Aliens Could Time Messages To Earth, And Scientists Are Listening

Search For Alien Signals Finds Only Earthly Noise So Far

A pilot search using a radio telescope built for alien hunting found no sign of deliberate alien signals in a narrow slice of sky. The test was sensitive enough to catch a transmitter as powerful as the retired Arecibo radar, once the strongest on Earth, from more than 100 light-years away, under the best conditions.

Finding an alien message is a needle-in-a-haystack problem, with countless directions, times, and radio channels to check. A civilization hoping to be heard would likely save energy by aiming a beam at a target and sending it only occasionally, which makes the signal easy to miss unless both sides know when to tune in.

Game theory offers a shortcut. Aliens living near the flat plane that Earth orbits in could watch Earth cross the Sun’s face, the same way astronomers here spot planets crossing distant stars. The middle of that crossing is a moment both sides could work out without ever talking, what game theorists call a Schelling point, an obvious meeting place two strangers might pick independently. From Earth, the matching position is a point in the sky directly opposite the Sun. Light’s travel time would force a timing correction, most likely by the sender, a feat an advanced civilization could manage.

Scientists have discussed this idea for decades, yet large earlier radio surveys likely spent only 0.0004 percent of their time on the spot, all by accident. A team from the SETI Institute and partner universities set out to fix that with the Allen Telescope Array, a 42-dish radio observatory in Hat Creek, California, starting with a pilot test in summer 2023. The study was published in The Astronomical Journal.

Two Beams Separate Alien Signals From Earth Interference

Because that spot sits opposite the Sun, it is above the horizon only at night. Of the array’s 42 dishes, the pilot used 20, tuned to a lower band of radio frequencies than the full survey will use. That band included the frequency hydrogen gas emits, a long-proposed channel for cosmic conversation.

To tell alien signals from Earth interference, the telescope points two beams at once. One aims straight at the target spot, while a second aims a short distance away as a control. A real signal from deep space should appear strongly in the first beam and far weaker in the second. Interference from satellites and other human technology shows up in both. Signals from distant worlds should also slide slowly in frequency as Earth rotates, a shift like the changing pitch of a passing siren, while most local interference holds steady.

Three-Night Pilot Search Finds No Alien Signals

Over three nights, the telescope logged about 17 hours of observations, less than 1 percent of what the full survey will gather. Software flagged 1.9 million narrow radio blips. About 1.3 million showed up identically in both beams, a sign of Earth interference, leaving roughly 580,000 candidates.

Researchers then examined the most promising candidates by eye, more than 2,100 in all, and every one turned out to be noise or Earth-based interference. One drifting signal looked interesting at first, but its strength differed too little between beams to come from a distant source, and it sat in a band used for satellite communications.

Coming up empty still produced a measurement. Depending on how a signal was positioned and how much it drifted in frequency, the test could have spotted an Arecibo-class beam squeezed onto one narrow radio channel, out to 44 light-years in the worst case and 124 in the best. That holds only for the small slice of sky the pilot covered, roughly one twenty-fourth of the way around the full circle.

Pilot Results Reshaped the Four-Year Alien Signal Search

Lessons from the pilot changed the plan. The lower radio frequencies were so crowded with interference that the search software ran about ten times slower, so the full survey moves to higher frequencies, at the cost of covering less area. The full survey tackles satellite interference, with a satellite company pausing or limiting transmissions as its satellites cross the telescope’s beam, since phone-service satellites can otherwise swamp the entire band for about 15 seconds.

Sweeping the entire band of sky where such an alignment could occur should take about four years, since the array also serves other projects. By the full survey’s end, the team estimates it could catch the same kind of transmission from roughly 32 to 79 light-years away, depending on conditions, a shorter reach than the pilot’s but across far more sky. A promising signal would trigger saving that night’s recording and a recheck on later nights. More than 121 additional nights of data have already been recorded, with first-year results due in a future paper.

Three quiet nights cannot say whether anyone is out there, and the approach rests on a guess about how aliens think. Still, if an alien civilization is timing messages to Earth’s transit across the Sun, scientists are now listening in the right neighborhood at the right time.

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Disclaimer: This article summarizes a single pilot study and is intended for informational purposes only. Three nights of observing cannot show that alien transmitters are absent, and conclusions may change as the full survey is completed. Readers interested in the science should consult the original paper and qualified astronomers.

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