A matrioshka brain is a hypothetical megastructure proposed by Robert Bradbury (1956-2011), based on the Dyson sphere, of immense computational capacity. It is an example of a Class B stellar engine, employing the entire energy output of a star to drive computer systems. This concept derives its name from the nesting Russian Matryoshka dolls. The concept was deployed by its inventor, Robert Bradbury, in the anthology Year Million: Science at the Far Edge of Knowledge.
The concept of a matrioshka brain comes from the idea of using Dyson spheres to power an enormous, star-sized computer. The term "matrioshka brain" originates from matryoshka dolls, which are wooden Russian nesting dolls. Matrioshka brains are composed of several Dyson spheres nested inside one another, the same way that matryoshka dolls are composed of multiple nested doll components. The innermost Dyson sphere of the matrioshka brain would draw energy directly from the star it surrounds and give off large amounts of waste heat while computing at a high temperature. The next surrounding Dyson sphere would absorb this waste heat and use it for its computational purposes, all while giving off waste heat of its own. This heat would be absorbed by the next sphere, and so on, with each sphere giving off less heat than the one before it. For this reason, matrioshka brains with more nested Dyson spheres would tend to be more efficient, as they would waste less heat energy. The inner shells could run at nearly the same temperature as the star itself, while the outer ones would be close to the temperature of interstellar space. The engineering requirements and resources needed for this would be enormous.
The term "matrioshka brain" was invented by Robert Bradbury as an alternative to the "Jupiter brain"—a concept similar to the matrioshka brain, but on a smaller planetary scale and optimized for minimal signal propagation delay. A matrioshka brain design is concentrated on sheer capacity and the maximum amount of energy extracted from its source star, while a Jupiter brain is optimized for computational speed.
Some possible uses of such an immense computational resource have been proposed. One idea suggested by Charles Stross, in his novel Accelerando, would be to use it to run perfect simulations or uploads of human minds into virtual reality spaces supported by the Matrioshka brain. Stross even went so far as to suggest that a sufficiently powerful species utilizing enough raw processing power could launch attacks upon, and manipulate, the structure of the universe itself. In Godplayers (2005), Damien Broderick surmises that a matrioshka brain would allow simulating entire alternate universes. The futurist and transhumanist author Anders Sandberg wrote an essay speculating on implications of computing on the massive scale of machines such as the matrioshka brain, published by the Institute for Ethics and Emerging Technologies. Matrioshka brains and other megastructures are a common theme in the fictional Orion's Arm universe where they are used by superintelligences as processing nodes connected via artificial wormholes.
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