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Searching for extraterrestrial signals in more than 1,300 galaxies has helped limit expectations of how many communicating, technological civilizations might exist beyond Earth.
The search, conducted with the Murchison Widefield Array (MWA) in Australia, focused on low radio frequencies in the 80-300 MHz range. For comparison, SETI (short for Search for Extraterrestrial Intelligence) typically looks for alien signals in the 1,420 MHz hydrogen emission frequency. In fact, lower frequencies are relatively uncharted real estate for SETI.
The search was led by Chenoa Tremblay of the SETI Institute in California and Steven Tingay, director of the MWA, of Curtin University in Australia. The team focused on a 30-degree field of view in the constellation Vela, the Sails, which encompasses 2,880 galaxies. The redshifts, and hence distances, to 1,317 of these galaxies had been measured with high accuracy before—so Tremblay and Tingay targeted those galaxies specifically. By knowing the distances to the galaxies, the pair could place constraints on the strength of any transmitters in those galaxies.
Although they were unable to detect an extraterrestrial signal in their initial search, Tremblay and Tingay concluded in their paper that they could detect a signal at a frequency of 100 MHz with a transmitter power of 7 x 10^22 watts.
“This work represents a significant step forward in our efforts to detect signals from advanced extraterrestrial civilizations,” Tremblay said in a statement. “The MWA’s wide field of view and low frequency range make it an ideal tool for such investigations, and the limits we set will guide future studies.”
Relating to: Are We Alone? Intelligent Aliens Could Be Rare, According to New Study
For most of its 64-year history, SETI has focused on stars in our Milky Way galaxy, but in recent years the net has begun to expand.
For example, in 2015, the Thermal Imaging from Alien Technologies (G-HAT) project surveyed 100,000 galaxies with NASA’s Wide-field Infrared Survey Telescope (WISE) to look for civilizations that might have formed “Dyson swarms” around all the stars in their respective galaxies. None were found. In 2023, a team led by Yuri Uno of National Chung Hsing University in Taiwan suggested that there could be no more than one civilization three billion light-years away, aiming a radio transmitter with a power of over 7.7 x 10^26 watts at the Milky Way.
That same year, Michael Garrett of the Jodrell Bank Centre for Astrophysics and Andrew Siemion of Breakthrough Listen conducted a search of background galaxies to constrain the maximum detectable power, and arrived at a range of about 10^23 watts to 10^26 watts. (The exact maximum power of a potential signal will depend on the distance from the galaxy from which it originated.) Finally, Carmen Choza of the SETI Institute recently led a team that conducted a targeted search of 97 galaxies with the Green Bank Telescope — but detected nothing.
Where does all this power come from?
To achieve these transmitter powers, technological aliens would need to harness the power of a star, or perhaps even several stars.
In 1964, Soviet astronomer Nikolai Kardashev developed a classification scale for extraterrestrial civilizations based on the amount of energy they had at their disposal. A type 1 civilization would be able to harness all the energy available on a planet, typically 10^16 watts or more. A type 2 civilization would be able to harness the full power of a star, which would be 10^26 watts for a Sun-like star. A type 3 civilization would be able to harness the full power output of every star in its galaxy, which would be about 10^36 watts.
The zero detections so far don’t mean that technological, communicative extraterrestrial life doesn’t exist, it just means that our observations aren’t comprehensive enough to say either way about its existence. We’re just not sure. Estimates suggest there are up to 2 trillion galaxies in the observable universe, and we’ve only searched a tiny fraction of them, and only for a short time.
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Running an intergalactic radio beacon wouldn’t be cheap either; it’s possible that any radio beacons were turned off to save power while we were looking. Or maybe they were pointed toward other galaxies. Perhaps Kardashev type 2 and 3 civilizations are rare, meaning we can’t see transmitters of that power, so given the constraints, the radio beacons could be out there, but operating at a lower power than we can detect. What’s more, this new research was working at lower frequencies — but transmitters at higher frequencies can’t be ruled out.
Tremblay and Tingay note that the few powerful radio transmitters on Earth, and some of our earliest transmissions, are in the low frequency range, making it justifiable to search in this range. Also, given the relative scarcity of SETI searches at these low frequencies, there is always a chance of finding something unexpected. For SETI to be successful, radio searches need to cover a large number of frequencies to ensure we don’t miss that elusive signal.
“Continuing to work together across the frequency domain will be crucial in the future,” Tremblay and Tingay conclude in their paper.
The study was published on August 26 Astrophysics Journal.