by J. N. Nielsen

Nick Nielsen thinks big. In fact, today’s essay, which ranges over vast stretches of time and space and places human civilization in a continually expanding context, reminds me of nothing so much as the Olaf Stapledon of Starmaker. As with Stapledon, the questions are deeply philosophical: If we find a way to travel arbitrarily close to the speed of light, thus creating a civilization Carl Sagan once envisioned — one spread not only over space but over aeons — how will we cohere as a species? And what forms will our migrations take after the first pioneers have left our niche in the cosmos behind? For more of Nielsen’s work, see his blogs Grand Strategy: The View from Oregon and Grand Strategy Annex.


In my previous Centauri Dreams post, Cosmic Loneliness and Interstellar Travel, I argued that our cosmic loneliness is the reason we seek peer species and peer civilizations in the universe, that interstellar travel is a more practicable way to explore the universe for intelligent life than SETI/METI communication, and that such travel will eventually result as a consequence of the development of a 1G starship (a spacecraft that can accelerate or decelerate at a rate equal to terrestrial gravity). In my Centauri Dreams post prior to that, SETI, METI, and Existential Risk, I argued that SETI efforts will find technological civilizations if they are out there, and, by the same token, we will be found whether we want to be found or not, but we ought not to shrink from this possibility because the potential risk is at the same time a civilizational opportunity.

In this present post I would like to explore what kind of large-scale spacefaring civilization would emerge from the positions I have taken in the previous posts, specifically, the idea that we would be found by advanced civilizations if there were any, but we haven’t been found. I would like for these three Centauri Dreams posts to be understood as one long argument (as Darwin said of his Origins), and the argument is this: if it makes more sense to travel than to communicate, and if there is no sign of travel to Earth by extraterrestrial civilizations, then we are alone, or very nearly alone, in the cosmos. We may not be absolutely alone in the universe, but we are likely to be sufficiently alone that we can embark upon the initial stages of building a spacefaring civilization without the likelihood of finding any peer civilization in our initial voyages.

On the basis of physics as we understand it today, the spacefaring civilization we are capable of building will be subject to the constraints of a relativistic universe (except in the case of a disruptive breakthrough in science or technology), but we must learn to see this limitation as being at the same time an opportunity. Relativistic interstellar travel, and the spacefaring civilization that emerges from such voyages, will contribute materially to the existential viability of a civilization. This is the great opportunity that lurks within the limitations imposed by relativistic travel.

In my first post to Centauri Dreams, Existential Risk and Far Future Civilization, I argued that the existential viability of civilization is contingent upon three conditions: 1) knowledge, 2) redundancy, and 3) autonomy. Knowledge transforms uncertainties into calculable risks that can be managed; redundancy assures that if one center of civilization succumbs to an existential risk, other centers of civilization will remain to continue the life of civilization; autonomy among centers of civilization assures that distinct centers will pursue distinct existential risk mitigation strategies, therefore lowering the likelihood that multiple redundant centers of civilization would all succumb to the same existential risk.

A spacefaring civilization established by relativistic interstellar travel secures all three of these conditions in an especially robust manner. And it does so, as Carl Sagan said, in virtue of, “another and quite unexpected method.” The unexpected method of securing existential risk mitigation is what I call the establishment of a temporally distributed civilization, i.e., a civilization that is distributed not only in space, but also in time.


The idea of a temporally distributed civilization is something that I began to develop in my 2012 100YSS presentation, “The Large Scale Structure of Spacefaring Civilization.” There I observed that we tend to think of human expansion into the universe as distribution in space, but in a relativistic universe, distribution in time cannot be separated from distribution in space. As the engineering innovations of industrial-technological civilization bring us gradually ever closer to the possibility of a 1G starship, our spatial distribution in the universe will at the same time become a temporal distribution through time dilation: the faster we go and the farther we go, the more time will have elapsed at our point of origin.

Time dilation is not only subject to misconception, but the way in which the “twin paradox” has been commonly presented seems almost as though calculated to elicit a poignant response: one of two twins remains on Earth, while another flies to a distant star at relativistic velocities. When the traveling twin, who has experienced acceleration, returns, the twin who has remained on Earth is now old, while the returning traveler is yet young. The greater the speed of travel, the greater the time dilation effect, so that a 1G starship would allow a traveler to not only exceed the age of a twin, but to exceed the age of the Earth, and this would be sad indeed. It sounds like madness to undertake such a voyage.


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Even if many or most individuals are dissuaded from interstellar travel by time dilation, or are risk averse and would never fly to the stars out of fear, it would ludicrous to argue that no one will take the risk, or that no one would be willing to leave the world behind. Even under the conditions of total separation I strongly suspect that there would be a few individuals and small groups willing to cut their ties to their homeworld and go on a one-way journey to the stars. And it is highly likely that among the first to make the journey to the stars, some will never be heard from again (Some of these may prove to be questions as fascinating to the public as the fate of Amelia Earhart.) They will be as lost to us as early ships lost at sea. Perhaps they will go farther than any of us; perhaps they will be waiting for us at the edge of the universe when we finally arrive, en masse, as a civilization.

This, however, represents a limiting case of relativistic travel. To give an accurate picture of the large scale structure of spacefaring civilizations, the twins paradox must be recast in terms of populations rather than individuals. As in evolutionary biology, it is the population that is the unit of selection, although in this context it is a human population that is temporally selected, or rather selects itself. The vanguard of spacefaring civilization will consist of populations who desire their differentiation from the civilization of source (eocivilization); this process will be strongly selective of those who feel alienated from the civilization of their birth and are willing to abandon it.

