A matrioshka brain is a hypothetical megastructure proposed by Robert Bradbury, 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 Russian Matrioshka dolls.
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 minimum 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 more optimized for computational speed.
Such a structure would be composed of at least two but typically more Dyson spheres built around a star, and nested one inside another. A significant percentage of the shells would be composed of nanoscale computers (see molecular-scale computronium). These computers would be at least partly powered by the energy exchange between the star and interstellar space. A shell (or component, should a Dyson swarm be the design model used) would absorb energy radiated onto its inner surface, utilize that energy to power its computer systems, and re-radiate the energy outwards. The nanoscale computers of each shell would be designed to run at different temperatures; shells or components at the core could be nearly as hot as the central star, while the outer layer of the matrioshka brain could be almost as cool as interstellar space.
The ideal mechanism for extracting usable energy as it passes "through" a shell or component, the number of shells (or orbital levels) that could be supported in such a manner, the ideal size of the shells to be constructed, and other details, are all issues of speculation.
The idea of the matrioshka brain violates none of the currently known laws of physics, although the engineering details of building such a structure would be staggering, as such a project would require the "disassembly" of significant portions (if not all) of the planetary system of the star for construction materials.
Possible uses 
The possible uses of such an immense computational resource are nearly infinite. 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.
A whole class of transcomputational problems would require (at least) planet-sized computers and aeons of time for solution.
The concept was deployed by its inventor, Robert Bradbury, in the anthology Year Million: Science at the Far Edge of Knowledge, and attracted interest from reviewers in the Los Angeles Times and the Wall Street Journal.
The idea of immensely powerful computing devices was discussed in an essay by Nick Bostrom in The Philosophical Quarterly. Bostrom speculates that if humans deliberately evolved to a post-human stage the species would run massive computer simulations before each stage using machines such as the Matrioshka brain. Going further, Bostrom speculates that humans may in fact be actors in a massive computer simulation. Raymond Kurzweil mentions the concept several times in his book The Singularity Is Near (2005), following a similar train of thought. He makes the point that existence within a computer simulation could be as "real" as within the conventional biosphere - if indeed the distinction can be made. An article in the April 2003 journal of the British Interplanetary Society also discussed the concept.
See also 
- Megascale engineering
- Strong AI
- Technological Singularity
- "Robert Bradbury's matrioshka brain site". Robert J. Bradbury. Retrieved 2009-08-28.
- "Matrioshka Brains – Some Intermediate Stages in the Evolution of Life". Department of Astronomy, University of Virginia. Retrieved 2009-08-28.
- "Jupiter & Matrioshka Brains: History & References". Robert Bradbury. Retrieved 2009-08-28.
- Charles Stross (2006). Accelerando, Singularity Series. Ace Books. ISBN 0-441-01415-1.
- Charles Stross. "Nightfall". Asimov's Science Fiction. Retrieved 2009-08-28.
- Damien Broderick (2005). Godplayers. Thunder's Mouth. ISBN 1-56025-670-2.
- Anders Sandberg (December 22, 1999). "The Physics of Information Processing: Superobjects: Daily Life Among the Jupiter Brain". Institute for Ethics and Emerging Technologies. Retrieved 2009-08-28.
- Levy, Brett (August 26, 2008). "Book Review: 'Year Million: Science at the Far Edge of Knowledge,' edited by Damien Broderick". Los Angeles Times. Retrieved 2009-08-28.
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- Nick Bostrom. Are we living in a computer simulation?. The Philosophical Quarterly. JSTOR 3542867.
- Ray Kurzweil (2005). The singularity is near: when humans transcend biology. Viking. p. 360ff. ISBN 0-670-03384-7.
- JBIS: Journal of the British Interplanetary Society, Volume 56. British Interplanetary Society. 2003. p. 277.
- "Matrioshka Micronode". Orion's Arm. Retrieved 2009-08-28.
- Sandberg, A (22 Dec. 1999). "The Physics of Information Processing Superobjects: Daily Life Among the Jupiter Brains".