And life is getting noisier, as SETI scientists battle against increasing radio frequency interference (RFI) while straining to hear any whispers from possible technological life elsewhere in the Universe.

There’s so much radio interference these days that any detected signal has to be assumed to be RFI until proven otherwise. Scientifically, taking this skeptical approach is the right thing to do, lest you make a fool of yourself for prematurely declaring triumph in the search for ET. But emotionally, there’s always a tingle of excitement whenever a candidate signal is found. 

Yet for SETI scientists, those tingles have been growing increasingly subdued.

“I think at this point, people rarely get excited, no matter what the signal looks like at the beginning, because there have been so many false alarms in the past,” Seth Shostak, Senior Astronomer at the SETI Institute in California, tells Supercluster. 

And RFI has been growing worse over the years. Mobile phones, passing aircraft, signals from satellites and spacecraft, military transmissions, electronic machinery, and even microwave ovens, all produce radio emissions that SETI has to contend with. 

“There are so many transmitters now,” laments Shostak.

The prevalence of RFI means that SETI is picking up signals all the time. Usually, sophisticated algorithms are able to sift through the terabytes of data that are typically being collected by radio telescopes as they scan across billions of narrowband channels, and identify the terrestrial interference. When that fails, if the signal is still transmitting, then another simple check is a spatial one: if the signal is real, turning your telescope away from its apparent point of origin in the sky should have the effect of seeing the signal go away, to return when you point back at its source. If you can still hear the signal when turning your telescope off-target, then it must be local interference.

But every now and then a signal comes along that beats these filters. These false alarms can be a double-edged sword, says Shostak.

“On the one hand, false alarms are kind of frustrating because you spend a lot of effort on them and you get people’s hopes up, but personally, if I had to vote, I’d say that false alarms are valuable because they test the system.”

For one thing, the detection of false alarms shows that your equipment is working. In SETI that’s not automatically obvious; unlike radio astronomers who know to expect a signal from neutral hydrogen if they point at a galaxy, SETI astronomers don’t expect to hear anything, at least not right away.

Identifying the origin of false alarms and the patterns they leave in the data allows the filters to recognize them in the future, making the search more efficient.

The most exciting false alarms also often have an interesting story to tell as they become part of the fabric of SETI’s own mythology. So without further ado, here are six of the best false alarms in SETI.

The very first false alarm came right at the beginning, during the early days of the very first SETI experiment back in April 1960. This was, of course, Frank Drake’s Project Ozma, performed on the 85-foot Howard Tetel radio telescope at Green Bank. Over the course of two months, Drake listened to two nearby stars thought to be promising candidates for hosting habitable planets (of course, in 1960, no actual exoplanets were known) — tau Ceti and Epsilon Eridani. While slewing the telescope between the two, Drake detected a powerful, pulsed signal at 1420MHz. The signal was there the next day too, but it became apparent that it was moving across the sky far too fast to be on the celestial sphere. Instead, it was a military transmission from an airplane at an altitude greater than any known plane had ever flown. Although it has never been officially confirmed, Drake likely detected a secret U2 spy plane a month before one was shot down over the Soviet Union, revealing its existence to the world.

Seth Shostak was at the center of a false alarm in 1997 that demonstrated how, despite best efforts, news of a SETI detection is always going to leak out.

It was June 24th and scientists at the SETI Institute were in the middle of Project Phoenix, the first survey to take place after private donations had resurrected SETI following the cancellation of NASA’s alien-hunting program in 1993. It was all routine until a persistent signal was detected apparently coming from the red dwarf star YZ Ceti, 12 light years away.

“It looked good,” Shostak recalls. “It survived all the checks that we made.”

Well, almost. The team was hurriedly ringing around other observatories, trying to get independent confirmation of the signal, when an on-off check suggested that the signal was still being detected six arc-minutes away from the star — a red flag that it was RFI.

