Could Some Radio Signals Be Messages From Extraterrestrial Civilizations?

Cameron White
5 hours ago
6 min read

For more than a century, humanity has been using radio waves to communicate across the Earth. Television broadcasts, radar, satellites, military transmissions, and radio stations have filled space around our planet with electromagnetic signals. If another technological civilization exists somewhere in the universe, it is reasonable to ask a fascinating question: could we someday detect its radio transmissions?

Scientists have been asking exactly that through the Search for Extraterrestrial Intelligence (SETI). Rather than searching directly for alien spacecraft, SETI researchers look for technosignatures—observable evidence of technology that could indicate an intelligent civilization. Radio signals are among the most important possibilities. (NASA Science)

Why Radio Waves?

Radio waves are a form of electromagnetic radiation that can travel enormous distances through space. A technologically advanced civilization could theoretically use radio transmissions to communicate between planets or even between different star systems.

Radio is particularly interesting to SETI because artificial transmissions can potentially have characteristics that distinguish them from many naturally occurring astronomical signals. Scientists often search for narrow-band signals, meaning signals concentrated within a very small portion of the radio spectrum. SETI considers such signals potentially interesting because many natural radio sources produce broader emissions. (SETI Institute)

However, detecting something unusual is only the beginning. Scientists must determine whether the signal actually came from space, whether it repeats, whether it can be detected by another telescope, and whether ordinary human technology or an astronomical phenomenon can explain it.

That requirement has produced some remarkable mysteries.

The Famous Wow! Signal

Perhaps the most famous unexplained radio signal in SETI history is the Wow! Signal, detected on August 15, 1977, by Ohio State University's Big Ear radio telescope.

The signal was exceptionally strong and narrow-band. It appeared near the frequency of the hydrogen line, around 1420 MHz, a frequency of particular interest to radio astronomers. The signal lasted approximately 72 seconds, corresponding to the period during which the telescope's beam swept across its apparent source. (SETI Institute)

Astronomer Jerry Ehman later examined the printed data and saw the unusual sequence:

6EQUJ5

He circled it and wrote “Wow!” beside the signal. That handwritten reaction gave the mysterious detection its name.

What made the event particularly intriguing was that its rise and fall appeared consistent with a distant celestial source passing through the telescope's beam. In other words, it did not immediately look like an ordinary nearby terrestrial transmission. (SETI Institute)

But there was a major problem.

It never happened again.

Despite decades of interest and repeated attempts to find the signal again, the Wow! Signal has not been conclusively redetected. The SETI Institute therefore does not consider it evidence of extraterrestrial intelligence. A single unexplained detection is not enough to establish that an alien civilization transmitted it. (SETI Institute)

BLC1: A Signal From Proxima Centauri

The story became even more interesting decades later.

In 2019, the Breakthrough Listen project was observing Proxima Centauri, the nearest star to our Solar System. Researchers discovered a narrow-band radio signal around 982 MHz that appeared to have several characteristics expected of a possible technological transmission.

The signal became known as BLC1, short for Breakthrough Listen Candidate 1.

Initially, it attracted considerable scientific attention because it appeared during observations pointed toward Proxima Centauri and showed characteristics that made it worthy of further investigation. (Nature)

If BLC1 had turned out to be extraterrestrial, it would have been extraordinary. Proxima Centauri is only about 4.2 light-years away, making it one of the most tantalizing locations from which humanity could potentially receive a technological transmission.

But further investigation produced a much less exotic explanation.

Researchers concluded that BLC1 was human-made radio interference, specifically a complicated intermodulation product produced by local electronic signals. (Nature)

This case illustrates one of the biggest challenges facing SETI: Earth itself is extremely noisy.

The Problem With Listening to Space

Modern civilization produces enormous quantities of radio-frequency energy.

Satellites, aircraft, radar systems, telecommunications equipment, cell towers, electronics, spacecraft, and other technologies can all create signals that appear in astronomical observations.

A signal can therefore look alien simply because scientists don't initially recognize its terrestrial origin.

This is why serious SETI research requires extensive verification.

A promising signal might need to:

Appear to come from a particular astronomical direction.
Exhibit characteristics inconsistent with ordinary natural sources.
Survive tests designed to eliminate terrestrial interference.
Potentially be observed by another telescope.
Repeat or otherwise provide independently verifiable evidence.
Remain unexplained after extensive scientific investigation.

