After the solar system is colonized, settled, and developed, interstellar colonization is the next step in mankind’s journey through the final frontier. The distances between planets within our own solar system are easily surmountable once enough space infrastructure is built out to allow the mass production of faster vehicles; among other methods of propulsion, nuclear thermal rockets and especially nuclear pulse propulsion will enable the entire solar system out through the Kuiper Belt to be traversed in weeks to months. While this is a long trip compared to inter-continental jetliners today, it is still comparable to inter-continental sea voyages during the Age of Discovery, and thus will be easily manageable. In particular migration and trade will not face any real technological barriers, excepting goods that are very time-sensitive. Even that obstacle may be surmounted after a while; as Philip Lubin points out laser beam propulsion in the near future may be able to propel microscopic or very small payloads to close to the speed of light, enabling rapid interplanetary cargo transportation, rapid as in Earth to Mars in less than an hour. Unfortunately the accelerations are too great to transport people, but the idea is extremely intriguing.
The distances between stars, however, are vast compared to the distances between planets. Proxima Centauri, the nearest star to us, is 4.221 light-years away; for comparison the distance between Earth and Mars is at most 22 light-minutes. Even Neptune is only 4 light-hours away from Earth, 9000 times closer than Proxima Centauri. A spacecraft that travels fast enough to reach Neptune in a month, an acceptable trip time even for many tourists, would take 740 years to reach Proxima Centauri, an acceptable trip time for virtually no one.
Practical interstellar Travel
Obviously if any travelers want to see the stars within their own lifetimes, much faster transportation will be needed. This is sometimes taken to mean that interstellar travel is impractical using current technology, but that is most emphatically not the case: nuclear pulse propulsion, a technology we have been capable of using since the 1950s, is capable of accelerating a spacecraft to 10% of light speed. Admittedly this is the theoretical top speed which would take considerable engineering advances to achieve, but the point is that achieving this sort of speed is only a matter of time if we decide to pursue it; we already have the technological base required to do so.
10% of light speed would of course enable explorers or colonists to reach Proxima Centauri in 42 years, which is certainly long, much longer than the average sailing voyage, but well within one human lifetime. The original crew, if young, will reach their destination by the time old age starts to set in. This is important as it gives them something concrete to look forward to, and avoids the potential pitfalls of generation ships, which are legion if science fiction is any indication. For worldbuilding purposes it also makes stories about exploration, travel, trade, and migration a lot easier if centuries’ worth of development isn’t occurring within each star system, and since such “fast” travel times are perfectly realistic anyway there’s no reason for even the hardest science fiction to center around generation ships unless one wants to for other reasons.
The 10% of light speed figure for nuclear pulse propulsion, which is the most energetic known technology that we could build now and will likely remain by far the easiest and cheapest method for fast spaceflight through the next several centuries, is for acceleration only; if half the pulse units must be kept in reserve for decelerating to one’s destination, the maximum speed drops to 5% of light speed. Fortunately solar and magnetic sails, which do not require fuel, can be used for deceleration, enabling full power to be devoted to acceleration. Within the next few centuries, even perhaps within the 21st century, nuclear pulse propulsion good enough to get us to the stars within our lifetimes will likely be available and affordable.
Hot-rodding a Starship with Solar Sails
Solar sails may also be used as a propulsion method; the maximum speed that can be reached depends upon the size of the sail that can be built and how close one can get to the sun without burning up. With currently known materials the theoretical maximum speed solar sails can attain with a solar “fry-by” trajectory is apparently 12% of light speed . This would require sails of titanic proportions, so although it is somewhat faster it may be less practical than nuclear pulse, at least for the next few centuries. However, the real gain from such methods come from the fact that they don’t need fuel, thus there is no drag from any added fuel mass. A maximally-efficient solar sail that reaches 12% of light speed may after its fry-by light up a maximally-efficient nuclear pulse drive as a booster to attain another 10% of light speed. This would enable speeds of up to 22% of light speed or so to be attained for even the largest ships within the next several centuries.
This more-than-doubling of travel speed would be extremely helpful; Proxima Centauri could be reached in only 18 years, opening up the possibility for a round trip with a 20 year stay to be completed well within a human lifetime (it would take 56 years in total). 22% of light speed is peppy enough for time dilation to shave 3 months off an 18 year journey from the point of view of the crew. While 18 years is still long enough to preclude temporary travel as opposed to permanent settlement, many possibilities open up with such short trip times.
