It is conceivable that a civilization advanced enough to see itself in a cosmological context would think of a way to make itself known to the universe. In “The Messenger: A Galactic Center Gravitational-Wave Beacon” pre-print on Arxix, an international team of researchers have proposed a hypothetical alien craft, a beacon at the Milky Way’s Galactic Center, “The Messenger”, that emits gravitational waves that are “unambiguously artificial”.
“In order to be recognized as such, a gravitational wave Messenger beacon must emit a clearly unnatural signal, such as a persistent emission of gravitational waves at a constant frequency,” they write.
The human species in 1977 with Voyager 1 and 2 sent two postcards from Earth out into the Galaxy. Destination: “addressee unknown”. The Voyagers carry not only instruments designed to study the planets in the outer solar system, but also two golden records with messages intended for extraterrestrial lifeforms or future humans. That was a random shot into the vast depths of outer space.
An electromagnetic beacon emitting at radio or infra-red wavelengths (in order to penetrate the dust of the spiral arms) is not be best way to draw attention, as such beams are strongly directional. The same arguments hold also for receiving alien signals on Earth. The search for extraterrestrial intelligence (SETI) is a program where since the 1980ies patterns in the narrow-band electromagnetic radio emission are searched. This is a needle in the haystack search.
A gravitational-wave observatory, however, thanks to its almost omnidirectional antenna characteristics, is not limited to search a specific area of the sky, as it practically receives all the signals within its range of sensitivity. Therefore, it is reasonable to assume that an advanced civilization might use the gravitational-wave channel for
Our existence in the universe resulted from a rare combination of circumstances. The same must be true for any advanced extraterrestrial civilization. If there exist any in the Milky Way, they are likely scattered over large distances in space and time, however, they must be aware of the unique property of the Galactic center: it hosts the closest massive black hole, SGR A* to anyone in the Galaxy.
A sufficiently advanced civilization may have placed material in orbit around this black hole to study it, extract energy from it, and for communication purposes. In either case, its orbital motion will necessarily be a source of gravitational waves. A Jupiter-mass “Messenger” on the innermost stable circular orbit around the black hole can be sustained for a few billion years by the energy output of a single star and emits an unambiguously artificial (continuous) gravitational wave signal that will be observable with LISA-type detectors –the European Space Agency’s planned Laser Interferometer Space Antenna (LISA), scheduled for launch in 2034.
Laser Interferometer Space Antenna (LISA)
A better way to ensure that the signal can be detected throughout the whole Galaxy is with a gravitational-wave beacon. As opposed to electromagnetic waves, gravitational waves are produced by the accelerated movement of macroscopic charges (masses) and once emitted, travel through space virtually unperturbed. Gravitational wave phenomena are omnipresent in the Universe and with sufficient technological prowess relatively straight forward to detect.
In the case of the Advanced LIGO and Advanced Virgo detectors signals emitted during the last stages of the inspiral and merger of binary systems of compact objects – stellar mass black holes and neutron stars – are detectable from distances of the order of 1 Gpc and 100 Mpc.
With only a minor technological tweak LISA, could also function as a detector for signals broadcast by advanced extraterrestrial civilizations existing somewhere in the Milky Way, according to Marek Abramowicz of Sweden’s University of Gothenburg.
LISA will comprise three spacecraft, arranged in a triangular formation, 2.5 million kilometers apart, designed to function as an enormous, orbiting gravitational wave detector.
While acknowledging that life on Earth arose from “a rare combination of circumstances,” Abramowicz and colleagues concede that similar constraints limits the chances of other technologically savvy civilizations arising.
To we humans, the team writes, the Milky Way is naturally a unique location in the Universe. Our emergence is the result of many rare circumstances and conditions of extremely low probability. Yet here we are, a (moderately) intelligent civilization on a small, rocky planet that is rich on silicates, oxygen, and water and has a protective atmosphere and magnetic field. We are orbiting together with a relatively large moon well inside the Goldilocks zone around a rather common, yellow main sequence star in a habitable region about two thirds from the center of a spiral galaxy.
There is a fair chance, they suggest, that the Milky Way is also home to someone else. The predictive power of the Drake equation, which estimates the number of extraterrestrial civilizations by a handful of parameters, is arguably not very great, its estimates range from zero to tens of thousands. The researchers decided to be mildly optimistic and suppose that there is, or was, at least one technologically highly developed civilization out there.
They argue that if a sophisticated extraterrestrial civilization would decide to construct a device to study the massive black hole in the Galactic center, or to extract energy from it, or even for intentions unfathomable to the human mind, this device can, like Voyager 1 and 2 also serve as a Messenger.
Any intentional gravitational-wave Messenger must be long lived and of an appropriate mass and density to produce a measurable signal in the least inhospitable parts of the Galaxy, about 6-10 kpc from the center, which means, it cannot be an arbitrary object on a “natural” orbit around a black hole, as the orbit would decay due to gravitational-wave emission and the Messenger-Sgr A* binary would merge.
The Messenger’s Specs
Instead, an apparatus able to spread an information through the Galaxy should necessarily have a few precisely determined, unique features:
1. The signal should be long lasting, periodic and provided by gravitational waves.
2. Specifically, the emission of gravitational waves should be due to orbital motion of the Messenger along the innermost stable circular orbit around Sgr A∗ , the massive black hole in the center of the Galaxy.
3. The Messenger should be continuously supplied with energy to compensate orbital energy loss due to the emission of gravitational waves.
4. The source of this energy supply should be a natural astronomical phenomenon, operating for a few billion years, with no need of maintenance or service.
5. The largest sustainable Messenger mass is of the order of a Jupiter mass, but the object itself has to be much more compact.
In the first year of its operation LISA will be able to verify if a Jupiter-mass orbiter is present in the Galactic
center. Smaller masses will require a longer interval of observation to reach a conclusive signal-to-noise ratio. The absence of a continuous signal from the direction of Sgr A* does not imply that extraterrestrial life has not evolved. A successful detection, however, will provide a definite and unambiguous proof that an intelligent civilization does or did exist in our Galaxy.
The Daily Galaxy, Sam Cabot via NASA and “The Messenger: a galactic center gravitational-wave beacon” – Marek Abramowicz, Michał Bejger, Eric Gourgoulhon, and Odele Straub