For more than half a century, the search for extraterrestrial intelligence has been built on the assumption that if aliens exist and try to communicate, their signals will be faint, scattered, and easy to miss. 

So astronomers have spent decades scanning narrow slices of the radio spectrum, hoping to catch a weak signal buried in cosmic noise. However, a new study suggests something totally different—if an advanced civilization actually wanted to be noticed, it would not broadcast weak, unfocused emissions. 

It would do the opposite—concentrate its power into tightly aimed, high-intensity beams directed at specific targets. 

“Our principal assumption is that a purposely communicative technological civilization will do its technological best to establish communication with other extraterrestrial technological intelligences (ETIs),” Benjamin Zuckerman, study author and an astrophysicist from the University of California, Los Angeles, said.

If this idea is even roughly right, then the silence in our data is not just a lack of evidence. It actually limits how many nearby civilizations could be sending signals we could detect.

From faint radio traces to laser-like beams

The traditional logic behind SETI comes from a simple constraint—interstellar communication is hard. If a civilization has limited power, the most efficient strategy is to broadcast in all directions. 

However, this makes any signal extremely weak by the time it reaches another star. This is why SETI searches have focused on extremely narrow frequency bands.

“Radio search programs have employed very narrow (few Hz) bandwidths (BWs)—because, if an ETI has a given (limited) amount of power to transmit, then the way to maximize the signal-to-noise ratio at the receiving antenna is to use very narrow transmission and reception BWs,” Zuckerman notes.

The difficulty is that no one knows which frequency to listen to, so even decades of work have covered only a tiny fraction of possibilities.

The study challenges this assumption directly. It suggests that aliens capable of interstellar technology would not necessarily choose inefficiency. So instead of broadcasting isotropically like a dim bulb, it could use highly directional transmission systems, more like a laser pointer than a lamp.

Power is not the constraint

Detectability depends less on total power and more on direction—whether Earth happens to lie inside the beam. So, power is no longer the limiting factor. Even a system drawing on the order of 60 megawatts could generate a signal that, if correctly aimed at Earth, would stand out dramatically above cosmic background noise.

For instance, at distances of roughly 200 parsecs, such a directed signal could appear with a strength of around 10¹⁰ Jansky. For comparison, modern radio telescopes can detect signals down to about 1 Jansky. 

In simple words, a well-aimed transmission would not be subtle—it would be obvious in routine astronomical data.

“The most uncertain factor in our communication with a nearby ETI will not be power starvation, but rather the wavelength of transmission; this may be radio, infrared, or optical,” Zuckerman said

This could also mean that we may already have observed such signals without recognizing them. Large-scale sky surveys, conducted over the past century for entirely different scientific purposes, have already scanned vast regions of the sky with sufficient sensitivity. Yet none have reported persistent, anomalous emissions from nearby Sun-like stars.

A quiet solar neighborhood, measured in missed encounters

To turn this idea into a constraint, Zuckerman builds a conservative model of where communicative civilizations might exist. The starting point is simple. Life as we know it requires liquid water, limiting potential habitats to planets in the habitable zones of their stars.

However, technological intelligence takes time to emerge—on Earth, roughly 4.5 billion years. This means only older, stable, Sun-like stars are realistic candidates. Within a sphere of about 200 parsecs, there are roughly 500,000 Sun-like stars. 

Of these, about 200,000 are old enough to potentially host advanced life. Statistical estimates suggest that around 60,000 of them could host habitable planets.

An advanced civilization would not need to search blindly. With sufficiently powerful telescopes, it could identify which of these planets show signs of life, then further narrow its focus to worlds with Earth-like conditions—oceans, continents, and long-term climate stability.

At that point, communication becomes targeted rather than universal. A civilization might direct signals toward only a few hundred carefully selected worlds. From our perspective, detecting such a signal would require monitoring many stars—but if even a single nearby civilization were actively communicating, its signal should stand out in existing data.

Even slow interstellar probes, traveling at just 1% of the speed of light, could reach us in about 10,000 years—an extremely short timescale in cosmic terms.

“The absence of evidence for alien probes in the solar system suggests that no alien civilization has passed within ∼100 lt-yr of Earth during the past few billion years,” Zuckerman notes.

The silence is no longer just silence

Taken together, the absence of both signals and physical visitation leads to a more quantitative conclusion than SETI has traditionally offered. Rather than implying ignorance, the silence becomes a constraint.

It suggests that technologically communicative civilizations are either extremely rare in our region of the galaxy or not actively attempting to communicate in ways we can detect.

According to Zuckerman’s study, the number of civilizations in the Milky Way that are both technologically advanced and actively transmitting may be fewer than 100,000—and possibly closer to 10,000.

However, these numbers come with important boundaries. “The limits to be derived apply only to ETIs that are doing their technological best to establish communication with other technological species in their vicinity,” the study notes.

So they apply only to civilizations using electromagnetic communication and deliberately attempting contact. A species that communicates differently—or chooses not to broadcast at all—would remain invisible to this approach.

SETI programs should change

The implication of this study is not that the search should stop, but that it should change. Instead of focusing narrowly on tiny frequency bands, future surveys may need to examine broader wavelength ranges across large populations of nearby, Sun-like stars. 

The goal would not just be to listen more carefully—but to search more completely. Such efforts could either tighten the limits further or finally reveal a signal that has been sitting in existing data all along.

“Thus, search programs should aim to cover as much of the electromagnetic (EM) spectrum as possible—this is very difficult to do with currently designed radio SETI programs.”

The study is published in The Astrophysical Journal.