In particular, the distances reachable within a century expand from 10 light-years (with 10% of light speed) to 22 light-years. 10 light-years restricts us to 14 stars, but a range of 22 light-years opens up 151 stars. This greatly enlarges the number of potential destinations. Dozens if not hundreds of stars being explored and settled brings us into the earliest stages of an interstellar space opera world, even just using technology likely to be available within a few centuries.
Contemplating interstellar Migration
Of course, space is big: as we saw in the previous post, under any realistic scenario even the inner part of our own solar system is likely to be only lightly populated even as far into the future as 2200. Human populations are not particularly likely to explode any time soon, and spreading one Earth’s worth of population across land area many times greater than Earth, which is provided within our own solar system, will obviously lead to every region having a rather sparse population. Even if human population does enter into a permanent baby boom it will take centuries longer to cover the planets of our own solar system in Earth-style density. The areas involved are just that immense.
Populations sometimes take centuries to migrate and reach the equilibrium indicated by the fundamentals. For instance, cities along the East Coast of the United States tend to be much more populous than comparably-situated cities further west. The reason is that the East Coast was settled first and it takes time for later-settled regions to catch up; the fundamentals would suggest comparable areas in the west would grow faster, and indeed they actually have been. Even after one to four centuries, though, the population still has not come to an equilibrium. Regions that went through this process earlier like northern Europe strongly suggest that will eventually happen, but the point is that this process takes time even with modern transportation technology.
The somewhat longer, but still modest, trip times across the solar system will likewise only enable something resembling equilibrium to be achieved on the order of centuries, the process quite possibly taking most of the coming millennium. Places that are better situated to host very large populations and economies but are remote and settled late will take a long time to fulfill their potential. This is important at the interstellar scale because the much longer trip times will also mean a longer time required to reach equilibrium. If it takes the better part of a millennium for remote parts of the solar system to reach their potential, a place as remote as the Alpha Centauri system (which includes Proxima Centauri) will take much longer. Alpha Centauri, being a more massive star system, might actually be a better site for a spacefaring civilization than our own solar system, but it would likely take at least several centuries to realize that potential even if it were as close as the outer planets in terms of travel time. If travel times remain on the order of decades, it will take even longer, perhaps thousands of years.
That doesn’t mean that Alpha Centauri won’t be a thriving colony with a large population and economy before then; if during the early phase of spacefaring civilization everything goes right with development of propulsion and long-term isolated space habitats, if there is a fervent desire to explore and settle the unknown, and if societies are flush with wealth Alpha Centauri may well be home to a large thriving colony as early as the 22nd century. In this scenario the first colonial explorers likely arrived sometime later in the 21st century and were the first settlers. In somewhat less rosy scenarios the system would still be settled well before the end of this millennium.
Of course most space opera settings take place over much more than two solar systems, and systems other than Alpha Centauri will also be targets. At 23% of light speed, Barnard’s Star could be reached in 27 years, Sirius in 39 years, Epsilon Eridani in 48 years, 61 Cygni and Procyon in 52 years, and Tau Ceti in 54 years. These are all bright stars within relatively easy travel distance from Earth at such speeds. Once established there, the denizens of these other solar systems will one day send out explorers and settlers to stars that are within easy travel distance from where they are. This “island hopping” pattern of settlement will within a few centuries reach destinations that are centuries away from Earth but are relatively close to the nearest civilized system. This is not unlike how Greenland was settled out of Iceland’s population and Vinland (now known as Newfoundland) in turn was settled out of Greenland’s population.
As long as travel times between stars are measured in decades, it seems likely that explorers and frontier settlers will move not too much slower than travel speeds will allow them to. Perhaps several decades or at most a century of development in each star system would be required before a new expedition leaves for another system. The frontier of the most developed “core” of interstellar civilization won’t progress nearly as fast, thus the future in this scenario will consist of a rapidly expanding “bubble” of extremely lightly settled frontier around a more-slowly expanding populous core. Eventually the pioneers may well become the dominant group numerically, though any coordination on their part would be limited due to the great distances and thus communication times involved.
Island-hopping through the Galaxy
At 22% of light speed our entire galaxy from end to end could be traversed in 455,000 years. This is an extremely long time compared to a human lifetime but is relatively short on a geological time scale; for comparison, humans and thus sapient life on Earth have been around in some form for at least 2 million years. Of course, an island-hopping strategy implies staying at each destination before sending out a new expedition, which will add time to that 455,000 years figure. Assuming an average of twenty years’ travel time and a century at each destination the time involved expands to 2.2 million years. This is once again an immense time, but without accounting for unknown technological developments, the continents on Earth won’t have moved much and the human species could easily not have changed much in that amount of time.
