Despite decades of monitoring across radio, optical, and infrared bands, no technosignature has been confirmed, a result often attributed to the fact that only a small fraction of the Galaxy has been explored so far.

A common idea is that extraterrestrial signals may already have passed through Earth without being recognized, implying that similar signals might still be crossing our planet today and could be detected as instruments become more sensitive.

A new theoretical study by EPFL physicist Claudio Grimaldi from the Laboratory of Statistical Biophysics examines this assumption by asking what undetected past contacts would imply for current and future searches for technosignatures.

In the work, technosignatures are modeled as emissions from technological species or their artifacts located somewhere in the Milky Way, with signals spreading at the speed of light and lasting from very short durations, like days, to very long durations of thousands of years.

Earth is considered to be in contact whenever one of these signals sweeps across its position in space, but actual detection requires that the source lies within a maximum distance at which its signal remains strong enough for present or near-future telescopes to pick it out from background noise.

That distance effectively summarizes both the intrinsic strength of the technosignature and the sensitivity of the receiving instruments, meaning that a signal could easily cross Earth and still remain too faint, too brief, or at the wrong wavelength to be noticed.

Using a Bayesian statistical framework, Grimaldi connects three key quantities: the number of times alien technosignatures have contacted Earth in the past, the typical lifetime of those technosignatures, and the range of distances that current or upcoming SETI observations can realistically probe.

The analysis includes both broadly emitted signals, such as waste heat or omnidirectional transmissions that spread over large regions, and highly focused signals, such as intentional beacons or laser flashes tightly directed across interstellar space.

By exploring this parameter space, the study tests how many undetected past contacts would be required to make a present-day detection within a few hundred to a few thousand light years highly probable.

The results challenge some optimistic expectations, showing that achieving a high probability of detection within such relatively nearby volumes would often require that an implausibly large number of technosignatures have already crossed Earth unnoticed since 1960.

In many scenarios explored, the number of required past contacts exceeds even the number of potentially habitable planets estimated to exist within the same region of the Galaxy, making these specific combinations of assumptions unlikely rather than strictly impossible.

The picture changes when searches extend to much larger distances and to long-lived technosignatures that can persist as they traverse the Milky Way.

If technosignatures last thousands of years and are distributed on galactic scales, the probability of detection becomes higher at distances of several thousand light years or more, but the model still predicts that only a few detectable signals would exist in the entire Galaxy at any given time.

In this framework, the idea that many signals have quietly passed us by does not automatically imply that detection is imminent; instead, it points toward scenarios in which extraterrestrial technologies are rare, far away, or emitting over long timescales rather than being numerous and nearby.

The work therefore reinforces the view that technosignature science is a long term, statistically driven endeavor, where progress depends on expanding searches to cover wider regions and deeper volumes of the Milky Way rather than expecting obvious signals to appear quickly in our cosmic neighborhood.

This perspective supports ongoing and planned efforts to conduct broad, sensitive surveys across different wavelengths, aiming to improve constraints on how often technosignatures might arise and how they might be distributed through the Galaxy.

Research Report:Undetected Past Contacts with Technological Species: Implications for Technosignature Science

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