In 2021, Robin Hanson, Daniel Martin, Calvin McCarter and Jonathan Paulson published a paper with the following conclusions in their abstract:
“If life on Earth had to achieve n ‘hard steps’ to reach humanity’s level, then the chance of this event rose as time to the n-th power. Integrating this over habitable star formation and planet lifetime distributions predicts >99% of advanced life appears after today, unless n<3 and max planet duration <50 Gyr. That is, we seem early. We offer this explanation: a deadline is set by ‘loud’ aliens who are born according to a hard steps power law, expand at a common rate, change their volumes’ appearances, and prevent advanced life like us from appearing in their volumes. ‘Quiet’ aliens, in contrast, are much harder to see…”
The authors assume that advanced civilizations could form around dwarf stars that live much longer than the Sun, and also that the expansion speed of grabby aliens in visiting new territories is a substantial fraction of the speed of light. Based on the argument that we had not witnessed grabby aliens in the sky, the authors argue that the ratio of quiet to loud alien civilizations must be over 10,000 for there to be even one alien civilization ever in our galaxy. Since our civilization is regarded as quiet for now, our chance of becoming grabby is the inverse of this ratio, namely less than 1/10,000.
Even though this publication references seven of my papers, I find its perspective on grabby aliens to be misguided. The first point to keep in mind is that the existence of our civilization 13.8 billion years after the Big Bang, can be regarded early in cosmic history only if intelligent life is possible around dwarf stars which live up to ten trillion years. However, we know that Sun-like stars make only 6% of the entire population of stars which are mostly dwarf stars, and so there might be a physical reason as to why we live near a rare star like the Sun.
The habitable zone around any star is the region where liquid water could exist on the surface of rocky planets with an atmosphere, enabling the chemistry of life-as-we-know-it. Since dwarf stars are faint, their habitable zone is closer-in than in the Solar System. For example, the nearest star to the Sun, Proxima Centauri, carries 12% of the mass of the Sun and hosts a rocky planet in its habitable zone which is 20 times closer than the Earth-Sun separation. At this proximity to the dwarf star, the atmosphere of the planet could be stripped by the stellar wind and ultraviolet flares, making the planet a lifeless desert like Mars. We discussed this atmospheric stripping in a paper with my former postdoc, Manasvi Lingam, leading us to conclude that perhaps only Sun-type stars could support life-as-we-know-it.
With that in mind, the timing of our civilization is not particularly early. We exist near the middle of the Sun’s lifespan. If you were to select a random day in your life, it would most likely be somewhere around your midlife because that is where most of the days in your life are. I quantified this argument in a paper about the timing of life in cosmic history, published in collaboration with Rafael Batista and David Sloan.
In addition, one must keep in mind that the expansion speed of grabby aliens depends on their propulsion technology. Our interstellar probes travel at a fraction 0.0001 of the speed of light, well below the speeds envisioned by the authors. Travel at relativistic speeds requires an unrealistic energy supply (in proportion to speed squared) and unrealistic shielding from the inevitable damage from the bombardment of interstellar dust and gas particles, as calculated in my collaboration with Thiem Hoang. With our current chemical propulsion technology, space travel across the Milky-Way disk takes billions of years, as calculated in a recent paper I wrote with my Harvard College student, Shokhruz Kakharov.
Grabby aliens who use chemical rockets to grab resources from exoplanets on the other side of the Milky-Way disk would need billions of years to travel. By then, the resources might be extinguished by the evolution of the star hosting the exoplanets. Given that the roundtrip time is a third of the age of the Sun, the absence of grabby aliens in our sky is completely reasonable.
One thing is clear: if we will not search the sky for aliens, we will never find evidence that they exist. New knowledge does not fall into our lap. Instead of performing theoretical gymnastics with unknown parameters, we better engage in an experimental search for technological objects from aliens in our sky. If these are functional devices, they would take the form of Unidentified Anomalous Phenomena (UAPs). On the other hand, if they are dysfunctional space trash, they would appear as anomalous interstellar objects like `Oumuamua or IM1. In either case, we must monitor the sky to find them.
This is the rationale behind the Galileo Project. Our research team already has interesting results from the first UAP observatory and IM1 expedition. These findings will be reported in the coming months. The breaking news on whether the aliens are quiet or grabby will originate from experimental data, not from theoretical arguments.
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. His new book, titled “Interstellar”, was published in August 2023.
