Civilizations Could Time Their Communications Based on the Movement of a Single Star

The Search for Extraterrestrial Intelligence has been ongoing for decades at this point. Despite that, we have yet to find any rock-hard evidence of a signal from an alien civilization. When asked about this, experts point out just how little of the overall signal space we’ve analyzed. A signal could be coming from anywhere in the sky, at any frequency, and might not be continuous. Constraining the “search space” could help us find a signal faster, but what could we use to constrain it? It’s hard to think like an alien intelligence, let alone to mimic them.

One of the most famous examples of the reverse of search is the Arecibo message, wherein humanity tried to announce, “We are here,” using scientific and mathematical standards like numbers and the atomic number of some elements (hydrogen and carbon, for example). Even so, it was still sent as a binary signal using a type of frequency modulation at a single point in time back in 1975. The likelihood that any civilization in the Messier 13 globular cluster, its intended target, will be able to both receive and interpret it is negligible. But it would help if they had a key to interpret it. But how can we convey a key to unlock the message without the key itself being interpretable only with the same key?

Naoki Seto of Kyoto University’s Department of Physics has spent a lot of time thinking about that question, and he came to a similar conclusion about the usefulness of scientific constants. In the past, he produced papers that suggested the time of a future binary star merger or a past supernova explosion to help narrow down a patch of sky to look at. However, with a new paper released on March 21st, he suggested a new idea – the orbital period of an exceptionally bright star around the Milky Way’s supermassive black hole.

The supermassive black hole at the center of our galaxy, known as Sgr A*, would be well known to any alien civilization advanced enough to send communication signals to announce their presence. It also, conveniently, has several super-bright stars that orbit it on regular periods. Dr. Seto selected one of those stars, known simply as S2.

S2 is a B-type star, is skewed toward the blue end of the stellar spectrum, and weighs in at about 15 times the mass of our own Sun. But most importantly, it is very, very bright and orbits Sgr A* with an orbital period of almost exactly 16 years. 

Those features are important because of their prominence but also because of the ease of calculations for something called the Schelling point. A Schelling point is derived from game theory – specifically, how two people can communicate about how to communicate without actually communicating. For example, someone wants to meet up with their partner but doesn’t want to tell them when or where they want to meet up. The other person is also interested in meeting up but equally interested in not communicating when or where. 

A Schelling point is thinking through reasonable touch points culturally to try to determine a place to meet without expressly saying it. In one example, knowing that we’re both Americans, if we were to pick one distinct time of year to meet up, and knowing that the other person is thinking the same thing, they might settle on something well known, such as midnight on New Year’s Eve. As for a place to meet, why not New York, the country’s largest city, and maybe Grand Central Station, the most common meeting place in that great city? That would be a Schelling point for two Americans, and the same inductive reasoning can be applied to communications with alien life forms.

S2 and its orbital period are something we would have in common with any alien life that develops in this galaxy – they would be able to see it from wherever they are. Dr. Seto thinks that using detailed characteristics of this one particular star, astronomers could start to search specific patches of sky for signals that use its orbital period as a basis for communication.

This is admittedly an arbitrary selection of a touch point for the Schelling point, but the general idea holds. The most likely way we can narrow down the absurd number of search parameters plaguing the search for extraterrestrial intelligence is to try to think like an alien and come up with some shared common experience that we can use as a basis to try to communicate without prior communication. It’s a tricky problem, and one that has lasted for decades, but, as with all things in science, the more people that get to thinking about them, the more likely they are to be solved.

Learn More:
Naoki Seto – A Proposal for Enhancing Technosignature Search toward the Galactic Center
UT – What is the Arecibo Message?
UT – What are the Best Ways to Search for Technosignatures?
UT – The 2nd Annual Penn State SETI Symposium and the Search for Technosignatures!

Lead Image:
Image of the galactic center. For the interferometric GRAVITY observations the star IRS 16C was used as a reference star, the actual target was the star S2. The position of the centre, which harbours the (invisible) black hole known as Sgr A*,with 4 million solar masses, is marked by the orange cross.
Credit – ESO / MPE / Gillessen et al.