Setting bounds on SETI

The Search for Extraterrestrial Intelligence (SETI) has a data scale problem. There are just too many places to look for an interstellar signal, and even if you're looking in the right place you could be looking at the wrong frequency or at the wrong time. Several strategies have come up to narrow the search given this overabundance of data, and a new paper posted to the arXiv preprint server from Naoki Seto of Kyoto University falls nicely into that category—by using the Brightest Of All Time (BOAT) gamma ray burst, with some help from our own galaxy.

When searching for SETI signals, a civilization has to choose three important factors: where to look, what frequency signal to look for, and when to do so. The same problem is faced by the transmitting side—sending signals strong enough to reach other stars coherently in all directions is extraordinarily energy-intensive. In other words, no sane civilization would do that intentionally for long periods of time. And what if you send the wrong frequency? Or worse yet, hop between frequencies? How would a receiving civilization ever know how to find your signal?

According to multiple papers on this very subject, the answer lies in game theory, a sub-discipline of economics. In game theory, there is a concept called a Schelling point, which is a solution that a player would default to when they aren't able to communicate with other players. Typically, this would represent something unique, or a "natural" fit for whatever information they are trying to convey.

This concept had previously been used to attempt to suggest we should send/listen to signals via an "anchor event" in our galaxy such as historical supernovae or future neutron star mergers. However, each of these anchor events has its own flaws. There aren't any binary neutron star systems that are close enough to merging that they could act as an anchor. And distinct supernovae, such as the one that created the Crab Nebula, don't have well-defined distances, so it wouldn't constrain the search area very well.

To solve these problems, Dr. Seto, who has also published a paper on anchoring events, suggests a "hybrid" strategy. Instead of using only one event, use two—a "spatial" reference and a "temporal" reference. In the paper, he suggests the spatial reference be the center of the Milky Way, while the temporal reference would be an extremely bright event somewhere outside the galaxy.

The underlying idea is to have a "search ring" centered on the event, in the case that your civilization is looking for signals, and a "transmit ring" exactly opposite the event in the case that your civilization is intending to send them. The diameter of each of these rings grows based on the time since the original burst and the distance from the location to the galactic center. Importantly, the angle between the burst and the galactic center is used to "normalize" the time delay at which a signal would be sent to a specific star system.

A civilization in the target star system would also then know when to look for a signal, given the same parameters of when the burst started, its distance to the galactic center, and the angle at which the burst came from towards the galactic center. The reason for using the galactic center as a spatial Schelling point is because civilizations of our level of technological sophistication have very accurate distance measurements about how far Sgr A* (the black hole at the center of the galaxy) is from their home system.

For a "bright event," Dr. Seto notes that an ideal one was recently found in the form of gamma ray burst GRB 221009A. It was nicknamed the BOAT because it was 40 times brighter than the next brightest GRB on record. It is also ideally positioned in the sky—low "galactic latitude" meaning many of the stars in the Milky Way would be in the search radius of the original ring. Dr. Seto calculates that this combination of brightness and ideal sky position likely only happens once in 100,000 years.

Unfortunately, all of this spatial and temporal timing doesn't necessarily coordinate the other variable—frequency. There are some theories that a Schelling point for frequency, such as the Hydrogen Line of 1,420 MHz, where hydrogen shines when undergoing a frequency transition, could be used, but realistically, the receiving civilization would still have to search multiple frequencies over that time.

The hybrid technique itself could limit the space needed to search for sending civilizations by a factor of 100. But, it is entirely based on the assumption that the other civilization would come up with the same methodology. Given the uniqueness of the opportunity represented by GRB 221009A, maybe it's worth it to take a look at some potentially interesting star systems—we might not get another chance like this for 100,000 years.