Black Star (semiclassical gravity)
Encyclopedia
A black star is a gravitational object composed of matter. It is a theoretical alternative to the black hole
concept from general relativity
. The theoretical construct was created through the use of semiclassical gravity
theory. A similar
structure should also exist for the Einstein-Maxwell-Dirac equations
system which is the (super)classical limit of quantum electrodynamics and for the Einstein-Yang-Mills-Dirac system which is the (super)classical limit of the standard model.
A black star need not have an event horizon
, and may or may not be a transitional phase between a collapsing star
and a singularity
. A black star is created when matter compresses at a rate significantly less than the freefall velocity of a hypothetical particle falling to the center of its star, because quantum processes create vacuum polarization
, which creates a form of degeneracy pressure, preventing spacetime
(and the particles held within it) from occupying the same space at the same time. This energy is theoretically unlimited, and if built up quickly enough, will stop gravitational collapse from creating a singularity. This may entail an ever-decreasing rate of collapse, leading to an infinite collapse time, or asymptotically approaching a radius less than zero.
A black star with a radius slightly greater than the predicted event horizon for an equivalent mass black hole will appear very dark, because almost all light produced will be drawn back to the star, and any escaping light will be severely gravitationally redshifted. It will appear almost exactly like a black hole. It will feature Hawking radiation
, as virtual particles created in its vicinity may still be split, with one particle escaping and the other being trapped. Additionally, it will create thermal Planckian radiation that will closely resemble the expected Hawking radiation of an equivalent black hole.
The predicted interior of a black star will be composed of this strange state of spacetime, with each length in depth heading inward appear the same as a black star of equivalent mass and radius with the overlayment stripped off. Temperatures increase with depth towards the centre.
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
concept from 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...
. The theoretical construct was created through the use of semiclassical gravity
Semiclassical gravity
Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical....
theory. A similar
structure should also exist for the Einstein-Maxwell-Dirac equations
Einstein-Maxwell-Dirac equations
Einstein-Maxwell-Dirac equations are related to quantum field theory. The current Big Bang Model is a quantum field theory in a curved spacetime. Unfortunately, no such theory is mathematically well-defined; in spite of this, theoreticians claim to extract information from this hypothetical theory...
system which is the (super)classical limit of quantum electrodynamics and for the Einstein-Yang-Mills-Dirac system which is the (super)classical limit of the standard model.
A black star need not have an event horizon
Event horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
, and may or may not be a transitional phase between a collapsing star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
and a singularity
Gravitational singularity
A gravitational singularity or spacetime singularity is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system...
. A black star is created when matter compresses at a rate significantly less than the freefall velocity of a hypothetical particle falling to the center of its star, because quantum processes create vacuum polarization
Vacuum polarization
In quantum field theory, and specifically quantum electrodynamics, vacuum polarization describes a process in which a background electromagnetic field produces virtual electron–positron pairs that change the distribution of charges and currents that generated the original electromagnetic...
, which creates a form of degeneracy pressure, preventing spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
(and the particles held within it) from occupying the same space at the same time. This energy is theoretically unlimited, and if built up quickly enough, will stop gravitational collapse from creating a singularity. This may entail an ever-decreasing rate of collapse, leading to an infinite collapse time, or asymptotically approaching a radius less than zero.
A black star with a radius slightly greater than the predicted event horizon for an equivalent mass black hole will appear very dark, because almost all light produced will be drawn back to the star, and any escaping light will be severely gravitationally redshifted. It will appear almost exactly like a black hole. It will feature Hawking radiation
Hawking radiation
Hawking radiation is a thermal radiation with a black body spectrum predicted to be emitted by black holes due to quantum effects. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after the physicist Jacob Bekenstein...
, as virtual particles created in its vicinity may still be split, with one particle escaping and the other being trapped. Additionally, it will create thermal Planckian radiation that will closely resemble the expected Hawking radiation of an equivalent black hole.
The predicted interior of a black star will be composed of this strange state of spacetime, with each length in depth heading inward appear the same as a black star of equivalent mass and radius with the overlayment stripped off. Temperatures increase with depth towards the centre.
Sources
- Carlos Barceló, Stefano Liberati, Sebastiano Sonego and Matt Visser, Scientific American (October 2009) Black Stars, Not Black Holes
- Visser, Matt; Barcelo, Carlos; Liberati, Stefano; Sonego, Sebastiano (2009) "Small, dark, and heavy: But is it a black hole?", ,