You may wish to review Part 1, Part 2, Part 3, and Part 4 of this series before proceeding, if you are not yet conversant in its basis and terminology. In particular, review the caveats listed at the beginning of Part 4. Now, in Part 5, we extend the time horizon to 10,000 years and examine humanity's energy-driven cycles and pathways over the next ten millennia. This is 1% of the million-year horizon that will culminate the series.
Once again, this series is not speculation - it is hypothesis based on application of underlying physical principles to the large-scale evolution of human civilization.
Table of Contents
(Current part in bold)
I. The Energy History of Life (Part 1)
II. The Energy History of Humanity (Part 1)
III. The Next Decade (Part 2)
IV. The Next Century (Part 3)
V. The Next Millennium (Part 4)
VI. 10,000 Years (Part 5)
VII. 100,000 Years
VIII. Mark: One Million Years
At the conclusion of Part 4, one thousand years from now, humanity has bifurcated into two broad classes of civilization - the Root, which relies on passive harvesting of solar energy, and is concentrated along Earth's solar orbit - and the N+1 Spore driven by controlled fusion, which has spread humanity to the outer solar system. Root civilization is highly dense, complex, and intricately interdependent - the technological rainforest - whereas Spore civilizations (note the plural), though numerous and kaleidoscopic, are independent and widely distributed. Most of the human species and its wealth lies off Earth in Spore civilizations, though no single cohesive entity approaches Root in population.
However, despite how far humanity will have spread via N+1 Spore, its growth over 1 kiloyear (1 kyr) will still be dwarfed by the enormity of the solar system: Spore civilizations, though wealthy and rapidly growing, will be only the tiniest of specks within their overall resource environments. Additional millennia (between 1 and 2, is my guess) will need to pass before even the most limited of the major societies reach a point of diminishing returns and begin to value internal complexification (e.g., emphasis on density, efficiency, and stability) over linear growth, but the richest will transition last.
Once this occurs, a given society will enter a transitional period between Spore and Root in which it either attempts to augment its resources through (possibly contentious) local colonization, or by pursuing economic/political unification among states within a given system. We are therefore likely to see the history of Earth - perhaps even its modern history - reenacted in macrocosm in the gas giant planetary systems over the next 10 kyr, with various states inhabiting (or sharing) individual bodies and object swarms (such as the Jupiter Trojans).
The bulk resources of these systems so vastly exceed those of Earth in both quantity and accessibility that they will likely each come to have more independent political states than Earth has today. I would thus not be at all surprised if the transitional periods of major states are accompanied by wars and upheavals among them, but the most limited (i.e., least powerful) would be forced to transition first, so eras of conflict and disorder would be strewn out over centuries and millennia. It is unlikely, however, that these problems would significantly affect civilization outside a given system.
Mars would be the first major Spore locus to reach this boundary, and its history and intimate contact with the Root system (Earth + Luna + 1 AU solar orbit) probably means that the transition would be undramatic - it would grow to increasingly adopt and resemble Root until it had effectively merged with it. The details of how this transition would occur in the Martian case are a matter of speculation, but I will take a shot at it: With growing internal complexity and efficiency, energy needs would be met with increasingly large, centralized sources of fusion, but at some point the marginal cost of expanding such infrastructure would exceed that of simply concentrating and passively harvesting the otherwise dim solar energy.
This may already have been the case far earlier, but it would not become economically relevant until Martian society is dense, complex, and interdependent - i.e., until it joins the Root system in form, at which point it increasingly joins it in function as well. As the sunlight available at Mars is insufficient, solar concentrators would have to be placed closer to the Sun - possibly at 1 AU (within the Root) or closer. Reliance on fusion would gradually phase out as this passive infrastructure expanded, and the Root would slowly grow outward to encompass Mars by virtue of having a common energy system. This does not mean that they would be one culture or political state, but they would indeed be a single civilization.
Main Belt asteroid civilizations would separate down three tracks, although a choice would not necessarily be permanent: (1) Those with a great deal of success would continue to incorporate and settle new objects, perhaps gathering them into a single solar orbit for ease of transportation between them, and would remain as highly mobile, fusion-based N+1 Spore. Societies with lesser prospects for growth in the Main Belt that had reached their internal limits would either join with more successful ones, (2) move their asteroid(s) into the Root (like old people moving to Florida, I suppose), or (3) put their asteroid(s) in elliptical "Trade" orbits that cross into the Root near perihelion (closest approach to the Sun) and outward beyond the Main Belt at aphelion (furthest from the Sun). They could create these new orbits very slowly and cheaply - the delta-v could be strung out over many years or even decades.
