327 research outputs found
Ether theory of gravitation: why and how?
Gravitation might make a preferred frame appear, and with it a clear
space/time separation--the latter being, a priori, needed by quantum mechanics
(QM) in curved space-time. Several models of gravitation with an ether are
discussed: they assume metrical effects in an heterogeneous ether and/or a
Lorentz-symmetry breaking. One scalar model, starting from a semi-heuristic
view of gravity as a pressure force, is detailed. It has been developed to a
complete theory including continuum dynamics, cosmology, and links with
electromagnetism and QM. To test the theory, an asymptotic scheme of
post-Newtonian approximation has been built. That version of the theory which
is discussed here predicts an internal-structure effect, even at the
point-particle limit. The same might happen also in general relativity (GR) in
some gauges, if one would use a similar scheme. Adjusting the equations of
planetary motion on an ephemeris leaves a residual difference with it; one
should adjust the equations using primary observations. The same effects on
light rays are predicted as with GR, and a similar energy loss applies to
binary pulsars.Comment: Standard LaTeX, 60 pages. Invited contribution to the book ``Ether,
Spacetime and Cosmology" (M. C. Duffy, ed.), to appear at Hadronic Press. v2:
minor improvements, new refs., post-scriptum summarizing later wor
Towards an interpretation of MOND as a modification of inertia
We explore the possibility that Milgrom's Modified Newtonian Dynamics (MOND)
is a manifestation of the modification of inertia at small accelerations.
Consistent with the Tully-Fisher relation, dynamics in the small acceleration
domain may originate from a quartic (cubic) velocity-dependence of energy
(momentum) whereas gravitational potentials remain linear with respect to mass.
The natural framework for this interpretation is Finsler geometry. The simplest
static isotropic Finsler metric of a gravitating mass that incorporates the
Tully-Fisher relation at small acceleration is associated with a spacetime
interval that is either a homogeneous quartic root of polynomials of local
displacements or a simple root of a rational fraction thereof. We determine the
low energy gravitational equation and find that Finsler spacetimes that produce
a Tully-Fisher relation require that the gravitational potential be modified.
For an isolated mass, Newton's potential is replaced by where is MOND's acceleration scale and is a yet
undetermined distance scale. Orbital energy is linear with respect to mass but
angular momentum is proportional to . Asymptotic light deflection
resulting from time curvature is similar to that of a singular isothermal
sphere implying that space curvature must be the main source of deflection in
static Finsler spacetimes possibly through the presence of the distance scale
that appears in the asymptotic form of the gravitational potential. The
quartic nature of the Finsler metric hints at the existence of an underlying
area-metric that describes the effective structure of spacetime.Comment: Revised version, 9 pages, 1 figure. Accepted for publication in
Monthly Notices of the Royal Astronomical Societ
Computationally Tractable Riemannian Manifolds for Graph Embeddings
Representing graphs as sets of node embeddings in certain curved Riemannian
manifolds has recently gained momentum in machine learning due to their
desirable geometric inductive biases, e.g., hierarchical structures benefit
from hyperbolic geometry. However, going beyond embedding spaces of constant
sectional curvature, while potentially more representationally powerful, proves
to be challenging as one can easily lose the appeal of computationally
tractable tools such as geodesic distances or Riemannian gradients. Here, we
explore computationally efficient matrix manifolds, showcasing how to learn and
optimize graph embeddings in these Riemannian spaces. Empirically, we
demonstrate consistent improvements over Euclidean geometry while often
outperforming hyperbolic and elliptical embeddings based on various metrics
that capture different graph properties. Our results serve as new evidence for
the benefits of non-Euclidean embeddings in machine learning pipelines.Comment: Submitted to the Thirty-fourth Conference on Neural Information
Processing System
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