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 this 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.
Lately, I had the great pleasure of joining a team of brilliant researchers called “Applied Physics” led by Gianni Martire, which published a novel paper that discusses the prospects of gravitational SETI. The team showed that LIGO is sensitive to a massive craft of Jupiter mass accelerating to a fraction of the speed of light throughout the Milky-Way galaxy or a Moon-mass craft at a distance of tens of light years. Future gravitational wave observatories like DECIGO, Cosmic Explorer, the Einstein Telescope and the Big Bang Observer (BBO) are expected to reach sensitivities that are at least two orders of magnitude better than LIGO, increasing the search volume by a factor of a million.
Gravitational SETI will target signals that are invisible to undeveloped civilizations. Only civilizations which reach a high level of scientific and technological level would notice the signals. This constitutes a good stealth strategy for the transmitting civilizations to avoid predators who are focused on physical strength rather than advanced science and technology. In addition, gravitational wave signals decline in amplitude inversely with distance from the source rather than distance squared as is the case for the intensity of electromagnetic signals. Finally, sensitive gravitational wave sensors with a broad spectral sensitivity can also serve as the ultimate alert system in mitigating the existential risk from dark objects.
The great benefit of gravitational sensors is that gravitational signals are inevitable.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
