Undetected past contacts with technological species: implications for technosignature science

TL;DR

This paper uses Bayesian inference to assess the likelihood of undetected extraterrestrial technosignatures reaching Earth, concluding that such undetected contacts are improbable unless certain unlikely conditions are met, and suggesting future search strategies.

Contribution

It introduces a Bayesian framework to estimate the detectability of past extraterrestrial contacts, highlighting the improbability of undetected signals within a few hundred light-years.

Findings

High detectability requires implausibly large contact numbers.

Detectability increases if emitters are clustered near Earth.

Detection prospects improve with searches over thousands of light-years.

Abstract

In the search for extraterrestrial intelligence (SETI), the highly incomplete sampling of the technosignature search space is often considered as a plausible explanation for the persistent lack of detections over six decades of searches. If correct, this would imply that technosignatures may already have reached Earth without being detected or correctly identified. Here, we explore this possibility using a Bayesian inference framework to estimate present-day detectability given $n\ge 1$ undetected contacts over the past 65 years -- the period since the first SETI experiment. We show that achieving high detectability of technosignatures emitted within a few hundred light-years of Earth would require implausibly large $n$ values, even exceeding the population of habitable planets within that range. More conservative estimates can be obtained only assuming that emitters are tightly clustered near Earth or that their population in the Milky Way has undergone a very recent and sudden boost. This tension is further exacerbated for short-lived technosignatures and persists whether they are omnidirectional, as in Dysonian megastructures, or directional, as in intentional communication attempts. These findings suggest that, if undetected past contacts from the Milky Way have indeed occurred, the best prospects of detection may lie in searches extending over several thousand light-years, though only a few detectable technoemissions would be expected.