coalescences

Coalescence is the process by which two or more distinct bodies, phases, or lineages merge to form a single entity. The term comes from the Latin coalescere, "to grow together." Coalescence occurs across disciplines, from fluid dynamics to genetics and astrophysics.

In fluids and materials science, droplet coalescence describes when nearby liquid droplets come into contact and merge due to surface tension, decreasing surface area. The process depends on viscosity, interfacial tension, and the intervening thin film between droplets; it is studied with capillarity theory, film rupture dynamics, and experimental imaging. Coalescence competes with stabilization mechanisms in emulsions and foams.

In population genetics, coalescent theory models the ancestral relationships of gene copies by tracing lineages backward in time until they meet at a common ancestor. Introduced by Kingman in the 1980s, it provides a probabilistic framework for inferring demographic history, migration, and selection from genetic data, often using coalescent times and genealogical trees.

In astronomy and gravitational wave science, coalescence denotes the inspiral and merger of compact objects such as black holes or neutron stars. The event produces gravitational waves and electromagnetic counterparts in some cases and is a key source for detectors like LIGO and Virgo. Analysis of coalescences informs relativistic physics, stellar evolution, and cosmology.