How Long Do Civilisations Last?

How Long Do Civilisations Last? By Mark Thompson - March 02, 2026 09:25 AM UTC | Astrobiology It is one of the most famous questions in science, and it was asked, as legend has it, over lunch. Enrico Fermi, the physicist who helped build the first nuclear reactor and whose name graces a unit of length so small it makes an atom look generous, was chatting with colleagues about the possibility of alien life when he suddenly asked ‘where is everybody?’ The universe is thirteen billion years old. Our Galaxy alone contains hundreds of billions of stars, a significant proportion of which host planets. Many of those planets sit in the right temperature range for liquid water. The numbers, by any reasonable estimate, suggest that life should have emerged many times over, in many places, long before our own planet had even formed. And yet. No signals. No visitors. No evidence of anyone at all. This is the Fermi paradox, and it has remained unresolved for seventy-five years. Enrico Fermi (Credit : Los Alamos National Laboratory) Now two physicists from Sharif University of Technology in Tehran have approached it from a new angle. Rather than asking why we have not found other civilisations, Sohrab Rahvar and Shahin Rouhani have asked what the silence itself tells us and the answer places a hard mathematical ceiling on how long technologically advanced civilisations are likely to survive. If we assume, optimistically, that intelligent life emerges relatively readily on Earth like planets, and the sheer number of such planets in our Galaxy means there would be enormous numbers of civilisations, then the absence of any contact with those civilisations must mean they are not there anymore. The Galaxy is old enough, and space is well connected enough, that a long lived technological civilisation would eventually have made itself known. We would have detected their signals, or encountered their probes, or found some trace of their engineering. We have found none of these things. Th...