The SETI Institute emphasizes that no confirmed extraterrestrial signal has yet been detected. (SETI Institute)

Could an Alien Civilization Actually Communicate With Radio?

Absolutely—at least according to the laws of physics.

Humans have already demonstrated that radio communication can cross enormous distances. We routinely communicate with spacecraft millions or billions of kilometers away.

An extraterrestrial civilization with technology comparable to—or significantly more advanced than—ours could theoretically construct powerful transmitters capable of sending radio signals across interstellar distances.

In fact, Earth itself is producing detectable technological signatures.

A 2025 study discussed by the SETI Institute examined how detectable Earth might appear to an extraterrestrial civilization. Researchers found that some of Earth's strongest technological radio emissions, particularly powerful planetary radar transmissions, could potentially be detectable from extremely large distances under favorable circumstances. (SETI Institute)

That creates an intriguing possibility:

If another civilization possesses radio telescopes powerful enough to detect Earth, perhaps it could already know that a technological civilization exists here.

The Difference Between a Strange Signal and an Alien Message

One of the most important distinctions in SETI is the difference between an unexplained signal and an extraterrestrial signal.

They are not the same thing.

An unexplained signal simply means scientists don't yet know what caused it.

An extraterrestrial signal would require evidence that the source is genuinely beyond Earth and that its characteristics cannot reasonably be explained by natural astrophysical processes or human technology.

The Wow! Signal remains unexplained, but that doesn't automatically make it extraterrestrial.

BLC1 is an even better example. It initially appeared promising, but detailed analysis ultimately identified a human technological explanation. (Nature)

This is why scientists are cautious.

The goal isn't simply to find something strange.

The goal is to find something that survives every attempt to explain it away.

What Would a Real Alien Signal Look Like?

There is no guaranteed blueprint for extraterrestrial communication.

An alien civilization might use radio waves, lasers, or technologies humanity has not yet developed. NASA describes radio and laser pulses as among the possible technological signatures that astronomers could search for. (NASA Science)

A particularly compelling radio detection could potentially contain:

A highly structured signal.

Artificial communication could contain mathematical patterns, repeated sequences, or other evidence of deliberate construction.

A precise astronomical origin.

The signal might consistently originate from a particular star system or planet.

A detectable frequency pattern.

Scientists could observe a narrow-band transmission or another unusual radio signature.

Repetition.

A signal that returns predictably would be dramatically more convincing than a one-time event.

Information.

The ultimate discovery would be a transmission containing an unmistakable artificial message.

Imagine detecting a repeating sequence of prime numbers coming from another star system.

That would be very difficult to explain as a random natural phenomenon.

The Most Important Signal May Be One We Haven't Heard Yet

The absence of a confirmed extraterrestrial signal does not demonstrate that extraterrestrial civilizations don't exist.

The universe is enormous.

There are billions of stars in the Milky Way alone, and many stars have planets. Humanity has only examined a tiny fraction of the possible locations, frequencies, signal types, and time periods in which an extraterrestrial civilization might be transmitting.

There is another problem: timing.

If a civilization 1,000 light-years away transmitted a radio signal today, we wouldn't hear it for another 1,000 years.

Likewise, if we detect a signal coming from 1,000 light-years away, we're seeing something that happened approximately 1,000 years in the past.

The universe therefore functions as a giant cosmic archive.

A Message Across the Stars

Perhaps the most fascinating possibility is that humanity's first undeniable evidence of extraterrestrial intelligence won't arrive in the form of a spacecraft.

It could arrive as a faint signal buried inside astronomical data.

A sequence of radio waves.

A repeating pattern.

A transmission coming from a distant star.

And somewhere on Earth, a scientist could eventually look at a computer screen and realize that the signal isn't random.

It isn't interference.

It isn't a satellite.

It isn't a natural astronomical phenomenon.

Someone—or something—is transmitting.

That moment would fundamentally change humanity's understanding of its place in the universe.

For now, however, the scientific answer remains cautious: we have found intriguing signals, but no radio transmission has yet been confirmed as extraterrestrial communication. The mystery remains open, and that is precisely what makes the search so compelling. (SETI Institute)