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.
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.
The researchers worked through the mathematics carefully, building on the famous Drake equation (the formula that attempts to estimate the number of communicating civilisations in the Galaxy at any one time.
They also introduced a particularly powerful constraint from electromagnetic communication.
Our radio telescopes have now been listening long enough that our ‘light cone’ (the region of space from which signals could have reached us) encompasses the entire Galaxy’s history going back roughly 100,000 years. Any civilisation that existed within our Galaxy during that window and was broadcasting detectable signals should, in principle, have been heard by now.
The silence, the authors argue, is not a matter of our technology being too primitive. It is a genuine absence. Running the numbers, the team concluded that if intelligent life is common, technological civilisations must typically survive for no more than around 5,000 years. Not millions of years. Not even tens of thousands. Five thousand years… a figure that puts the entirety of recorded human history inside the danger zone. We have been a technological civilisation, in any meaningful sense, for only around 200 years. We are, statistically speaking, at the very beginning of the most vulnerable period of our existence.
The paper lists the threats with uncomfortable frankness: asteroid impacts, super volcano eruptions, climate change, pandemics, nuclear war, artificial intelligence, rogue biotechnology.
The authors note that history is littered with collapsed civilisations from the Romans, the Maya and Easter Island that never recovered. In a world as interconnected as ours, a civilisation ending catastrophe could, for the first time, be truly global.
As Rahvar and Rouhani are careful to state, their results “should be interpreted as upper bounds derived from the Fermi paradox, not as predictions of actual lifespans.”
The mathematics does not say civilisations must die at 5,000 years, only that they cannot, on average, survive much longer than that if we are to explain the silence we observe. Other explanations remain entirely possible, perhaps civilisations choose not to communicate, perhaps we are among the first intelligent species to arise, perhaps the distances are simply too vast. The study does not rule any of these out.
But the implication that sits beneath the equations is hard to ignore. The Galaxy may be, or may have been, full of civilisations that rose, built remarkable things, reached for the stars and then fell silent before they could reach anyone else.
Whether through war, environmental collapse, or the misuse of their own technology, the universe appears to impose a strict limit on how long intelligence persists. We do not yet know which category we belong to.
Written by Mark Thompson/Universe Today.
