Galaxy Simulations Offer a New Solution to the Fermi Paradox

Galaxy Simulations Offer a New Solution to the Fermi Paradox

Greg Rakozy

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 said.

But the fact that no interstellar visitors are here now — what Hart called “Fact A” — does not mean they do not exist, the authors say. While some civilizations might expand and become interstellar, not all of them last forever. On top of that, not every star is a choice destination, and not every planet is habitable. There’s also what Frank calls “the Aurora effect,” after Kim Stanley Robinson’s novel Aurora, in which settlers arrive at a habitable planet on which they nonetheless cannot survive.

When Carroll-Nellenback and his coauthors included these impediments to settlement in their model and ran many simulations with different star densities, seed civilizations, spacecraft velocities and other variations, they found a vast middle ground between a silent, empty galaxy and one teeming with life. 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. The solar system may well be amid other settled systems; it’s just been unvisited for millions of years.