It would be extremely naïve, of course, to assume the technology we will have by the 24th century will remain the state of the art for millions of years. Physical laws far beyond our understanding could easily be (and likely will be) ancient knowledge by then. This is one reason why science fiction, even space opera, doesn’t often go far beyond the next few centuries to millennia of the human future: past a few thousand years it just becomes too unpredictable, and keeping the setting the same for that long strains plausibility. Dune, which takes place around 20,000 years in the future, is perhaps the furthest-out world of any prominence; the Orion’s Arm worldbuilding project at 10,000 years in the future also deserves an honorable mention. Star Wars is among the most advanced and well-developed of the more-famous worlds, but it takes place in another galaxy “a long time ago”, although even if you discount the expanded universe it is clearly the result of millennia of interstellar civilization.
Toward truly fast interstellar Travel
22% of light speed is hardly a cosmic barrier: only the speed of light itself is the ultimate speed limit for an object with mass traveling through space. Antimatter propulsion could easily reach perhaps twice this speed, with corresponding reduction in travel times. Theoretically antimatter propulsion could reach well over 90% of light speed, though a titanic quantity of fuel would be required. If 95% of light speed could be reached constant acceleration of 1g could be employed, which is the practical maximum for human comfort. This would enable 6 years to Proxima Centauri, though time dilation shortens the trip to 3.5 years for the travelers.
More exotic and speculative methods like the construction of artificial wormholes could be employed; although we have little idea how to do such a thing, the laws of physics seem to permit it, making it ultimately an engineering problem that given sufficient advancement will be surmounted. Before a wormhole can be constructed a ship has to get to the other end, limiting a wormhole network’s expansion to just under light speed from Earth’s perspective. Thus it would take at least 100,000 years to cover the galaxy with them, almost certainly considerably longer. Once constructed, though, they would function as a shortened path to their destinations, compressing interstellar distances to interplanetary scales, with a corresponding increase in travel intensity and economic integration. This assumes that naturally-occurring wormholes don’t exist; if they do, as the hypotheses of quantum foam and ER=EPR suggest, then stabilizing and harnessing them may enable rapid travel to any point in the universe and perhaps even beyond. There is also the possibility that an alien precursor species may have already done the work of constructing wormholes that are still usable.
A possibility not often mentioned is the gravitational catapult idea first proposed by Robert L. Forward. Gravity can generate fields just as electromagnetism can; it’s just that to do so extremely dense material (like neutronium or micro black holes) needs to be moved around at near the speed of light. Since this is well beyond our technology not much is said about this, but such fields could be made either attractive or repulsive, and can provide far greater acceleration and deceleration than any other method. Even better, the field would act on all parts of a ship (like the hull and the passengers) simultaneously, so they could be accelerated even at thousands of times Earth gravity and not feel a thing. Such catapults could almost instantly send ships rocketing off at relativistic speeds toward another star system, where a catapult there would decelerate them.
A good speed to send ships at using this method might be the point where the cosmic microwave background is blue-shifted into the visible range. This would provide a nice natural glow for the passengers without subjecting them to too much hard radiation. More importantly the photons of the cosmic background won’t drag the ship’s speed down much at this level. The speed in question is apparently 99.999835% of light speed. This yields a Lorentz factor (a measure of time dilation) of 550. From Earth’s perspective you’ll arrive in 4.22 years, but from your perspective you’ll arrive in a bit under 3 days.
Of course it is unknown when developments that appear feasible from a scientific perspective like wormhole construction or harnessing, gravitational catapults, or even the much more famous Alcubierre warp drive will become technically feasible to engineer, but it is extremely plausible at least one of these methods will become a viable method of propulsion within the next few million years; given even a modest rate of scientific advancement the scale would be more like a few thousand years. Once these ultra-relativistic or effectively faster-than-light methods of propulsion become viable the splendid isolation frontier colonies enjoyed during earlier stages will be broken by the availability of almost instant travel.
