The first-ever observations of merging binary stars stunned the world’s astronomy community, but not quite as stunning as the first-ever signal from extraterrestrial life will be. GW170817 is the name given to the gravitational wave signal seen by the LIGO and Virgo detectors on 17 August 2017. Lasting for about 100 seconds, the signal was produced by the merger of the two neutron stars. The observation was then confirmed – the first time this has happened for gravitational waves – by observations with light waves.
The light from the neutron star merger is produced by the radioactive decay of atomic nuclei created in the event. (Neutron star mergers do more than just produce optical light, by the way: they are also responsible for making most of the gold in the universe.)
A new paper discusses the possibility of receiving a radio signal from intelligence beyond the Milky Way Galaxy, around the time when we observe a binary neutron star merger in their galaxy. High-precession measurements of the binary parameters would allow them to send the signal ~104 years before they themselves observe the merger signal.
“We were really impressed by the rapid growth of multi-messenger astronomy associated with [the neutron star merger detected last August], and started thinking about interesting possibilities far beyond traditional astronomical studies,” lead author Yuki Nishino, a physicist at Kyoto University in Japan, wrote in an email to Space.com. “I think one of the basic grounds for developing an advanced civilization is a profound desire to leave behind information.” Adding that it may well be safer in a scenario like his, where alien and correspondent civilizations are in two separate galaxies buffered by vast distances.
Nishino and his co-author started plotting how a technologically advanced alien civilization beyond our galaxy might piggyback on the light signals created by colliding neutron stars to catch our attention, using the ability to predict the merger of a binary neutron star elsewhere in their own galaxy with plenty of warning. We can sometimes do that now because many neutron stars are pulsars, which produce a predictable pulsing jet of light. That means we can track where they are and how they are interacting in binary systems.
Obtaining evidence of extraterrestrial intelligence (ETI) is a long-term objective in astronomy. At present, considering the expected signal strength, signatures of Galactic ETI are the primary observational target. There is still the possibility that Galactic ETI signals (both active and passive types) might be difficult to detect. This difference might be caused by the following reasons. (1) The total number of Galactic civilizations might be small, e.g., because of their exceedingly short lifetimes. (2) From the security perspective, considering the travel time within the Galaxy, ETI might attempt to cloak their techno-signatures or even bio-signatures, rather than purposefully transmitting signals to attract other Galactic civilizations.
Extra-Galactic SETI could be a fruitful option, complementary to searches in the Milky Way, For a purposeful signal transmission outward from their galaxy, because of the required travel distances, the security issue would be considerably mitigated. However, at the same time, due to the dilution, detecting an extra-Galactic signal would be more difficult than detecting Galactic signals. This difficulty would also be understood by the extra-Galactic ETI, and they would carefully arrange the timing and direction of their intended signal transmissions, considering the mutually obvious points in the strategy space, namely the Schelling point in game theory, a focal point, a solution that people will tend to use in the absence of communication, because it seems natural, special, or relevant to them..
In this relation, various astrophysical systems have been proposed as a potential tool for synchronized signal transmission. The timing accuracy and the relative separation between the three points (the sender, the receiver, and the adopted astronomical system) are the key elements, limiting the range of the actual application.
On 2017 August 17, the LIGO-Virgo network detected an inspiral gravitational wave (GW) from a binary neutron star (BNS) system (GW170817). Soon after this highly energetic event, its host galaxy NCG4993 was identified by electromagnetic observations, at a distance of ~40 Mpc. It is an elliptical galaxy with a stellar mass similar to the Milky Way but a somewhat smaller.These Galactic BNSs contain recycled pulsars as one of the binary components, allowing high-precision measurements of the binary parameters.
If ETI can detect the recycled pulsar of an inspiraling BNS system in their galaxy, they can also make a high-precision measurement for the quantities required for the signal transmission to be synchronized with the BNS merger. This synchronization scheme seems reasonable to as a signal receiver, and could be on the Schelling point mentioned earlier.
Given the impacts of the ETI signal detection, we primarily regard ourselves as a receiver. For technological feasibility, a receiver is assumed to have a sensitivity up to phase 2 of the SKA (around 2030), and we specifically discuss our present situation in relation to GW170817. For a sender, we mainly presume a technological level that might be realized in 50–100 years on the Earth, taking into account the future plans currently under discussion.
To simplify, we deal with a model in which our universe is composed by single-species galaxies equivalent to the Milky Way. This is based on the fact that ~50% of the B-band luminosity density is contained in galaxies more luminous than the Milky Way.
The Search for Artificial Signals: Considering the successful follow-up observation for GW170817, it is likely that we can identify the host galaxy of a BNS merger at a distance beyond 40 Mpc, and can subsequently search for a synchronized artificial signal coming from somewhere in the galaxy.
Image credit top of page: Neutron star merger NSF/LIGO/Sonoma State University/A. Simonnet
The Daily Galaxy via IOP Science and Space.com
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