Back when SETI began, we expected intelligent extraterrestrials to make things easy for us.
That they’d go out of their way, with all the energy and know-how of a civilization millions, perhaps even billions of years older than us, to build brilliant beacons in our galaxy that would be unmistakable.
After more than six decades of searching, we've detected no such beacons in the Milky Way galaxy. If our extraterrestrial neighbors are transmitting, then they’re doing so quietly.
But our galaxy is just one of hundreds of billions in the observable universe. Are there beacons in those galaxies? And despite the vast distances, could we detect them?
According to two heavyweights in the field, yes, we could detect them, but there’s a problem. Michael Garrett, the Sir Bernard Lovell Chair in Astrophysics at the Jodrell Bank Centre for Astrophysics at the University of Manchester, and Andrew Siemion, Director of the Berkeley SETI Research Center and the Bernard M. Oliver Chair for SETI at the SETI Institute, point out that SETI has spent the past six decades mostly ignoring all these other galaxies. Even when they’re right there in the field of view of our radio telescopes.
When a radio telescope listens to a star of interest as part of a SETI investigation, it’s not just observing that star, say Garrett and Siemion in a paper published in the Monthly Notices of the Royal Astronomical Society. The telescope’s field of view not only includes other stars, both in the foreground and background, but a multitude of distant galaxies.
“Some of those background objects are stars in our galaxy, some of them are nearby galaxies, and some of them are incredibly distant galaxies,” Garrett tells Supercluster. “There’s a lot of interesting exotica in any given patch of sky: quasars and active galactic nuclei, radio galaxies, interacting galaxies, groups and clusters of galaxies, you name it.”
Garrett and Siemion broke down observations by the 100-meter-diameter Green Bank radio telescope in West Virginia to get a sense of what SETI is missing out on. In 469 pointings of the telescope, they counted 143,024 extragalactic objects, including 28,405 galaxies, 87,841 infrared sources, 8,016 ultraviolet sources, 401 X-ray sources, 398 radio sources, and 44 quasars, plus other assorted cosmic paraphernalia.
Yet when a radio telescope focuses on a chosen target star during a SETI survey, scientists tend to ignore everything around it.
“One thing that SETI has never really owned up to is that when we present things to the public and each other, we act as though we’re just observing one target star, and that’s not true,” says Garrett. “We’re doing a lot more than what we say we’re doing, and it’s annoyed me for a long time that we don’t recognize that.”
So, say Garrett and Siemion, why not start recognizing it? If our galaxy is sounding a little quiet on the extraterrestrial front, let’s expand our horizons and listen to all those faraway galaxies that are in the field of view too.
Welcome to extragalactic SETI.
But, you might ask, aren’t those galaxies really, really far away? We’re talking tens of millions, if not hundreds of millions, or even billions, of light years away. Could a society of technologically gifted aliens really build a beacon powerful enough to span those distances?
Turns out it’s not beyond the realm of possibility. To understand how, we need to look at what astronomers call radio galaxies — galaxies with energetic cores, usually as a result of a supermassive black hole stirring things up and prompting the eruption of lobes of radio emission. Some of these radio galaxies have an effective isotropic radiated power (EIRP) of 7.5 x 10^23 watts. The EIRP is an estimation of the isotropic output of a source of radio waves — in other words, how much energy it radiates in any given direction.
The smart money says aliens don’t use isotropic transmitters. They’d require too much energy to radiate that power in every direction. But a transmitter only needs to radiate in one direction. If the transmissions are targeted, or cycle across a number of different directions, then they can take advantage of what’s called ‘antenna gain’.
The gain is an energy boost that the transmitter gives the signal in one direction. A transmitter the size of the Square Kilometer Array would have an antenna gain of about 108 (the Square Kilometer Array, which is being built from millions of small dishes in South Africa and Australia, will be a receiver, not a transmitter, but the principle still works) so a transmitter that size would need only an intrinsic power of 1015–1016 watts, multiplied by the gain, to equal the typical EIRP of a radio galaxy.
All this means that if we can detect those radio galaxies — then we can detect artificial signals — even at intergalactic distances.
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Even so, it’s going to be costly for an extraterrestrial society to keep continuously transmitting at this power. Too often SETI hasn’t really considered the limitations of alien economics. But in 2010 microwave physicist Jim Benford, his twin brother and science-fiction author Gregory Benford, and Jim’s son, NASA astronomer Dominic Benford, wrote a paper regarding the cost optimization of SETI signals. In other words, exploring how costly interstellar, and indeed intergalactic, signals are in terms of energy and resource, and what options aliens have for transmitting as inexpensively as possible. They concluded that short-duration pulsed beacons, perhaps cycling back and forth between different stars, was the way to go.
So, to get a grasp of what is actually feasible over extragalactic scales, Garrett and Siemion went to Jim Benford and asked for his opinion.
“We saw Jim a few months previously in California, where we talked to him about it,” says Garrett. “He’s worked on these very high-powered radio transmitters, so we asked him about the feasibility of creating something with these large powers, and he thinks we can get quite close to those powers with our current technology.”