In the middle of all this, Shostak received a phone call from a New York Times journalist asking about the signal. Shostak was puzzled since no announcement had been made, but the journalist was well-connected and had managed to find out through the grapevine. The signal turned out to be communications from the joint NASA–ESA Solar and Heliospheric Observatory (SOHO) mission, but it was an important reminder that no signal, real or false, could be kept secret for long.

“It’s safe to say the media will always get hold of it,” says Shostak. “The media are often on top of the story long before the scientists know.”

Sometimes, the RFI call comes from inside the house.

In the early 2000s, astronomers at the Parkes Radio Observatory in Australia began detecting puzzling pulsed signals that lasted just a few milliseconds and appeared when the telescope was pointed in a particular direction.

Radio astronomer Sarah Burke-Spolaor of West Virginia University led the initial investigation into these signals, which she called ‘perytons’. It became gradually obvious that they must be RFI because they were being seen all over the field of view rather than coming from one specific position. Furthermore, they tended to happen around midday, and peaked in July, which is the middle of winter in the Southern Hemisphere. These facts were significant clues: the clincher was when microwave physicist Jim Benford pointed out that the observatory’s visitor center was directly behind the radio dish when it was detecting perytons. The radio emissions were coming from the old microwave oven in the visitor’s center, every time the door was opened prematurely!

This was a false alarm of a different kind: not a radio signal, but an astrophysical phenomenon that was weird enough that it looked like it could be a techno-signature.

Astronomer Tabetha Boyajian of Louisiana State University noticed unusual dips in light coming from the star KIC 8462852. It seemed like huge swarms of objects were transiting the star, and at times the transiting swarm was so dense that it blocked 22 percent of the star’s light. Compare this to a transit by a Jupiter-sized planet, which blocks just 1 percent of the light.

Speculation was rife that this could be evidence for some kind of partially built Dyson swarm, or some other mega-structure. Ultimately, observations showed that the magnitude of the transits were not equal when observed through different filters. In other words, the transiting objects were blocking more light at certain wavelengths than at others, meaning they could not be solid objects. Instead, they are enormous, dense clouds of dust, although it is still not clear whether this dust is actually orbiting the star, or is in interstellar space and just happens to be crossing our line of sight.

With the mystery still to be completely solved, Boyajian’s Star shows that there aren't always quick answers, and it could potentially take years for some false alarms to be resolved.

The most exciting recent false alarm was BLC-1 (Breakthrough Listen Candidate 1), detected in 2019 by the Parkes telescope as part of the Breakthrough Listen SETI search. Signals were observed apparently coming from Proxima Centauri, which is the closest star to our Solar System. What’s remarkable is that BLC-1 passed all the tests that are meant to filter out RFI, including the on-off test.

When a particularly compelling signal comes along, it gets assigned to a small group of Breakthrough Listen scientists to ‘red team’ it, or in other words try and disprove it. Normally they’re able to do this fairly quickly, but BLC-1 proved more resistant to being discredited. 

“In the case of BLC-1, we red-teamed it for a couple of days and we weren’t able to disprove it, and that’s the point that it became a bonafide candidate signal,” says Andrew Siemion, who is Director of Breakthrough Listen at the University of California, Berkeley. This red teaming usually happens about once per year, which gives an indication of how rare strong candidates are. But BLC-1 was a tough nut to crack.

The signal had a characteristic Doppler shift, and the Breakthrough Listen team explored every avenue to try and explain it as RFI.

“We thought for sure that we would be able to ascribe the Doppler signature to the acceleration of some known moving object around the observatory,” says Siemion. “But we weren’t able to do that.”

Excitement began to grow among the team.

“The best way to describe it was that my excitement was oscillatory,” says Siemion, as he reflected on how things would look good, and then swing the other way, and then back again as options were discussed and ruled out. It was only when one crucial discovery was made that Siemion accepted that the signal wasn’t extraterrestrial in nature.

