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### Einstein's Theory Of Gravitation | Center For Astrophysics

Our modern understanding of gravity comes from Albert Einstein's theory of general relativity, which stands as one of the best-tested theories in science.
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### Gravity - Gravitational Theory And Other Aspects Of Physical Theory ...

The Newtonian theory of gravity is based on an assumed force acting between all pairs of bodies—i.e., an action at a distance. When a mass moves, the force acting on other masses had been considered to adjust instantaneously to the new location of the displaced mass. That, however, is inconsistent with special relativity, which is based on the axiom that all knowledge of distant events comes from electromagnetic signals. Physical quantities have to be defined in such a way that certain combinations of them—in particular, distance, time, mass, and momentum—are independent of choice of space-time coordinates. This theory, with the The Newtonian theory of gravity is based on an assumed force acting between all pairs of bodies—i.e., an action at a distance. When a mass moves, the force ...
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### Quantum Theory Of Gravity. I. The Canonical Theory

Following an historical introduction, the conventional canonical formulation of general relativity theory is presented. The canonical Lagrangian is expressed in terms of the extrinsic and intrinsic curvatures of the hypersurface ${x}^{0}=\mathrm{constant}$, and its relation to the asymptotic field energy in an infinite world is noted. The distinction between finite and infinite worlds is emphasized. In the quantum theory the primary and secondary constraints become conditions on the state vector, and in the case of finite worlds these conditions alone govern the dynamics. A resolution of the factor-ordering problem is proposed, and the consistency of the constraints is demonstrated. A 6-dimensional hyperbolic Riemannian manifold is introduced which takes for its metric the coefficient of the momenta in the Hamiltonian constraint. The geodesic incompletability of this manifold, owing to the existence of a frontier of infinite curvature, is demonstrated. The possibility is explored of relating this manifold to In the quantum theory the primary and secondary constraints become ... The classical phenomenon of gravitational collapse shows that the force term is not ...
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### Is Gravity A Theory Or A Law? | The Happy Scientist

"Every point mass attracts every single point mass by a force pointing along the line intersecting both points. The force is directly proportional to the ...
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### Gravitational Theories

A theory of gravitation is a description of the long range forces that electrically neutral bodies exert on one another because of their matter content.
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### Stochastic Gravity: Theory And Applications

Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel. In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime, compute the two-point
correlation functions of these perturbations and prove that Minkowski spacetime
is a stable solution of semiclassical gravity. Second, we discuss structure
formation from the stochastic gravity viewpoint. Third, we discuss the
backreaction of Hawking radiation in the gravitational background of a black
hole and describe the metric fluctuations near the event horizon of an
evaporating black hole Feb 5, 2008 ... ... semiclassical gravity is based on the Einstein-Langevin equation, ... we describe three applications of stochastic gravity theory.
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### A Gravity Theory On Noncommutative Spaces

A deformation of the algebra of diffeomorphisms is constructed for
canonically deformed spaces with constant deformation parameter theta. The
algebraic relations remain the same, whereas the comultiplication rule (Leibniz
rule) is different from the undeformed one. Based on this deformed algebra a
covariant tensor calculus is constructed and all the concepts like metric,
covariant derivatives, curvature and torsion can be defined on the deformed
space as well. The construction of these geometric quantities is presented in
detail. This leads to an action invariant under the deformed diffeomorphism
algebra and can be interpreted as a theta-deformed Einstein-Hilbert action. The
metric or the vierbein field will be the dynamical variable as they are in the
undeformed theory. The action and all relevant quantities are expanded up to
second order in theta. Aug 16, 2005 ... Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Mathematical Physics (math-ph).
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### How A Weird Theory Of Gravity Could Break Cause-and-effect | Live ...

May 21, 2021 ... While most astronomers believe that dark matter exists, some still think that we need to modify our theory of gravity. However, new research has ...
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### Semiclassical Gravity Theory And Quantum Fluctuations

We discuss the limits of validity of the semiclassical theory of gravity in
which a classical metric is coupled to the expectation value of the stress
tensor. It is argued that this theory is a good approximation only when the
fluctuations in the stress tensor are small. We calculate a dimensionless
measure of these fluctuations for a scalar field on a flat background in
particular cases, including squeezed states and the Casimir vacuum state. It is
found that the fluctuations are small for states which are close to a coherent
state, which describes classical behavior, but tend to be large otherwise. We
find in all cases studied that the energy density fluctuations are large
whenever the local energy density is negative. This is taken to mean that the
gravitational field of a system with negative energy density, such as the
Casimir vacuum, is not described by a fixed classical metric but is undergoing
large metric fluctuations. We propose an operational scheme by which one can
describe a fluctuating gravitati Apr 6, 1993 ... We discuss the limits of validity of the semiclassical theory of gravity in which a classical metric is coupled to the expectation value of the ...
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### Effective Field Theory For Massive Gravitons And Gravity In Theory ...

We introduce a technique for restoring general coordinate invariance into
theories where it is explicitly broken. This is the analog for gravity of the
Callan-Coleman-Wess-Zumino formalism for gauge theories. We use this to
elucidate the properties of interacting massless and massive gravitons. For a
single graviton with a Planck scale Mpl and a mass mg, we find that there is a
sensible effective field theory which is valid up to a high-energy cutoff
Lambda parametrically above mg. Our methods allow for a transparent
understanding of the many peculiarities associated with massive gravitons,
among them the need for the Fierz-Pauli form of the Lagrangian, the presence or
absence of the van Dam-Veltman-Zakharov discontinuity in general backgrounds,
and the onset of non-linear effects and the breakdown of the effective theory
at large distances from heavy sources. The natural sizes of all non-linear
corrections beyond the Fierz-Pauli term are easily determined. The cutoff
scales as Lambda ~ (mg^4 Mpl)^(1/5) for t This is the analog for gravity of the Callan-Coleman-Wess-Zumino formalism for gauge theories. We use this to elucidate the properties of interacting massless ...
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### Gravity: It's Only A Theory | National Center For Science Education

Furthermore, gravity theory suggests that the planets have been moving in orderly orbits for millions and millions of years, which wholly contradicts the Second ...
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