1,173 research outputs found
Reply to ``Comment on Model-dependence of Shapiro time delay and the `speed of gravity/speed of light' controversy''
To determine whether the Shapiro time delay of light passing near a moving
object depends on the ``speed of gravity'' or the ``speed of light,'' one must
analyze observations in a bimetric framework in which these two speeds can be
different. In a recent comment (gr-qc/0510048), Kopeikin has argued that such a
computation -- described in gr-qc/0403060 -- missed a hidden dependence on the
speed of gravity. By analyzing the observables in the relevant bimetric model,
I show that this claim is incorrect, and that the conclusions of gr-qc/0403060
stand.Comment: 3 page reply to gr-qc/051004
Near-horizon Bondi-Metzner-Sachs symmetry, dimensional reduction, and black hole entropy
In an earlier short paper [Phys. Rev. Lett. 120, 101301 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.101301], I argued that the horizon-preserving diffeomorphisms of a generic black hole are enhanced to a larger three-dimensional Bondi-Metzner-Sachs symmetry, which is powerful enough to determine the Bekenstein-Hawking entropy. Here, I provide details and extensions of that argument, including a loosening of horizon boundary conditions and a more thorough treatment of dimensional reduction and meaning of a "near-horizon symmetry.
A Note on Black Hole Entropy in Loop Quantum Gravity
Several recent results have hinted that black hole thermodynamics in loop
quantum gravity simplifies if one chooses an imaginary Barbero-Immirzi
parameter . This suggests a connection with
or conformal field
theories at the "boundaries" formed by spin network edges intersecting the
horizon. I present a bit of background regarding the relevant conformal field
theories, along with some speculations about how they might be used to count
black hole states. I show, in particular, that a set of unproven but plausible
assumptions can lead to a boundary conformal field theory whose density of
states matches the Bekenstein-Hawking entropy.Comment: v2: added references; v3: slight addition to discussion of 3d
gravity; v4: more references, typos fixe
Four-Dimensional Entropy from Three-Dimensional Gravity
At the horizon of a black hole, the action of (3+1)-dimensional loop quantum
gravity acquires a boundary term that is formally identical to an action for
three-dimensional gravity. I show how to use this correspondence to obtain the
entropy of the (3+1)-dimensional black hole from well-understood conformal
field theory computations of the entropy in (2+1)-dimensional de Sitter space.Comment: 8 pages; v2: more references, typos fixed, minor rewording; v3: some
clearer explanations in response to referees, more reference
Kinetic Energy and the Equivalence Principle
According to the general theory of relativity, kinetic energy contributes to
gravitational mass. Surprisingly, the observational evidence for this
prediction does not seem to be discussed in the literature. I reanalyze
existing experimental data to test the equivalence principle for the kinetic
energy of atomic electrons, and show that fairly strong limits on possible
violations can be obtained. I discuss the relationship of this result to the
occasional claim that ``light falls with twice the acceleration of ordinary
matter.''Comment: 11 pages, LaTeX; pedagogical paper sent to archive at students'
reques
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