To help visualize what I'm talking about, here is an image - a grossly simplified image, I assure you - of the various asteroid populations between Earth and Jupiter:
The third path described above would serve as the basis for formation of N+2 Spore - civilizations ultimately capable of sustained (sublight) interstellar travel. While there would have been many continuing N+1 Spore attempts to reach nearby stars, their prospects would be limited, so I would expect that either none would have succeeded or that Alpha Centauri A or B might have a very small human presence from the N+1 iteration. Within the class of "Trader" asteroids, however, there would be a cyclical process that could logically evolve much more versatile and ecologically robust systems. Those with the most eccentric orbits, carrying them the furthest away from the Sun before returning, would develop uniquely resilient ecosystems capable of sustaining the long periods of resource-isolation.
We can imagine that some of these Trader civilizations would be small, nomadic stereotypes, but others could be quite large and regard passage through occupied systems as nothing more than periodic festivities with a chance to mingle with strange foreigners. Still others might be like luxury cruise ships, picking up rich retirees from wherever, or carrying travelers looking to "take the scenic route" to wherever else. Whatever the case may be, they would serve to reacquaint far-flung humanity with itself to some extent, spreading ideas, technologies, cultures, languages, and (not an insignificant thing on such timescales) genes.
The numbers of such Trader states would increase over time, as new asteroid civilizations reached their internal limits and chose migratory orbits in order to expand economic potential. This is the definition of N+2 Spore: Entire self-contained civilizations that migrate as a whole in order to exploit far-flung resources, rather than consisting of mobile individuals and enterprises that serve an immobile core. Some Traders would eventually join the Root, but some would pursue greater and greater eccentricities for reasons typical of Spore evolution: To expand the size and diversity of accessible markets, see new places, and perhaps in some cases to pursue longer periods of isolation for political or religious reasons.
The N+2 iteration would be highly energetic and evolve rapidly, because it would have access both to the vast raw materials of the outer solar system at aphelion and intense solar energy at perihelion. But because the peak inputs of energy and raw materials would be separated across years or even decades, their economies and ecosystems would become even more robust and efficient. This will allow them to go for even longer periods in isolation, and eventually the minority who choose extremely eccentric orbits reaching out to the most distant Kuiper Belt societies (of which there would be hundreds to thousands by this point) would be capable of going for decades on end without any kind of external input.
Sustained interaction with Kuiper Belt societies (if not creation of new ones) by Trader civilizations would breed even more N+2 Spore, and their relative isolation would further intensify this iteration's evolution. Some new Traders originating in the Kuiper Belt would choose orbits only dipping into the outer planets; some would choose radically eccentric orbits that cross even into the Root before returning to the KB, but some would pursue an even more distant aphelion to exploit resources beyond their original orbit.
Very few of these civilizations are themselves likely to pursue interstellar travel, but their numbers would grow promiscuously over time, and their exploration of the furthest reaches of the solar system would create both the ecological technology and social mentality for it. There would still be N+1 attempts at interstellar colonization occurring, probably with miserable results, but they would have returned plenty of information about risks, pitfalls, and the physical environments of interstellar space and the Alpha Centauri system. So when an N+2 society chose to migrate out of the solar system or was founded for that purpose (the most likely scenario), they would not be operating blindly - they would be the first viable wave of interstellar colonization.
We can imagine how this might reasonably occur: A genetically viable population (a few thousand individuals) is established on/in a small body or swarm of bodies by an existing civilization, with all the tools and technologies they will need to expand into their resource environment over decades or centuries (depending on maximum speed). They would then be sent on their way, and live as a burgeoning civilization for the entire trip rather than a slowly-dying one. As growth of some kind is necessary to both social and ecological robustness, it would be planned to occur at an optimal rate so that they arrive at Alpha Centauri A or B with some margin remaining.
N+2 interstellar colonists would not especially care about planets, but would mainly be interested in asteroids. However, they would probably find Trojan swarms and icy dwarf planets useful. This is interesting because the first interstellar colonies are going to grow in a pattern directly opposite to that of expansion into the solar system - the icy outlands will be settled long before they take an interest in warm rocks with significant g-fields like Mars. Even a second Earth would probably be ignored except as a scientific curiosity.
The pace and diversity of interstellar expansion will depend largely on the maximum practical sub-light speed: If the max is 1% of c, which I consider an improbably low figure, then reaching Alpha Centauri would take hundreds of years - still attainable for an N+2 civilization, but yields a depressingly slow rate of expansion to other stars. If it's 10% of c, then the same journey would take only decades. And if speeds are attainable that reach a majority fraction of c, such as 50%, 60%, or 70%, then it could be less than a single decade. Technological concepts exist that envision such speeds, but we know far too little to say they can be realized, so I will assume the middle course and base interstellar expansion predictions on minority fractions of c. The same expansion pattern would occur regardless of which is the case, it would just occur faster or slower.
