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In futurology, computronium refers to material engineered to maximize its use as a computing substrate. While futurists usually use it to refer to hypothetical materials engineered on the molecular, atomic, or subatomic level by some advanced form of nanotechnology, the term can also be applied both to contemporary computing materials, and to theoretical physics constructs that are unlikely to ever be practical to build.
Conventional Integrated Circuits
Contemporary integrated circuits can be considered a form of computronium. The density and speed of integrated circuit computing elements has increased roughly exponentially for a period of several decades, following a trend described by Moore's Law. While it is universally accepted that this exponential improvement trend will end, it is unclear exactly how dense and fast integrated circuits will get by the time this point is reached. Working devices have been demonstrated that were fabricated with MOSFET transistor gate width of 6.3 nanometres using conventional semiconductor materials, and devices have been built that used carbon nanotubes as MOSFET gates, giving a gate width of approximately 1 nanometre.
The ultimate density and computing power of integrated circuits are limited primarily by power dissipation concerns. An integrated circuit chip contains many capacitive loads, formed both intentionally (as is the case with gate to channel capacitance) and unintentionally (between any conductors that are near each other but not electrically connected). Changing the state of the circuit causes a change in the voltage across these parasitic capacitances, which involves a change in the amount of stored energy. As the capacitive loads are charged and discharged through resistive devices, an amount of energy comparable to that stored in the capacitor is dissipated as heat.
The result of heat dissipation on state change is to limit the amount of computation that may be performed on a given power budget. While device shrinkage can reduce some of the parasitic capacitances, the number of devices on an integrated circuit chip has increased more than enough to compensate for reduced capacitance in each individual device.
Two other approaches exist to lowering the power cost of state changes. One is to reduce the operating voltage of the circuit, and to reduce the voltage change involved in a state change (making a state change only change node voltage by a fraction of the supply voltage). This approach is limited by thermal noise within the circuit. There is a characteristic voltage proportional to the device temperature and to the Boltzmann constant, which the state switching voltage must exceed in order for the circuit to be resistant to noise. This is typically on the order of 50-100 mV, for devices rated to 100 [[centigrade|degrees C] external temperature (about 4 kT, where T is the device's internal temperature and k is the Boltzmann constant).
The second approach is to attempt to provide charge to the capacitive loads through paths that are not predominately resistive. This is the principle behind adiabatic circuits. The charge is supplied either from a variable-voltage inductive power supply, or by other elements in a reversible logic circuit. In both cases, the charge transfer must be primarily regulated by the non-resistive load. As a practical rule of thumb, this means the rate of change of a signal must be much slower than that dictated by the RC time constant of the circuit being driven. In other words, the price of reduced power consumption per unit computation is reduced absolute speed of computation.
In practice, while adiabatic circuits have been built, it has proven very difficult to use it to reduce computation power substantially in practical circuits.
Lastly, there are several techniques used to reduce the number of state changes associated with any given computation. For clocked logic circuits, the technique of clock gating is used, to avoid changing the state of functional blocks that aren't required for a given operation. As a more extreme alternative, the asynchronous logic approach implements circuits in such a way that an explicit externally supplied clock is not required. While both of these techniques are used to varying extents in integrated circuit design, the limit to practical applicability of each appears to have been reached.
In summary, conventional integrated circuits and their anticipated future descendants provide a form of high-density but power-hungry computronium limited by heat dissipation concerns.
Molecular Nanotechnology
Excited Atoms and Nuclei
Limits to Computation
Use of Computronium
The degree of application of computronium depends on the societal conditions that one assumes, and on the characteristics of computronium. Conditions that would see widespread use are:
- Computronium that is cheaper than or comparable in cost to other computing substrates. Limiting case: computronium is defined as matter optimized for best price/computation ratio.
Silicon-based microchips arguably fall into this category, as they are cheaper than other contemporary computing substrates that offer superior absolute performance.
- Computronium that offers computing speeds greater than alternative substrates, combined with a demand for high computing speed.
Fine-linewidth microchips arguably fall into this category. Development is expensive, but market forces demand (and pay for) faster chips. Similarly, Gallium Arsenide and Silicon Germanium semiconductor substrates offer higher speed at increased cost.
- Computronium that offers greater storage or computing density than alternative substrates, combined with a demand for small computing devices.
Various forms of computer storage device (including hard disks and random access memory chips) can be considered forms of density-optimized computronium.
- Computronium that offers more computing or data storage capacity per unit mass, combined with a demand for low-mass computing devices.
While it is difficult to pinpoint contemporary examples where mass is the driving force behind computronium development, the switch from vacuum tube computers to transistor computers, and from discrete transistor computers to integrated circuit computers, was in considerable part driven by mass concerns, as lower mass computing devices had a wider range of applications, and hence a wider market.
- Computronium that offers more computing operations per unit energy consumed, coupled with a demand for low-power computing devices.
The move to lower voltages and smaller devices in conventional integrated circuits are in part a result of the demand for greater computing power while maintaining a fixed power budget. The transition from emitter coupled logic to CMOS logic in high-speed computers was also driven by power concerns.
Many futurists speculate about futures where demand for computing power grows to the point where very large amounts of computronium are desired. Examples of applications include Jupiter Brains, planet-sized constructs made of computronium, and Matrioshka Brains, concentric Dyson spheres designed to extract all possible energy from the host star for use towards computation.
References
External links
- Matrioshka Brain Home Page (http://www.aeiveos.com/~bradbury/MatrioshkaBrains/) -- A website with speculations concerning advanced supercomputers and computronium.
- Orion's Arm (http://www.orionsarm.com/) -- A collaborative fiction / world building web site with a strong futurist/posthumanist emphasis.
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