Since the first Search for Extraterrestrial Intelligence (SETI) experiment in 1960, scientists have been scanning the Milky Way for signs of advanced alien civilizations.
They have looked for unusual radio signals, brief flashes of laser light, and even excess heat that might come from massive alien engineering projects.
Despite decades of searching, no confirmed signal has ever been found.
Many researchers have explained this silence by saying we have explored only a tiny fraction of the galaxy. But another possibility is more intriguing: what if alien signals have already reached Earth, yet we failed to notice them?
Scientists call any detectable evidence of alien technology a “technosignature.” This could include artificial radio transmissions, laser pulses, or heat from giant structures built around stars. For us to detect such a signal, two things must happen.
The signal must first travel across space and reach Earth. Then our instruments must be sensitive enough, pointed in the right direction, and able to distinguish the signal from natural cosmic noise.
Even if a signal arrives, it could still go unnoticed if it is too weak, too short, or sent at a wavelength we are not monitoring.
Some researchers have suggested that this may already have happened—that alien signals could have crossed Earth’s path during the past sixty years without being detected.
If so, improved technology might soon reveal signals that are passing by even now.
A new study by theoretical physicist Claudio Grimaldi from EPFL in Switzerland challenges this hopeful idea.
Using statistical modeling, he examined how many alien signals would need to have reached Earth since 1960 for us to have a good chance of detecting one today, and how far away those signals likely originated.
Grimaldi’s model treated technosignatures as emissions from distant technological species somewhere in the galaxy. These signals travel at the speed of light and could last anywhere from a few days to thousands of years. Earth is considered “contacted” if a signal passes through our location in space, but detection only occurs if the signal is strong enough for our telescopes.
His results suggest that if we want a high chance of detecting nearby alien technology today, an enormous number of signals would have had to pass Earth unnoticed in the past—so many that the scenario becomes highly unlikely. Detection becomes more plausible only if signals come from much farther away and last for very long periods. Even then, only a handful of detectable signals might exist across the entire Milky Way at any given time.
The findings suggest that if alien technologies exist, they are probably rare, distant, or long-lasting rather than frequent and nearby. Instead of expecting an obvious signal to appear soon, scientists may need to conduct patient, large-scale searches that scan vast regions of the galaxy over long periods.
In other words, the silence we hear may not mean we are alone. It may simply mean that finding cosmic neighbors is far harder than we once imagined.
