Implosion is when an object collapses into itself because of a difference in pressure or the force of gravity. It is the opposite of an explosion, which makes an object take up more space. Implosion makes an object take up less space and brings matter and energy together. This happens when the pressure inside an object is much lower than the pressure outside, or when forces pulling inward are much stronger than those pushing outward. The object then collapses into itself or into the space it once occupied if it is not completely solid. Examples of implosion include a submarine being crushed by water pressure and a star collapsing under its own gravity.

Sometimes, implosion can send material outward. This may happen when material falling inward bounces back or when outer parts are pushed outward as the inside collapses. If the object was solid before the implosion, it usually becomes more dense after the collapse. This means it becomes more compact or changes into a material that is harder to compress.

Examples

In an implosion-type nuclear weapon, a ball of plutonium, uranium, or another special material is squeezed together using explosives arranged in a circle. This makes the material smaller and denser, increasing its density by two to three times. When the material becomes dense enough to start a nuclear reaction, it creates a powerful explosion.

In some thermonuclear weapons, the energy from this explosion is used to compress a capsule filled with fusion fuel, which then starts a fusion reaction (see Teller–Ulam design). Using radiation to compress materials, as in hydrogen bombs or laser-driven fusion, is called radiation implosion.

Cavitation is a process where bubbles form and collapse in a liquid. For example, when a fast-moving water propeller creates a bubble, the surrounding liquid quickly forces the bubble to collapse, which is a type of implosion.

Implosion plays a major role in the collapse of large stars. When a star’s core stops producing energy, the forces that keep its particles apart weaken. The core collapses rapidly, forming a neutron star or black hole. The outer parts of the star may then fall inward and rebound, creating a supernova.

Cathode-ray tubes have a very low pressure inside. If the glass covering breaks, the tube may implode, causing sharp glass pieces to fly at high speeds. Older cathode-ray tubes used special coatings or materials to prevent breaking, but tubes removed from devices must be handled carefully to avoid injury.

Sometimes, people incorrectly call the controlled collapse of large buildings using timed explosions "implosion." In reality, the structure falls inward due to the explosions, not because of a true implosion.