The Fermi paradox questions why aliens have never visited Earth despite the Universe being so old and so vast that races should have evolved interstellar travel and come calling by now.
Now two scientists believe they may have the answer.
Astrobiologists Dr Michael Wong, of the Carnegie Institution for Science in Washington, and Dr Stuart Bartlett, of California Institute of Technology, have hypothesised that civilisations burn out when they grow too large and technical.
Faced with an ever-growing population and eye-watering energy consumption, worlds hit a crisis point known as a "singularity" where innovation can no longer keep up with demand.
The only alternative to collapse is to abandon "unyielding growth" and adopt a balance that allows survival but prevents the society moving any further forward, or venturing far from its own spot in the universe.
Writing in the Royal Society Open Science, Dr Wong and Dr Bartlett said: “We propose a new resolution to the Fermi paradox: civilisations either collapse from burnout or redirect themselves to prioritising homeostasis, a state where cosmic expansion is no longer a goal, making them difficult to detect remotely.
“Either outcome — homeostatic awakening or civilisation collapse — would be consistent with the observed absence of (galactic-wide) civilisations.”
Large civilisations reach crisis points
The pair argue that the general principles of life are universal and that although the emergence and evolution of life on other planets remains speculative, it may be inevitable.
Once on the path, life is likely to follow a similar trajectory to the civilisations of Earth, they claim, eventually organising into a globally connected state, with technology that needs increasing amounts of energy to maintain growth.
Using city growth equations, which sets limits on how far societies can scale up, the experts show how large civilisations eventually hit crisis points, which, once recognised, causes a halt in further growth.
The pair point to similar ‘mini-awakenings’ on Earth which have prevented global crises, such as the de-escalation of weapons of mass destruction since the Cold War, and the ban on Chlorofluorocarbons (CFCs) to mend the hole in the ozone layer.
However alien civilisations which are close to burnout may be the most easy to detect, according to the research, because they would be using energy in a ‘wildly sustainable manner’ which would provide a good signal emanating from their world.
“This presents the possibility that a good many of humanity’s initial detections of extraterrestrial life may be of the intelligent, though not yet wise, kind,” they conclude.
Previous suggestions for why intelligent aliens have not already visited Earth, include the discovery of a physical difficulty which makes space travel infeasible, whether related to astronomy, biology or engineering.
Some scientists have suggested that aliens have simply never chosen to visit us, or if they have we have not noticed, perhaps arriving before humans had evolved.
It is also possible that advanced civilisations arose too recently, and too far away, for aliens to have reached us yet.
In 2015, scientists analysing data from the Hubble and Kepler space telescopes concluded that around 92 per cent of potentially habitable worlds did not exist when Earth formed, so it may be that our Solar System is simply ahead of the game.