Detecting extraterrestrial signals may require searching farther and longer than previously expected.
For decades, scientists have scanned the sky for evidence of extraterrestrial technology. A new study from EPFL takes a different angle, asking what we should realistically expect to detect today if signals from alien civilizations have already passed by Earth without being noticed.
Since the first SETI experiment in 1960, astronomers have searched the Milky Way for signs of advanced civilizations using radio waves, optical flashes, and infrared emissions. Despite decades of effort, no confirmed signals have been found.
Decades of searches yield silence
This lack of detection is often explained by the limited portion of the galaxy that has been explored so far. However, another possibility is that signals have already reached Earth but went undetected.
A technosignature refers to any observable signal or physical evidence of advanced technology beyond Earth, such as artificial radio transmissions, laser pulses, or excess heat from large engineered structures.
Detecting such a signal requires two conditions to be met. The signal must first reach Earth, and our instruments must then be capable of identifying it. Even if a signal arrives, it may go unnoticed if it is too faint, too brief, transmitted at an unexpected wavelength, or obscured by background noise. This means a signal could reach Earth without ever being recognized.
It is therefore plausible that technosignatures have already passed through our region of space over the past several decades without being detected. If so, additional signals could still be passing by today as observational technology continues to improve.
Modeling what missed signals imply
Claudio Grimaldi, a theoretical physicist at EPFL’s Laboratory of Statistical Biophysics, investigated what these missed signals would imply for current and future searches. Using statistical modeling, he examined how many signals would need to have crossed Earth since 1960 to give a reasonable chance of detecting one today, and how far away those signals are likely to originate.
The study, published in The Astronomical Journal, treats technosignatures as emissions produced by distant technological species or their artifacts within the Milky Way. These signals travel at the speed of light and can persist for durations ranging from days to thousands of years.
In this framework, Earth is considered “contacted” whenever such a signal passes through its location. Detection occurs only if the signal originates from within a range where it remains strong enough for our instruments to register it.
To connect the key variables, Grimaldi applied a Bayesian statistical method that incorporates new information to refine predictions. His analysis focused on three central factors:
- How many signals have intersected with Earth in the past
- How long these technosignatures typically persist
- How far current or upcoming instruments are capable of detecting them
The study considers both signals that spread in all directions, such as waste heat from large-scale structures, and tightly focused emissions like laser beams or beacons, treating both scenarios within the same framework.
Detection may require distant signals
The results challenge a commonly held optimistic view. Achieving a high probability of detecting technosignatures within a few hundred or even a few thousand light-years would require an extremely large number of signals to have already passed Earth undetected. In many modeled scenarios, this number becomes unrealistically high, sometimes exceeding the number of potentially habitable planets in that region of the galaxy.
Detection becomes more plausible only when searches extend to much greater distances. If technosignatures last for long periods and are distributed across the Milky Way, then signals from several thousand light-years away may be detectable. Even in that case, the expected number of detectable signals at any given time remains very small.
Search shifts toward long-term strategy
These findings suggest that the absence of past detections does not mean discovery is imminent. If extraterrestrial technologies exist and have already sent signals that reached Earth, they are likely to be rare, distant, or long-lived rather than frequent and nearby.
This perspective shifts the focus of SETI toward long-term, wide-area searches that cover larger regions of the galaxy. Instead of waiting for clear signals close to Earth, future efforts may depend on sustained observation across vast distances.
Reference: “Undetected Past Contacts with Technological Species: Implications for Technosignature Science” by and Claudio Grimaldi, 16 February 2026, The Astronomical Journal.
DOI: 10.3847/1538-3881/ae394b
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