There will be long-term socio-political consequences of the establishment of initial interstellar settlements by those least attached to the civilization that made spacefaring possible, but these socio-political consequences will be limited by the small size of the communities in question. In the same way that any communicating civilization would be overtaken by any traveling civilization, because of the technological innovations that would occur during the period of time while waiting for a signal to travel interstellar distances and then return, so too any civilization that experiences less time as a result of time dilation would be overtaken by a non-accelerated center of civilization. Later technological innovations from the original center of civilization will eventually overtake the vanguard, but the scales of space, time, and technology involved will mean that it will be an open question whether these vanguard communities will have transformed themselves into something unrecognizable in the interim.

It is likely that the greater part of innovation in propulsion technologies (and therefore that attainment of greater velocities) will occur wherever the greater part of the human population is to be found, which in the initial stages of a spacefaring civilization means that most advances in propulsion technologies will occur on a given species’ home planet while its initial starships will be isolated from these innovations. Thus technologies on the home world will surpass those who have made the first interstellar journeys.

In so far as such interstellar travel is continually improved and refined, it would not be isolated groups, but rather large groups that will eventually travel, or many individuals or small groups who could rendezvous at an appointed place and time. Collectively, such a group of travelers would bring its contemporaneous civilization along with it—they would both travel to the stars, and have their familiar civilization, although that civilization would not continue to mirror the civilization of source indefinitely as it independently developed, though later voyages are likely to diverge less from the source of civilization than earlier voyages.

Beyond a certain threshold, when off-planet population clusters approach the levels of density required for innovative scientific research on an industrial scale, and as innovations from the original center of the civilization result in cheaper, more effective, and more widely distributed transportation technologies, the bias will shift from the certainties of settled planetary life to the possibilities of life on a larger scope and scale that represents an increase in an order of magnitude of the choices and opportunities available. For the same reasons that populations have steadily moved from rural areas to urban centers, driving further urbanization once urbanization had become a viable way of life for the formerly-rural masses, populations will steadily migrate from settled planetary life to an accelerated life that joins the time-dilated community once this becomes a viable way of life for the civilized masses.

As civilization enlarges in scope, it is subject to a greater degree to the natural forces that govern the large scale structures of space-time. At the largest cosmological scale, the theory of relativity would prove to be constitutive of civilization. The farther a civilization extends in space, the greater number of frames of reference it encounters and the greater the diversity of these frames of reference. It is this process that will yield a temporally distributed civilization.

In the context of a temporally distributed civilization, a distinction must be made between the chronological age of a civilization and the temporal span that a civilization covers, since the cosmological distribution of centers of civilization at relativistic velocities means that accelerated populations cover a greater span of time than unaccelerated populations remaining at the original source of civilization. The temporal span of a civilization is the total portion of the age of the universe occupied by a given spacefaring civilization. This temporal span will be much larger in its scale of time than the chronological age of the civilization.

More significantly, distinct centers of civilization widely dispersed in space and time (i.e., separated by a significant temporal span) may be chronologically very close in age. When velocities close to the speed of light are attainable, this temporal span may be dramatic. Centers of civilization separated by thousands or even millions of light years may be chronologically only a few years apart, so that essentially the same civilization exists millions of years apart in terms of its temporal span. Separation of months or years or even decades amounts to little more than a rounding error in terms of the scale of time involved.

This distribution of essentially the same civilization throughout widely separated spans of time will result in a very high degree of existential risk mitigation, since these temporally distributed centers of civilization will not even be subjected to the same natural disasters occurring on a cosmological scale, as they will inhabit different ages of the universe. A sterilizing gamma ray burst may doom the unfortunate center of civilization coeval with that disaster, but other coeval centers of civilization will be spared this particular risk, though they may be subject to other existential risks.

These coeval centers of civilization, approximately the same chronological age, but widely separated across the universe as faster and farther travel takes us ever greater distances from the original source of our civilization, will be like stepping stones across the cosmos. An interstellar traveler might pass from one arm of the Milky Way galaxy to another, always having a familiar center of civilization to stop and to pause. Some would choose to stay and maintain that center of civilization (and its coeval character), while others may choose to go farther. Such stepping stones across the cosmos might eventually take us from galaxy to galaxy, cluster to cluster, and supercluster to supercluster.

The new centers of civilization that result from interstellar voyaging, and which can serve as stepping stones across the cosmos, will be connected to each other as peer civilizations. It is only when you seek to retrace your steps that you come “back” to a near-peer civilization (i.e., a civilization sufficiently removed in time that it is no longer a peer simpliciter), now removed by degrees of separation in time, or even to a non-peer civilization, the farther and faster you go back to the former source of civilization. If homeworld civilization stagnates, one might even return to something like a peer civilization, but even a peer civilization would be unrecognizable, as the continents of Earth rearrange themselves and all our cities and monuments disappear and are replaced by new structures, until the Earth is no longer habitable.

The cosmos itself forces us to confront the fact that you can’t go home again. Earth is our cosmological home, and once we leave it for the stars we will not be able to return to the world that we left behind. But we will take our terrestrial civilization with us to the stars, and these new centers of civilization established by interstellar voyaging will possess the knowledge, redundancy, and autonomy requisite to mitigating any existential risk.

The civilization that I have described will be both strikingly similar to and radically different from the civilization that we know today, and we will have to formulate new modes of self-understanding in order to conceptualize our place within such a cosmic order. The advent of temporally distributed civilization will mean that the historical consciousness that human civilization has laboriously constructed, and which we have greatly expanded since the formulation of scientific historiography, will have to be expanded and extended once again by the expanded and extended human experience of a civilization that spans the geometry of spacetime across the galaxy and eventually across the universe, step by step.