Light passes through, but at certain frequencies the atmosphere can smother, interfere with or completely block out some of the light.

That’s certainly true for the radio part of the electromagnetic spectrum. Signals ranging from about 5 MHz up to a terahertz can pass through the atmosphere, but at either extreme the signal is heavily diminished. At low frequencies the radio waves can be reflected back into space by the upper part of our atmosphere, called the ionosphere, and at the highest frequencies the radio waves are absorbed by water and carbon dioxide molecules in our atmosphere. 

The clearest part of the radio window is between about 1 and 10 GHz, so for much of the history of radio astronomy it has been these frequencies that have been focused on. And where radio astronomy goes, the search for extraterrestrial intelligence (SETI) is usually there right beside it.

There’s a practical reason for that: since Giuseppe Cocconi and Philip Morrison’s seminal 1959 paper in Nature in which the concept of searching for alien radio signals first appeared, SETI has traditionally been a radio search (although SETI is beginning to broaden out more now). Until recently, funding in SETI has been lacking, so researchers in the field have been at the mercy of what the radio astronomers want to do with their telescopes, often piggybacking on more traditional scientific observations and the frequencies those observations are conducted at.

Things are now changing, and SETI has been able to broaden its scope as it mirrors developments in regular radio astronomy. Higher and lower frequency domains are now opening up, providing fresh hunting ground in the search for ET.

The assumption is that an artificial signal from aliens is almost certainly going to be a narrowband radio transmission, perhaps a hertz wide or even smaller. That’s because nature doesn’t produce narrowband signals, so there’s no way one could be mistaken for some astrophysical phenomenon such as a pulsar. There’s not actually any reason why an alien signal couldn’t be broadband – it would certainly be more noticeable – but a narrowband signal is less susceptible to dispersion, where electrons in the interstellar medium interact with radio waves, delaying the shorter frequency ones. A narrowband signal has only a small difference between its highest and lower frequency wave, tempering the degree by which they disperse.

The problem with narrowband signals is that there are billions upon billions of narrowband channels that ET could broadcast on. How do we know which channel to tune into? Back during Project Ozma, which was Frank Drake’s pioneering first ever SETI search in 1960, Drake was limited to a receiver tuned to a single, 100 Hz wide channel centered on the 1420 MHz (1.42 GHz) emission of neutral hydrogen in the Universe, which the 26-meter (85 foot) radio telescope that he was using at Green Bank, West Virginia, was designed to observe. 

Fast forward 35 years and Project Phoenix, which was the SETI effort that arose from the ashes of NASA’s cancelled SETI program used a multi-channel receiver co-designed by Jill Tarter that could simultaneously detect millions of narrowband channels. Today, Breakthrough Listen’s back-end receivers scrutinize billions of channels, each just 3Hz wide.

While modern-day SETI experiments can cover a large sweep of the radio spectrum, no telescope can observe every frequency, meaning it still comes back to the question of which channels should be prioritized. Any aliens out there transmitting messages will be faced with a similar problem – which is the best frequency to transmit on?

Chenoa Tremblay, who is a radio astronomer at California’s SETI Institute, tells Supercluster that “we’re now exploring a much larger frequency space, so we don’t have to play as many of those guessing games.”

Nevertheless, light years apart, alien transmitter and human receiver have to cooperate on an almost telepathic level in order to converge on the same frequency for communication. Fortunately, game theory – for that is what this problem essentially is – has a few tricks up its sleeve to even the odds.

Named by economist Albert Schelling in 1960, you can think of Schelling points as common solutions to problems that people (or aliens) gravitate to without communication with one another. This works because these solutions prove to be convenient, accessible or relevant in some way.

“Schelling points are places that others might find interesting, they might also be trying to study those regions, or they might post a beacon there,” Tremblay says. “It’s about trying to find common ground.”

