In 1950 Enrico Fermi asked: “Where is everybody?” His sense of cosmic loneliness is not necessarily a testimony to the small number of intelligent partners in our cosmic neighborhood but could instead be a reflection on how large are the cosmic scales of time and space.
To appreciate the range of scales in the Universe, we can use a logarithmic measure of powers of ten. The size of a small human cell is 5 orders of magnitude larger than the diameter of a hydrogen atom and 5 orders of magnitude smaller than the scale of our body. Hence, a cell is roughly halfway logarithmically between the scales of an atom and our body.
The scale of the human body is 10 orders of magnitude larger than the diameter of an atom and 9 orders of magnitude smaller than the diameter of the Sun. Therefore, our body is roughly halfway logarithmically between the scales of an atom and the Sun.
The radius of the Sun is 19 orders of magnitude larger than the radius of an atom and 18 orders of magnitude smaller than the distance that light has travelled since the Big Bang, 46 billion light years. Although the age of the Universe is 13.8 billion years, cosmic expansion stretched the largest scale we can probe, namely our cosmic horizon. Altogether, the size of the Sun is roughly halfway logarithmically between the size of an atom and the scale of the observable Universe.
Another important point is that most space is empty. The radius of an atom is 5 orders of magnitude larger than the radius of its nucleus, where 99.95% of the atomic mass is concentrated. This means that matter within our body is concentrated in nuclei which are separated from each other like tennis balls separated by the size of a city, 6.5 kilometers.
We can now address Fermi’s question. The distance from the Sun to the nearest star is 10 orders of magnitude larger than the radius of the Sun. This is equivalent to tennis balls that are separated from each other by the radius of the Earth. Given these vast interstellar separations, advanced civilizations must trash interstellar space with ten quadrillion unguided relics per star in order for one of these objects to collide with Earth once per decade. Humanity launched only 5 of our own probes to interstellar space over the last 5 decades. At this rate, we might launch only a hundred million relics over the next billion years before the brightening Sun will boil off all oceans on Earth. This will be halfway logarithmically from the requirement of ten quadrillion interstellar launches, so that meteor enthusiasts on a nearby habitable exoplanet might notice one of our relics as an interstellar meteor during their career.
And even if extraterrestrial analogs of Elon Musk from nearby stars exceed our interstellar output by more than a factor of a hundred million, the resulting interstellar space trash has a very small chance of landing at the Los Alamos National Laboratory during lunchtime in the summer of 1950 when Fermi asked his question.
Of course, the detection probability could increase dramatically if functional probes targeted Earth because the planet developed its Last Universal Common Ancestor (LUCA) of life as early as 4.2 billion years ago. Our imagination of what these probes might be is limited by a century of modern science and technology. It is also limited by the materials we have access to with our current technologies. These materials are drawn from Earth and not from the cosmos at large. When my parents got married on a farm, they were poor. And so, my father constructed their first furniture out of the wooden boxes that he used for packaging oranges in their backyard. Are our rockets similarly limited by terrestrial resources and appear primitive from an interstellar perspective? In my latest paper, I suggested that bottling “dark energy” which is abundant throughout the Universe could potentially be used to launch payloads to space without rocket fuel.
If functional interstellar probes employ artificial intelligence (AI) and stealth “dark” technologies that are beyond our imagination, then we would classify them as either “dark comets” or as “empty trash bag objects” or as “Unidentified Anomalous Phenomena (UAPs)”, possibilities that I will discuss in a Congressional briefing on May 1, 2025.
Fermi’s question could be explained by the rarity of technological space trash as a source of interstellar meteors and by the potentially anomalous nature of advanced interstellar probes. Electromagnetically loud civilizations might have been invaded and silenced long ago by interstellar predators. In that case, it is just a matter of time before we will witness a response to the electromagnetic signals we leaked out over the past century. They only reached a hundred light years by now, a spherical volume that includes only ten thousand stars — merely one part in ten million of the total number of stars in the Milky Way galaxy. The response may take millions of years, long by our standards, but short from a cosmic perspective where time is measured in billions of years.
The fundamental question is whether our technologies are at the logarithmic midpoint in the cosmic class of technological civilizations that ever existed over the past 13.8 billion years of cosmic history. There is no better way to find the answer to this question than to search for the best students in our class of technological civilizations.
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.
