In February 2020, four distinguished astrophysicists —Jonathan Carroll-Nellenback, Adam Frank, Jason Wright, Caleb Scharf— suggested that Earth may have remained unvisited by space-faring civilizations all the while existing in a galaxy of interstellar civilizations seeded by moving stars that spread alien life, offering a solution to the perplexing Fermi paradox. They concluded that a planet-hopping civilization could populate the Milky Way in as little as 650,000 years.
“It’s possible that the Milky Way is partially settled, or intermittently so; maybe explorers visited us in the past, but we don’t remember, and they died out,” says Jonathan Carroll-Nellenback, an astronomer at the University of Rochester and his collaborators in a 2019 study that suggests it wouldn’t take as long as thought for a space-faring civilization to planet-hop across the galaxy, because the orbits of stars can help distribute life, offering a new solution to the Fermi paradox. “The solar system may well be amid other settled systems; it’s just been unvisited for millions of years.”
Very long times between close-enough encounters with other habitable worlds
In an email to The Daily Galaxy, Carroll-Nellenback expanded on his conjecture that our solar system may have been unvisited for millions of years:
“If the nearest settled star system is currently out of range of our best technology (and future technology) – then we may have to wait for the movement of the stars themselves to bring other star systems closer. This can lead to very long times between close-enough encounters with other habitable worlds… approaching millions of years (or longer depending on assumptions about future technology and abundance of suitable planets).
“So the fact that we haven’t apparently been visited/settled in as far back as we can tell (10 Myr) doesn’t mean that we are necessarily alone in the galaxy – or that we have never been settled… It may just be that settlements are short lived – and happen very infrequently because the rate of settlement is tied to the motion of the stars themselves. The density of systems (and presumably the density of systems that are suitable for settlements) increases as one moves inward in galactic radius – or in globular clusters – and in those regions – nearby settle- able systems may always be in range – so one does not have to wait for stellar motions (or millions of years) between encounters.”
Or, as Seth Shostak, an astronomer at the SETI Institute thinks the Fermi Paradox is likely to be explained by something more complex than distance and time, that is, perception: “The click beetles in my backyard don’t notice that they’re surrounded by intelligent beings — namely my neighbors and me, but we’re here, nonetheless.”
“The sun has been around the center of the Milky Way 50 times,” noted Carroll-Nellenback and his collaborators Jason Wright at Penn State, Adam Frank of Rochester and Caleb Scharf of Columbia University. “Stellar motions alone would get you the spread of life on time scales much shorter than the age of the galaxy.”
According to research by Carroll-Nellenback and his colleagues, reported Rebecca Boyle for Quanta, their simulations show that natural variability will mean that some galaxies will be settled, but often not — solving Fermi’s quandary with a solution that avoids assumptions about the intent and motivation of any exo-civilization seeking to settle other planetary systems.
The team calculated the speed of an advancing settlement via probes of finite velocity and range to determine if the galaxy can become inhabited with space-faring civilizations on timescales shorter than its age. They included the effect of stellar motions on the long-term behavior of the settlement front which adds a diffusive component to its advance.
Humans must, therefore, be the only technological civilization in the galaxy.
Their models showed that the Milky Way can be readily ‘filled-in’ with settled stellar systems under conservative assumptions about interstellar spacecraft velocities and launch rates. We then consider the question of the galactic steady-state achieved in terms of the fraction of settled planets. They calculated a range of parameters for which the galaxy supports a population of interstellar space-faring civilizations even though some potential star systems are uninhabited.
The results point to ways in which Earth might remain unvisited in the midst of an inhabited galaxy, breaking the link between astrophysicist Michael Hart’s famous calculation that a single space-faring species could populate the galaxy within a few million years, and maybe even as quickly as 650,000 years and their absence, given the relative ease with which they should spread, which means they must not exist and that humans must, therefore, be the only technological civilization in the galaxy.