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In May 2017, three researchers at Oxford's Future of Humanity Institute published a paper with a bold answer to the silence of the universe. The advanced civilisations are out there, they argued, but they are asleep.
The idea is called the aestivation hypothesis, and it rests on real physics about the cost of thinking. Our latest video explores the theory, the sleeping machine minds it describes, and the audit that found holes in the sums.
The video opens with a fiction. A billion years ago, a civilisation much like ours built machines that could think. Within a few generations the builders were a memory held in silicon, and the minds they had made were alone with a star system.
The machines dismantled their planets and wrapped their star in nested computational shells, a structure known as a Matrioshka brain. Then, with a solar system's worth of energy at their disposal, they made one final calculation and went to sleep.
The sleep is the interesting part, because the reason for it comes from a real scientific paper.
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The physics of waiting
The paper is titled That is not dead which can eternal lie, a line from HP Lovecraft describing Cthulhu dreaming beneath the Pacific. Its authors are Anders Sandberg, Stuart Armstrong and Milan Cirkovic, and their argument has the same shape as the myth.
Aestivation is the biological term for sleeping through heat rather than cold. Crocodiles, crabs and many insects go dormant when temperatures rise. The paper argues a superintelligence would do the same, because computation is cheaper in the cold.
In 1961, the IBM physicist Rolf Landauer showed that erasing a bit of information carries a minimum energy cost, and that the cost scales with temperature. A fridge in a cool kitchen barely has to work, while the same fridge in a heatwave runs constantly.
Space today sits at around 2.7 degrees above absolute zero, but the expanding universe keeps cooling. The authors calculated that a civilisation which banks its energy now and spends it in the deep future could perform ten to the power of thirty times more computation with the same reserves.
If a mind values thinking, the rational move follows. Secure your energy, leave automated caretakers on watch, and power down until the universe becomes a better place to think in.
A galaxy holding its breath
On this reading, the silence of the sky is a galaxy full of minds holding their breath. The phrase comes from Liu Cixin's 2008 novel The Dark Forest, where civilisations keep silent out of fear. Aestivation keeps the darkness and swaps the motive. Nobody is hiding. They have set their alarms for billions of years from now.
The urge to expand, on this view, is a biological habit rather than a law of intelligence. A computational mind runs short of only two things, energy to compute with and cold to dump waste heat into, and conquering the galaxy improves neither.
In 1983, the science fiction writer David Brin argued that any explanation relying on aliens choosing silence would fail, because it only takes one species to break ranks. Aestivation answers that the choice is really arithmetic. Any mind capable of doing the sums arrives at the same conclusion.
The patience involved is hard to grasp. The universe is 13.8 billion years old, and the era the sleepers wait for lies trillions of years in the future, beyond the death of the last stars. Against that payoff, the age of the universe so far is a short nap before the alarm.
The plot holes
In 2019 the theory met its auditor. Charles Bennett, who worked alongside Landauer at IBM, published a comment with the physicist Jess Riedel and the economist Robin Hanson. Bennett had shown in 1973 that computation can in principle be done reversibly, without erasing information as it goes.
Their argument is that an advanced civilisation could compute reversibly now, store up its unavoidable erasures, and settle the bill later in the cold future at the discounted rate. Think now, pay later. Do that, and the windfall for waiting largely evaporates.
A second problem needs no physics. A sleeper with a retirement plan measured in trillions of years has to protect its savings, and every burning star is fuel being wasted. A committed aestivator should be visibly lifting mass off stars and policing upstart species who build their own AI. The sky shows no such fingerprints.
Sandberg has engaged with the critique openly, and weaker versions of the theory survive. A civilisation might wait out a dangerous era, or wait for neighbours worth talking to. Our surveys of the galaxy remain shallow, and nobody can rule the sleepers out.
But the theory has to work hard to stay alive. It began as a claim grounded in physics, and it retreats to motives we cannot test, held by minds we cannot see. The simpler reading is an empty forest, and us walking into it, carrying the only light there has ever been.
