Geon (physics)
Encyclopedia
In theoretical
general relativity
, a geon is an electromagnetic
or gravitational wave
which is held together in a confined region by the gravitational attraction of its own field
energy
. They were first investigated theoretically in 1955 by J. A. Wheeler
, who coined the term as a contraction of "gravitational electromagnetic entity."
Since general relativity is a classical field theory
, Wheeler's concept of a geon does not treat them as quantum-mechanical
entities, and this generally remains true today. Nonetheless, Wheeler speculated that there might be a relationship between microscopic
geons and elementary particle
s. This idea continues to attract some attention among physicists, but in the absence of a viable theory of quantum gravity
, the accuracy of this speculative idea cannot be tested. However recently Sundance Bilson-Thompson
, Fotini Markopoulou, and Lee Smolin
, in the context of Loop Quantum Gravity
discovered some objects very similar to the Wheeler idea of geon.
Wheeler did not exhibit explicit geon solutions to the vacuum Einstein field equation, a gap which was partially filled by Brill and Hartle
in 1964 by the Brill-Hartle geon. This is an approximate solution which exhibits the features expected by Wheeler—at least temporarily. 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, which would lay to rest Wheeler's initial hope that a geon might serve as a classical model for stable elementary particles.
Theoretical physics
Theoretical physics is a branch of physics which employs mathematical models and abstractions of physics to rationalize, explain and predict natural phenomena...
general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, a geon is an electromagnetic
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
or gravitational wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
which is held together in a confined region by the gravitational attraction of its own field
Field (physics)
In physics, a field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector, a spinor or, more generally, a tensor,...
energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
. They were first investigated theoretically in 1955 by J. A. Wheeler
John Archibald Wheeler
John Archibald Wheeler was an American theoretical physicist who was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission...
, who coined the term as a contraction of "gravitational electromagnetic entity."
Since general relativity is a classical field theory
Classical field theory
A classical field theory is a physical theory that describes the study of how one or more physical fields interact with matter. The word 'classical' is used in contrast to those field theories that incorporate quantum mechanics ....
, Wheeler's concept of a geon does not treat them as quantum-mechanical
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
entities, and this generally remains true today. Nonetheless, Wheeler speculated that there might be a relationship between microscopic
Microscopic
The microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...
geons and elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s. This idea continues to attract some attention among physicists, but in the absence of a viable theory of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
, the accuracy of this speculative idea cannot be tested. However recently Sundance Bilson-Thompson
Sundance Bilson-Thompson
Sundance Osland Bilson-Thompson is an Australian theoretical particle physicist. He has developed the idea that certain preon models may be represented topologically, rather than by treating preons as pointlike particles. His ideas have attracted interest in the field of loop quantum gravity, as...
, Fotini Markopoulou, and Lee Smolin
Lee Smolin
Lee Smolin is an American theoretical physicist, a researcher at the Perimeter Institute for Theoretical Physics, and an adjunct professor of physics at the University of Waterloo. He is married to Dina Graser, a communications lawyer in Toronto. His brother is David M...
, in the context of Loop Quantum Gravity
Loop quantum gravity
Loop quantum gravity , also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the theories of quantum mechanics and general relativity...
discovered some objects very similar to the Wheeler idea of geon.
Wheeler did not exhibit explicit geon solutions to the vacuum Einstein field equation, a gap which was partially filled by Brill and Hartle
James Hartle
James Burkett Hartle is an American physicist. He has been a professor of physics at the University of California, Santa Barbara since 1966, and he is currently a member of the external faculty of the Santa Fe Institute...
in 1964 by the Brill-Hartle geon. This is an approximate solution which exhibits the features expected by Wheeler—at least temporarily. 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, which would lay to rest Wheeler's initial hope that a geon might serve as a classical model for stable elementary particles.