Searching for Partners Around the Milky Way

Avi Loeb
5 min readFeb 2, 2023
History of Earth (Credit: Wouldloper)

What is the chance that a technological gadget with an extraterrestrial version of ChatGPT would visit Los Alamos for lunch in 1950?

Given the vast extent of the orbital radius of the Earth around the Galactic center, 24,000 light years, and the huge age of the Earth, 4.6 billion years, the chance of a random encounter during a particular month in a kilometer cubed volume near Earth would be of order a part in 10 to the power of 62. This represents a tiny chance even if we multiply it by the billions of Milky Way stars from where the gadget could have originated or by the additional number of Musk-like entrepreneurs that might have existed on each habitable planet.

The likelihood is minuscule because our window of opportunity represents a tiny real-estate footprint on Earth and a negligible timeline on the Galactic scales. We would have not noticed most visits because only over the past decade we had the technology to detect football field-sized interstellar objects, like `Oumuamua — which was discovered as a near-Earth object on October 2017 based on its reflection of sunlight within the orbit of the Earth around the Sun, or meter-sized interstellar meteors — which were discovered on January 2014 and March 2017 from the fireball they created by friction in the Earth’s atmosphere.

It is therefore premature to be puzzled by Enrico Fermi’s question: “where is everybody?”, posed at lunchtime in the company of Emil Konopinski, Edward Teller and Herbert York at Los Alamos National Laboratory in the summer of 1950. This was 7 decades before astronomers developed the capability to detect the first interstellar objects. Remarkably, the first two interstellar meteors exhibited material strength in excess of solar system rocks and the first near-Earth interstellar object `Oumuamua exhibited a push from the Sun without showing any evidence for a cometary tail. This should make us curious whether these messengers from afar provide an affirmative answer to Fermi’s question.

The observed star formation history of the Universe implies that most sun-like stars formed 10 billion years ago, 5.4 billion years before the Earth. This means that another technological civilization could have predated ours by billions of years, allowing for chemical rockets to reach Earth billions of years ago.

In the same way that Earth circles the Sun once a year, the Sun circles the center of the Milky Way galaxy every 200 million years.

A Galactic-orbital-period ago, the dinosaurs started living on Earth. They went extinct by an asteroid impact 66 million years ago, about a third of an orbital period ago. The sky looked different to the eyes of the dinosaurs because the Milky Way disk was oriented in a different part of our sky. If dinosaurs were visited by technological probes, they would have ignored the strange objects in their sky as much as they ignored the deadly asteroid that approached them. Enrico Fermi would have never been informed about their encounter.

Two Galactic orbital periods ago, the first vertebrate land animals with a spinal column appeared on Earth. If they were visited, they would have watched the artificial intelligence (AI) astronauts with awe and continued with their daily routines without recording the incident.

About 2.7 Galactic periods ago, the Cambrian explosion of species occurred on Earth when practically all major animal phyla started appearing in the fossil record. An AI astronaut visiting before that time would have been disappointed to encounter only primitive forms of life. The time since the Cambrian explosion to now represents only the last 12 percent in the Earth’s history. In other words, the chance of timing an extraterrestrial probe to witness complex terrestrial life is only 1/8.

The Earth froze over in snowball events about 3.2–3.6 orbital periods ago, making a visit by interstellar cameras very boring. Any extraterrestrial watchers from a distance would have considered Earth as hostile to life as the frozen moons of Jupiter and Saturn appear to our telescopes.

Since the Earth formed, it has gone around the Galactic center 23 times. The Earth scooped a lot of space and potentially collided with non-functional spacecraft like the interstellar meteors represented by a billions-year old versions of our own interstellar probes: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons.

But the interesting question is whether the Earth encountered a functional device with AI. Our best hope is to imagine that the extraterrestrials did not lose interest in Earth because of its long history of primitive life. If probes visited Earth in the distant past they are likely to also do so in our future. Given the tremendous astronomical dimensions of space and time relative to human scales, it is our duty use our best telescopes and monitor the sky with an open mind and not regard a visit as an “extraordinary claim” in the words of Carl Sagan, while not allocating the funds to gather the “extraordinary evidence” to check for it.

The Galileo Project brings a fresh perspective to this curiosity-driven search. It is funded by private donations, demonstrating that the general public is excited by the prospects. The recent reports on Unidentified Aerial Phenomena (UAP) to the US Congress by the Director of National Intelligence in 2021 and 2022 imply that the US Government is also interested. It is time for the mainstream of the academic community to catch up and attend to the public and governmental interests. The subject resonates more with taxpayers than the speculations about extra dimensions, the multiverse, string theory, supersymmetry, cosmic inflation or the nature of dark matter and dark energy, which consume most of the oxygen in the halls of mainstream academia. Extraordinary significance requires common sense and a shift in priorities from ego-driven goals of showing off with mathematical virtuosity or abstract concepts to transparent child-like curiosity about our actual cosmic neighborhood.

The time is ripe to replace Steven Weinberg’s dictum: “The more the universe seems comprehensible, the more it also seems pointless”, with the ambition to obtain meaning to our short life on this tiny rock we call Earth by finding partners in the vast extents of space and time that the Universe offers us. Dating requires action on both sides, and success would greatly benefit us if the partner represents our technological future.


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.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".