It is conceivable that the Earth-mass planet Proxima b, which lies in the habitable zone of the nearest star to the Sun, Proxima Centauri, hosts a civilization similar to ours.
Owing to minor differences between their and our history, it is possible that they had reached our current technological state slightly in advance. Consider a tiny margin of head start for Proxima b science, of order 0.00002 of the age of the Earth. This amounts to 100,000 years, meaning that Proxima’s astronomers had used telescopes to discover Earth a hundred millennia ago, in the way that we discovered Proxima b on August 24, 2016. How would that discovery be reflected in their space age?
Driven by long-term curiosity, the rocket builders of the most advanced space agency on Proxima b could have chosen the habitable planet Earth as an exciting destination for their chemical rockets. This would be an admirable vision that NASA did not reciprocate as of yet. But Proxima b residents might have realized that science is better than politics and hence chose to invest more in space exploration than in military budgets. In that case they can serve as better role models than our politicians.
At a speed of 30 kilometers per second, or 0.0001 of the speed of light c, the chemical rockets launched from Proxima b a hundred millennia ago could have reached us by now. Around the time of the launch, humans on Earth left Africa based on an early-modern human fossil found in a cave in Israel. Taking into account the fact that Proxima Centauri is moving at a speed of tens of kilometers per second relative to the Sun and was farther away in the past, the trip could have taken of order 100,000 years, implying that these chemical rockets could have arrived at Earth around the present time. But are these chemical rockets the first spacecraft to arrive at our backyard from Proxima b?
Probably not. In the centuries that elapsed after the first attempt to launch chemical rockets towards Earth, the Proxima b scientists could have developed more advanced propulsion technologies. Suppose it took them a millennium after the invention of chemical rockets to develop hydrogen fuel propulsion which brings their spacecraft to a speed that is ten times faster, 0.001c. If the later generation of Proxima b scientists maintained the ambition to reach Earth, the second wave of spacecraft launched towards Earth would have completed the trip in just 10,000 years and could have arrived 89,000 years at Earth earlier than the first wave of chemical rockets. A millennium later, when nuclear propulsion technologies might enable a travel speed 0.01c or 3,000 kilometers per second, the trip to Earth would have taken only 1,000 years. This wave of nuclear spacecraft could have reached the Earth 97,000 before the first chemical rockets. Finally, if after another millennium the Proxima b scientists end up developing light-sails pushed by powerful laser beams to 0.1c, in the spirit of the Breakthrough Starshot Project which I led over the past decade, these light-sails would have reached the Earth within a century, suggesting that they could have been here 96,900 years before the first chemical rockets from Proxima b.
In summary, assuming an improvement by a factor of 10 in spacecraft speed per millennium of research and development (R&D) on Proxima b, nuclear spacecraft would have reached Earth first, about 97,000 years ago, followed by light-sails merely a century later, followed by hydrogen-propelled spacecraft 8,000 years later, and chemical rockets arriving around the present time. In that scenario, hydrogen-propelled spacecraft would have reached us a few millennia before the Great Pyramids were constructed in Giza, Egypt. The more advanced spacecraft would have arrived as humans were migrating out of Africa.
By examining the equipment arriving at Earth as a function of time, terrestrial archaeologists can infer its speed and decipher the history of propulsion technologies on Proxima b based on the time it took for it to arrive at Earth from the same planet of origin. This analysis would constitute a new field of research: archaeology of interstellar propulsion.
This thought experiment implies that we should expect advanced technologies to visit us before we detect any chemical rockets arriving at our backyard. As I showed in a recent paper with my student Shokhruz Kakharov, even the slowest chemical rockets take about a few billion years to reach us from the opposite side of the Milky-Way galaxy. Since most stars formed billions of years before the Sun, most Galactic spacecraft could have made the trip to Earth by now.
Documented human history is only 8 millennia long, but it is possible that archaeologists or geologists will find traces of propulsion technologies far more advanced than we developed so far. Is it also possible that such a visit triggered human intelligence? In that case, biologists should find a related anomaly in human DNA.
A new study reported that Proxima Centauri flares frequently, with flares carrying the energy output of the Hiroshima atomic bomb every few minutes. The stability of the Sun might appear more attractive to a technological civilization on Proxima b. Their recognition of Earth as a favorite destination would have triggered mass migration of aliens towards Earth, an interstellar exodus of far bigger proportions than that of the Israelites from Egypt to the Promised Land, celebrated this week in the Jewish holiday of Passover. The arrival times of the waves of Proxima b migrants would depend on the timing of their departure, reflected in the propulsion technologies they use.
If the shortening of the travel time exceeds the delay in departure time, the generation of migrants arriving first might include the descendants of those who had the patience to depart last from Proxima b. The descendants of the impatient migrants who used chemical propulsion will find the Promised Land of Earth to be inhabited not only by humans but also by later generations of aliens from Proxima b. Earth would appear to them like a time machine, reversing the technological history of their home planet.
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.
