My wife woke me up at 1:45AM on our visit to Tel Aviv as sirens alerted local residents to an approaching missile, launched by the Iran-backed Houthis in Yemen. The dark missile was demolished in a luminous fireball and never reached its destination. As an astrophysicist, I am less impressed by human-made objects like this one, even if they threaten my life. To discuss my true passion, I joined a few hours later a podcast interview titled “Fade to Black” with Jimmy Church, in which we discussed the least understood constituents of the Universe.
Astronomers tend to label things they do not understand as “dark”. One example is dark extraterrestrial civilizations, which remain invisible out of fear from predators. We cannot see them, and so we do not know if they exist. Another example is dark comets, a population of mysterious near-Earth objects which exhibit a non-gravitational acceleration but no coma. We do not see a cometary tail, so we do not know if they truly are comets. On spatial scales that are a billion times larger, there is evidence for dark matter which induces attractive gravity. We do not see dark matter, so we do not know whether it truly exists or instead gravity or inertia are modified at low accelerations. On scales that are a thousand to a million times, there is evidence for dark energy, whose repulsive gravity triggers the accelerated expansion of the Universe. We do not know its nature either. Altogether, the observed universe is full of “dark turtles all the way down.”
Jimmy wondered where the energy of cosmic acceleration comes from. I explained that it originates from the repulsive gravity of the vacuum. In the simplest “cosmological constant” case, the mass density of the vacuum is constant in space and time. But if an advanced technological civilization was able to bottle dark energy, then its repulsive gravity could be used to propel a spacecraft without fuel. Such a gadget would match the appearance of a dark comet without a coma. If we let our imagination run wild, sunlight pressure or dark energy could be used to propel objects classified as “dark comets” which were manufactured by dark civilizations.
This may sound far-fetched. A more conservative take is that dark comets are water icebergs with low levels of dust evaporation, that dark matter is composed of weakly-interacting particles — like the elusive neutrinos, that dark energy is a constant characterizing the vacuum — related to a combination of fundamental constants such as the electron charge, Newton’s constant, Planck’s constant and the speed of light — as I suggested in a paper with Mark Hertzberg, and that dark civilizations are simply dead.
The last point is particularly reasonable. Most civilizations throughout cosmic history are likely dead by now, for the same reason that most people who ever lived on Earth are dead. This is a simple consequence of the fact that most Sun-like stars formed billions of years before the Sun, and the Sun will boil off all liquid water on Earth’s surface in a billion years. Therefore, most sun-like stars must have extinguished life-as-we-know-it on Earth-like planets near them by now. We were not around to hear the cries for help from these civilizations and by now they went dark. Finding them would require archaeological expeditions to exo-planets.
These conservative assertions about the dark sectors define the mindset of mainstream scientists. But reality is sometimes more imaginative than scientists. If we never check imaginative alternatives, we might never find them. Given how little we know, we should encourage out-of-the-box thinking about the dark sectors. Scientific data collection is worth pursuing because it offers the opportunity to make unexpected discoveries. When I led an expedition to retrieve materials from the crash site of the first known interstellar meteor in the Pacific Ocean, some scientists argued that they did not believe the localization data from U.S. government satellites, that what we found was coal ash and that the meteor was a truck. Jimmy asked what motivated my research and I explained: “I am seeking a higher intelligence in interstellar space, because I cannot find it on Earth.”
Jimmy asked how skeptics could be eventually convinced. I explained that all we need is to collect clear evidence beyond a reasonable doubt. Imagine finding a functional device far more advanced than human-made technologies — akin to ChatGPT version 1000, or a direct image of an interstellar object with buttons on its surface. Once we discover indisputable evidence for an extraterrestrial technological artifact, commentators who discredit related scientific work would be quick to claim that there is nothing fundamentally new here because other people discussed the same ideas long ago.
But credit is secondary to the progress associated with illuminating the dark sectors of our knowledge base through the hard work of collecting new data. The Galileo Project under my leadership is currently assembling three observatories in Massachusetts, Pennsylvania, and Nevada, to search for unfamiliar objects in the sky. It also prepares the pipeline for analyzing data in 2025 concerning new interstellar objects from the Rubin Observatory in Chile.
Knowing that we have cosmic neighbors will remove the last sentiment of privilege that the mainstream of science embraces for the human condition in the cosmos. Encountering a super-human intelligence will return the sense of humility that was advocated by traditional cultures.
After my first podcast interview (out of three) today, I had my morning jog along the Tel Aviv beachfront promenade. The crashing waves of the Mediterranean Sea resembled unsubstantiated critical comments which dissipate into foam and are forgotten after they crash on our shores of knowledge. Upon my return, the old hotel concierge told me that the sirens did not get him out of bed last night. “One way or another, I only have a short time to live, as I will soon fade into darkness,” he noted. In anticipation of the first day of Hanukkah, the Jewish festival of lights, I replied: “but as long as we live — we better shed light and fight the darkness surrounding us.”
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.
