A survey of some physical limitations on computer elements

Some limitations imposed on the basic attributes of computer elements that arise from physical law rather than from consideration of technology or cost are surveyed in the light of recent literature. In particular, discussions are given of the reliability of storage of information, of the physical size, and of the speed of response of elements of the memory of a large, high-speed, digital computer. The various factors affecting the reliability considered include: logical and physical redundancy, crosstalk and the resulting difficulties of single-particle memory elements; storage lifetime in a nonequilibrium (steady-state) device is also discussed. The limits on size of semiconductor devices include: the minimum size of the voltage pulse needed for nonlinear operation, the limitations set by the need for cooling, by fluctuations in impurity content, and by the resolving power of fabrication processes. The effects of the inertia of matter on the speed of operation are discussed generally, noting the different magnitudes of response of transitions involving electronic rearrangements as compared to spin rotations or nuclear motions. Finally, comments are made on the possibility in principle of ultradense packing because of the extreme smallness of the atomic scale, and on the biological examples of such ultradense packing that exist in the genetic code and the mammalian brain.