##
computable structure

Overview page. Subjects: Physics.

A mathematical structure for which a systematic algorithm exists for solving all problems with that structure. It has been postulated that fundamental physics should involve computable...

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cosmic censorship

Overview page. Subjects: Physics.

A hypothesis concerning singularities and black holes in the general theory of relativity. It was suggested in 1969 by the British physicist Roger Penrose (1931– ). The **cosmic censorship...**

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differential geometry

Overview page. Subjects: Physics — Mathematics.

The area of mathematics which uses differential calculus in the study of geometry. For example, to prove that the area of a circle is exactly *πr*
^{2}.

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General relativistic phenomena

Overview page. Subjects: Astronomy and Astrophysics.

A range of astrophysical phenomena are distinctively relativistic, and hence require general relativity. Most are dealt with elsewhere in this book. An incomplete list is

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graviton

Overview page. Subjects: Astronomy and Astrophysics — Physics.

A hypothetical particle or quantum of gravitational energy, predicted by the general theory of relativity. Gravitons have not been observed but are predicted to travel at the speed of light...

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holographic hypothesis

Overview page. Subjects: Physics.

A key principle of quantum gravity postulating that it is possible to obtain information about the bulk of a system, such as a black hole, from the surface of that system. This hypothesis...

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index theorems

Overview page. Subjects: Physics.

Theories in mathematics that relate the solutions of an equation to topology associated with the equation. There are many applications of index theorems to gauge theories, quantum gravity,...

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knot theory

Overview page. Subjects: Physics.

A branch of mathematics used to classify knots and entanglements. Knot theory has applications to the study of the properties of polymers and the statistical mechanics of certain models of...

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loop variable

Overview page. Subjects: Physics.

A quantity that is used to characterize field theories as an alternative to characterizing them in terms of fields and potentials. A loop variable is like a line of force or flux line that...

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Planck units

Overview page. Subjects: Physics.

A system of units, used principally in discussions of quantum theories of gravity, in which length, mass, and time are expressed as multiples of the Planck length, mass, and time...

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quantum cosmology

Overview page. Subjects: Astronomy and Astrophysics.

The study of the very earliest stages of the Universe after the Big Bang, including the properties of the initial singularity and of the Planck era in general. This field is highly...

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singularity

Overview page. Subjects: Astronomy and Astrophysics — Literature.

A mathematical point at which certain physical quantities reach infinite values. For example, according to general relativity, the curvature of spacetime becomes infinite in a black hole....

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spin network

Overview page. Subjects: Physics.

A quantum theory of space–time, first suggested by Roger Penrose in the 1960s, that classical space–time emerges from a network that is like a polymer network in rubber. The spin network...

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wormhole

Overview page. Subjects: Astronomy and Astrophysics — Physics.

A solution of Einstein's equation for the general theory of relativity that connects two distant regions in one asymptotically flat universe. These solutions have also been considered in...