The ability to walk through walls or be otherwise transported through solid matter is a wish-fulfilment fantasy less prevalent than Invisibility, perhaps owing to its greater scientific implausibility. Inventions enabling such movement appear in Murray Leinster's "The Mole Pirate" (November 1934 Astounding), where "earth planes" travel through the substance of the Earth; Colin Kapp's "Lambda I" (December 1962 New Worlds), with similar passenger ships employing "tau space" for routine intra-Earth Transportation; Barrington J Bayley's "The Radius Riders" (July 1962 Science Fiction Adventures UK) as by P F Woods, whose "subterrene" voyagers discover Earth to be (in effect) internally infinite; and Damon Knight's Beyond the Barrier (1964), whose protagonist falls helplessly through the planet in a penetrator which lacks the ability to manoeuvre, eventually emerging on the far side. Subterranean war machines move through deep strata in Philip E High's The Time Mercenaries (1968 dos). Larry Niven's Ringworld (1970) mentions a device called the "cziltang brone" which supposedly makes the eponymous world's superstrong construction material temporarily permeable, allowing passage without physical airlocks.
Matter penetration is also posited as a personal Superpower possessed by such beings as the titular Monster of A E van Vogt's "Discord in Scarlet" (December 1939 Astounding), the human title character of Marcel Aymé's "Le Passe-Muraille" ["The Walker through Walls"] (1943; in Across Paris and Other Stories coll trans 1957; coll vt The Walker through Walls 1962), the protagonist of 4D Man (1959), the Alien Hlat of James H Schmitz's "Lion Loose" (October 1961 Analog), the eponym of Warren Murphy's Destroyer sequence, and the Comics Superhero The Flash – whose power of super-speed includes the ability to walk through walls by adjusting his vibrational rate, a typical invocation of Imaginary Science in this context. Special human commandos in Colin Kapp's The Ion War (1978) are temporarily transformed into a gaslike "para-ion" state through which projectile or energy Weapon fire passes harmlessly.
A "natural" example of interpenetration appears in A Wreath of Stars (1976) by Bob Shaw, whose antineutrino world coexists in the same space as our own, each intangible to the other. The living "echoes" produced by Matter Transmission in Thomas M Disch's Echo Round his Bones (December 1966-January 1967 New Worlds; 1967) have the essential intangibility of ghosts. We generally reserve the term "matter penetration" for cases where both the barrier and that which penetrates it remain solid, or return to the identical solid state, when the transit is complete. [DRL]
see also: Evgeny Voiskunsky.
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