There are tens of billions of galaxies in the universe, each with tens of billions of stars. Many stars have planets, and a healthy fraction of those are rocky and can sustain liquid water on their surfaces (like Earth). Even frozen moons circling frigid gas giants generate and retain enough energy to heat huge subsurface oceans.
Meanwhile, the ingredients for life as we know it are everywhere. Four of the six most abundant elements in the universe—carbon, hydrogen, oxygen, and nitrogen—also happen to be crucial players in biochemistry. We’ve even spotted complex organic molecules on asteroids, comets, and in interstellar clouds of gas.
In short, the more we learn about the universe, the more likely it seems that life in its simplest forms ought to be somewhat common. And if simple life is common and even a small fraction of life-sustaining planets develops a technological civilization, there ought to be many such civilizations in the galaxy. So…where is everybody?
Welcome to the Fermi Paradox
Though attributed to famed physicist, Enrico Fermi, who reportedly posed the question at lunch in 1950, other researchers dug out the implications in published works after he died.
The thinking goes like this: If there are other technological, spacefaring civilizations in the universe, they will eventually develop interstellar travel and settle nearby stars. Those societies will settle new stars, and in the fullness of cosmological time, they’ll hop from system to system until they’ve settled the whole galaxy.
The Milky Way is around 13.6 billion years old, but we don’t see evidence of any other technological civilizations in our neighborhood. So what gives?
There’s been no shortage of speculative answers to that question. But some of the earliest arguments took umbrage at the claim there’s been enough time. That is, it would take a very, very long while to build interstellar spacecraft and travel the vast, empty spaces between stars.
The galaxy isn’t old enough to have been fully settled yet…or is it?
A Formula for Galactic Empire
According to a recent addition to the Fermi debate, space and speed are no barrier to galactic empire—even without fancy tech like warp drives.
A 2019 paper, authored by Penn State and University of Rochester astronomers Jonathan Carroll-Nellenback, Adam Frank, Jason Wright, and Caleb Scharf, laid out an intricate model of galactic settlement, including the motion of stars, the fraction of habitable systems, the speed and range of ships, and other factors.
Now, in a new research note, the team present a visualization of just what that process might look like in action.
The simulation shows a significant fraction of a galaxy can be settled in a relatively short period of time, even with ships traveling no faster than the Voyager spacecraft. Further, the center of the galaxy could be a cosmic cantina that’d make even George Lucas blush.
The researchers found a key to speed was the motion of the stars themselves. Whereas older simulations relied on static configurations of stars, the galaxy is anything but stationary. Stars are in constant motion relative to the galactic center and each other.
In the simulation, ships (white cubes) in settled star systems (magenta spheres) wait for new systems (white points) to pass within their limited range (around 10 light-years) before launching a mission. Not all systems have habitable planets, and some that are habitable turn out to be unsettleable upon arrival (an outcome the researchers dub the “Aurora effect” after Kim Stanley Robinson’s novel Aurora). And finally, settlement ships launch no more frequently than every 100,000 years.
These are all quite conservative assumptions—especially the frequency of launches—and they don’t rely on some future (possibly fantastical) propulsion technology.
“This means we’re not talking about a rapidly or aggressively expanding species, and there’s no warp drive or anything,” Wright told Gizmodo.
“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.”
And yet, despite all this, the timescale covered by the simulation is “just” a billion years, under 10 percent the age of the galaxy. Not only does leveraging star motion accelerate the process, so too does the density of stars—note the explosive exponential growth in the center of the galaxy (a place they suggest is ripe for SETI’s attention).
How does the team account for the fact we’ve not found evidence of other technological civilizations yet?
In their 2019 paper, they explored a wide range of possible scenarios, given the model. The recently released simulation is only one. If the parameters are tweaked—for example, the fraction of settleable worlds is especially low—the outcomes may look different, including scenarios where the galaxy is empty. Likewise, they note that in fully settled galaxies, the finite lifetime of civilizations may result in densely populated neighborhoods surrounded by population voids.
This range of possibilities is notable. A good model helps frame the debate, but many unknowns remain. Which is, of course, why the Fermi paradox is fun. Absent hard evidence, it’s fertile ground for speculation. Our knowledge of the galaxy is far from complete.
Answering Fermi
Still, like the stars, science isn’t stationary. Unknowns in Fermi’s day are now on firmer footing. We know many star systems have planets resembling Earth and have evidence that liquid water may not be so rare.
NASA’s Perseverance rover is preparing to drill into Martian soil in search of life. The James Webb Space Telescope will study exoplanet atmospheres for biosignatures. And NASA plans to send probes to promising outer-solar-system moons.
Finding even a few microbes in another part of our own solar system would be ground-shaking. If life has occurred more than once around the same star—what are the odds it’s never arisen anywhere else?
More complicated are questions of how often simple life gives rise to technological civilizations, whether such cultures would be motivated and capable of settling other stars, and if they persist long enough to spread across an entire galaxy.
Still, every year, as we reach further into our solar system and look deeper into the universe, the answer to Fermi’s famous question gets a little bit clearer.
Image Credit: Jason T. Wright, Jonathan Carroll-Nellenback, Adam Frank, and Caleb Scharf / The American Astronomical Society
