Grabby aliens hypothesis: only 30% chance we'll ever meet them

David Matolcsi, writing on LessWrong, critically examines Robin Hanson's 'grabby aliens' hypothesis and concludes the probability of ever encountering another powerful alien civilization in the observable universe is only about 30%. Hanson's original argument posits that intelligent life must emerge early in cosmic history because later, 'grabby' alien civilizations will spread at near-light speed, conquering all habitable planets. This would explain why Earth appears unusually early (13 billion years after the Big Bang) on a relatively short-lived planet. Hanson uses this to argue that aliens very likely exist—otherwise our earliness would be a statistical anomaly.

Matolcsi presents two objections. First, he distinguishes between two theories of life's rarity: Theory A (extremely rare, requiring anthropic selection) and Theory B (rare but not that rare). Under Theory A, we should expect zero other intelligent species in the observable universe—our existence is a fluke. Under Theory B, other species are expected, but their emergence later is suppressed by grabby aliens. Matolcsi applies Bayesian reasoning: before knowing Earth's properties, an observer should expect to be on a long-lived later planet if Theory B holds. That we find ourselves on an early short-lived planet updates beliefs, but not overwhelmingly toward aliens. He estimates the posterior probability at ~30%, far below Hanson's confident conclusion. The debate hinges on the true rarity of life, which remains unknown.