35,369 research outputs found
Apparent Superluminal Behavior
The apparent superluminal propagation of electromagnetic signals seen in
recent experiments is shown to be the result of simple and robust properties of
relativistic field equations. Although the wave front of a signal passing
through a classically forbidden region can never move faster than light, an
attenuated replica of the signal is reproduced ``instantaneously'' on the other
side of the barrier. The reconstructed signal, causally connected to the
forerunner rather than the bulk of the input signal, appears to move through
the barrier faster than light.Comment: 8 pages, no figure
Mode summation approach to Casimir effect between two objects
In this paper, we explore the TGTG formula from the perspective of mode
summation approach. Both scalar fields and electromagnetic fields are
considered. In this approach, one has to first solve the equation of motion to
find a wave basis for each object. The two T's in the TGTG formula are
T-matrices representing the Lippmann-Schwinger T-operators, one for each of the
objects. The two G's in the TGTG formula are the translation matrices, relating
the wave basis of an object to the wave basis of the other object. After
discussing the general theory, we apply the prescription to derive the explicit
formulas for the Casimir energies for the sphere-sphere, sphere-plane,
cylinder-cylinder and cylinder-plane interactions. First the T-matrices for a
plane, a sphere and a cylinder are derived for the following cases: the object
is imposed with general Robin boundary conditions; the object is
semitransparent; and the object is magnetodielectric. Then the operator
approach is used to derive the translation matrices. From these, the explicit
TGTG formula for each of the scenarios can be written down. Besides summarizing
all the TGTG formulas that have been derived so far, we also provide the TGTG
formulas for some scenarios that have not been considered before.Comment: 42 page
Stimulated Raman adiabatic passage analogs in classical physics
Stimulated Raman adiabatic passage (STIRAP) is a well established technique
for producing coherent population transfer in a three-state quantum system. We
here exploit the resemblance between the Schrodinger equation for such a
quantum system and the Newton equation of motion for a classical system
undergoing torque to discuss several classical analogs of STIRAP, notably the
motion of a moving charged particle subject to the Lorentz force of a
quasistatic magnetic field, the orientation of a magnetic moment in a slowly
varying magnetic field, the Coriolis effect and the inertial frame dragging
effect. Like STIRAP, those phenomena occur for counterintuitively ordered field
pulses and are robustly insensitive to small changes in the interaction
properties
Performance, Politics and Media: How the 2010 British General Election leadership debates generated ‘talk’ amongst the electorate.
During the British General Election 2010 a major innovation was introduced in part to improve engagement: a series of three live televised leadership debates took place where the leader of each of the three main parties, Labour, Liberal Democrat and Conservative, answered questions posed by members of the public and subsequently debated issues pertinent to the questions. In this study we consider these potentially ground breaking debates as the kind of event that was likely to generate discussion. We investigate various aspects of the ‘talk’ that emerged as a result of watching the debates. As an exploratory study concerned with situated accounts of the participants experiences we take an interpretive perspective. In this paper we outline the meta-narratives (of talk) associated with the viewing of the leadership debates that were identified, concluding our analysis by suggesting that putting a live debate on television and promoting and positioning it as a major innovation is likely to mean that is how the audience will make sense of it – as a media event
Heat conduction in the disordered harmonic chain revisited
A general formulation is developed to study heat conduction in disordered
harmonic chains with arbitrary heat baths that satisfy the
fluctuation-dissipation theorem. A simple formal expression for the heat
current J is obtained, from which its asymptotic system-size (N) dependence is
extracted. It is shown that the ``thermal conductivity'' depends not just on
the system itself but also on the spectral properties of the fluctuation and
noise used to model the heat baths. As special cases of our heat baths we
recover earlier results which reported that for fixed boundaries , while for free boundaries . For other choices we
find that one can get other power laws including the ``Fourier behaviour'' .Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Energy Density-Flux Correlations in an Unusual Quantum State and in the Vacuum
In this paper we consider the question of the degree to which negative and
positive energy are intertwined. We examine in more detail a previously studied
quantum state of the massless minimally coupled scalar field, which we call a
``Helfer state''. This is a state in which the energy density can be made
arbitrarily negative over an arbitrarily large region of space, but only at one
instant in time. In the Helfer state, the negative energy density is
accompanied by rapidly time-varying energy fluxes. It is the latter feature
which allows the quantum inequalities, bounds which restrict the magnitude and
duration of negative energy, to hold for this class of states. An observer who
initially passes through the negative energy region will quickly encounter
fluxes of positive energy which subsequently enter the region. We examine in
detail the correlation between the energy density and flux in the Helfer state
in terms of their expectation values. We then study the correlation function
between energy density and flux in the Minkowski vacuum state, for a massless
minimally coupled scalar field in both two and four dimensions. In this latter
analysis we examine correlation functions rather than expectation values.