Bands wandering the Cosmos Primeval
This may disrupt the pleasant stomping grounds of the frontier races and expose them to colonization by galactic civilization, but once again space is a very big place. Although it would be arrogant to assume the entire galaxy is unoccupied, consider that there are currently as many as 50 stars in the Milky Way for every human currently alive. With a naturally-provided surface area per solar system equivalent to 10,000 Earths, Freeman Dyson’s estimate for our own solar system (the vast majority of this being asteroids and comets), this leads to a “galactic population density” of almost 100 trillion square miles per person. At this density even an extremely advanced culture may be faced with more star and planet than they know what to do with.
This is the population de-concentration that will be enabled by very rapid or even instant travel across the galaxy. Hunter-gatherer bands, the ancestral form of social organization, average a few dozen in population; scientific research has confirmed that humans can only manage around 150 real social relationships, and this only with constant maintenance, so it seems likely that given the choice humans will cluster in these bands again in the future, especially as nomads in a resource-abundant environment. Given a band size of about 50 humans each band would have 2500 stars in the galaxy to themselves, a territory tens if not hundreds of light-years across. Virtually all of this is likely to remain wilderness for a very long time.
Of course all of this assumes there are no alien tribes already roaming these regions, but consider that to fill the galaxy to the current density of our solar system 400 billion times the current human population would be needed. To fill all the galaxy’s naturally-occurring surface area to the density of Earth 4 quadrillion times as many people would be needed. 10 trillion times these numbers would be needed for a rough estimate of the whole universe. While this hardly precludes strings of city-planets like a pearl necklace (after all, an average city-planet would only require about a trillion people), unless there are populations far larger than even most space opera authors and worldbuilders contemplate these city-planets will be drops of population in an ocean of wilderness.
Most speculatively of all, if we consider a rough estimate of the whole universe, which contains perhaps 10 trillion galaxies, then the current human population dispersed evenly over the whole universe yields a mindbogglingly low density of 1 octillion square miles per person. Each person’s share is 500 trillion stars, or the equivalent of 1250 galaxies. For a band their territory would span over 60,000 galaxies. Even per-capita energy usage consistent with a Type III culture on the Kardashev scale would be undetectable at such low densities. Of course such a culture given virtually instantaneous travel would still have gathering places which would have a much hotter energy signature, but good luck finding them even among thousands of stars, let alone thousands of galaxies. There’s a solution to the Fermi Paradox for you: the aliens are so spread out they just don’t give off an obvious sign of their presence, much like the “leave no trace” method of wilderness adventuring.
So we see that unless most star systems are already occupied by an indigenous civilization or other aliens, the advancement of interstellar propulsion and colonization ensures that a population, unless it grows fairly rapidly, will disperse to a very light population level. Given low-relativistic interstellar travel, the sort that may be attained by advanced nuclear pulse or solar sail, this will lead to a set of “core worlds”, likely surrounding the Sun and Alpha Centauri, with most of the population in classic space-opera-style density, and a peripheral “frontier” which will expand much faster and be very sparsely populated. This will likely be the case for centuries following the development of spacefaring civilization, most likely millennia, and would be a fascinating setting to worldbuild in. It is close enough to our own time for our history to be as exotic but also as remembered as the Greeks and Romans are to us.
Each star system due to the great distances involved will remain relatively isolated but it will only take a few years one-way to send information or small cargo, such as alien biological specimens, between each solar system. People will take several decades to travel between stars, meaning permanent migration will be the primary market, and perhaps a relatively large one.
This landscape will change considerably once ultra-relativistic or effectively faster-than-light propulsion becomes available, as it will greatly accelerate exploration of the galaxy and even the wider universe, along with colonization of the stars. The population won’t grow fast enough to keep up with this development (in fact it may even shrink), and so density will drop immensely over time, leading to a demography of nomadic bands of a few dozen people each separated by many light-years and having hundreds of star systems to themselves, at least as far as other humans are concerned.
While rapid travel will enable bands to meet each other and even have large-scale gatherings at will (permanently-maintained gathering places may even resemble classic city-planets, albeit with the vast majority being temporary as opposed to permanent residents) the dominant theme of this setting and this future will be the cosmos primeval, an immense wild. Although bands may not be technically isolated due to easy travel, the extremely low population densities will enable any band to be as isolated as it wishes, due to the difficulty of actually finding one band among hundreds of stars or even galaxies. This lifestyle will actually be similar to what is likely to occur in our own solar system at the dawn of spacefaring civilization, and the development of interstellar civilization smoothly transitions into this sort of setting. This perhaps is an unsung form of realistic space opera, awaiting a definitive adaptation, which would provide a very rich setting for stories and other forms of art.