Robin Hanson on Grabby Aliens

Perhaps the solution to the Fermi paradox is a question: “Why are humans so early?” The Grabby Aliens hypothesis, presented in the Astrophysical Journal paper, “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare” (full text at link), is summarised in its abstract.

If life on Earth had to achieve n “hard steps” to reach humanity’s level, then the chance of this event rose as time to the nth power. Integrating this over habitable star formation and planet lifetime distributions predicts >99% of advanced life appears after today, unless n < 3 and max planet duration <50 Gyr. That is, we seem early. We offer this explanation: a deadline is set by loud aliens who are born according to a hard steps power law, expand at a common rate, change their volume appearances, and prevent advanced life like us from appearing in their volumes. Quiet aliens, in contrast, are much harder to see. We fit this three-parameter model of loud aliens to data: (1) birth power from the number of hard steps seen in Earth’s history, (2) birth constant by assuming a inform distribution over our rank among loud alien birth dates, and (3) expansion speed from our not seeing alien volumes in our sky. We estimate that loud alien civilizations now control 40%–50% of universe volume, each will later control \sim 10^53\times 10^7 galaxies, and we could meet them in ∼200 Myr–2 Gyr. If loud aliens arise from quiet ones, a depressingly low transition chance (<\sim 10^{−4} ) is required to expect that even one other quiet alien civilization has ever been active in our galaxy. Which seems to be bad news for the Search for Extraterrestrial Intelligence. But perhaps alien volume appearances are subtle, and their expansion speed lower, in which case we predict many long circular arcs to find in our sky.

Or, in the “elevator pitch” on the home page of the Grabby Aliens Web site:

Advanced aliens really are out there, and we have enough data to say roughly where they are in space and time, and when we will see or meet them.

But what about those funny things in the sky that pilots are reporting?

Source code for the simulations of propagation of alien civilisations across the universe is available on GitHub.

Or, perhaps, we’re looking in the wrong places.