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Particularly interesting is the fact that the vacuum, being a dynamical entity, gravitates, which using the concepts of general relativity (GR) means that it affects and is affected by the geometry of the spacetime. This rests at the root of the Hawking effect [4,5], according to which black holes should emit a thermal bath of particles.

Contrary to James Woodward's objection: The speed of light in vacuum index of refraction is mostly from the forward elastic scattering of photons off virtual electron positron pairs (Kramers-Kronig dispersion relations). In materials real on-shell charges contribute. As far gravity is concerned, the equivalence principle demands that off-shell virtual or on-shell real makes no difference to how active sources bend spacetime. Therefore, just as in graphene the effective emergent collective long wave fine structure constant is predicted to be (index of refraction)(vacuum EM coupling) ~ 2, so too, the effective emergent collective long wave gravity coupling should be (index of refraction)^4GNewton/c(vacuum)^4.

Background independence is simply gauge invariance under local T4(x) LNIF --> LNIF' frame transformations. Only the local field equations need obey it not their solutions + pre and post-selected boundary conditions from past and future event horizons etc.

On Oct 8, 2010, at 9:16 AM, JACK SARFATTI wrote:

Phys. Rev. Lett. 105, 151102 (2010) [4 pages]
Awaking the Vacuum in Relativistic Stars
Abstract
William C. C. Lima1,*, George E. A. Matsas2,†, and Daniel A. T. Vanzella1,‡
1Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 15980-900, São Carlos, SP, Brazil
2Instituto de Física Teórica, Universidade Estadual Paulista, Rua Dr. Bento Teobaldo Ferraz, 271 - Bl. II, 01140-070, São Paulo, SP, Brazil
Received 18 March 2010; published 7 October 2010

Void of any inherent structure in classical physics, the vacuum has revealed to be incredibly crowded with all sorts of processes in relativistic quantum physics. Yet, its direct effects are usually so subtle that its structure remains almost as evasive as in classical physics. Here, in contrast, we report on the discovery of a novel effect according to which the vacuum is compelled to play an unexpected central role in an astrophysical context. We show that the formation of relativistic stars may lead the vacuum energy density of a quantum field to an exponential growth. The vacuum-driven evolution which would then follow may lead to unexpected implications for astrophysics, while the observation of stable neutron-star configurations may teach us much on the field content of our Universe.
© 2010 The American Physical Society
URL:    http://link.aps.org/doi/10.1103/PhysRevLett.105.151102
DOI:    10.1103/PhysRevLett.105.151102
PACS:    04.40.Dg, 04.62.+v, 95.30.Sf
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