The search for extraterrestrial intelligence (SETI) is slowly evolving from a fringe endeavor to a more mainstream one thanks to improvements in the capability of astronomical surveys, detector sensitivity, and greater philanthropic financial support. Still, because of the vastness of the universe and the scarcity of resources, scientists must develop strategies around where, when, and how to discover alien civilizations.
Much of SETI involves trying to receive signals broadcast by other civilizations. However, it could be that every civilization in the universe has decided that transmitting messages for other civilizations to receive is unwise or dangerous, but that listening for messages sent by others is a safe and worthwhile pursuit.
This “SETI Paradox” would leave all SETI efforts doomed to failure because for any civilization’s SETI efforts to succeed, some other civilization must engage in messaging extraterrestrial intelligence (METI). One important question is how two civilizations should coordinate their efforts to discover each other, given that they are not certain about each other’s existence. Another is which civilization should send a message, and which should listen.
Onus to transmit
In a recent preprint, astronomer Eamonn Kerins of the UK’s University of Manchester has developed a game-theory framework that determines not only how and where civilizations should target their efforts, but also which of two such civilizations has the onus to send a message, and which should be listening for that message.
As Jason Wright, professor of astronomy and astrophysics at Penn State, summarizes, “Kerins’ idea is that the symmetry that keeps us all from transmitting can be broken by recognizing that some species have access to more information about other planets, and that the ‘onus to transmit’ should lie with them”. He adds, “It’s a neat approach and suggests that there are systems towards which we have the onus to transmit and should be contacting, while other [planets] are better targets for listening. If other species are following the same logic, then this should make SETI programs more efficient and likely to succeed.”
Kerins considers the scenario in which both civilizations can gather data that suggests the existence of the other. Ideally, each civilization should gather similar data because only when the data are comparable would the civilizations possess “mutual” information – which is key to Kerins’ framework. Because the civilizations may vastly differ in their technological capabilities, it is important that they both consider the simplest possible evidence of the other’s existence.
“Common denominator information”
Kerins proposes that civilizations should use “common denominator information” (CDI) to find potential SETI/METI targets. CDI is evidence that both parties can recognize, and that is independent of either party’s particular method of acquiring information. Kerins offers the example of the amount of starlight blocked by a planet as it transits across its host star – which is called the transit’s signal strength.
This quantity is simple enough that any civilization engaged in SETI/METI efforts should be capable of measuring it, and it is also independent of how the measurement is made. In this sense, a transit’s signal strength is “intrinsic” and therefore can be compared by two civilizations who are looking for one another. Crucially, each party should be able to determine not only their own signal strength as the other party would measure it, but also the signal strength of the other planet. Then, each party would know what the other knows, and therefore both parties know who has the superior evidence about the other’s potential existence.
Where to look for signals from alien astronomers with a good view of Earth
Kerins argues that whichever party has superior information has the greater incentive to send a message to the other – the onus to transmit – while the party with inferior information should listen for a signal.
On where and how to implement his game-theory framework, Kerins points to planets in the “Earth’s transit zone” (ETZ), a slice of space in which an observer can watch Earth transit the Sun. “From the basic idea of transits, and with technology comparable to ours, [extraterrestrial civilizations in the ETZ] can work out that we are a potentially habitable planet,” says Kerins. “The transit method is among the first methods that any civilization capable of finding other planets would establish. Therefore, if there are SETI-capable civilizations out there, by using the transit method, we’ll hopefully embrace most of them, because they’ll also have knowledge of the transit method. The situation in which we’re looking at a region of the sky where we can see them in transit and they can see us in transit maximizes our chances of success.”
The preprint has been accepted for publication in The Astronomical Journal.