771 research outputs found

### The fundamental role of the retarded potential in the electrodynamics of superluminal sources

We calculate the gradient of the radiation field generated by a polarization
current with a superluminally rotating distribution pattern and show that the
absolute value of this gradient increases as R^(7/2) with distance R within the
sharply focused subbeams constituting the overall radiation beam. This result
not only supports the earlier finding that the azimuthal and polar widths of
these subbeams narrow with distance (as R^(-3) and R^(-1), respectively), but
also implies that the boundary contribution to the solution of the wave
equation governing the radiation field does not always vanish in the limit
where the boundary tends to infinity. There is a fundamental difference between
the classical expressions for the retarded potential and field: while the
boundary contribution for the potential can always be made zero via a gauge
transformation preserving the Lorenz condition, that for the field may be
neglected only if it diminishes with distance faster than the contribution of
the source density in the far zone. In the case of a rotating superluminal
source, however, the boundary term in the retarded solution for the field is by
a factor of order R^(1/2) larger than the source term of this solution in the
limit, which explains why an argument based on the solution of the wave
equation governing the field that neglects the boundary term (such as that
presented by J. H. Hannay) misses the nonspherical decay of the field. Given
that the distribution of the radiation field of an accelerated superluminal
source in the far zone is not known a priori, the only way to calculate the
free-space radiation field of such sources is via the retarded solution for the
potential. Finally, we apply these findings to pulsar observational data: the
more distant a pulsar, the narrower and brighter its giant pulses should be

### Spectral properties of the nonspherically decaying radiation generated by a rotating superluminal source

The focusing of the radiation generated by a polarization current with a
superluminally rotating distribution pattern is of a higher order in the plane
of rotation than in other directions. Consequently, our previously published
asymptotic approximation to the value of this field outside the equatorial
plane breaks down as the line of sight approaches a direction normal to the
rotation axis, i.e., is nonuniform with respect to the polar angle. Here we
employ an alternative asymptotic expansion to show that, though having a rate
of decay with frequency (mu) that is by a factor of order mu^(2/3) slower, the
equatorial radiation field has the same dependence on distance as the
nonspherically decaying component of the generated field in other directions:
it, too, diminishes as the inverse square root of the distance from its source.
We also briefly discuss the relevance of these results to the giant pulses
received from pulsars: the focused, nonspherically decaying pulses that arise
from a superluminal polarization current in a highly magnetized plasma have a
power-law spectrum (i.e., a flux density proportional to mu^alpha) whose index
(alpha) is given by one of the values -2/3, -2, -8/3, or -4

### A new mechanism for generating broadband pulsar-like polarization

Observational data imply the presence of superluminal electric currents in
pulsar magnetospheres. Such sources are not inconsistent with special
relativity; they have already been created in the laboratory. Here we describe
the distinctive features of the radiation beam that is generated by a rotating
superluminal source and show that (i) it consists of subbeams that are narrower
the farther the observer is from the source: subbeams whose intensities decay
as 1/R instead of 1/R^2 with distance (R), (ii) the fields of its subbeams are
characterized by three concurrent polarization modes: two modes that are
'orthogonal' and a third mode whose position angle swings across the subbeam
bridging those of the other two, (iii) its overall beam consists of an
incoherent superposition of such coherent subbeams and has an intensity profile
that reflects the azimuthal distribution of the contributing part of the source
(the part of the source that approaches the observer with the speed of light
and zero acceleration), (iv) its spectrum (the superluminal counterpart of
synchrotron spectrum) is broader than that of any other known emission and
entails oscillations whose spacings and amplitudes respectively increase and
decrease algebraically with increasing frequency, and (v) the degree of its
mean polarization and the fraction of its linear polarization both increase
with frequency beyond the frequency for which the observer falls within the
Fresnel zone. We also compare these features with those of the radiation
received from the Crab pulsar.Comment: 8 pages, 8 figure

### Morphology of the nonspherically decaying radiation generated by a rotating superluminal source: reply to comment

The fact that the formula used by Hannay in his Comment is "from a standard
text on electrodynamics" neither warrants that it is universally applicable,
nor that it is unequivocally correct. We have explicitly shown [J. Opt. Soc.
Am. A 25, 543 (2008)] that,since it does not include the boundary contribution
toward the value of the field, the formula in question is not applicable when
the source is extended and has a distribution pattern that rotates faster than
light in vacuo. The neglected boundary term in the retarded solution to the
wave equation governing the electromagnetic field forms the basis of
diffraction theory. If this term were identically zero, for the reasons given
by Hannay, the iffraction of electromagnetic waves through apertures on a
surface enclosing a source would have been impossible. If this term were
identically zero, for the reasons given by Hannay, the diffraction of
electromagnetic waves through apertures on a surface enclosing a source would
have been impossible

