Unobtainium

Unobtainium is a term used to describe any hypothetical material with properties that are unlikely or impossible for any real material to possess. Such materials often arise in the context of science fiction. For example, the material forming the foundation of a ringworld requires a tensile strength on the same order as that binding an atomic nucleus together. Since no such material is thought to be possible, a ring world is therefore said to be built out of unobtainium. Unobtainium can be used in a disparaging context (eg., "that idea is silly, you need unobtainium wires to hold the planet up!") or a hypothetical one ("if one were to build an unobtainium shell around a black hole's event horizon, what would happen to the material piling up on it?").

The word "unobtainium" is an informal one, apparently developed within science fiction fandom, and probably in ironic reaction to invented element names in, for example, Star Trek; maybe also by analogy with the naming system for the heaviest actual elements.

An alternative source for "unobtainium" exists within the aerospace industry, which has frequently encountered design problems beyond the capabilities of the available materials. Engineers working for Lockheed Corporation at the Skunk works refer to the SR-71 Blackbird as being being made of "unobtainium" because of the radical decision to use an untried new material, titanium in the construction of this remarkable aeroplane. At the time, "Unobtainium" was required because no known material could withstand the extreme temperatures and stresses of exposure to a Mach 3 airstream.