1,931,015 research outputs found
The existence of light-like homogeneous geodesics in homogeneous Lorentzian manifolds
In previous papers, a fundamental affine method for studying homogeneous
geodesics was developed. Using this method and elementary differential topology
it was proved that any homogeneous affine manifold and in particular any
homogeneous pseudo-Riemannian manifold admits a homogeneous geodesic through
arbitrary point. In the present paper this affine method is refined and adapted
to the pseudo-Riemannian case. Using this method and elementary topology it is
proved that any homogeneous Lorentzian manifold of even dimension admits a
light-like homogeneous geodesic. The method is illustrated in detail with an
example of the Lie group of dimension 3 with an invariant metric, which does
not admit any light-like homogeneous geodesic
Breaking the light speed barrier
As it is well known, classical special relativity allows the existence of
three different kinds of particles: bradyons, luxons and tachyons. Bradyons
have non-zero mass and hence always travel slower than light. Luxons are
particles with zero mass, like the photon, and they always travel with
invariant velocity. Tachyons are hypothetical superluminal particles that
always move faster than light. The existence of bradyons and luxons is firmly
established, while the tachyons were never reliably observed. In quantum field
theory, the appearance of tachyonic degrees of freedom indicates vacuum
instability rather than a real existence of the faster-than-light particles.
However, recent controversial claims of the OPERA experiment about superluminal
neutrinos triggered a renewed interest in superluminal particles. Driven by a
striking analogy of the old Frenkel-Kontorova model of a dislocation dynamics
to the theory of relativity, we conjecture in this note a remarkable
possibility of existence of the fourth type of particles, elvisebrions, which
can be superluminal. The characteristic feature of elvisebrions, distinguishing
them from tachyons, is that they are outside the realm of special relativity
and their energy remains finite (or may even turn to zero) when the elvisebrion
velocity approaches the light velocity.Comment: 37 pages, no figures, two last sections extended, to be published in
Acta Physica Polonica
Light sterile neutrinos
The theory and phenomenology of light sterile neutrinos at the eV mass scale
is reviewed. The reactor, Gallium and LSND anomalies are briefly described and
interpreted as indications of the existence of short-baseline oscillations
which require the existence of light sterile neutrinos. The global fits of
short-baseline oscillation data in 3+1 and 3+2 schemes are discussed, together
with the implications for beta-decay and neutrinoless double-beta decay. The
cosmological effects of light sterile neutrinos are briefly reviewed and the
implications of existing cosmological data are discussed. The review concludes
with a summary of future perspectives.Comment: 41 pages; final version to be published as a Topical Review in
Journal of Physics
On a characteristic initial value problem in Plasma physics
The relativistic Vlasov-Maxwell system of plasma physics is considered with
initial data on a past light cone. This characteristic initial value problem
arises in a natural way as a mathematical framework to study the existence of
solutions isolated from incoming radiation. Various consequences of the
mass-energy conservation and of the absence of incoming radiation condition are
first derived assuming the existence of global smooth solutions. In the
spherically symmetric case, the existence of a unique classical solution in the
future of the initial cone follows by arguments similar to the case of initial
data at time . The total mass-energy of spherically symmetric solutions
equals the (properly defined) mass-energy on backward and forward light cones.Comment: 16 pages. Version in pres
Light rings as observational evidence for event horizons: long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects
Ultracompact objects are self-gravitating systems with a light ring. It was
recently suggested that fluctuations in the background of these objects are
extremely long-lived and might turn unstable at the nonlinear level, if the
object is not endowed with a horizon. If correct, this result has important
consequences: objects with a light ring are black holes. In other words, the
nonlinear instability of ultracompact stars would provide a strong argument in
favor of the "black hole hypothesis," once electromagnetic or
gravitational-wave observations confirm the existence of light rings. Here we
explore in some depth the mode structure of ultracompact stars, in particular
constant-density stars and gravastars. We show that the existence of very
long-lived modes -- localized near a second, stable null geodesic -- is a
generic feature of gravitational perturbations of such configurations. Already
at the linear level, such modes become unstable if the object rotates
sufficiently fast to develop an ergoregion. Finally, we conjecture that the
long-lived modes become unstable under fragmentation via a
Dyson-Chandrasekhar-Fermi mechanism at the nonlinear level. Depending on the
structure of the star, it is also possible that nonlinearities lead to the
formation of small black holes close to the stable light ring. Our results
suggest that the mere observation of a light ring is a strong evidence for the
existence of black holes.Comment: 10 pages, RevTeX
Resonance states below pion-nucleon threshold and their consequences for nuclear systems
Regular sequences of narrow peaks have been observed in the missing mass
spectra in the reactions pp --> p pi^+ X and pd --> ppX_1 below pion-production
threshold. They are interpreted in the literature as manifestations of
supernarrow light dibaryons, or nucleon resonances, or light pions forming
resonance states with the nucleon in its ground state. We discuss how existence
of such exotic states would affect properties of nuclear systems. We show that
the neutron star structure is drastically changed in all three cases. We find
that in the presence of dibaryons or nucleon resonances the maximal possible
mass of a neutron star would be smaller than the observational limit. Presence
of light pions does not contradict the observed neutron star masses. Light
pions allow for the existence of extended nuclear objects of arbitrary size,
bound by strong and electromagnetic forces.Comment: preprint ECT*-02-18, 6 pages, 3 figure
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