10,755 research outputs found
Principles of Discrete Time Mechanics: IV. The Dirac Equation, Particles and Oscillons
We apply the principles of discrete time mechanics discussed in earlier
papers to the first and second quantised Dirac equation. We use the Schwinger
action principle to find the anticommutation relations of the Dirac field and
of the particle creation operators in the theory. We find new solutions to the
discrete time Dirac equation, referred to as oscillons on account of their
extraordinary behaviour. Their principal characteristic is that they oscillate
with a period twice that of the fundamental time interval T of our theory.
Although these solutions can be associated with definite charge, linear
momentum and spin, such objects should not be observable as particles in the
continuous time limit. We find that for non-zero T they correspond to states
with negative squared norm in Hilbert space. However they are an integral part
of the discrete time Dirac field and should play a role in particle
interactions analogous to the role of longitudinal photons in conventional
quantum electrodynamics.Comment: 27 pages LateX; published versio
Representation in Westminster in the 1990s : The ghost of Edmund Burke
Why are 'trustee' notions of representation still invoked in the UK House of Commons in the 1990s? In answering this question this article analyses the premises of Burkean theory and the arguments that these premises are of little relevance in the late twentieth century. Despite these dismissals of trusteeship, Burkean ideas are still articulated in the Commons some 200 years after they were first voiced. The idea of trusteeship can prove extremely useful to justify the actions of representatives when those actions conflict with constituency 'opinion', party policy or the wishes of interest groups. Examples of the occasions when Burkean notions have been invoked in the 1990s are provided
Evidence for polar jets as precursors of polar plume formation
Observations from the Hinode/XRT telescope and STEREO/SECCHI/EUVI are
utilized to study polar coronal jets and plumes. The study focuses on the
temporal evolution of both structures and their relationship. The data sample,
spanning April 7-8 2007, shows that over 90% of the 28 observed jet events are
associated with polar plumes. EUV images (STEREO/SECCHI) show plume haze rising
from the location of approximately 70% of the polar X-ray (Hinode/XRT) and EUV
jets, with the plume haze appearing minutes to hours after the jet was
observed. The remaining jets occurred in areas where plume material previously
existed causing a brightness enhancement of the latter after the jet event.
Short-lived, jet-like events and small transient bright points are seen (one at
a time) at different locations within the base of pre-existing long-lived
plumes. X-ray images also show instances (at least two events) of
collimated-thin jets rapidly evolving into significantly wider plume-like
structures that are followed by the delayed appearance of plume haze in the
EUV. These observations provide evidence that X-ray jets are precursors of
polar plumes, and in some cases cause brightenings of plumes. Possible
mechanisms to explain the observed jet and plume relationship are discussed.Comment: 10 pages, 4 figures, accepted as APJ Lette
Comment on ``Superconducting PrBa_2Cu_3O_x''
Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the
observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single
crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of
this Comment is to show the inconsistency of the value of effective magnetic
moment \mu_{eff} reported by Zou et al. (2.92\mu_B) with their magnetic
susceptibility data. The estimation made directly from their data points gives
a considerably smaller value of \mu_{eff}=2.09\mu_B. At the same time the
values of mu_{eff}=2.9\mu_B and 3.1\mu_B were obtained for our Pr123 single
crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This
suggests that Pr occupies only about a half of the RE sites in TSFZ crystal.
The other half of the RE sites is occupied most probably by the nonmagnetic Ba.
Noteworthy, SC with T_c=43 K was observed earlier for
Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius
than Pr^{3+} and so the substitution of Ba for Pr could give a natural
explanation not only for the SC in TSFZ Pr123 but also for the elongation of
the distance between the CuO_2 planes observed by Zou et al.Comment: Slightly extended version of Comment accepted to Phys. Rev. Lett.