In the early days of SETI, most radio telescopes were set up to study the 1.42 GHz line of neutral hydrogen, which fills the galaxy and the Universe at large, so Drake and his associates had to settle for searching on that frequency. As it turns out, 1.42GHz is not deemed a bad choice, for a number of reasons, and that’s because it’s potentially a Schelling point – a ‘magic frequency’ as it were, that to this day has remained a mainstay of SETI. Consider that much radio astronomy is conducted at this frequency; if aliens wanted to transmit on a frequency that they knew we’d be looking at, then 1.42GHz is the one.

But there’s more to this magic frequency. Consider this: a water molecule is formed of two atoms of hydrogen and one atom of oxygen. Put another way, water is made from an atom of hydrogen plus a hydroxyl molecule, which is made from one atom of hydrogen and one oxygen atom. Hydroxyl radiates at four different frequencies between 1.612 and 1.720 GHz. Barney Oliver called the range between the emission frequencies of hydrogen and hydroxyl the ‘water hole’, because as he put it, in an arid land the water hole is where all life comes to meet. Just as it is in the desert, so might it be in interstellar communication. It’s also a pretty quiet part of the radio spectrum; as 1971’s Project Cyclops, which was concept study for a massive phased array of radio telescopes for SETI led by Oliver, suggested, the water hole almost “seems especially marked for interstellar contact.”

The range of frequencies in the water hole form another Schelling point, as Tremblay explains.

“In the case of the water hole, not only is it relevant to life, but hydrogen is also prevalent in the Universe,” she says. “If another civilization is also interested in studying the galaxy and looking at neutral hydrogen, they might think it is common ground.”

The water hole isn’t the only source of possible magic frequencies. On Earth, 1.42 GHz is a protected frequency reserved for radio astronomy. Aliens could also recognize that their astronomers might want a clear channel to study the Universe, and so they might choose to transmit on another frequency instead rather than flooding it with radio frequency interference (FRI).

So over the years SETI scientists have suggested alternative magic frequencies, such as integer multiples of the hydrogen line, or multiplying it by pi, which is 4.462 GHz.

Jason Wright of Penn State University has proposed that aliens may utilize the Planck time as the basis for their transmission frequencies. The Planck time is a tiny amount of time, just 5.39 x 10^–44 seconds, and is defined as the time it takes for light to travel a distance equal to the Planck length, which quantum physics tells us is the smallest unit of length possible, 1.62 x 10^–35 meters. Wright suggests multiplying the Planck time with multiples of another fundamental number, the fine-structure constant that helps describe the strength of the electromagnetic force between two particles, to create a list of frequencies that Wright thinks would act as Schelling points because of the fundamental nature of the numbers involved. He calls this list a ‘frequency comb,’ because on a graph the spikes at each calculated frequency look like the teeth of a comb.

The 1.42 GHz frequency isn’t the only frequency that radio astronomy is conducted at. For example, before its catastrophic collapse, the Arecibo Observatory operated in what’s referred to as the ‘S-band,’ between 2.655 and 3.353 GHz, where radio emission produced by synchrotron radiation – when charged particles spiral around magnetic fields at high acceleration and release energy – in supernova remnants and star-forming regions can be observed. (The water hole is considered L-band, between 1 and 2 GHz.) These frequencies may also draw the interest of humans and aliens.

Radio astronomy is now also trying to broaden out and push at the extremes of that radio window, opening up new spaces for SETI play in at the same time.

For example, one trick to get around the ionospheric interference of low frequency radio waves is to try and observe them on days when space weather, powered by solar activity, is quiet. The charged particles on the solar wind can stir up the ionosphere, rending observing conditions impossible, but thanks to constant solar monitoring, we now know when conditions are going to be ideal.

However, SETI hasn’t previously done much listening at low frequencies simply because of the availability of instruments designed to operate at those frequencies.

“Then around 2013 we saw a new resurgence in low-frequency telescopes,” she says, citing the LOFAR (LOw-Frequency ARray) telescope in the Netherlands, the Murchison Wide-field Array in her Australian homeland, NenuFAR that operates between 10 and 80 MHz in France, and the Long Wavelength Array of 265 antennas in New Mexico and Texas.

“People are beginning to understand that there’s a lot of astronomy that can be done at these lower frequencies, so [SETI] can take advantage of that,” says Tremblay.

