Synchronized SETI—The Case for "Opposition"

Abstract

If the signals being sought in search for extraterrestrial intelligence (SETI) programs exist but are brief (for example, they are produced intermittently to conserve energy), then it is essential to know when these signals will arrive at the Earth. Different types of transmitter/receiver synchronization schemes are possible, which vary in the relative amount of effort required by the transmitter and the receiver. The case is made for a scheme that is extremely simple for the receiver: Make observations of a target when it is at maximum angular distance from the Sun (i.e., "opposition"). This strategy requires that the transmitter has accurate knowledge of the distance and proper motion of the Sun and the orbit of the Earth. It is anticipated that within the next 10-20 years it will be possible to detect directly nearby extrasolar planets of approximately terrestrial mass. Since extraterrestrial transmitters are expected to have significantly more advanced technology, it is not unreasonable to expect that they would be able to detect the presence of the Earth and measure its orbit at even greater distances. This strategy is simple to implement, and opposition is also typically the time when observations are easiest to make. Limited opposition surveys contained in a number of all-sky surveys have already been performed. However, full-sky opposition surveys are best suited to detectors with very large fields of view.

Searching for Cost-Optimized Interstellar Beacons

“…Nussinov (2009) suggested that inhabitants of star systems that lie close to the plane of the Earth's orbit around the sun could detect eclipsing by our annual transit across the face of the sun. That would tell them that Earth lies in a liquid water stable habitable zone (Corbet, 2003). Through spectroscopic analysis of our atmosphere, they might see traces of chemistry indicating that Earth likely bears life, particularly the prominent ozone line.…”

Interstellar Communication Network. I. Overview and Assumptions

“…A set of schemes have been proposed for the scenario where the participants are usually located on planets, and are not aware of each other's existence. To determine the location and time for initiating a communication, numerous coordination schemes have been proposed, synchronized through the gravitational waves of binary neutron star mergers (Seto 2019), Gamma-Ray bursts (Corbet 1999), maximum angular distance from the Sun (Corbet 2003), supernovae (Tang 1976;McLaughlin 1977;Makovetskii 1980;Lemarchand 1994), pulsars (Edmondson & Stevens 2003;Vidal 2017Vidal , 2018, binary emphemerides (Pace & Walker 1975), and exoplanet transits (Filippova & Strelnitskij 1988;Arnold 2005;Kipping & Teachey 2016;Heller & Pudritz 2016;Wells et al 2018;Forgan 2019).…”

The Breakthrough Listen Search for Intelligent Life: A 3.95-8.00 GHz Search for Radio Technosignatures in the Restricted Earth Transit Zone

“…The ETZ method can also be used in conjunction with the idea of "synchrosignals" (Makovetskii 1977) or temporal Schelling Points (Wright et al 2018b). For a given object within the ETZ, there is a "special" time, the Earth's transit from their vantage point (or, from Earth, "opposition" as in Corbet 2003), that is known to both the observer on Earth and the hypothetical transmitter on the object. This provides a lower-energy alternative to a continuous signal and has been suggested for the ecliptic specifically in Filippova (1992).…”