Should We Worry About Interstellar Predators?
The news about our technological era has been propagating at the speed of light for roughly 120 years, and reached only 20,000 nearby star systems. This constitutes a tiny fraction of only one part in ten million of the total number of stars in the Milky-Way galaxy.
But it is important to recognize that in the next few centuries this number will grow in proportion to that time cubed, meaning that in a millennium — it will be about a thousand times larger. The growth will slow down to a scaling with time squared after the next millennium because most Milky-Way stars are distributed in a two-dimensional disk whose thickness is of order a thousand light-years.
If there are predator civilizations out there, this volume growth could have dramatic implications for the long-term survival of humanity because they may regard us as a threat and arrive at Earth to destroy us.
Our risk of being noticed increases rapidly with the number of technological centuries we endure. And after being noticed by a predator, we might still not hear from a predator for a period corresponding to their travel time to reach us, which could be as long as millions to billions of years if they use rocket technologies or hundreds of years if they operate weapons, such as a powerful laser beam, that travel at the speed of light.
The saving grace is that as our signals move out, they also get fainter, reducing the chance of them being noticed in the background noise of the Milky-Way galaxy.
In a paper (accessible here) that I co-authored with Matias Zaldarriaga in 2007, we showed that the new generation of radio observatories on Earth could detect radio broadcast leakage from an Earth-like civilization out to a distance of order a hundred light years. Such a radio signal will show up as a series of narrow spectral lines that do not coincide with known atomic or molecular lines. For example, the high spectral resolution attainable with the forthcoming Square-Kilometer-Array will allow us to monitor the periodic Doppler shift of the broadcast lines over the planet’s orbital period around the parent star. Determination of the parent star mass through observations of its spectrum could then be used to infer the inclination, semi-major axis and eccentricity of the planet’s orbit. This, in turn, will allow us to estimate the temperature on the planet’s surface and to assess whether it can support liquid water or life as we know it.
In other words, a twin-civilization could eavesdrop on us right now if it is located within our radio bubble out to 120 light years. But many more extraterrestrial civilizations may be aware of the habitability of planet Earth as they can observe it transiting the Sun. The Earth’s transit of the Sun could be seen from up to a billion stars, about 0.5% of all stars in the Milky-Way galaxy.
The main scenario that we should lose sleep about is of alien predators who noticed Earth a long time ago and decided to monitor the situation from up close, forecasting that it may lead to the development of space technologies that pose a threat to them. If they have camped in the outer solar system, they can respond to anything happening on Earth within decades at the speed of chemical rockets. We might notice their gadgets flying around in the form of anomalous solar-system or interstellar objects.
For all these reasons, we must stay alert to the cosmic news delivered to us on a daily basis by the NSF-DOE Rubin Observatory. We are used to thinking about the Universe as an independent entity that has nothing to do with our actions here on Earth. But this paradigm could break down as soon as we realize that someone else out there may be watching us and responding to what we do.
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.
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