Tremblay is referring to commensal SETI, in which SETI instruments piggyback on telescopes doing regular astronomical work. Tremblay herself is the project lead on two commensal SETI searches, one on the MeerKAT radio telescope in South Africa, and the other on the mighty Very Large Array (VLA) of radio telescopes in New Mexico, USA. Between the two projects, the search is incredibly comprehensive covering almost every frequency from 0.5 GHz all the way up to 50 GHz. 

Tremblay has also gone low in the past too, conducting a very low frequency search with the Murchison Wide-field Array. That search listened to more than 1,300 galaxies at frequencies between just 80 to 300 MHz (0.08 and 0.3 GHz). SETI searches have also been conducted in the past with LOFAR, while Tremblay is supervising a student who is going as low as 30–70 MHz in a search utilizing the Long Wavelength Array.

Tremblay points out that some of our earliest loud radio transmissions from Earth were at low frequencies. She cites Space Fence, which was a United States Air Force space surveillance system back in the Cold War, designed to detect foreign satellites passing overhead. Its powerful 1-megawatt radars operated at 216.9 MHz (0.2169 GHz). The original Space Fence was decommissioned in 2013 and a new one was initiated in 2020 that operates at much higher frequencies.

And indeed, radio astronomy itself began at very low frequency, when Karl Jansky detected radio waves coming from the galactic centre at 20.5 MHz with his ‘merry go-round’ rotating antenna in 1933.

Low frequencies can be attractive for SETI because lower frequencies have correspondingly larger wavelengths, and if the wavelength is larger than the micron-sized dust that inhabits space, a low frequency radio signal can pass through the interstellar medium more easily and not be absorbed. In other words, low frequency signals can permeate through much greater densities of cosmic dust. So there is a logical reason to transmit in the low megahertz range, and a logical reason to search there – another Schelling point.

At the other extreme is the very high frequency range that the Atacama Large Millimeter/submillimeter Array explores, observing at frequencies between 35 and 950GHz. It’s able to do so because it is seated on the Chajnantor Plateau, 5,000 meters above sea level in the Atacama Desert, above much of the atmospheres water vapour that can absorb those high frequency radio waves that exist at millimeter and submillimeter wavelengths. 

PhD student Louisa Mason of the University of Manchester has recently conducted the first ever SETI search using archive ALMA data. Mason and her colleagues – Manchester’s Michael Garrett and Kelvin Wandia, and Breakthrough Listen’s Andrew Siemion – focused on observations of 28 handpicked stars in a narrow range of frequencies centred on 90.642 and 93.152 GHz. The reason for these frequencies is that they fall into a band between 84 and 116 GHz where ALMA has the largest field of view. Mason’s survey was able to rule out transmitters from those stars with a power of 7 x 10^17 watts (70,000 trillion watts), which is a little more powerful than a Kardashev type I civilization (a species that is able to consume every scrap of energy on its home planet), a not unreasonable assumption for a civilization more technologically advanced than we.

Interestingly, the frequency range observable by ALMA does not include any frequencies in Jason Wright’s frequency comb based on the Planck time. But that doesn’t mean we shouldn’t still observe it, and there’s a very good reason to continue to do so.

A so-called ‘magic frequency’ is only a true Schelling point if it is the solution towards which both parties gravitate. So far, there’s no evidence that ET has arrived at the same conclusions that we have. Maybe their logic takes them to different solutions. Maybe their biology is different and they don’t place as much importance on the water hole. Perhaps their technology is different and their electronics and transmissions operate at different frequencies to what we might expect. Maybe they’re not using radio at all – optical and near-infrared lasers are a viable alternative, or they can somehow utilize neutrinos or gravitational waves. Or possibly they don’t exist at all, but that’s something that we could probably never prove. All we can keep doing is casting the net as far and as wide as possible and continue to consider all possibilities, waiting and hoping for the day when we finally find something. And when that day comes, it might be just what we expected, or it might be something that had never occurred to us.

Until that day, all our hailing frequencies continue to be open.