When walking down the street, one cannot escape the realization that dogs pay attention to dogs, babies focus on babies, and adults greet adults. This brings home the sense that when looking out we will most likely recognize extraterrestrial technological signatures that resemble those that we possess. Of course, our imagination for what’s possible technologically evolves as fast as our innovation. But while watching our cosmic neighborhood, we can refine Enrico Fermi’s question: “where is everybody?” to mean “how far can we detect our known technological footprint coming from a twin civilization?
Obviously, we can identify extraterrestrial technological debris on Earth, the Moon or Mars. So far, we have not recovered computer terminals or cell phones in archaeological digs, nor in geological surveys. The Moon offers the benefit of lacking geological mixing or atmospheric erosion and serves as the ideal museum for technological relics. The footprints of our own astronauts on the Moon survive tens of millions of years before they will be washed out by micrometeorites. In the context of NASA’s Artemis program to settle on the Moon, we should check if there are any visible footprints from extraterrestrial visits millions of years ago.
The nearest habitable exoplanet is Proxima Centauri b at a distance of 4.25 light years. Given its closeness to its host star, the planet is likely tidally locked — facing the star with a permanent dayside. In a paper I wrote with the Stanford undergraduate student, Elisa Tabor, we showed that if the nightside of Proxima b is illuminated by powerful artificial lights, they might be detectable by JWST.
A new calculation I just completed shows that thermonuclear explosions which release the energy equivalent of more than a megaton-of-TNT on Proxima b would be detectable in the deepest exposures of our most sensitive space telescope, JWST. The first such exposures were celebrated in the White House on July 11, 2022. Although these focused on detecting the first stars in the early universe, we could also point the telescope in search for powerful thermonuclear explosions on the nearest habitable planet.
There are two challenges to a thermonuclear search that come to mind. First, we need the explosion to overlap in time — after taking account of the light travel time, with the observing period which is unlikely given the rarity of thermonuclear wars on Earth. And second, impacts of meteors bigger than thirty meters would release similar amounts of energy. On Earth, such impacts are rare and occur once per two centuries. Unless Proxima b is embedded in a dense cloud of 30-meter-sized asteroids, it will not be hit by one of them during a JWST observing run.
Another unusual source of light comes from rocket launches. Our biggest rocket, Starship of Elon Musk’s SpaceX, is expected to consume about a few kilotons of methane fuel per launch. This is equivalent to a few percent of a megaton of TNT over a long launch time and will not be detectable by JWST even at the distance of the nearest star, Proxima Centauri.
Detectability has more promising prospects for radio signals. As I calculated with Matias Zaldarriaga in a 2006 paper, the most powerful radar signals that our Ballistic Missile Early Warning Systems (BMEWS) transmitted after World War II, are detectable by our most sensitive radio telescope out to a distance of 300 light years. This forecast includes the improved radio sensitivity expected over the next decade with the Square Kilometer Array, which is designed to detect the imprint of the first stars on the intergalactic medium. Given our most optimistic forecasted capabilities, radio astronomers will be sensitive to twin civilizations near a million nearby stars. Given that we transmitted the powerful BMEWS signals over a period that lasted less than one part in a hundred million of the age of the Sun, our chance of finding twin civilizations through their BMEWS signals is only a percent. It is therefore not surprising that no radio signal from a hypothetical twin was discovered so far.
What about our chemical impact on the composition of the atmosphere of our planet? Our current level of industrial pollution of the Earth’s atmosphere is also not detectable with our existing telescopes. As I calculated in a 2014 paper with my former undergraduate student, Henry Lin, detection is not feasible with existing telescopes even for the ideal setting of a habitable planet around an Earth-sized white dwarf.
These detection challenges may provide a satisfying answer to Fermi’s paradox. Technological signals that are more easily detectable, such as those imagined for Kardashev Type II and III civilizations which harvest all the energy from their host star or galaxy, should be regarded as wishful thinking akin to Disneyworld’s Prince Charming or the view through the metaverse goggles — where virtual reality meets our desires.
Of course, the best prospects for an actual detection are offered by a terrestrial visit of a technological gadget. In case the extraterrestrial device was launched billions of years ago and is non-functional by now, it would appear as a meteor that burns up in the Earth’s atmosphere and has unusual composition. In the unlikely case that Kardashev Type II civilizations do exist, some interstellar objects with unusually high material strength might represent pieces from old broken Dyson spheres.
The first interstellar meteor, IM1, was documented by the US Government on January 8, 2014. Based on the stress exerted on its surface when it exploded, I concluded in a 2022 paper with Amir Siraj that its material strength was higher than all other 272 space rocks listed in the CNEOS fireball catalog. In the late spring of 2023 we plan to have an expedition to retrieve fragments of this meteor with the hope of learning whether it was natural or artificial in origin.
If the composition of IM1 implies an artificial origin, such a realization would represent the first step in our journey to find members of our technological twin in our cosmic neighborhood. The older civilizations walking down the street which are far more advanced than we, might be invisible to us at this time in much the same way that adults are ignored in the eyes of babies on strollers in the street.
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 chairs the advisory board for the Breakthrough Starshot project, and 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. His new book, titled “Interstellar”, is scheduled for publication in August 2023.