(v.81, N24, 1998), tentatevely to be publ. 14Dec98. 1 page, REVTex; 1 EPS fi
Fly-by-light flight control system technology development plan
The results of a four-month, phased effort to develop a Fly-by-Light Technology Development Plan are documented. The technical shortfalls for each phase were identified and a development plan to bridge the technical gap was developed. The production configuration was defined for a 757-type airplane, but it is suggested that the demonstration flight be conducted on the NASA Transport Systems Research Vehicle. The modifications required and verification and validation issues are delineated in this report. A detailed schedule for the phased introduction of fly-by-light system components has been generated. It is concluded that a fiber-optics program would contribute significantly toward developing the required state of readiness that will make a fly-by-light control system not only cost effective but reliable without mitigating the weight and high-energy radio frequency related benefits
Energy Requirement of Control: Comments on Szilard's Engine and Maxwell's Demon
In mathematical physical analyses of Szilard's engine and Maxwell's demon, a
general assumption (explicit or implicit) is that one can neglect the energy
needed for relocating the piston in Szilard's engine and for driving the trap
door in Maxwell's demon. If this basic assumption is wrong, then the
conclusions of a vast literature on the implications of the Second Law of
Thermodynamics and of Landauer's erasure theorem are incorrect too. Our
analyses of the fundamental information physical aspects of various type of
control within Szilard's engine and Maxwell's demon indicate that the entropy
production due to the necessary generation of information yield much greater
energy dissipation than the energy Szilard's engine is able to produce even if
all sources of dissipation in the rest of these demons (due to measurement,
decision, memory, etc) are neglected.Comment: New, simpler and more fundamental approach utilizing the physical
meaning of control-information and the related entropy production. Criticism
of recent experiments adde
Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs
A theoretical study of photonic bands for one-dimensional (1D) lattices
embedded in planar waveguides with strong refractive index contrast is
presented. The approach relies on expanding the electromagnetic field on the
basis of guided modes of an effective waveguide, and on treating the coupling
to radiative modes by perturbation theory. Photonic mode dispersion, gap maps,
and intrinsic diffraction losses of quasi-guided modes are calculated for the
case of self-standing membranes as well as for Silicon-on-Insulator structures.
Photonic band gaps in a waveguide are found to depend strongly on the core
thickness and on polarization, so that the gaps for transverse electric and
transverse magnetic modes most often do not overlap. Radiative losses of
quasi-guided modes above the light line depend in a nontrivial way on structure
parameters, mode index and wavevector. The results of this study may be useful
for the design of integrated 1D photonic structures with low radiative losses.Comment: 9 pages, 8 figures, submitted to Physical Review
One, two, or three stars? An investigation of an unusual eclipsing binary candidate undergoing dramatic period changes
We report our investigation of 1SWASP J234401.81-212229.1, a variable with a 18 461.6 s period. After identification in a 2011 search of the SuperWASP archive for main-sequence eclipsing binary candidates near the distribution's short-period limit of ~0.20 d, it was measured to be undergoing rapid period decrease in our earlier work, though later observations supported a cyclic variation in period length. Spectroscopic data obtained in 2012 with the Southern African Large Telescope did not, however, support the interpretation of the object as a normal eclipsing binary. Here, we consider three possible explanations consistent with the data: a single-star oblique rotator model in which variability results from stable cool spots on opposite magnetic poles; a two-star model in which the secondary is a brown dwarf; and a three-star model involving a low-mass eclipsing binary in a hierarchical triple system. We conclude that the latter is the most likely model
Principles of Discrete Time Mechanics: II. Classical field Theory
We apply the principles discussed in an earlier paper to the construction of
discrete time field theories. We derive the discrete time field equations of
motion and Noether's theorem and apply them to the Schrodinger equation to
illustrate the methodology. Stationary solutions to the discrete time
Schrodinger wave equation are found to be identical to standard energy
eigenvalue solutions except for a fundamental limit on the energy. Then we
apply the formalism to the free neutral Klein Gordon system, deriving the
equations of motion and conserved quantities such as the linear momentum and
angular momentum. We show that there is an upper bound on the magnitude of
linear momentum for physical particle-like solutions. We extend the formalism
to the charged scalar field coupled to Maxwell's electrodynamics in a gauge
invariant way. We apply the formalism to include the Maxwell and Dirac fields,
setting the scene for second quantisation of discrete time mechanics and
discrete time Quantum Electrodynamics.Comment: 23 pages, LateX, To be published in J.Phys.A: Math.Gen: contact email
address: [email protected]
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