Inferring the rate of technosignatures from 60 yr of nondetection

TL;DR

This paper estimates the upper limits on extraterrestrial technological emissions over 60 years of non-detection, suggesting such signals are rare and providing benchmarks for future SETI strategies.

Contribution

It introduces a probabilistic model to infer upper bounds on extraterrestrial emission rates based on long-term non-detections, considering directional signals and spatial distributions.

Findings

Less than 1-5 emissions per century with 95% credibility

Waiting times until next crossing event are 60-1800 years with 50% probability

Directional signals increase emission rate bounds without affecting waiting times

Abstract

For about the last 60 years the search for extraterrestrial intelligence has been monitoring the sky for evidence of remotely detectable technological life beyond Earth, with no positive results to date. While the lack of detection can be attributed to the highly incomplete sampling of the search space, technological emissions may be actually rare enough that we are living in a time when none cross the Earth. Here we explore the latter possibility and derive the likelihood of the Earth not being crossed by signals for at least the last 60 years to infer upper bounds on their rate of emission. Under the assumption that technological emitters are distributed uniformly in the Milky Way and that they generate technoemissions at a constant rate, we find less than about one to five emissions generated per century with 95% credible level. This implies optimistic waiting times until the next crossing event of no less than 60-1800 years with a 50% probability. A significant fraction of highly directional signals increases the emission rates upper bounds, but without systematically changing the waiting time. Although these probabilistic bounds are derived from a specific model and their validity depends on the model's assumptions, they are nevertheless quite robust against weak time dependences of the emission rate or nonuniform spatial distributions of the emitters. Our results provide therefore a benchmark for assessing the lack of detection and may serve as a basis to form optimal strategies for the search for extraterrestrial intelligence.