Given this speed assumption, we can reasonably say that N+2 expansion originating from Sol in the 10 kyr time horizon would expand the human sphere out to about 10 light-years in radius, encompassing 11 stellar systems with 15 stars among them. Taking into account time spent accelerating in both directions, if the maximum speed is 10% of c, then Epsilon Eridani (at 10.5 light-years) could be reachable with an ETA inside a couple of centuries. Here is a diagram of nearby stars, encompassing a somewhat larger distance:
Of course, not all of the stars within reach will be equally attractive, though the parameters for such a determination might not be obvious. We today would aim for stars like the Sun, but a Trader-derived civilization might prefer a maximum richness and accessibility of metallic asteroids, so a different kind of star might be preferable. A hot, intense star like Sirius A with deep metallic lines might have more and heavier asteroids - we don't know yet whether binaries are poor in asteroids due to gravitational perturbation - but coping with the radiation from the star might be more bother than it's worth. And a small, cold M dwarf like Barnard's Star might have fewer metal-rich asteroids, but the smallness of the system and radiative output might make the environment easier to deal with.
Whatever is preferred, humanity will go wherever it can: The only difference between a juicy target and a less attractive one is the proportion of interstellar movement sent in its direction. By this time, these star systems will have been well-characterized both by robotic probes and remote sensing, so people will have a pretty good idea of what they're aiming for - they will not be leaping into shadow. In fact, the horizon of robotically-explored stellar systems will be larger than 10 light-years, and probably several times as large, encompassing an order of magnitude more stars.
We will know as much about these other star systems as we know today about the other planets in ours. We can call the distance reached by probes the "Technology Horizon," and that reached by humans (insofar as they are defined today) as the "Human Horizon" - although by contemporary standards they may view the technology as a form of "human." There is also the possibility that technology will travel without a crew and then generate humans from scratch on-site - in that case, the Technology Horizon and the Human Horizon would be the same, but I want to keep my assumptions conservative and not rely on any specific technology being developed.
To give a sense of perspective of these horizons, let us put it in the context of the Milky Way galaxy:
Meanwhile, back at Sol, the Root would be expanding both inward toward the Sun - growing toward the source of its energy - and outward as former Spore loci fill their economic niches and join the Root in a similar manner to Mars. First, they begin to encounter resource limits; then they centralize and expand energy infrastructure; then they reach a point of diminishing returns, and depend increasingly on far-flung trade. Since raw materials are far more abundant beyond Mars, the resources of outer system civilizations will be in high demand within the Root, while solar energy beamed or focused from the Root will be in increasing demand by transitioning Spore societies: Like Mars, they will find it progressively cheaper to passively harvest such energy than to build ever-more-gargantuan fusion reactors.
As a result, the raw materials of the outer solar system will go into the inward expansion of the Root - churning out solar harvesting material, concentrator systems, and habitat volume by the bushel. It probably will not bother trying to settle Venus or Mercury, but more likely engage in highly-evolved versions of strip-mining - perhaps using directed energy to ionize and suck up material from the atmosphere and surface into orbit, then deploy it elsewhere to be used as manufacturing feedstocks.
The Root at 1 AU (Earth's solar orbit) will be packed with habitat, but most of the intra-Root migratory growth will be inward. Due to the greater intensity of power per unit area the closer one is to the Sun, a given area facing the Sun is worth proportionally more along closer orbits up to the point that higher temperature makes the existing technology inefficient. Human population will expand inward to this point, and then subsequent rounds of innovation occur that move the boundary of efficiency closer to the Sun, yielding additional inward migration.
Root power infrastructure, however, will move inward much more rapidly than the human population, driving the construction of monstrously-sized arrays of concentrators, harvesting area, and perhaps "routers" that direct energy to numerous locations in the solar system with pinpoint accuracy and predetermined intensity on arrival. I won't speculate about whether this will be beamed (i.e., coherent) energy or just focused like a common lens, but either or both are easily conceivable.
For instance, if one wished to increase the solar energy that reaches Titan by a factor of 5 (just to throw out a number), sunlight could be focused by a Root solar router to arrive at Titan with precisely that amount of energy - it wouldn't affect anything else in the Saturn system except by secondary radiation of Titan's atmosphere, and the router could just turn off that particular stream when Titan passed behind Saturn.
Concentrator architecture in the inner solar system would be able to direct solar energy in abundance to an increasing number of simultaneous, respectively moving locations. Secondary, tertiary, etc. routers along the way or in planetary orbit could "multiplex" the energy further, dynamically directing it not only to civilizations on fixed bodies, but even to individual ships. This is the application that will ultimately cause the entire solar system to turn Root: The large-scale concentration, routing, and local multiplexing of solar energy. The scale of such a thing is daunting, but the underlying technology is utterly trivial and would saturate the outer solar system. This would also conveniently double as a high-density communications system.
