Undensity

Undensity is a hypothetical concept referring to a state of matter or a region of space where the density is significantly lower than its surrounding environment or a baseline value. The term is not widely established in scientific literature and is often used in speculative contexts or as a descriptive term for phenomena that exhibit extreme emptiness. For instance, in theoretical physics, undensity might be invoked when discussing the properties of voids in the cosmic web, which are vast regions containing very few galaxies.

The opposite of undensity is density, which describes the mass of a substance per unit volume. Extremely low-density materials, such as aerogels, exhibit properties that might be informally referred to as undense. These materials are largely composed of air, making them exceptionally lightweight and porous.

In the realm of science fiction, undensity might be explored as a means of propulsion, where a craft creates a region of low density behind it, or as a defensive mechanism, where a ship generates a field of undensity to deflect incoming projectiles. However, within established scientific frameworks, the term lacks a precise definition or measurable quantity. The concept of a vacuum, representing the absence of matter, is the closest established scientific notion to undensity, though a perfect vacuum is difficult to achieve and may still contain quantum fluctuations. The term undensity remains largely conceptual and is not a standard term in physics or chemistry.