For more than 60 years, scientists have been scanning the skies for signs of intelligent life beyond Earth.
Using powerful telescopes, they have searched for unusual radio signals, brief laser flashes, and even excess heat that might come from massive alien technology.
Despite decades of effort, no confirmed signal from extraterrestrial civilizations has been found.
A new study from the Swiss Federal Institute of Technology in Lausanne (EPFL) asks a fascinating question: what if alien signals have already reached Earth, but we simply didn’t notice them?
Scientists call any evidence of advanced alien technology a “technosignature.” This could be an artificial radio broadcast, a laser beam used for communication, or heat produced by enormous space structures.
For us to detect such a signal, two things must happen. First, the signal must physically arrive at Earth. Second, our instruments must be capable of detecting it at the right time, in the right direction, and at the right wavelength.
Even if a signal passes by our planet, we could still miss it if it is too weak, too short-lived, or buried in cosmic noise.
Some researchers have suggested that alien signals may already have crossed Earth at some point since the first SETI search began in 1960.
If that were true, it might mean more signals are passing by right now, waiting to be discovered as technology improves. However, EPFL physicist Claudio Grimaldi wanted to test whether this optimistic idea actually makes sense.
Grimaldi used statistical modeling to estimate how many alien signals would need to have reached Earth in the past for us to have a good chance of detecting one today.
He imagined signals coming from distant civilizations scattered across the Milky Way, traveling at the speed of light and lasting anywhere from a few days to thousands of years.
Earth would be “contacted” whenever one of these signals happened to pass through our region of space. Detection, however, would only occur if the source was close enough and strong enough for our telescopes to pick up.
His results challenge the hopeful belief that detection is likely just around the corner.
If we expect to find signals from civilizations within a few hundred or even a few thousand light-years, then an enormous number of signals must have passed Earth unnoticed in the past.
In many cases, the required number would be unrealistically high — even greater than the number of potentially habitable planets nearby. This makes the idea of frequent, nearby alien broadcasts unlikely.
The picture changes only when scientists consider signals that come from much farther away or last for extremely long periods. If advanced civilizations produce long-lived technosignatures that spread across the galaxy, then detecting a few of them becomes more plausible. Even so, the study suggests there would likely be only a handful of detectable signals across the entire Milky Way at any given time.
Rather than discouraging the search for extraterrestrial intelligence, the findings suggest that patience is essential.
If alien technology exists, it is probably rare, distant, or subtle. The best strategy may be to scan huge areas of the galaxy over long periods instead of focusing only on our immediate cosmic neighborhood.
In other words, the silence we hear today does not necessarily mean no one is out there — it may simply mean the universe is vast, quiet, and easy to miss if you are not listening in exactly the right way at exactly the right moment.
