A New Way to Search for Alien Civilizations

For decades, radio SETI searches have focused on one important question: if another technological civilization is trying to contact us, how would they send that message?

In a recent SETI Live conversation, host Dr. Lauren Sgro spoke with UCLA astronomer emeritus Dr. Ben Zuckerman about his paper, Broadband Searches for Extraterrestrial Technological Intelligence: A New Strategy to Find Alien Civilizations. Their discussion explored a different way of thinking about SETI searches, and challenged one of the assumptions that has shaped many traditional strategies.

For Dr. Zuckerman, the motivation behind the paper comes back to a fundamental question. “Are we alone in our Milky Way galaxy?” he explained. “And how best might one go about trying to answer this question?”

Rethinking the Assumptions Behind Radio SETI

Traditional radio SETI searches often focus on narrow-band signals. The reasoning is based on an idea Dr. Zuckerman calls “power starvation.”

If a civilization does not know where to send a signal, it may need to broadcast in many directions. Because the available energy is spread out, the signal arriving at Earth would be weak. To make such a signal easier to detect, a transmitter could concentrate its energy into a very narrow range of wavelengths.

However, Dr. Zuckerman suggests that this assumption may not apply to a sufficiently advanced civilization.

The key question is: what if they already know where to look?

An advanced technological society, he explained, could have built powerful space telescopes and interferometers capable of studying distant planets. Using these instruments, they could identify worlds with characteristics that suggest the presence of life.

“They will know that Earth is special,” Dr. Zuckerman said.

They may not know that humans exist, but they could recognize Earth as a planet with oceans, an atmosphere, changing seasons, and other signs that make it an interesting target.

If a civilization knows where to direct a message, then the need to conserve power changes. Instead of sending weak signals in every direction, they could send powerful signals directly toward promising worlds.

Looking for Signals We May Have Missed

One of the most interesting ideas discussed during SETI Live was that scientists on Earth may not need to build a new telescope to begin searching this way.

Dr. Zuckerman pointed to existing broadband radio surveys carried out for other astronomical purposes. These surveys were designed to study objects such as distant galaxies, but they also captured data from many nearby Sun-like stars.

Because older Sun-like stars are not expected to produce strong radio emissions, an unusual signal coming from one of these systems could stand out. Additionally, we know of no natural sources that can produce narrowband signals, and so they would be attributed to ET life.

The challenge is that researchers were not necessarily looking for these kinds of signals when the observations were collected.

“There might be something we’ve just overlooked,” Dr. Zuckerman said.

Rather than only collecting new observations, scientists could return to existing archives and examine the data with a different question in mind: could there be evidence of a technological civilization hidden among these measurements?

Modern computational tools, including artificial intelligence, could make this type of search more practical by helping researchers analyze large astronomical datasets for unusual patterns.

Why Broadband Signals Could Change the Search

A major difference between traditional narrow-band searches and the strategy discussed by Dr. Zuckerman is the amount of information that could be carried.

Narrow signals can be easier to detect when the correct frequency is known, but they carry limited information. Broadband signals, on the other hand, could allow a civilization to transmit much more complex information.

The challenge is that we do not know which wavelengths another civilization would choose.

As Dr. Zuckerman explained, extraterrestrial communication could potentially occur across different parts of the electromagnetic spectrum, including radio, infrared, or optical wavelengths.

That uncertainty is one reason SETI searches continue to explore many different approaches. At the SETI Institute, we use both radio signature and LASER searches.

What If We Still Find Nothing?

During the conversation, Lauren asked what a continued lack of detections could tell us.

Dr. Zuckerman emphasized that a null result would not prove we are alone in the universe. Instead, it would place limits on how many communicative technological civilizations could exist.

In his paper, he uses a search region extending about 650 light-years around the Sun as an arbitrary example. If researchers searched this volume thoroughly across relevant wavelengths and found no evidence of communication, it would suggest that the number of purposely communicative civilizations in the Milky Way is lower than some earlier estimates.

The calculation also depends on an important assumption: that our local region is representative of the galaxy.

Dr. Zuckerman noted that larger search volumes in the future could provide even stronger constraints.

What About Alien Probes?

The discussion also explored another possibility: instead of sending signals, could an advanced civilization have sent physical probes?

Dr. Zuckerman explained that if technological civilizations were common and had existed for a long time, some may have had opportunities to send probes toward nearby stars, including our own solar system.

The fact that we have not found evidence of such technology provides another way to estimate how common advanced civilizations might be.

As with radio searches, the absence of a discovery does not answer every question, but it does provide valuable information.

According to Dr. Zuckerman, the fact that we have found no evidence of past contact—whether in the form of signals, probes, or visiting spacecraft—can be used to place an upper limit on N, the number of technologically communicative civilizations in the Milky Way. Over the past two billion years, roughly two million old, single Sun-like stars have passed within 100 light-years of Earth. Compared to the roughly 20 billion similar stars in the Galaxy, this "null result" suggests that such civilizations must be rare, placing an upper limit of about 10,000 on N.

Searching the Galaxy While Protecting Earth

The conversation ended with a reminder that the search for life beyond Earth also changes how we think about life here.

Dr. Zuckerman emphasized that if technological civilizations are rare, then life on Earth is especially valuable.

“Life on Earth is really precious,” he said, highlighting the importance of finding ways to live sustainably and protect our planet’s biosphere.

The search for other civilizations is ultimately also a reflection on our own.

As we continue looking for signs of life elsewhere, we are reminded of the responsibility to understand and preserve the life we already know exists.

Watch the full SETI Live conversation here. Read the published paper.

Final questions

1. What is a space-based interferometer?

An interferometer combines observations from multiple telescopes to act like one much larger telescope. In the discussion, Dr. Zuckerman explained that an advanced civilization could use such instruments to study distant planets and identify worlds with characteristics that suggest the presence of life.

2. Why are broadband searches different from traditional SETI searches?

Traditional radio SETI searches often focus on narrowband signals because they are efficient for a transmitter that does not know where a receiver is located. Broadband searches explore the possibility that an advanced civilization could know which planets are interesting targets and send stronger, information-rich signals directly toward them.

3. Could existing astronomical data contain missed technosignatures?

Possibly. Dr. Zuckerman suggested that some archival surveys may contain unusual signals that were not recognized because researchers were not searching for broadband technosignatures at the time. Revisiting these datasets with new tools could reveal signals that deserve further investigation.