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Some Implications of the Cosmological Constant to Fundamental Physics
In the presence of a cosmological constant, ordinary Poincare' special
relativity is no longer valid and must be replaced by a de Sitter special
relativity, in which Minkowski space is replaced by a de Sitter spacetime. In
consequence, the ordinary notions of energy and momentum change, and will
satisfy a different kinematic relation. Such a theory is a different kind of a
doubly special relativity. Since the only difference between the Poincare' and
the de Sitter groups is the replacement of translations by certain linear
combinations of translations and proper conformal transformations, the net
result of this change is ultimately the breakdown of ordinary translational
invariance. From the experimental point of view, therefore, a de Sitter special
relativity might be probed by looking for possible violations of translational
invariance. If we assume the existence of a connection between the energy scale
of an experiment and the local value of the cosmological constant, there would
be changes in the kinematics of massive particles which could hopefully be
detected in high-energy experiments. Furthermore, due to the presence of a
horizon, the usual causal structure of spacetime would be significantly
modified at the Planck scale.Comment: 15 pages, lecture presented at the "XIIth Brazilian School of
Cosmology and Gravitation", Mangaratiba, Rio de Janeiro, September 10-23,
200
Cosmological Term and Fundamental Physics
A nonvanishing cosmological term in Einstein's equations implies a
nonvanishing spacetime curvature even in absence of any kind of matter. It
would, in consequence, affect many of the underlying kinematic tenets of
physical theory. The usual commutative spacetime translations of the Poincare'
group would be replaced by the mixed conformal translations of the de Sitter
group, leading to obvious alterations in elementary concepts such as time,
energy and momentum. Although negligible at small scales, such modifications
may come to have important consequences both in the large and for the
inflationary picture of the early Universe. A qualitative discussion is
presented which suggests deep changes in Hamiltonian, Quantum and Statistical
Mechanics. In the primeval universe as described by the standard cosmological
model, in particular, the equations of state of the matter sources could be
quite different from those usually introduced.Comment: RevTeX, 4 pages. Selected for Honorable Mention in the Annual Essay
Competition of the Gravity Research Foundation for the year 200
Improved Transverse Crack Detection in Composites
A modified ultrasonic C-scan technique was implemented for improving the detection of a certain type of damage in composite specimens. The type of damage being studied is transverse (through the thickness) cracking of unidirectional off-axis graphite-epoxy specimens. These cracks are difficult to detect using standard through-transmission C-scan techniques. The modification is based on mode conversion to produce transmitted shear waves from incident longitudinal waves. While mode conversion is used extensively with isotropic materials, its use with composites is more limited. This is largely because the computation of wave propagation parameters is significantly more complicated with highly anisotropic materials than with isotropic materials. The appropriate incident angles to produce the desired mode conversion were computed based on the mechanical properties of the composite. Once the angles were computed the technique was simple to implement and resulted in marked improvement in detection of the transverse cracks being studied
Gravity and the Quantum: Are they Reconcilable?
General relativity and quantum mechanics are conflicting theories. The seeds
of discord are the fundamental principles on which these theories are grounded.
General relativity, on one hand, is based on the equivalence principle, whose
strong version establishes the local equivalence between gravitation and
inertia. Quantum mechanics, on the other hand, is fundamentally based on the
uncertainty principle, which is essentially nonlocal in the sense that a
particle does not follow one trajectory, but infinitely many trajectories, each
one with a different probability. This difference precludes the existence of a
quantum version of the strong equivalence principle, and consequently of a
quantum version of general relativity. Furthermore, there are compelling
experimental evidences that a quantum object in the presence of a gravitational
field violates the weak equivalence principle. Now it so happens that, in
addition to general relativity, gravitation has an alternative, though
equivalent description, given by teleparallel gravity, a gauge theory for the
translation group. In this theory torsion, instead of curvature, is assumed to
represent the gravitational field. These two descriptions lead to the same
classical results, but are conceptually different. In general relativity,
curvature geometrizes the interaction, while torsion in teleparallel gravity
acts as a force, similar to the Lorentz force of electrodynamics. Because of
this peculiar property, teleparallel gravity describes the gravitational
interaction without requiring any of the equivalence principles. The
replacement of general relativity by teleparallel gravity may, in consequence,
lead to a conceptual reconciliation of gravitation with quantum mechanics.Comment: 15 pages, 2 figures. Talk presented at the conference "Quantum
Theory: Reconsideration of Foundations-3", June 6-11, 2005, Vaxjo University,
Vaxjo, Swede
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