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posted by janrinok on Sunday January 19 2020, @12:31PM   Printer-friendly

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.


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  • (Score: 0) by Anonymous Coward on Sunday January 19 2020, @07:36PM

    by Anonymous Coward on Sunday January 19 2020, @07:36PM (#945427)

    I think it *may* be becoming clear that progress is not incremental, but punctuated. Physics at the start of the 20th century was seen as basically solved. It's somewhat ironic that it was Michelson, of the Michelson-Morley experiment (which set the stage for the discovery of relativity), who said that 'the future of physics rests primarily in the 6th decimal place.' Physics then just exploded into a vast revolution from the 1920s until somewhere around the 70s. Since then we haven't really done a whole heck of a lot in the field, in spite of an exponential increase in computational and observational resources.

    We tend to forget that such explosions can and do happen which is natural since none have happened in our lifetime. But take something like the EM Drive. It probably does not work, yet if it had it would have been an unimaginable revolution in our species. Because of punctuated technological evolution, it's not really possible to predict the future. People 100 years ago would think it completely unimaginable that we'd have a space station orbiting over head with humans inside it today, to say nothing of landing on the moon, and that would have been just 50 years in the future from that time.

    And indeed we can even regress. In 1969 we managed to go from 0 to putting a man on the moon. Today the exact same organizations, though with entirely different people of course, are desperately trying to send a man around the moon. On a substantially lengthier schedule. Starting from all the knowledge we collected from 1969. And they're failing. The Linear Interpolators from 50 years ago would have expected us today to have colonized most of our solar system and be taking holidays to get some moon poon. To them our complete regression today would have been unthinkable. And today phenomenal success in the future is equally unthinkable. It's all because LERPing doesn't work when talking about an exponential + punctuated system, such as we are.