One of the most exciting breakthroughs in astronomy over the past decade was the detection of gravitational waves. Since the days of Galileo Galilei, astronomy was about the detection of electromagnetic signals with telescopes. As it turns out, the main constituents of the Universe are not observable in that way.
Our current data indicates that 85% of the matter in the Universe is invisible electromagnetically, constituting dark matter. In addition, 70% of the energy budget of the Universe is dark energy. Cosmologists infer these constituents because they affect visible matter gravitationally. Can we build a detector of near-Earth objects that would sense the gravitational signal of passing dark objects?
If dark matter is made of asteroid-mass objects, like primordial black holes, our telescopes would not notice them even when they pass near Earth. In a recent paper, I showed that the LIGO-Virgo-KAGRA gravitational wave observatories could detect a dark object if it moves close to the speed of light and its mass is larger than a hundred million tons. Such an object would cross the radius of the Earth within two hundredths of a second and produce a gravitational tidal signal in the frequency band of LIGO-Virgo-KAGRA. Needless to say, no such object was detected so far.
Within a decade, the LISA space observatory will expand gravitational wave detection to the frequency range between milli- and micro-Hertz and a smaller spacetime strain. This will usher in a new era of sensitivity to dark near-Earth objects in the asteroid mass range. It could also open the door to the detection of Unidentified Anomalous Phenomena (UAP) gravitationally, which the Galileo Project observatories are attempting to detect electromagnetically. Pulsar Timing Arrays(PTAs) probe a frequency range of a few nano-Hertz, but so-far they were only sensitive to the cumulative gravitational wave background at these frequencies – which constitute the noise floor for the detection of individual sources.
Gravitational wave detectors are the most exciting telescopes of the next millennium as they will open the door for detecting objects that we had never noticed before. As I showed in another recent paper, it is impossible to block or dissipate gravitational wave signals. They offer the optimal communication method, detectable through Earth or the Sun.
It is conceivable that extraterrestrial technological civilizations communicate in gravitational signals, and our failure to notice them so far is because traditional SETI relied on seeking electromagnetic signals with traditional telescopes. If so, the silence that triggered Fermi’s question: “Where is everybody?” stems from our blindness to gravitational signals at the appropriate frequency.
Aliens would choose a communication channel that does not interfere with the frequencies of the loudest natural sources of gravitational waves in the cosmos. These are black hole binaries of stellar mass – to which LIGO-Virgo-KAGRA is tuned, as well as supermassive black hole pairs – to which LISA and PTAs are tuned. In that case, gravitational-SETI will need to develop sensitivity in other frequency bands.
The main challenge in producing detectable gravitational signals is the requirement to move large masses at high speeds. To within an order of magnitude, the gravitational wave strain is of order the gravitational potential produced by the transmitter divided by the speed of light squared times the square of the characteristic speed by which its mass moves in units of the speed of light. For context, the gravitational wave strain produced by the nearest stellar binary, Alpha-Centauri A & B – as the two stars orbit each other every 80 years, is only of order 10^{-24} and extremely challenging to detect.
Five years ago, a team led by Marek Abramowicz published a paper on the possibility that an advanced technological civilization harvests energy from the supermassive (4 million solar-mass) black hole Sagittarius A* at the center of the Milky Way and uses it for communication. They found that a Jupiter-mass structure in the innermost stable circular orbit around the black hole would emit an unambiguous gravitational wave signal that could be observed by LISA.
Hellbound and Down 