any star that resides beyond our horizon is useless to us. Over the next 100 billion years or so the vast majority of the stars and galaxies in our universe will become unreachable. Nothing but a handful of galaxies will remain.
Without intervention this outcome is inevitable. But in a recent paper I argued it would be possible for a sufficiently advanced civilization to tame the expansion of space, and to resist its effects. Facing such a situation, it’s not hard to imagine such a civilization would decide to take aggressive countermeasures. In particular, they could choose to expand rapidly outward, building Dyson spheres around the stars they reach and using the energy captured to accelerate and propel those stars toward the Milky Way and away from the approaching horizon.
Over a period of billions of years our progeny could secure many trillions of stars that would have otherwise fallen victim to the expansion of space, beyond our reach forever. These stars could ultimately become gravitationally bound to the future version of the Milky Way, leading over time to the formation of an enormous supergalaxy
The stars in such a system could be used in any number of ways. In addition to the energy produced via normal stellar evolution, stars that have run out of hydrogen could be merged together to form more massive stars that could burn helium or other nuclear elements. Some stars might even be converted into black holes, from which greater quantities of energy could potentially be extracted.
For thousands of years human beings have manipulated their environment to accomplish their goals—plowing fields, domesticating plants and animals, and redirecting waterways with pumps and levees. One day our progeny may very well begin to terraform the structure of the universe itself, forcing it to conform to their needs and desires, enabling life to thrive for trillions