Limits to computation

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There are several physical and practical limits to the amount of computation or data storage that can be performed with a given amount of mass, volume, or energy:

• The Bekenstein bound limits the amount of information that can be stored within a spherical volume to the entropy of a black hole with the same surface area.
• The temperature of the cosmic microwave background radiation gives a practical lower limit to the energy consumed to perform computation of approximately 4kT per state change, where T is the temperature of the background (about 3 kelvins), and k is the Boltzmann constant. While a device could be cooled to operate below this temperature, the energy expended by the cooling would offset the benefit of the lower operating temperature.

Several methods have been proposed for producing computing devices or data storage devices that approach physical and practical limits:

• A Matrioshka Brain is a set of concentric Dyson spheres that attempts to capture as much usable energy as possible from the host star, to make it available for computation.
• A cold degenerate star could conceivably be used as a giant data storage device, by carefully perturbing it to various excited states, in the same manner as an atom or quantum well used for these purposes. Such a star would have to be artificially constructed, as no natural degenerate stars will cool to this temperature for an extremely long time. It is also possible that nucleons on the surface of neutron stars could form complex "molecules"[1] which some have suggested might be used for computing purposes[2], creating a type of computronium based on femtotechnology which would be faster and denser than computronium based on nanotechnology.
• It may be possible to use black hole as a data storage and/or computing device, if a practical mechanism for extraction of contained information can be found. Such extraction may in principle to be possible (Stephen Hawking's proposed resolution to the black hole information paradox). This would achieve storage density exactly equal to the Bekenstein Bound. The scientist Seth Lloyd calculated the computational abilities of an "ultimate laptop" formed by compressing a kilogram of matter into a black hole of radius 1.485 x 10^-27 meters, concluding that it would only last about 10^-19 seconds before evaporating due to Hawking radiation, but that during this brief time it could compute at a rate of about 5 x 10⁵⁰ operations per second, ultimately performing about 10³² operations on 10¹⁶ bits. Lloyd notes that "Interestingly, although this hypothetical computation is performed at ultra-high densities and speeds, the total number of bits available to be processed is not far from the number available to current computers operating in more familiar surroundings."[3]

None of these methods are expected to be practical in the near future.

[edit] References

1. Lloyd, S. (2000-08-31). "Ultimate physical limits to computation" (PDF). Nature 406: 1047–1054.