A Dyson swarm catches a star’s light. A Matrioshka brain puts that light to work thinking. The idea was put forward in 1997 by Robert J. Bradbury, an American researcher who asked what the most powerful possible computer would look like if a civilisation could rebuild an entire solar system for the purpose.
Shells within shells
Bradbury’s answer was a set of nested shells around a star, each one a swarm of computers. The innermost shell runs on sunlight and, like any machine, gives off waste heat. The next shell out runs on that heat and gives off its own, at a lower temperature, and so on outward. Each layer harvests the energy of the layer inside it and radiates it away cooler still, until the outermost shell glows faintly in infrared.
The name comes from the matryoshka, the Russian doll with smaller dolls nested inside. Bradbury chose a stellar design because a single solid computer the size of Jupiter, the so-called Jupiter brain, could not use a star’s output without melting.
Bradbury’s numbers
In his write-up, Bradbury estimated that such a structure would use the entire power output of a star, about 10²⁶ watts, and all the useful building material of a solar system, about 10²⁶ kilograms. A single layer, he reckoned, could perform about 10⁴² operations per second. He put the human brain at about 10¹³ operations per second on roughly 10 watts. By his own figures, one Matrioshka brain would match the thinking power of around 10²⁹ human brains.
He also considered what the shells could be made of, from diamond for the cooler layers to heat-resistant materials such as aluminium oxide and titanium carbide close to the star. Bradbury was careful to say that his paper had not been peer reviewed.
In fiction and in the search for life
The Matrioshka brain appears in science fiction, most famously in Charles Stross’s 2005 novel Accelerando. For astronomers it is another version of Dyson’s idea: whatever a civilisation does with a star’s energy, the heat has to go somewhere. A Matrioshka brain would look like a star that has vanished from view and been replaced by a large, cool infrared glow.