But here’s the rub: it’s unlikely that an extragalactic signal would be in the form of a message, thinks Benford. There are a number of reasons why, such as detecting a bit rate that would be very weak over intergalactic distances, but principally because:
Why would they want to wait millions of years for a reply?
More likely, any powerful radio emission would be in the form of leakage from power beaming. Writing in the Astrophysical Journal Letters in 2016, Jim and Dominic Benford argued that the most observable emissions from an extraterrestrial civilization might be beams of microwave energy transmitting power to drive spacecraft forwards or to supply energy to a space station or asteroid habitat. In 2020, Jim Benford even suggested that the famous Wow! signal could have been an example of leakage from power beaming.
“This can explain the observed features of the Wow! signal: the power density received, the signal’s duration, and its frequency,” Benford told me in 2020. “It also explains why the Wow! source has not been observed again, despite many attempts.”
Suppose, though, that aliens did want to transmit a message from one galaxy to another. It’s one thing to have the ability, but to which of the hundreds of billions of galaxies should they send the message? And, on the flip side, how do those receiving the message — i.e., us — know which galaxy to listen to?
In game theory, the solution to this is known as a Schelling point, which is a kind of common focal point that multiple groups will arrive at without even knowing the other’s intentions. Naoki Seto and Yuki Nishino of Kyoto University in Japan realized that a big enough cosmic event could act as a Schelling point since astronomers will be guaranteed to be looking in the right direction. If an extraterrestrial civilization times its transmission just right, then those astronomers might also stumble across the signal while observing this cosmic event.
Seto and Nishino arrived at the conclusion that the best event, visible across the electromagnetic spectrum and indeed beyond in gravitational waves and maybe even neutrinos, is a binary neutron-star merger. Such events create a black hole and unleash a tremendous amount of radiation called a kilonova. One was observed on 17th August 2017 in the galaxy NGC 4993, which is 140 million light-years away. Gravitational waves were also detected from the event, as was a short gamma-ray burst (GRB). It’s the only time so far that the light of a kilonova, the burst of gamma rays, and gravitational waves have all been detected coming from the same event.
Seto and Nishino think that extraterrestrial life might ‘piggyback’ a signal on the emissions from a binary neutron star merger. The timing of when such collisions will occur could be calculated extremely precisely by monitoring the in-spiral of the two neutron stars, and how long it will take the light and gravitational waves to arrive at different galaxies at different distances can also be calculated with precision. The binary neutron-star merger doesn’t have to occur in the alien’s home galaxy — indeed, if it did and they were too close it might be bad news for them — they just have to beam a message to other galaxies in the opposite direction, knowing that their galaxy would be in the field of view when astronomers come to observe the neutron-star merger. They can also time their signals to arrive at the same time as the kilonova’s emissions.
“Receivers just need to search for signals around the time of the merger,” says Seto, who has also written about using a similar technique to search for SETI signals in the vicinity of supernova explosions.
It’s not a perfect plan. Binary neutron-star mergers with detectable afterglows and gravitational waves are rare — usually, we only detect the short blast of gamma rays, which can be seen from the other side of the Universe — perhaps a bit too far to detect an extraterrestrial signal.
“Considering the typical distances of short GRBs and their observed rate, it would be reasonable for us to limit the search only to nearby mergers, because of the required transmission power,” says Seto.
Seto’s not aware of anyone having analyzed the 17th August 2017 event for SETI signals, but suspects that the data is not in a ready format for SETI.
“For SETI, we need high-resolution data — both in frequency and time — which would require a huge amount of data storage, but this is not the case for afterglow-related studies,” says Seto. “Most of the existing radio data seem to be compressed in some form that is not suitable for SETI.”
Intriguingly, detecting an extragalactic SETI signal wouldn’t just tell us that we’re not alone in the universe. It would also tell us about how technological life develops because the amount of power required would need a particular type of society.
In 1964, the Soviet radio astronomer Nikolai Kardashev developed an energy scale that he believed extraterrestrial civilizations could be measured by, and it’s a scale that has caught hold of the imagination ever since. A type I Kardashev civilization is able to harness and consume all the energy available to it on a single planet, approximately 1016 watts, give or take depending upon the planet and the star it orbits. (Note that humankind has not yet achieved type I status.) A type II Kardashev civilization, on the other hand, is able to harness the entire power output of a star, which is approximately 1026 watts for a Sun-like star.
A type I civilization would have to use all its energy to maintain an intergalactic beacon. For a type II civilization, however, it would be just a fraction of its energy budget.
“You would expect that beacons that are really powerful would come from a Kardashev type II society,” says Garrett.
So, the detection of an extraterrestrial signal from another galaxy would be very strong evidence that the Kardashev scale is indeed a real trajectory for technological life, and that it is possible to reach at least type II, perhaps by building a Dyson sphere around a star. (Type III would involve interstellar travel, and the ability to harness the energy output of an entire galaxy.)
There’s one other corollary to all this. If we detect a signal from technologically intelligent life in our galaxy, then because of the light travel time, the signal would be hundred, thousands, or tens of thousands of years old in the worst-case scenario.
Especially for the closer stars, it is eminently possible that the senders would still be around. For extragalactic SETI, however, the signals will be many millions, even billions, of years old. Could societies survive that long, or would we be receiving a signal from the ghosts of the past?