A recent video by Kurzgesagt asked if modern physics supplied a boundary, an absolute limit, to human expansion into the universe. The answer, apparently, is “yes:” humans cannot travel* beyond our own Local Group of galaxies, because “dark energy” propels other galactic clusters away from us at velocities too great to match. If lightspeed imposes an upper limit on spacecraft – and it must, because faster-than-light objects propagate backward in time, violating causality – we can explore our galaxy and Andromeda and a couple of dozen nearby dwarf galaxies, but no further.
The author presented this as a sad story, from the standpoint of a future “Type III Civilization” that has colonized and exploited the entire Milky Way galaxy. Permit me to say, though, that we twenty-first-century primitives need not mourn, because even creating a galaxy-spanning civilization probably lies beyond humans' capacity. We may develop the means, but I doubt we will ever have the motives to colonize an entire galaxy, let alone the rest of our Local Group.
Sci-fi writers, and historians, have adduced several possible motives for interstellar colonization, none of which seems likely to push us all the way through the Milky Way (100 billion star systems, remember) except perhaps in a distant, post-human future. Let us look in this post at one of them: population growth.
Overpopulation sounds like a reasonable motive for extra-solar colonization, especially when one realizes how rapidly human population has grown in the past couple of centuries – from one billion people in 1800 to more than seven billion today. SF writers like Harry Harrison and Larry Niven, and nonfiction authors like Paul Ehrlich, opined in the 1960s and '70s that the global population was headed for the 20 or 30 billion mark, and that war and famine would necessarily result. Since then, however, human fertility rates have declined, not for Malthusian reasons but as a result of economic growth in poor countries. In a modernizing, industrializing country, it makes more economic sense to have fewer children and educate them than it does to have many untrained children to work in the fields or mines. Smaller families also make more practical sense when access to modern medicine relieves couples of the need for “replacements” for those who die, and when birth control gives women more reliable control over their own fertility.
We have no reliable way of predicting fertility rates in the future, but we can speculate based on historical evidence from the last few thousand years. On that basis high population growth looks more like an anomaly than an inevitable trend. It usually resulted from innovations in food production and medicine, like the adoption of American crops in Europe and China in the seventeenth and eighteenth centuries, the development of medical antisepsis and antibiotics in the early twentieth century, and the “Green Revolution” in agriculture later in that century. More common is slow growth or stasis,** punctuated by crashes that wipe out several centuries of growth, like the epidemics that beset the early-modern Americas. I think it unlikely that we'll have another pandemic on the scale of the Black Death, but I do note that there are quite a few industrial countries that are losing population to parents' practical economic decisions: Japan, Russia (though poor health is also to blame there), and much of western Europe. Growth is leveling off in formerly underdeveloped nations like India and China. Within the next century, I suspect that the world's demographic growth rate will regress to the historical norm: slow to nonexistent. (The “hockey stick” of demographic growth will, in short, prove part of a sigmoid curve.)
In the longer term, we may see occasional population spurts like those mentioned above, but human historical trends and technological advances both militate against sustained high growth. Short-term growth episodes may eventually leave planet Earth with 20 or 30 billion people, but not for several more centuries. By then, humans should have found ways to house those people here on Earth: constructing denser and more livable cities, environmentally re-engineering desert or tundra for human settlement, and/or building artificial islands and reclaiming land from the sea. Further forward in the future, we will probably find ways to render some of the other worlds in our Solar System, like Venus and Mars, inhabitable by human beings.***
Or, by then, more imminent disasters than population growth will have destroyed us and our works. More on those in another post.
* Technically, spacecraft can travel outside the Local Group, but they cannot reach other galactic clusters – only empty intergalactic space.
** In medieval and early-modern Europe, for instance, any multi-generational population growth at all was considered high.
*** This assumes that space travel becomes as inexpensive in the future as trans-oceanic travel became in the eighteenth and nineteenth centuries, the heyday of global travel and colonization.