The ability to walk through walls or be otherwise transported through solid matter is a wish-fulfilment fantasy less prevalent thanInvisibility, perhaps owing to its greater scientific implausibility. The first sf example is perhaps "The Ray of Displacement" (October 1903The Metropolitan Magazine) by Harriet PrescottSpofford, whose titularRay diffuses an individual's atoms and allows them to slide through solid matter (also granting effectiveInvisibility). FurtherInventions enabling such movement appear in MurrayLeinster's "The Mole Pirate" (November 1934Astounding), where "earth planes" travel through the substance of the Earth; HarryHarrison's first story "Rock Diver" (February 1951Worlds Beyond), in which a vibratory device allows prospectors to exploreUnderground without digging; Sydney JBounds'sThe World Wrecker (1956), whereCities are destroyed by planting phase-shifted rocks beneath them which are then restored as normal matter; ColinKapp's "Lambda I" (December 1962New Worlds), with passenger ships employing "tau space" for routine intra-EarthTransportation; Barrington JBayley's "The Radius Riders" (July 1962Science Fiction Adventures UK) as by P F Woods, whose "subterrene" voyagers discover Earth to be (in effect) internally infinite; and DamonKnight'sBeyond the Barrier (1964), whose protagonist falls helplessly through the planet in a penetrator which lacks the ability to manoeuvre, eventually – thanks to a difference in local heights – emerging on the antipodean side. Subterranean war machines move through deep strata in Philip EHigh'sThe Time Mercenaries (1968 dos) and are attacked by rock-penetrating equivalents of depth charges. LarryNiven'sRingworld (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 personalSuperpower possessed by such beings as the titularMonster of A Evan Vogt's "Discord in Scarlet" (December 1939Astounding), the human title character of MarcelAymé's "Le Passe-Muraille" ["The Walker-through-Walls"] (1943; inAcross Paris and Other Stories coll trans1957; coll vtThe Walker-through-Walls1962), the protagonist of4D Man (1959), theAlien Hlat of James HSchmitz's "Lion Loose" (October 1961Analog), the eponym of WarrenMurphy'sDestroyer sequence, and theComicsSuperheroThe Flash – whose power of super-speed includes the ability to walk through walls by adjusting his vibrational rate, a typical invocation ofImaginary Science in this context. Special human commandos in ColinKapp'sThe Ion War (1978) are temporarily and painfully transformed into a gaslike "para-ion" state through which projectile or energyWeapon fire passes harmlessly.

A "natural" example of interpenetration appears inA Wreath of Stars (1976) by BobShaw, whose antineutrino world coexists in the same space as our own, each intangible to the other. The living "echoes" produced byMatter Transmission in Thomas MDisch'sEcho Round His Bones (December 1966-January 1967New 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: EvgenyVoiskunsky.

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