“When we found other signals in the data that had the same specific drift signature to BLC-1 but were clearly interference, I think that was, for me, the moment where the pendulum never really swung back in the other direction,” he says.

Ultimately, a study led by Breakthrough’s Sofia Sheikh concluded that the signal was being produced by the intermodulation of radio emissions from multiple clock oscillators commonly found in terrestrial electronics. But it was the first time a signal had passed all the regular filters that are relied on to weed out RFI. 

As fascinating as the false alarms can be, SETI scientists would naturally prefer to avoid them altogether. To that end, Siemion and Berkeley graduate student Bryan Brzycki have come up with a way to test whether a detected signal really has come from deep space without having to resort to the spatial on-off test.

This has great advantages. You need to do the on-off test while the signal is still transmitting, which isn’t always possible if the signal is short or doesn’t repeat. The famous Wow! signal of 1977 is a good example of a short, non-repeating signal, and that is why we don’t chalk it down as a false alarm — we don’t know if it was.

With Brzycki and Siemion’s new method, the test can be done long after the fact (though alas, there’s not enough data in the Wow! Signal to further test it). 

The new method takes advantage of the fact that filling the not-so-empty space between us and the stars is a fog of interstellar plasma — basically a sea of ionized atoms and free electrons. It’s turbulent and patchy, denser in some directions than in others. The free electrons in the plasma interact with radio waves passing through it, refracting the waves to differing degrees depending on their frequency. A narrowband signal will still contain some variances in frequency, and this interstellar refraction results in radio waves of slightly different frequencies arriving at Earth at slightly different times, prompting constructive and destructive interference between them. This has the effect of causing the amplitude of the signal to rise and fall, often rendering it too faint to be detected.

This effect is called ‘scintillation’ and to astronomers conducting astrophysical observations, it can be a headache. Brzycki and Siemion, however, realized that it could be a boon for SETI analysis.

That’s because the modulation that it imposes on the signal is an undeniable sign that the signal has traveled through interstellar space and therefore cannot be RFI. Brzycki has developed an algorithm to look for scintillation in signals to distinguish them from the noise of radio Earth.

“The effect that we’re looking for is that the signal is modulated by the plasma throughout the galaxy, and that would indicate that the signal is coming from far away and not just above or in Earth’s atmosphere from RFI,” Brzycki tells Supercluster. “There’s a lot of interference that we detect on a regular basis and really this is a way to differentiate and prove that the signal is coming from outer space.”

The method does have some limitations. The greater the density of plasma a signal passes through, the stronger the modulation, but that means it doesn’t really work for any signals from neighboring stars — we’d have been out of luck for BLC-1, for example. Brzycki and Siemion estimate that a signal would need to have traveled, on average, 10,000 light years to have a detectable modulation caused by interstellar scintillation. This distance, however, will vary along different lines of sight. In some directions where there is more plasma closer to us, the distance is reduced. In others, the distance would be greater.

Using the 100-meter Green Bank radio telescope in West Virginia, Brzycki is testing his algorithm by surveying stars in the direction of the densely packed galactic center, a line of sight along which are the most stars and the most plasma. He’s currently working on developing his algorithm further so that ultimately it can be used with ease by anyone in the SETI community. So long as the signal is continuous — either a carrier tone or a constant pulsing — the method should be able to identify it as interstellar in origin.

“This is the first time in radio SETI where we have had a potential way of telling the difference between interference and a techno-signature without spatial information for a one-off signal,” says Siemion. “Being able to intrinsically look at the signal and tell whether or not it’s coming from a distant source is a revelation.”

The algorithm will not be able to eradicate false alarms completely. Besides not being suitable for nearby star systems, it’s possible although unlikely that RFI could mimic the modulation produced by the scintillation. However, it should cut down quite drastically on the false alarms. 

Now that SETI astronomers know what to look out for, the hope is that they won’t get fooled again.