It will not happen instantly, but the extension of the Root power/communications infrastructure to the outer solar system - a region exceedingly rich in raw materials and previously requiring some effort to expand the energy supply - would be economically explosive. Wealthy civilizations that had still been in N+1 Spore would suddenly be swamped with cheap, trivially expansible energy with which to exploit their abundant material economies, and would thus be accelerated to the point of Root transition and beyond. Meanwhile, beyond the outer boundaries of the expanded infrastructure - i.e., the Kuiper Belt - the formation of N+2 Spore would accelerate beyond notice or control. Those going interstellar would be visiting the same group of nearby stars as previous waves, but there would be a lot more of them.
Closer to the Sun, the inner boundary of the Root power infrastructure would be increasingly dense to the point of forming something similar to a Dyson Ring - a swarm of artificial objects sharing a solar orbit so closely that they are effectively one object. The density of these developments would be such that it would noticeably affect the Sun's spectrum as measured from outside the solar system1.
At some point the inward-migration of the power infrastructure would reach a lasting equilibrium, allowing the Sun-directed growth of habitat to catch up with it. The human infrastructure of the Root would not exactly be solid, but it would be extraordinarily dense (from an orbital mechanics standpoint) and visible everywhere within the Root - an entire sky full of humanity, blazing more brilliantly in reflected sunlight than any starry night ever seen up to today. I would expect that by this point the energy-collection boundary would be within the orbit of Mercury, and the ecliptic plane from that boundary out to Mars would be swarming with monumental inhabited constructs.
Since habitats closer to the Sun would not have to be energy-efficient - in fact, they would need to throw off quite a lot of energy by necessity - they might do so profitably by selling it to outer solar system buyers and beaming it to distant routers. And so a process will have begun at 10 kyr whereby the outward direction of energy - whether through dedicated infrastructure or as the waste radiation of Root civilization - will cause the massive economic inflow of the solar system's resources, forming the basis of what will ultimately become a fluidly-interconnected Root habitat - not exactly a solid disk, but something that is connected in energy terms while still being mechanically (and thus orbitally) independent. But that is for the next Part.
I said in Part 4 that the Root civilization would probably be authoritarian, and that will not change with inward-migration: In fact, I imagine that it would become monstrously technocratic - energy economics becoming indistinguishable from moral values. Recall the society depicted in George Lucas's brilliant film THX-1138, and imagine every event in it occurring much more quickly, on a far vaster number of levels, and in more constrained but also more intricate (and far more aesthetically pleasing) environments. It would be unimaginably Byzantine, but I don't think the people would suffer or feel oppressed, for the most part - they would feel themselves to be part of a much greater whole, and possibly have religious sentiments surrounding the inward expansion of their civilization toward the Sun.
As for Earth, I imagine at this point it must from hereon out be referred to as "Earth" with quotation marks - I honestly don't expect there to be a climate or ecosystem worth mentioning. A natural, free-flowing, open-sky atmosphere in an economy that dense would just be regarded as a wasteful and stupid barrier to mobility. What's more, the strong gravity field would be seen as a disadvantage, so I would expect "Earth" to be more of an industrial mining operation than a human habitat - although plenty of habitat would be attached to it, either solidly or gravitationally. The same is likely of the Moon - the two bodies would just be treated as raw material sinks, with everything of historical or biological significance either preserved in parks or broken up and shipped out.
The sheer size and complexity of the human species at that point makes "Earth" not analogous to Africa as a point of origin, but more like a single village in Tanzania: You might - might, mind you - be aware of its significance in anthropology, but you are unlikely to regard it with much more than semi-curious indifference, and certainly not with the sort of wistful longing we in our narcissism usually assume off-world civilizations would feel toward the home planet. There would be no more yearning for the world we know than Canadians yearn to live in the Great Rift Valley - they might see images and feel a strange familiarity, but it wouldn't exactly gnaw on their psyche. They will not regard anything as having been lost.
Now, this is just speculation, but I imagine the "Earth" at this point resembling a dandelion seed head, with a vast fractal profusion of delicate tendrils extending upward and branching outward. My earlier description may have sounded horrible to you, but what I see in my mind is beautiful, and I believe the reality will be even more beautiful than we can imagine.
Update: It just occurred to me why people have such difficulty dealing with predictions beyond a handful of years - they can't handle the concept of equilibrium. If you asked people in 1960 to imagine 2010, they would either have depicted a charred nuclear wasteland or a Space Age utopia with people running all around the solar system. They simply could not imagine middle cases, where people are both limited and empowered. The same seems to be true on longer timescales - they either think humanity will be extinct, or that it will be godlike and roaming the universe free of the currently understood laws of physics, and I just don't buy either.
Footnote:
1 I will address the implications of this fact and later developments for Fermi's Paradox in the concluding Part of this series.