How would we recognize a signal from another civilization?

A strange signal is a reason to investigate, not an announcement that someone is calling. A technosignature would be an observable trace of technology beyond Earth. To recognize one, astronomers must show why a measured pattern fits that explanation better than natural sources, human interference, or a fault in the instrument.

A signal has to survive competing explanations

Searches can look for concentrated radio emission, brief laser flashes, or other possible effects of technology. These are different experiments, each with its own sensitivity and possible impostors. No special frequency or unusual shape is a universal stamp of extraterrestrial origin. NASA 2023, “Searching for Signs of Intelligent Life,” technosignature examples

Imagine that a telescope records a narrow stripe in a graph of radio frequency against time. Its frequency changes slowly. Before choosing an explanation, separate three statements:

Statement	What it establishes	What it leaves open
The recorded power rises above the measured background	The instrument detected a feature under the analysis rules	Whether it came from the sky or nearby equipment
The feature changes frequency	The received pattern is changing	Motion, drifting electronics, and processing errors can have different causes but similar appearances
It appears while pointing toward a star	A relation to the pointing or observing time	A telescope also responds outside its main beam; local interference may switch on at convenient times

This is a reasoning exercise, not a new observed signal. To make a stronger case, ask for a test whose result would differ between the alternatives. Pointing away is useful, but a local transmitter that happens to switch off during that interval can pass that one test.

The signal that looked promising near Proxima Centauri

In 2019, the Parkes Murriyang telescope recorded a narrow radio feature near 982 MHz during observations toward Proxima Centauri. It became known as blc1. Follow-up analysis found similar patterns at other frequencies, connected to local electronic interference. The researchers identified it as an intermodulation product: signals combined in electronics to produce another frequency. It was not an extraterrestrial technosignature. Sheikh et al. 2021, §§1, 4.4 and 7

The lesson is useful even without the instrument details: an initially unusual feature became understandable when investigators compared it with a wider population of signals. A failed extraterrestrial interpretation can still improve an experiment.

Design the next observation

Here is an invented follow-up problem. A signal appears in three short pointings toward one star and disappears in three alternating pointings away. All six observations use the same receiver. You have time for two further checks. Which pair would most directly challenge a local-interference explanation?

1. Repeat the same schedule and display the signal in brighter colors.
2. Change the pointing schedule, inspect nearby frequencies, and compare with local interference monitors.
3. Ask a geographically separated observatory to observe simultaneously with independent equipment.

Answer and explanation

Choose 2 and 3. A changed schedule tests whether local activity happened to line up with the old one; related features or a local monitor may reveal an electronic origin. Independent observations help test whether the source is associated with the same sky direction rather than one site or receiver. Neither check guarantees an extraterrestrial explanation: compare timing, frequency, sensitivity, pointing response, satellites and plausible natural mechanisms. Brighter display colors in option 1 add no independent evidence.

Real verification needs instrument-specific checks rather than a universal pass/fail recipe. Sheikh and colleagues organize such follow-up around signal properties, repetition and potential interference, and recommend simultaneous observations at separate sites when practicable. Sheikh et al. 2021, §§5–6

Silence is a result with a boundary

A search covers particular directions, times, frequency bands, signal shapes and strengths. Detecting nothing constrains transmitters that would have been detectable within that coverage. It cannot rule out every civilization, a transmitter aimed elsewhere, a different technology, or a signal too faint for the instrument. The right conclusion names the experiment’s reach. Wright, Kanodia and Lubar 2018, §2.1, dimensions of a radio search

Likewise, finding a potentially habitable planet is not evidence that a civilization uses it. The life-evidence article distinguishes environmental suitability from signs of biology. Technology adds another inference, not an automatic final step.

Try the mathematics: how long would a reply take?

Preparation: distance divided by speed. Consider an invented nearby system 20 light-years away, at rest relative to us for this calculation. A light-year is the distance light travels in one year. Radio waves travel at the speed of light in vacuum, so a message takes about 20 years each way:

$$t_{\rm one\ way}=\frac{20\ {\rm ly}}{1\ {\rm ly/yr}}=20\ {\rm yr}.$$

An immediate response would return about 40 years after our transmission. Processing delays lengthen the wait; relative motion can change it. Cosmic expansion is negligible for this local example. Receiving a signal today also means receiving information emitted in the past. The arithmetic sets a communication delay, not a probability that anyone is there.

For classrooms and clubs

Preparation: distinguish a claim from a measurement; no radio equipment is required. Objective: propose a check that could change a conclusion. For a 10-minute demonstration, give learners the six invented pointings above and ask them to defend two follow-ups. In a 25-minute investigation, have pairs invent a local source that passes one test, then design a second test to expose it. Use a paper table for the no-device version.

An effective answer states what each test can exclude and what remains uncertain. Avoid treating “unexplained” as synonymous with “alien.” Finish by explaining why one non-detection cannot answer the whole question of extraterrestrial life.

References

• Brennan, Pat. “Searching for Signs of Intelligent Life: Technosignatures.” NASA Science, 11 July 2023; updated 3 July 2025. Overview of search methods.
• Sheikh, Sofia Z., et al. “Analysis of the Breakthrough Listen signal of interest blc1 with a technosignature verification framework.” Nature Astronomy 5, 1153–1162 (2021). DOI: 10.1038/s41550-021-01508-8. Open manuscript.
• Wright, Jason T., Shubham Kanodia and Emily G. Lubar. “How Much SETI Has Been Done? Finding Needles in the n-Dimensional Cosmic Haystack.” The Astronomical Journal 156, 260 (2018). DOI: 10.3847/1538-3881/aae099. Open manuscript.