### Inadequacies in the conventional treatment of the radiation field of moving sources

There is a fundamental difference between the classical expression for the
retarded electromagnetic potential and the corresponding retarded solution of
the wave equation that governs the electromagnetic field. While the boundary
contribution to the retarded solution for the {\em potential} can always be
rendered equal to zero by means of a gauge transformation that preserves the
Lorenz condition, the boundary contribution to the retarded solution of the
wave equation governing the {\em field} may be neglected only if it diminishes
with distance faster than the contribution of the source density in the far
zone. In the case of a source whose distribution pattern both rotates and
travels faster than light {\em in vacuo}, as realized in recent experiments,
the boundary term in the retarded solution governing the field is by a factor
of the order of $R^{1/2}$ {\em larger} than the source term of this solution in
the limit that the distance $R$ of the boundary from the source tends to
infinity. This result is consistent with the prediction of the retarded
potential that part of the radiation field generated by a rotating superluminal
source decays as $R^{-1/2}$, instead of $R^{-1}$, a prediction that is
confirmed experimentally. More importantly, it pinpoints the reason why an
argument based on a solution of the wave equation governing the field in which
the boundary term is neglected (such as appears in the published literature)
misses the nonspherical decay of the field

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### Fundamental role of the retarded potential in the electrodynamics of superluminal sources: reply to comment

Neither Eq. (6.52) of Jackson [Classical Electrodynamics, 3rd ed. (Wiley, 1999)], or Hannay's derivation of that dquation in the preceding Comment [J. Opt. Soc. Am. A, ... (2009)], are applicable to a source whose distribution pattern moves faster than light in vacuo with nonzero acceleration. It is assumed in Hannay's derivation that the retarded distribution of the density of any moving source would be smooth and differentiable if its rest-frame distribution is. By working out an explicit example of a rotating superluminal source with a bounded and smooth density profile, we show that this assumption is erroneous. The retarded distribution of a rotating source with a moderate superluminal speed is, in general, spread over three disjoint volumes (differing in shape from each other and from the volume occupied by the source in its rest frame) whose boundaries depend on the spacetime position of the observer. Hannay overlooks the fact that the limits of integration in his expression for the retarded potential (which delineate the boundaries of the retarded distribution of the source) are not differentiable functions of the coordinates of the observer at those points on the source boundary that approach the observer, along the radiation direction, with the speed of light at the retarded time. In the superluminal regime, derivatives of the integral representing the retarded potential are well defined only as generalized functions

### DNA methylation predicts age and provides insight into exceptional longevity of bats

This work was supported by a Paul G. Allen Frontiers Group grant to S.H., the University of Maryland, College of Computer, Mathematical and Natural Sciences to G.S.W., an Irish Research Council Consolidator Laureate Award to E.C.T., a UKRI Future Leaders Fellowship (MR/T021985/1) to S.C.V. and a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada to P.A.F. S.C.V. and P.D. were supported by a Max Planck Research Group awarded to S.C.V. by the Max Planck Gesellschaft, and S.C.V. and E.Z.L. were supported by a Human Frontiers Science Program Grant (RGP0058/2016) awarded to S.C.V. L.J.G. was supported by an NSERC PGS-D scholarship.Exceptionally long-lived species, including many bats, rarely show overt signs of aging, making it difficult to determine why species differ in lifespan. Here, we use DNA methylation (DNAm) profiles from 712 known-age bats, representing 26 species, to identify epigenetic changes associated with age and longevity. We demonstrate that DNAm accurately predicts chronological age. Across species, longevity is negatively associated with the rate of DNAm change at age-associated sites. Furthermore, analysis of several bat genomes reveals that hypermethylated age- and longevity-associated sites are disproportionately located in promoter regions of key transcription factors (TF) and enriched for histone and chromatin features associated with transcriptional regulation. Predicted TF binding site motifs and enrichment analyses indicate that age-related methylation change is influenced by developmental processes, while longevity-related DNAm change is associated with innate immunity or tumorigenesis genes, suggesting that bat longevity results from augmented immune response and cancer suppression.Publisher PDFPeer reviewe

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