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Once superintelligent AI has settled another solar system or galaxy, bringing humans there is easy — if humans have succeeded in programming the AI with 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 in a low-tech way by simply transmitting the 2 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 assemble 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 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. 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.
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