A new computer simulation shows that a technologically advanced civilisation, even when using slow ships, can still colonise an entire galaxy in a modest amount of time. The finding presents a possible model for interstellar migration and a sharpened sense of where we might find alien intelligence.
Space, we are told time and time again, is huge, and that’s why we have yet to see signs of extraterrestrial intelligence. For sure, the distances between stars are vast, but it’s important to remember that the universe is also very, very old. In fact, I’d go so far as to say that, in terms of extremes, the Milky Way galaxy is more ancient than it is huge, if that makes sense.
It’s for this reason that I tend to dismiss distances as a significant variable when discussing the Fermi Paradox — the observation that we have yet to see any evidence for the existence of alien intelligence, even though we probably should have.
New research published in The American Astronomical Society is bolstering my conviction. The new paper, co-authored by Jason Wright, an astronomer and astrophysicist at Penn State, and Caleb Scharf, an astrobiologist at Columbia University, shows that even the most conservative estimates of civilisational expansion can still result in a galactic empire.
A simulation produced by the team shows the process at work, as a lone technological civilisation, living in a hypothetical Milky Way-like galaxy, begins the process of galactic expansion. Grey dots in the visualisation represent unsettled stars, magenta spheres represent settled stars, and the white cubes are starships in transit. Jonathan Carroll-Nellenback and Adam Frank, both astronomers at the University of Rochester, also participated in the study.
Rise of empires
Things start off slow in the simulation, but the civilisation’s rate of spread really picks up once the power of exponential growth kicks in. But that’s only part of the story; the expansion rate is heavily influenced by the increased density of stars near the galactic centre and a patient policy, in which the settlers wait for the stars to come to them, a result of the galaxy spinning on its axis.
The whole process, in which the entire inner galaxy is settled, takes one billion years. That sounds like a long time, but it’s only somewhere between 7% and 9% the total age of the Milky Way galaxy.
Another neat aspect of the video is that it shows a civilisation transitioning from Kardashev II status — in which it harnesses the power of entire star systems — to a full-blown Kardashev III civilisation, which has tapped into the energy output of the entire galaxy (more about the Kardashev scale here).
That a civilisation might want to embark on such an ambitious enterprise might seem implausible, but it’s important to remember Steven J. Dick’s Intelligence Principle, which states that the “maintenance, improvement and perpetuation of knowledge and intelligence is the central driving force of cultural evolution, and that to the extent intelligence can be improved, it will be improved,” as the science historian wrote in his 2003 paper, “Cultural Evolution, the Postbiological Universe and SETI.” Our civilisation keeps pushing the envelope of what’s possible, and we have no reason to believe this urge will cease any time soon. Hence the assumption that advanced civilisations will eventually seek to occupy every corner of the galaxy and set up camp around precious energy sources, namely stars.
My personal hero, the late Carl Sagan, thought it possible that alien civilisations might spawn colonisation waves, but he argued that it would take an inordinate amount of time for a civilisation to reach Kardashev III status, if ever. The new paper, however, imagines the transition as happening far faster than previously assumed, even under some unreasonably conservative assumptions.
Slow and steady wins the race
The starting point for the new paper was a model of expansion devised by the same team in 2019, in which the scientists showed “how even slow-moving ships, like the ones humanity makes today, can settle the galaxy in much less time than the galaxy has been around — if they can last long enough to make the trip to the nearest stars,” as Wright explained in an email.
The new simulation presents one example of what this process might actually look like, showing the pace and physical shape of the galactic-scale spread. It also shows how stellar motions are likely to play a significant role. The “stars themselves are moving around, so once you settle a nearby system, the star moves you to another part of the galaxy where more new stars will wander by and give you another nearby star to settle,” said Wright.
As noted, the new model is constrained by some very conservative rules. Migration ships are launched once every 10,000 years, and no civilisation can last longer than 100 million years. Ships can travel no farther than 10 light-years and at speeds no faster than 10 km per second , which is comparable to human probes like the Voyager and New Horizons spacecraft.
“This means we’re not talking about a rapidly or aggressively expanding species, and there’s no warp drive or anything,” said Wright. “There’s just ships that do things we could actually manage to do with something like technology we can design today, perhaps fast ships using solar sails powered by giant lasers, or just very long-lived ships that can make journeys of 100,000 years running on ordinary rockets and gravitational slingshots from giant planets.”
Aliens to the core
Even under these conditions, the entire inner part of the simulated galaxy became settled in a billion years. But as Wright reminded me, our “galaxy is over 10 billion years old, so it could have happened many times over, even with those parameters.”
Interestingly, the simulation also shows “how rapidly expansion occurs once the settlement front reaches the galactic bulge and centre,” as the astronomers wrote in their paper. Stars near the core are much closer to each other, so that tracks, but it’s “striking to see it happen in the video,” said Wright. The new video “confirms and validates previous work showing that the centres of galaxies are promising search directions for SETI [the search for extraterrestrial intelligence],” according to the paper.
Wright admitted that the simulation relies on very simple assumptions. Accordingly, the team is now working on a more realistic simulation that will allow for greater variation in these parameters, while also allowing civilizations to get both better and worse at interstellar travel.
The new paper enhances our understanding of the Fermi Paradox in that, “if there are spacefaring species, then they have had plenty of time to settle the entire galaxy by now, even without sci-fi tricks like warp drive,” Wright explained. Unlike me, however, Wright is not troubled by the Fermi Paradox, saying there are “lots of reasons why” we haven’t noticed aliens yet. (I will dig into this further for a future article, so stand by.)
“In fact, I’d be surprised if we had!” he added. “Rather, this is very good for SETI optimists who feel there should be lots of places in the galaxy that have technology we might be able to detect from Earth.”
Indeed, SETI scientists are already wise to the fact that the galactic centre might be a good place to look. Unfortunately, a recent survey of the galactic centre came up empty handed, but there’s a lesson in the new paper: It’s important to be methodical and patient. We just have to keep looking, regardless of whether aliens are out there or not.