In 2023, Breakthrough Listen, the largest ever astronomical programme searching for ‘technosignatures’ – evidence of past or present technology that would signal the presence of life beyond planet earth – moved their headquarters to the Department of Physics at the University of Oxford. Breakthrough Listen’s Dr Brian C Lacki, a theoretical astronomer with the Breakthrough Listen Initiative, has recently published two papers that look at the search for extraterrestrial intelligence in a new light: what if, thanks to interstellar travel, there are vast extraterrestrial intelligence populations? How would that affect SETI or the search for extraterrestrial intelligence?
One of the core debates in SETI is the plausibility and role of interstellar travel. It has been argued that interstellar travel would allow an extraterrestrial intelligence to expand across a galaxy in about ten million years, far shorter than the age of the Universe. The most profound consequence, in my opinion, is that it would allow ETIs to replicate, amplifying their populations by a factor of thousands, millions, or even billions. Little decisions and quirks from early in an ETI's history might be boosted and locked in across a galaxy. Similar galaxies then could have dramatically different technosignatures because of these histories. I have written two papers that use this idea as a starting point. They introduce the idea of a metasociety, the collection of societies in a galaxy. Metasocieties that are the result of interstellar travel may be very dense. If heavily populated metasocieties are out there, they raise important questions about how we do SETI while creating new opportunities. Instead of viewing ETIs as isolated planets, we can consider them as part of a galactic population.
The most common method of SETI is to look for radio frequency ‘beacons’. It has been argued that such beacons are rare or too expensive to be practical. But if every single star in a galaxy is populated, there could be a hundred billion societies, each with their own goals and quirks. It becomes much more likely that someone, somewhere decides to do something that we can detect – in fact, there may be a lot of someones in a densely populated galaxy leaving detectable traces. Normal SETI searches work under the assumption that radio beacons or other technosignatures are rare – for example, that a high-resolution radio image would show a single artifical ‘star’, the radio beacon, shining against a dull background, typically at a single frequency. At other frequencies, we would see nothing but the background.
But what if instead we looked at a densely populated galaxy where radio beacons are very common? At every frequency, the galaxy is filled with these artificial ‘stars’. Viewed with the low resolution of single radio telescopes, they would all blur together into a glow, like how the Milky Way looks like a nebulous ribbon of light on a dark night. And since there are so many, this glow will be present in every frequency channel. By the laws of statistics, and assuming the beacons are all the same brightness, there would be no ‘spikes’ in the spectrum that our usual methods look for. This is called confusion. When confusion prevails, we fail to detect technosignatures, not because they are too rare or too faint, but because there are so many that they all pile on top of each other. Confusion can be a big problem when observing distant galaxies. When we look at distant galaxies with a radio telescope, a trillion stars may all fall within the beam. If only one in a few hundred has a radio beacon, then since we typically observe a few billion channels, there will typically be more than one per channel and confusion sets in.
Although individual beacons no longer ‘stick out’, the collective emission of all the beacons in the galaxy is still producing a seemingly diffuse glow that pervades the galaxy, just as the visible light of billions of stars blends into the optical glow we see. So can we just look for the radio glow of ETI beacons? Many galaxies have natural radio emission, the result of cosmic ray electrons in magnetic fields, but it turns out that this emission is actually very faint in most cases, about one millionth of the total luminosity. Some galaxies, particularly elliptical galaxies with no star formation and no active nuclei, lack even that. This is a new collective bound: the total luminosity of artificial broadcasts cannot exceed the luminosity of the galaxy itself. This seems like an obvious thing to say. Yet we can actually get a few interesting results from it. First, it is strong enough to mostly rule out the possibility that confusion is preventing us from finding ET. But, second, some galaxies are really radio-faint despite being massive. In the Virgo Cluster of galaxies, M59 is estimated to have over half a trillion stars, yet when we look in the radio, we see nothing at all. Its luminosity at a frequency of one gigahertz is less than about forty Suns. The faintest radio transmitter we could detect individually in M59 with our current methods has an apparent luminosity of 3 × 10^24 Watts. If there were about ten thousand such transmitters in that galaxy, it would light the galaxy up enough in radio to have been seen in normal radio astronomy surveys. The brightest radio transmitters we can build would appear to have a luminosity of about ten terawatts. If every star in M59 had a gigahertz transmitter about a thousand times brighter, their collective glow would likewise be visible.
The papers develop a mathematical formalism to make these kinds of predictions. SETI researchers sometimes talk about the ‘haystack’, an abstract space of all the different possible characteristics of ETIs and their broadcasts. Each of our surveys are only sensitive to broadcasts in a small slice of the electromagnetic spectrum, with a certain range of luminosities, active at a particular time, and so on. This has mainly been used to argue for how little we know, that the amount of this space we have explored has been like a bathtub compared to an ocean. The papers take this concept further: we can imagine each broadcast as a speck in the haystack, and where it is determines its observable properties. By applying a branch of mathematics called point process theory, we can derive the statistical properties of the population, in particular, the total emission. The framework also provides a way to deal with the effects of different selections. Each survey samples a different piece of the haystack, each introducing its own biases. One example I give is longevity bias, where we are more biased in favour of things that last for a long time than things that quickly come and go. With this framework, and the concepts developed in the papers, my hope is that we can develop a better sense of how much of the haystack we really have explored, and to use statistical methods to look for the signs of ETIs in parts of the haystack that are beyond our direct reach.
Artificial broadcasts as galactic populations I
A point process formalism for extraterrestrial intelligences and their broadcasts, Brian C Lacki, The Astrophysical Journal, 966, 182
Artificial broadcasts as galactic populations II
Comparing individualist and collective bounds on broadcast populations in single galaxies, Brian C Lacki, The Astrophysical Journal, 966, 183