27 February 2017
In my previous post, Do the clever animals have to die?, I considered the “ultimate concern” (to borrow a phrase from Paul Tillich) of existential risk mitigation: the survival of life and other emergent complexities beyond the habitability of its homeworld or home planetary system. While a planetary system could be inhabited for hundreds of millions of years in most cases, and possibly for billion or tens of billions of years (the latter in the case of red dwarf stars, as in the recently discovered planetary system at TRAPPIST-1, which appears to be a young star with a long history ahead of it), there are yet many events that could occur that could render a homeworld or an entire planetary system uninhabitable, or which could be sufficiently catastrophic that a civilization clustered in the vicinity of a single star would almost certainly be extirpated by them (e.g., a sufficiently large gamma ray burst, GRB, from outside our solar system, or a sufficiently large coronal mass ejection, CME, from within our solar system).
Because any civilization that endures for cosmologically significant periods of time must have established multiple independent centers of civilization, and will probably have survived its homeworld having become uninhabitable, mature advanced civilizations may view this condition as definitive of a mature civilization. Having ensured their risk of extinction against existential threats through establishing multiple independent centers of civilization, these advanced civilizations may not regard as a “peer” (i.e., not regard as a fellow advanced civilization) any civilization that still remains tightly-coupled to its homeworld.
It nevertheless may be the case (if there are, or will be, multiple examples of advanced civilizations) that some civilizations choose to remain tightly-coupled to their homeworlds. We can posit this as the condition of a certain kind of civilization. In the question and answer segment following my 2015 talk, What kind of civilizations build starships? a member of the audience, Alex Sherwood, suggested, in contradistinction to the expansion hypothesis, a constancy hypothesis, according to which a civilization does not expand and does not contract, but rather remains constant; I would prefer to call this the equilibrium hypothesis. One way in which a civilization might exemplify the constancy hypothesis would be for it to remain tightly-coupled to its homeworld.
Some subset of homeworld-coupled civilizations will probably experience extinction due to this choice. Such a homeworld-coupled civilization might choose, instead of establishing multiple independent centers of civilization as existential risk mitigation, to instead establish de-extinction and backup measures that would allow civilization to be restored on its homeworld despite any realized existential risks. However, while this approach to civilizational longevity may ensure the existence of a civilization over the billions of years of the life of its parent star, if a civilization does not want the historical accident of the age of its parent star to determine its ongoing viability, then such a civilization must abandon its homeworld and eventually also its home planetary system.
A civilization might continue to exemplify the equilibrium hypothesis by maintaining the unity and distinctiveness of its civilization despite needing to pursue megastructure-scale projects in order to ensure its ongoing existential viability. The idea of constructing a Shkadov thruster to move a star was partly inspired by this particular conception of the equilibrium hypothesis, as a star might, by this method, be moved to another, younger star, and the homeworld transferred into the orbit of that younger star. In this way, the relationship to the parent star is de-coupled, but the relationship to homeworld remains exclusive. At yet another remove, an entire civilization might simply choose to pick up from its homeworld and transfer itself to another chosen world. (As an historical analogy, consider the ancient city of Knidos, which was founded on the Datça Peninsula, but as the city grew in size and wealth, the city fathers decided that they needed to start again, so they built themselves a new and grander city nearby, and moved the entire city to this new location.) This conception of the equilibrium hypothesis would de-couple a civilization from both parent star and homeworld, but could still maintain the civilization as a unique and distinctive whole, thus continuing that civilization in its equilibrium condition.
A civilization that establishes multiple independent centers of civilization (and thus, to some degree, exemplifies the expansion hypothesis) might still retain strong connections to its homeworld — only not the connection of dependency. Such civilizations fully independent of a homeworld might be said to be loosely-coupled to their homeworld, in contradistinction to civilizations tightly-coupled to their homeworld and exemplifying the equilibrium hypothesis. Expansionary civilizations might remain in close contact with a homeworld for as long as the homeworld was habitable, only to fully abandon it when the homeworld could no longer support life.
Eventually, as the climate changes and the continents move and the surface of Earth is entirely rearranged, as would be experienced by a billion-year-old civilization, almost all terrestrial cities and monuments will disappear, and even the familiar look of Earth will change until it eventually becomes unrecognizable. The heritage of terrestrial civilization might be preserved in part by moving entire monuments to other worlds, or to no world at all, but perhaps to a permanent artificial habitat that is not a planet. Terrestrial places might be recreated on other worlds (or, again, on no world at all) in a grand gesture of historical reconstruction.
There might be other surprising ways of preserving our terrestrial heritage, such as building projects that were never realized on Earth. For example, some future civilization might choose to build Étienne-Louis Boullée’s design for an enormous cenotaph commemorating Isaac Newton, or Antoni Gaudí’s unbuilt skyscraper, or indeed any number of countless projects conceived but never built. An entire city of unbuilt buildings could be constructed on other worlds, which would be new cities, cities never before built, but cities in the tradition of our terrestrial heritage, maintaining the connection to our homeworld even while looking to a future de-coupled from that homeworld.
A civilization that outlasts its homeworld could be said to be de-coupled from its homeworld, though the homeworld will always be the origin of the intelligent agent that is the progenitor of a civilization, and hence a touchstone and a point of reference — like a hometown that one has left in order to pursue a career in the wider world. One would expect historical reconstruction and reenactment in order to maintain our intimacy with the past, which is, at the same time, our intimacy with our homeworld, should we become de-coupled from Earth. If humanity goes on to expand into the universe, establishing multiple independent centers of civilization, including gestures of respect to our terrestrial past in the form of reconstruction, the eventual loss of the Earth to habitability may not come as such a devastating blow if some trace of Earth was preserved.
When the uninhabitability of the Earth does become a definite prospect, and should civilization endure up to that time, that future civilization’s opportunities for historical preservation and conservation will be predicated upon the technological resources available at that time, and what conception of authenticity prevails in that future age. A civilization of sufficiently advanced technology might simply preserve its homeworld entire, as a kind of museum, moving it to wherever would be convenient in order to maintain it in some form that it would be visited by antiquaries and eccentrics. Or such a future civilization might deem such preservation to be undesirable, and only certain artifacts would be removed before the planet entire was consumed by the sun as it expands into a red giant star. In an emergency abandonment of Earth, what could be evacuated would be limited, and principles of selection therefore more rigorous — but also constrained by opportunity. In the event of emergency abandonment, there might also be the possibility of returning for salvage after the emergency had passed.
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