In an explosion (top), force radiates away from a source. With implosion (bottom), the object collapses upon itself (generally being crushed by an outside force).
Implosion is a process in which objects are destroyed by collapsing in on themselves. The opposite of explosion, implosion concentrates matter and energy. An example of implosion is a submarine being crushed from the outside by the hydrostatic pressure of the surrounding water.
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In an implosion-type nuclear weapon design, a sphere of plutonium, uranium, or other fissile material is imploded by a spherical arrangement of explosive charges. This decreases the material's volume and thus increases its density by a factor 2 to 4, causing it to "go critical" and create a nuclear explosion. In some forms of thermonuclear weapons, the energy from this explosion is then used to implode a capsule of fusion fuel before igniting it, causing a fusion reaction (see Teller–Ulam design).
In the context of fluid dynamics, implosion was a term used by Victor Schauberger to describe the suctional process that causes matter to move inwards, not outwards as is the case with explosion. This inward (centripetal) motion, however, does not follow a straight (radial) path to the centre; it follows a spiralling, whirling path. This is called a vortex. This usage is unique to Schauberger, whose theories are not widely accepted by scientists.
Cavitation also involves an implosion process. When a cavitation bubble forms in a liquid (for example, by a high-speed water propeller), this bubble is typically rapidly collapsed—imploded—by the surrounding liquid.
Implosion is a key part of the gravitational collapse of large stars, which can lead to the creation of supernovae, neutron stars and black holes.
Large buildings of various structural types such as masonry, steel frame, or reinforced concrete may be reduced to an easily removed pile of rubble by selective destruction of supporting elements by sequenced and confined explosions. The goal is to confine the materials to specific areas, usually to avoid harm to nearby structures. The technique involves the firing of precisely placed demolition charges in specific timed intervals that use gravity to cause the center of the building to fall vertically while simultaneously pulling the sides inward, a process often erroneously described as an implosion.