Remarkably, we see qualitatively similar behavior to that in the Helfer state.
More specifically, an initial negative energy vacuum fluctuation in some region
of space is correlated with a subsequent flux fluctuation of positive energy
into the region. We speculate that the mechanism which ensures that the quantum
inequalities hold in the Helfer state, as well as in other quantum states
associated with negative energy, is, at least in some sense, already
``encoded'' in the fluctuations of the vacuum.Comment: 21 pages, 7 figures; published version with typos corrected and one
added referenc
Phase transition and hybrid star in a SU(2) chiral sigma model
We use a modified SU(2) chiral sigma model to study nuclear matter at high
density using mean field approach. We also study the phase transition of
nuclear matter to quark matter in the interior of highly dense neutron stars.
Stable solutions of Tolman-Oppenheimer-Volkoff equations representing hybrid
stars are obtained with a maximum mass of 1.69 , radii around 9.3
kms and a quark matter core constituting nearly 55-85 % of the star radii.Comment: 19 pages, 9 figures, accepted for Mod. Phys. Letts.
Monopole Oscillations and Dampings in Boson and Fermion Mixture in the Time-Dependent Gross-Pitaevskii and Vlasov Equations
We construct a dynamical model for the time evolution of the boson-fermion
coexistence system. The dynamics of bosons and fermions are formulated with the
time-dependent Gross-Pitaevsky equation and the Vlasov equation. We thus study
the monopole oscillation in the bose-fermi mixture. We find that large damping
exists for fermion oscillations in the mixed system even at zero temperature.Comment: 16 pages text and 12 figure
Canonical quantization of macroscopic electromagnetism
Application of the standard canonical quantization rules of quantum field
theory to macroscopic electromagnetism has encountered obstacles due to
material dispersion and absorption. This has led to a phenomenological approach
to macroscopic quantum electrodynamics where no canonical formulation is
attempted. In this paper macroscopic electromagnetism is canonically quantized.
The results apply to any linear, inhomogeneous, magnetodielectric medium with
dielectric functions that obey the Kramers-Kronig relations. The prescriptions
of the phenomenological approach are derived from the canonical theory.Comment: 21 pages, additional reference
Legacy data and cosmological constraints from the angular-size/redshift relation for ultra-compact radio sources
We have re-examined an ancient VLBI survey of ultra-comact radio sources at
2.29 GHz, which gave fringe amplitudes for 917 such objects with total flux
density >0.5 Jy approximately. A number of cosmological investigations based
upon this survey have been published in recent years. We have updated the
sample with respect to both redshift and radio information, and now have full
data for 613 objects, significantly larger than the number (337) used in
earlier investigations. The corresponding angular-size/redshift diagram gives
Omega_m=0.25+0.04/-0.03, Omega_\Lambda=0.97+0.09/-0.13 and K=0.22+0.07/-0.10.
In combination with supernova data, and a simple-minded approach to CMB data
based upon the angular size of the acoustic horizon, our best figures are
Omega_m=0.298+0.025/-0.024, Omega_\Lambda=0.702+0.035/-0.036 and K=
0.000+0.021/-0.019. We have examined simple models of dynamical vacuum energy;
the first, based upon a scalar potential V(phi)=omega_C^2 phi^2/2, gives
w(0)=-1.00+0.06/-0.00, (dw/dz)_0=+0.00/-0.08; in this case conditions at z=0
require particular attention, to preclude behaviour in which phi becomes
singular as z -->infinity. For fixed w limits are w=-1.20+0.15/-0.14. The above
error bars are 68% confidence limits.Comment: 24 pages, 9 figure
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