prevacuum

Prevacuum is an historical term used in 19th- and early 20th-century physics to describe a state of space that lies between ordinary matter-filled space and a true vacuum. In this usage, prevacuum refers to space that has not yet been evacuated of matter or, conversely, to a transitional state in which the properties of space are between a dense medium and an ideal vacuum. The term is most often encountered in discussions of the luminiferous ether and theories of light propagation in an all-pervading medium; some authors described the prevacuum as the otherwise unexplored or imperfectly evacuated region through which light travels before a complete vacuum is achieved.

As the concept of the ether was abandoned and the modern understanding of vacuum matured, prevacuum fell out of routine use. In historical sources, it can appear in arguments about how electromagnetic phenomena would differ in a pre-evacuated space vs. a perfect vacuum, or in analyses of the transition from bare matter to the vacuum state.

In contemporary physics, the term is largely obsolete. The standard vocabulary distinguishes between vacuum, the absence of matter; and the quantum vacuum, the lowest-energy state of a quantum field, which can exhibit fluctuations. Prevacuum is sometimes noted in historical overviews to illustrate early preconceptions about space, matter, and light.