After decades of searching, we still haven’t discovered a single sign of extraterrestrial intelligence. Probability tells us life should be out there, so why haven’t we found it yet?

The problem is often referred to as Fermi’s paradox, after the Nobel Prize–winning physicist Enrico Fermi, who once asked his colleagues this question at lunch. Many theories have been proposed over the years. It could be that we are simply alone in the universe or that there is some great filter that prevents intelligent life progressing beyond a certain stage. Maybe alien life is out there, but we are too primitive to communicate with it, or we are placed inside some cosmic zoo, observed but left alone to develop without external interference. Now, three researchers think they think they may have another potential answer to Fermi’s question: Aliens do exist; they’re just all asleep.

According to a new research paper accepted for publication in the Journal of the British Interplanetary Society, extraterrestrials are sleeping while they wait. In the paper, authors from Oxford’s Future of Humanity Institute and the Astronomical Observatory of Belgrade Anders Sandberg, Stuart Armstrong, and Milan Cirkovic argue that the universe is too hot right now for advanced, digital civilizations to make the most efficient use of their resources. The solution: Sleep and wait for the universe to cool down, a process known as aestivating (like hibernation but sleeping until it’s colder).

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