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Galactic Settlement and the Fermi Paradox:
A spacefaring species could easily settle the entire Milky Way given billions of years. Yet the fact is that there is no obvious one in our solar system right now. The supposed inconsistency between these statements is the Fermi Paradox, named for the Nobel Prize-winning physicist who supposedly first formulated it. In a trenchant formulation of the Fermi Paradox, American astrophysicist Michael H. Hart called the lack of extraterrestrial beings or artifacts on Earth today "Fact A." He showed that most objections to his conclusion—that a spacefaring civilization could have crossed the galaxy by now—stem from either a lack of appreciation for the timescales involved (it takes a small extrapolation from present human technology to get interstellar ships, and even slow ships can star-hop across our galaxy in less time than the galaxy's age) or else the dubious assumption that all members of all extraterrestrial species will avoid colonizing behaviors forever (an example of what I've called the monocultural fallacy).
William Newman and Carl Sagan later wrote a major rebuttal to Hart's work, in which they argued that the timescales to populate the entire galaxy could be quite long. In particular, they noted that the colonization fronts Hart described through the Milky Way might move much more slowly than the speed of the colonization ships if their population growth rates were so low that they only needed to spread to nearby stars very rarely. They also argued that being a long-lived civilization is inconsistent with being a rapidly-expanding one, so any species bent on settling the galaxy would not last long enough to succeed. In other words, they reasoned that the galaxy could be filled with both short-lived rapidly expanding civilizations that don't get very far and long-lived slowly expanding civilizations that haven't gotten very far—either way, it's not surprising that we have not been visited.
Being a long-lived civilization is inconsistent with being a rapidly-expanding one.
In a 2014 paper on the topic, my colleagues and I rebutted many of these claims. In particular, we argued that one should not conflate the population growth in a single settlement with that of all settlements. There is no reason to suppose that population growth, resource depletion, or overcrowding drives the creation of new settlements, or that a small, sustainable settlement would never launch a new settlement ship. One can easily imagine a rapidly expanding network of small sustainable settlements (indeed, the first human migrations across the globe likely looked a lot like this).
Another factor affects Newman and Sagan's numbers on timescales and colonization-front speeds. Most of the prior work on this topic exploits percolation models, in which ships move about on a static two-dimensional substrate of stars. In these models, a star launching settlement ships can quickly settle all of the nearby stars, limiting the number of stars it can settle. But real stars move in three dimensions, meaning that they can carry their orbiting settlements throughout the galaxy, and that a settlement will always have fresh new stars to settle if it waits long enough.
Jonathan Carroll-Nellenback, at the University of Rochester with Adam Frank, not long ago finished work, with Caleb Scharf and me, on analytic and numerical models for how a realistic settlement front would behave in a real gas of stars, one characteristic of the galactic disk at our distance from the galactic center. The big advances here are a few:
Carroll-Nellenback validated an analytic formalism for settlement expansion fronts with numerical models for a realistic gas of stars. He accounted for finite settlement lifetimes, the idea that only a small fraction of stars will be settle-able, and explored the limits of very slow and infrequent settlement ships. He also explored a range of settlement behaviors to see how galactic settlement fronts depend on them.
The idea that not all stars are settle-able is important to keep in mind. Adam Frank calls this the Aurora effect, after the Kim Stanley Robinson novel in which a system is "habitable, but not settle-able."
A very interesting read.
(Score: 2, Insightful) by khallow on Monday January 20 2020, @03:32PM
The laws of physics won't change. So yes, it will remain comparable in the future.
For example, aluminum costs about 15 MJ per kg to smelt [world-aluminium.org] from the nicest ores (bauxite) we have. Presently, it costs well over 100 MJ per kg to put in low Earth orbit (the energy cost of the propellant plus portion of vehicle construction). But even with perfect technologies that reduce that to energy cost alone, you're speaking of 10 MJ per kg - basically making Earth-based aluminum at least two thirds more expensive, energy-wise, than aluminum from elsewhere. Iron alloys would typically be much less energy intensive (since iron, nickel, and chromium would all be vastly easier to extract from iron-nickel asteroids) so the energy markup of launching from Earth would be much larger.
You can talk about how cheap energy will be in the future, but it'll still remain that it costs substantially less energy to move massive amounts of material from asteroids to where you want them to be than to launch from Earth, even to Earth orbit!