Geon

Geon is a term in theoretical physics used to describe a self-contained concentration of energy held together by its own gravitational and electromagnetic fields. The concept originated with John Archibald Wheeler in 1957 as a gravitational geon, a localized bundle of gravitational waves bound by spacetime curvature. The idea was later extended to include electromagnetic energy, leading to the notion of an electromagnetic geon. In the broad sense, a geon is a non-singular, localized wave packet of fields that could, in principle, propagate without dispersion because the field energy is held in place by the fields themselves.

In models of geons, the configuration is formed as a self-consistent solution of the coupled field equations, typically the Einstein-Maxwell equations for an electromagnetic geon or the Einstein equations for a gravitational geon. Geons are often envisioned as finite-energy, localized regions of space where the field energy does not radiate away instantly. However, such configurations are highly idealized.

Stability: In classical general relativity, geons are generally considered unstable. Small perturbations can cause the system to shed energy via gravitational or electromagnetic radiation, leading to dispersion. Despite extensive study, no stable, stationary geon has been demonstrated in realistic spacetimes; many results suggest geons would gradually decay, making them transient or quasi-stationary at best.

Relation to other concepts: The geon idea influenced later ideas about non-singular field configurations, solitons and compact objects like boson stars, though geons differ in relying on field energy of the gravitational and/or electromagnetic field rather than matter fields. The term is primarily of historical and theoretical interest in the study of nonlinear field dynamics in curved spacetime.