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Now comes a paper that rebuts Sagan and Newman, as well as Hart, and offers a new solution to the Fermi paradox that avoids speculation about alien psychology or anthropology.

The research, which is under review by The Astrophysical Journal, suggests that it wouldn’t take as long as Sagan and Newman thought for a spacefaring civilization to planet-hop across the galaxy, because the movements of stars can help distribute life. “The sun has been around the center of the Milky Way 50 times,” says Jonathan Carroll-Nellenback, an astronomer at the University of Rochester, who led the study. “Stellar motions alone would get you the spread of life on time scales much shorter than the age of the galaxy.” Still, although galaxies can become fully settled fairly quickly, the fact of our loneliness is not necessarily paradoxical. According to simulations by Carroll-Nellenback and his colleagues, natural variability will mean that sometimes galaxies will be settled, but often not—solving Fermi’s quandary.

The question of how easy it would be to settle the galaxy has played a central role in attempts to resolve the Fermi paradox. Hart and others calculated that a single spacefaring species could populate the galaxy within a few million years, and maybe even as quickly as 650,000 years. Their absence, given the relative ease with which they should spread, means they must not exist, according to Hart.

Sagan and Newman argued it would take longer, in part because long-lived civilizations are likelier to grow more slowly. Faster-growing, rapacious societies might peter out before they could touch all the stars. So maybe there have been a lot of short-lived, fast-growing societies that wink out, or a few long-lived, slowly expanding societies that just haven’t arrived yet, as Jason Wright of Pennsylvania State University, a co-author of the new study, summarized Sagan and Newman’s argument. But Wright doesn’t agree with either solution.

“That conflates the expansion of the species as a whole with the sustainability of individual settlements,” he says. “Even if it is true for one species, it is not going to be this iron-clad law of xenosociology where if they are expanding, they are necessarily short lived.” After all, he notes, life on Earth is robust, “and it expands really fast.”

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In their new paper, Carroll-Nellenback, Wright, and their collaborators, Adam Frank of Rochester and Caleb Scharf of Columbia University, sought to examine the paradox without making untestable assumptions. They modeled the spread of a “settlement front” across the galaxy, and found that its speed would be strongly affected by the motions of stars, which previous work—including Sagan and Newman’s—treated as static objects. The settlement front could cross the entire galaxy based just on the motions of stars, regardless of the power of propulsion systems. “There is lots of time for exponential growth, basically leading to every system being settled,” Carroll-Nellenback says.