Despite these efforts, no technosignature has been confirmed so far, a silence often attributed to the fact that only a tiny fraction of the cosmic search space has been explored.
A new theoretical study from EPFL physicist Claudio Grimaldi examines a different possibility: that alien technosignatures may already have crossed Earth's orbit since 1960 but escaped detection, and asks what this would imply for current and near-future searches.
Technosignatures are measurable signals or physical traces that point to the existence of advanced technology, including artificial radio emissions, laser beacons or waste heat from large-scale engineering projects.
For such a technosignature to be observed, its signal must first physically arrive at Earth and then be picked up by instruments that are sufficiently sensitive, correctly tuned and pointed, and able to distinguish the signal from natural background sources.
A signal can therefore contact Earth without being detected if it is too weak, too brief, emitted at an unmonitored wavelength or simply buried in noise.
The idea that such missed encounters may have occurred underpins a common optimistic view: if signals have already passed by without notice, more should be crossing Earth now and might soon be detected as technology improves.
Grimaldi's study, published in The Astronomical Journal, challenges that expectation by quantifying how many past contacts would be required to give a high probability of detecting a technosignature today and how far from Earth those signals would most likely originate.
In his model, technosignatures are emissions from distant technological species or their artifacts somewhere in the Milky Way, propagating at the speed of light and persisting over lifetimes that can range from days to thousands of years.
Earth experiences a contact whenever such a signal passes through its region of space, but an actual detection occurs only if the emitting source lies within a distance range where the signal remains strong enough for present or near-future telescopes.
To link the number of past contacts, the typical lifetime of technosignatures and the distance range accessible to searches, Grimaldi uses a Bayesian statistical framework that updates probabilities as new information is considered.
The analysis treats both omnidirectional emissions, such as diffuse waste heat from large structures, and highly focused transmissions, such as intentional beacons or targeted laser flashes, on the same footing.
The results indicate that achieving a high probability of detecting technosignatures within a few hundred to a few thousand light years would require an extremely large number of signals to have passed Earth unnoticed over the past six decades.
In many plausible scenarios, the implied number of technosignals becomes so high that it can exceed the estimated number of potentially habitable planets in the region, making those scenarios unlikely even if not strictly impossible.
The picture changes when searches extend to much larger galactic volumes: if technosignatures are long-lived and spread through the Milky Way, the chances of detecting one improve at distances of several thousand light years or more.
Even in that case, the work suggests that only a small number of detectable signals would be present across the entire Galaxy at any given time, reinforcing the idea that technosignatures are rare, distant or both.
The study concludes that the possibility of undetected past contacts does not by itself imply that detection is imminent; instead, it points to a search landscape in which encounters with technosignatures are infrequent and often far away.
This perspective reinforces the need for sustained, patient observing campaigns and supports strategies that emphasize deep, wide-field surveys of large swaths of the Milky Way over efforts that focus solely on nearby stars.
Research Report:Undetected Past Contacts with Technological Species: Implications for Technosignature Science
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