An old and seemingly discredited notion of ether has found its new incarnation in a "metric theory of gravity" by I. Schmelzer, who managed to find for his new ether a consistent and instructive interpretation in terms of condensed matter physics. Prof.V.A.Petrov

General Lorentz ether theory (GLET)

is a metric theory of gravity on Newtonian background with condensed matter interpretation and Lagrangian

L = -(8πG)^-1(Υg⁰⁰-Ξgⁱⁱ) (-g)^1/2 + L_GR(g_μν) + L_matter(g_μν,φ_m)

The Einstein equivalence principle holds exactly.
The preferred coordinates Xⁱ, T of the Newtonian background are harmonic.
The Einstein equations of GR in harmonic gauge appear in the natural limit Ξ,Υ→0.
The equations are identical to those of Logunov's "Relativistic Theory of Gravity" (RTG), but GLET has a less restrictive causality condition and allows different signs for the constants.

Papers for professionals:

A generalization of the Lorentz ether to gravity with general-relativistic limit (accepted for publication by the journal "Advances in Applied Clifford Algebras").

Appendix A of A Condensed Matter Interpretation of SM Fermions and Gauge Fields, arXiv:0908.0591, published in Foundations of Physics, vol. 39, nr. 1, p. 73 (2009) contains also a short introduction into GLET.

See also gr-qc/0205035, gr-qc/0001101, a quantum version of Einstein's hole argument and some solutions of GLET forbidden in RTG because of its causality condition.

The FAQ is, instead, written for laymen.

Condensed matter (ether) interpretation

In the preferred coordinates Xⁱ, T, we define a variant of the ADM decomposition. The metric g_μν(Xⁱ, T) decomposes into a scalar "ether density" ρ, an "ether velocity" vⁱ and "ether stress tensor" σ^ij:

g⁰⁰(-g)^¹⁄₂	=	ρ
gⁱ⁰(-g)^¹⁄₂	=	ρ vⁱ
g^ij(-g)^¹⁄₂	=	ρvⁱv^j - σ^ij

The harmonic equation

^∂⁄_∂X^μ g^μλ(-g)^¹⁄₂ ^∂⁄_∂X^λ X^ν = ^∂⁄_∂X^μ (g^μν(-g)^¹⁄₂) = 0

gives classical continuity and Euler equations for the ether variables:

^∂⁄_∂T ρ	+	^∂⁄_∂Xⁱ (ρvⁱ)	=	0
^∂⁄_∂T (ρv^j)	+	^∂⁄_∂Xⁱ (ρvⁱv^j-σ^ij)	=	0

The GLET Lagrangian can be derived from first principles of such an ether interpretation. The basic principle is that continuity and Euler equations of the ether appear as Noether conservation laws in some special variant of the Noether theorem.

Predictions

The limit Ξ, Υ → 0, gives the GR Einstein equations in harmonic coordinates. This allows to recover the predictions of GR which have been successfully tested (Solar system predictions, gravitational lenses, pulsar orbits). Nonetheless, even in this limit some qualitative differences remain:

A time-symmetric big bounce instead of the big bang, without big bang singularity, for arbitrary Υ > 0. This solves the horizon problem of GR cosmology.
Stable gravastars (or frozen stars) instead of black holes, with radius slightly greater than their Schwarzschild radius, for arbitrary Υ > 0.
The flat universe is preferred for symmetry reasons (different from GR).
The parameter Ξ leads to a cosmological term which may be used to fit some of the dark matter in cosmology.
The critical length, where an atomic ether structure becomes important, differs from the Planck length — it seems to increase together with the universe.