Inspiral

An inspiral is the gradual inward spiraling of two orbiting bodies as they lose energy and angular momentum, typically due to dissipative processes. The term is most often used in astrophysics to describe the evolution of compact binary systems, such as pairs of black holes or neutron stars, as they draw closer over time.

In general relativity, the dominant mechanism for compact objects is the emission of gravitational waves. The radiation carries away energy and angular momentum, causing the orbital radius to shrink and the orbital frequency to rise. The inspiral produces a characteristic gravitational-wave signal that increases in amplitude and frequency, often described as a chirp. The phase continues until the objects approach merger, after which the plunge and ringdown occur. Modeling uses post-Newtonian approximations during the early phase and numerical relativity near merger.

Timescales depend on the masses, separation, and orbital eccentricity. For stellar-mass binaries, the inspiral can span long periods but accelerates dramatically as the system enters the sensitive gravitational-wave band, ultimately terminating in a merger. The Peters equations provide a mathematical description of how orbits decay due to gravitational radiation in the simplest cases.

Observationally, inspiral signals were first detected by the LIGO and Virgo collaborations in 2015 (GW150914) and have since been observed in multiple black-hole mergers and in the neutron-star merger GW170817, enabling tests of gravity and insights into the production of heavy elements. In other contexts, inspiral can describe objects sinking through an accretion disk or dynamical-friction-driven migration in galaxies or star clusters, where dissipative forces similarly drive inward motion.