Undetected past contacts with technological species: implications for technosignature science

TL;DR

This work analyzes whether undetected past technosignature contacts could yield detectable signals today. By formulating a Bayesian Poisson framework over a Galaxy-wide emitter distribution and accounting for emission longevity and detectability distance, it shows that near-Earth detectability at high confidence would require unrealistically many past contacts, while a Milky Way-scale view lowers the requirement but still predicts only a handful of detectable events overall. The study demonstrates that detector sensitivity, directional vs isotropic emissions, and spatio-temporal emitter distributions jointly constrain present-day detectability, with results robust to prior choices. The findings suggest that if past contacts occurred, the best chances for detection lie in searches extending over several thousand light-years, but even then only a few technoemissions would be detectable across the Galaxy, informing strategic planning for next-generation technosignature searches.

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.

Figures (4)

• Figure 1: Temporal evolution of a spherical shell produced by an isotropic technoemission of duration $L$. In the present-day configuration (panel a), the outer and inner radii of the shell are $ct$ and $c(t - L)$, respectively; the example shown corresponds to the case in which Earth lies inside the hollow region of the shell, at a distance $d$ from the emitter. In panel b, the same shell is shown $\tau$ years ago, when the outer and inner radii were smaller by $c\tau$ and Earth was outside the shell. In panel c, the shell has intersected Earth at some point over the past $\tau$ years if $c(t - \tau - L) \le d \le ct$. Expressed in terms of the emitter appearance time $t$, this condition becomes $d/c \le t \le d/c + L + \tau$, yielding an appearance time window for intersection of $L + \tau$.
• Figure 2: Results assuming undetected contacts with technoemissions over the past $65$ years. a: Number $n$ of undetected past contacts required to achieve a present-day detectability of at least $95$ %, as a function of the search radius $R$. The thin black line shows the number of habitable planets $N_p$ as a function of R calculated from $N_p=10^{10}\int\!d\mathbf{r}\rho_\textrm{disk}(\mathbf{r})\theta(R-d)$, where $d$ is the distance from Earth. The grey region marks where $n>N_p$. b: Average emitter longevity versus $R$ for selected $n/N_p$ values. c: Upper and lower bounds on the expected number of present-day detections, $\langle k\rangle$, computed from Equation \ref{['bounds']} for detectabilities $\mathcal{P}=95$%, $99$%, and $99.9$% . For each $\mathcal{P}$, all possible $\langle k\rangle$ values lie within the corresponding colored areas.
• Figure 3: Effects of emitter clustering around Earth on past undetected contacts. a: Profile of the emitter PDF as a function the radial distance from the galactic center for different values of the standard deviation $\sigma$. b: Number $n$ of undetected past contacts required to achieve a present-day detectability $\mathcal{P}$ of at least $95$ %, as a function of the search radius $R$. c: Upper and lower bounds on $\langle k\rangle$ computed from Equation \ref{['bounds']} for $\mathcal{P}= 95$ %
• Figure 4: Effects of a temporal transition toward a high emitter birthrate on the number of undetected past contacts. a: Dependence of emitter appearance time for different transition times $t_c$. The growth rate is fixed at $1$ kyr$^{-1}$. b: Corresponding minimum number $n_\textrm{min}$ of undetected past contacts required to achieve a present-day detectability $\mathcal{P}$ of at least $95$ %. c: Upper and lower bounds on $\langle k\rangle$ computed from Equation \ref{['bounds']} for $\mathcal{P}= 95$ %.