For decades, scientists have been searching the skies for signs of extraterrestrial technology. A study by the University of Lausanne poses a pointed question: if signals from aliens have already reached Earth and we haven’t noticed them, what should we really expect to find today?

Why we still haven’t received signals from aliens.
Source: phys.org

Searching for intangible technosignatures

Since the first SETI experiment in 1960, astronomers have been scanning the Milky Way for signs of advanced extraterrestrial civilizations. These searches have included radio waves, optical flashes, and infrared thermal signals. So far, nothing has been confirmed. This silence is often explained by the fact that we have only explored a tiny fraction of outer space. But what if signals did reach Earth and passed us by unnoticed?

A technosignature is any measurable signal or physical trace that indicates the existence of advanced technology beyond Earth, such as artificial radio transmissions, laser flashes, or excess heat from large-scale engineering projects. 

For the technosignature to be detected, two conditions have to be met. First, the signal has to physically reach Earth. Secondly, our instruments should be sensitive enough, pointed in the right direction, and able to distinguish it from natural signals. This means that a signal may satisfy the first condition but still fail to meet the second. It may be too weak, too short, transmitted on an unsuitable wavelength, or lost in the noise.

It is often assumed that this may already have happened: extraterrestrial technosignatures could have reached Earth over the past six decades but remained undetected. If this is true, it means that even more signals may be passing us by right now, just waiting to be detected as our equipment improves.

Earth Technosignature Art Concept

SETI researchers investigated how detectable Earth is to alien civilizations, finding that our most visible technosignatures are radio emissions and atmospheric pollutants. If observed from nearby space, lasers, satellites, and city lights would also signal our presence. This study provides a new perspective on Earth’s visibility and helps refine SETI’s search strategies.

How many signals from aliens could have already reached Earth?

A new study challenges this assumption. Claudio Grimaldi, a theoretical physicist at the University of Lausanne’s Laboratory of Statistical Biophysics, has now investigated what unnoticed past contacts could mean for modern SETI searches. Using a statistical approach, Grimaldi asked the question: how many extraterrestrial signals would have had to cross Earth since 1960 for there to be a high probability of detecting one today, and from what distance from Earth would these signals most likely have come?

Grimaldi modeled techno-signatures as emissions from distant technological beings or their artifacts somewhere in the Milky Way. This radiation propagates at the speed of light and, according to this model, can last from very short periods, such as days, to very long periods, spanning thousands of years.

The Earth is considered “contacted” if such a signal passes through our location in space. Detection only occurs when the source is within range, where its signal is strong enough for our telescopes to pick up, reflecting both the signal and the sensitivity of the instruments.

Grimaldi used a Bayesian statistical approach, a method that updates estimates as new information becomes available, to combine three elements: the number of past contacts with Earth, the typical duration of technosignatures, and the range of distances that modern or future instruments can explore.

He considered both omnidirectional signals, such as heat radiation from large structures, and narrowly directed signals, such as beacons or laser flashes. The analysis treats these cases equally.

Research findings that challenge current views

The results of theoretical research cast doubt on the widespread optimistic view. If scientists today want to have a high probability of detecting technosignals at a distance of several hundred or even thousands of light years, this would require a very large number of technosignals to have passed Earth unnoticed in the past.

In many scenarios, this number becomes incredibly large, sometimes exceeding the number of potentially habitable planets in the region of the galaxy, making such scenarios highly improbable, but not strictly impossible.

This only changes when the search covers much greater distances. If technosignals are long-lived and widespread throughout the Milky Way, their detection becomes more plausible at distances of several thousand light-years or more. Even in this case, only a few detected signals can be expected across the entire galaxy at any given moment.

A change in approach to the search for extraterrestrial life

Research indicates that the fact that signals may have escaped detection in the past does not mean that their discovery is imminent. If extraterrestrial technologies exist and have contacted Earth, they are likely to be rare, distant, or long-lasting, rather than close and frequent.

This changes the perception of searching as a patient, long-term endeavor rather than a game of waiting for clear signals. This also strengthens the case for deep and wide surveys covering large parts of the Milky Way, rather than focusing solely on our cosmic neighborhood.