Life as We Know or Do Not Know It

Avi Loeb
5 min readNov 25, 2022


In preparation for Thanksgiving dinner (November 24, 2022)

The human spirit is admirably resilient. Life took hold on Earth as soon as the physical conditions enabled it. And it will continue as long as the physical conditions allow it. The high likelihood that the same physical conditions were replicated on billions of similar rocks within the Milky Way galaxy in the past ten billion years gives me hope that we are not alone.

Last night we hosted two dozen friends for the first post-pandemic Thanksgiving gathering in three years. It was as intellectually enriching as the fifteen previous Thanksgiving celebrations we held at our home. Life-as-we know-it is resilient as long as it is allowed to persist.

A decade ago, during the morning preparations for Thanksgiving dinner, I came across the realization that the Universe went through a habitable period 15 million years after the Big Bang, when the entire cosmos was at room temperature. My wife allowed me to start writing a paper on this idea a few hours before our guests arrived, in return for my promise to wash the dishes afterwards.

Earlier this week, on Sunday, November 20, 2022, NASA’s Orion spacecraft entered the sphere of influence of the Moon, making the Moon instead of Earth the main source of gravity acting on the spacecraft. During this holiday week, Orion captured images of the blue Earth from the far side of the Moon.

Earth setting from the far side of the Moon (right), captured by the Orion spacecraft (left) on November 21, 2022 during the Artemis I mission (Credit: NASA).

Earth’s blue color signals Rayleigh scattering by air molecules, named after the 19th-century British physicist Lord Rayleigh. The scattering probability scales inversely with the wavelength of light to the fourth power. This implies that blue light scatters more than red light — which is characterized by a longer wavelength, making the sky blue during the day. The oceans reflect the scattered light and share the color of the sky. When the Sun is low in the sky during sunrise and sunset, the light has to travel farther through the Earth’s atmosphere. As a result, the sky appears red at those times because the blue light is scattered away. In short, the colors of the Earth signal the existence of an atmosphere. And the existence of a terrestrial atmosphere allows for some forms of life that travel through this medium, such as birds. This, in turn, enables the ritual of eating a large bird during Thanksgiving. The holiday tradition relies on the existence of an atmosphere and so does the blue color in Orion’s image of Earth.

One wonders how widely celebrated is the ritual of eating an atmospheric animal on other blue planets.

Interstellar space is vast. The Milky Way galaxy is a trillion times larger than the Earth-Moon distance. Multiplied by a trillion, a spacecraft travel time of one day changes to 2.7 billion years. Much can happen to a destination during such a long travel time. Our interstellar targets for space exploration should not be based on what we observe when we launch them, but on their potential for being interesting in the long-term future.

We are very different now than our ancestors were when they left Africa, about 60 thousand years ago, the time it takes chemical rockets to reach the nearest star, Proxima Centauri. And from now on, our evolution will be accelerated over a timescale of decades as we enter a new phase with advanced technological devices, including artificial intelligence (AI), augmenting our bodily organs.

As we advance, we are likely to develop artificial interstellar habitats powered by our own nuclear reactors rather than rely on a natural source like the Sun — as we do now. The manufactured furnaces and lightbulbs of other civilizations emit much less light than stars and are not visible to us even in the deepest images from the Webb telescope. Finding them through a dedicated search for physical objects in our cosmic neighborhood, will constitute the next Copernican revolution.

Waiting for guests.

Probes arriving in our vicinity must have started their journey when life on Earth was far simpler. And if chemical rockets were to start the journey now from the edge of the Milky Way, they will not find any life forms on Earth when they arrive a billion years from now, because the Sun will boil off all liquid water on the surface of Earth by then.

For these reasons, we should not assume that interstellar probes have us in mind before they reach our cosmic neighborhood. These devices should be able to adapt to the new reality they find on Earth if they possess AI and machine learning (ML) capabilities.

This Thanksgiving holiday marks an historic week for the Galileo Project, which for the first time is collecting data on the sky from all of its instruments. The data will be analyzed by AI/ML algorithms in the coming weeks, to distinguish familiar atmospheric objects like birds from objects that “come extraterrestrially”, in the words of Avril Haines, the director of National Intelligence. Within a month, the Galileo research team plans to come up with a list of geographical locations where it will place copies of its first detector system in late spring 2023.

This development marks the beginning of a scientific search program for interstellar probes from intelligent civilizations which originated on other blue planets. With this in mind, the Orion image of Earth, the return to post-pandemic routines of life-as-we-know-it this holiday season, and the beginning of the search for life-as-we-do-not-know-it by the Galileo Project, all come together in an unprecedented way that makes me hopeful for an exciting new future awaiting our civilization.


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 June 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".