In theoretical

general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. ...

, a geon is a nonsingular

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...

gravitational wave Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in ...

which is held together in a confined region by the gravitational attraction of its own field

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

. They were first investigated theoretically in 1955 by J. A. Wheeler, who coined the term as a contraction of "gravitational electromagnetic entity".

Overview

Since general relativity is a

classical field theory A classical field theory is a physical theory that predicts how one or more physical fields interact with matter through field equations, without considering effects of quantization; theories that incorporate quantum mechanics are called quantu ...

, Wheeler's concept of a geon does not treat them as

quantum-mechanical Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, qua ...

entities, and this generally remains true today. Nonetheless, Wheeler speculated that there might be a relationship between geons and

elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, antiq ...

s. This idea continues to attract some attention among physicists, but in the absence of a viable theory of

quantum gravity Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the v ...

, the accuracy of this speculative idea cannot be tested. Wheeler did not present explicit geon solutions to the vacuum

Einstein field equation In the general theory of relativity, the Einstein field equations (EFE; also known as Einstein's equations) relate the geometry of spacetime to the distribution of matter within it. The equations were published by Einstein in 1915 in the form ...

, a gap which was partially filled by Brill and

Hartle Hartle is a surname. Notable people with the surname include: * Adam Hartle (born 1979), an American comedian * Barry Hartle (born 1939), an English footballer * Dean Hartle (1931-2018), an American politician * Enid Hartle (1935–2008), an Englis ...

in 1964 by the Brill–Hartle geon. In 1997, Anderson and Brill gave a rigorous proof that geon solutions of the vacuum Einstein equation exist, though they are not given in a simple closed form. A major outstanding question regarding geons is whether they are ''stable'', or must decay over time as the energy of the wave gradually "leaks" away. This question has not yet been definitively answered, but the consensus seems to be that they probably cannot be stable. This would lay to rest Wheeler's initial hope that a geon might serve as a classical model for stable elementary particles. However, this would not rule out the possibility that geons are stabilized by quantum effects. In fact, a quantum generalization of the gravitational geon using low-energy

shows that geons are stable systems even when quantum effects are turned on.. The quantum geon (called "graviball") is described as gravitons bound by their gravitational self-interaction. Since geons (classical or quantum) have a mass but are electromagnetically neutral, they are possible candidates for

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

Overview

See also

References

Further reading