1,389,750 research outputs found
What is the reference frame of an accelerated observer?
The general construction of extended refrence frames for noninertial
observers in flat space is studied. It is shown that, if the observer moves
inertially before and after an arbitrary acceleration and rotation, the region
where reference frames can coincide with an inertial system is bounded for
final velocities exceeding 0.6 c.Comment: 5 pages, 2 figures, published in Phys. Lett. A 215, 1 (1996
To what extent is Gluon Confinement an empirical fact?
Experimental verifications of Confinement in hadron physics have established
the absence of charges with a fraction of the electron's charge by studying the
energy deposited in ionization tracks at high energies, and performing Millikan
experiments with charged droplets at rest. These experiments test only the
absence of particles with fractional charge in the asymptotic spectrum, and
thus "Quark" Confinement. However what theory suggests is that Color is
confined, that is, all asymptotic particles are color singlets. Since QCD is a
non-Abelian theory, the gluon force carriers (indirectly revealed in hadron
jets) are colored. We empirically examine what can be said about Gluon
Confinement based on the lack of detection of appropriate events, aiming at an
upper bound for high-energy free-gluon production.Comment: 14 pages, 12 figures, version accepted at Few Body Physic
What is the Brillouin Zone of an Anisotropic Photonic Crystal?
The concept of the Brillouin zone (BZ) in relation to a photonic crystal
fabricated in an optically anisotropic material is explored both experimentally
and theoretically. In experiment, we used femtosecond laser pulses to excite
THz polaritons and image their propagation in lithium niobate and lithium
tantalate photonic crystal (PhC) slabs. We directly measured the dispersion
relation inside PhCs and observed that the lowest bandgap expected to form at
the BZ boundary forms inside the BZ in the anisotropic lithium niobate PhC. Our
analysis shows that in an anisotropic material the BZ - defined as the
Wigner-Seitz cell in the reciprocal lattice - is no longer bounded by Bragg
planes and thus does not conform to the original definition of the BZ by
Brillouin. We construct an alternative Brillouin zone defined by Bragg planes
and show its utility in identifying features of the dispersion bands. We show
that for an anisotropic 2D PhC without dispersion, the Bragg plane BZ can be
constructed by applying the Wigner-Seitz method to a stretched or compressed
reciprocal lattice. We also show that in the presence of the dispersion in the
underlying material or in a slab waveguide, the Bragg planes are generally
represented by curved surfaces rather than planes. The concept of constructing
a BZ with Bragg planes should prove useful in understanding the formation of
dispersion bands in anisotropic PhCs and in selectively tailoring their optical
properties.Comment: 9 pages, 6 figure
What is an integrable quench?
Inspired by classical results in integrable boundary quantum field theory, we propose a definition of integrable initial states for quantum quenches in lattice models. They are defined as the states which are annihilated by all local conserved charges that are odd under space reflection. We show that this class includes the states which can be related to integrable boundary conditions in an appropriate rotated channel, in loose analogy with the picture in quantum field theory. Furthermore, we provide an efficient method to test integrability of given initial states. We revisit the recent literature of global quenches in several models and show that, in all of the cases where closed-form analytical results could be obtained, the initial state is integrable according to our definition. In the prototypical example of the XXZ spin-s chains we show that integrable states include two-site product states but also larger families of matrix product states with arbitrary bond dimension. We argue that our results could be practically useful for the study of quantum quenches in generic integrable models
What is an internal groupoid?
An answer to the question investigated in this paper brings a new
characterization of internal groupoids such that: (a) it holds even when finite
limits are not assumed to exist; (b) it is a full subcategory of the category
of involutive-2-links, that is, a category whose objects are morphisms equipped
with a pair of interlinked involutions. This result highlights the fact that
even thought internal groupoids are internal categories equipped with an
involution, they can equivalently be seen as tri-graphs with an involution.
Moreover, the structure of a tri-graph with an involution can be further
contracted into a simpler structure consisting of one morphism with two
interlinked involutions. This approach highly contrasts with the one where
groupoids are seen as reflexive graphs on which a multiplicative structure is
defined with inverses
What is the impact of stock market contagion on an investor's portfolio choice?
Stocks are exposed to the risk of sudden downward jumps. Additionally, a crash in one stock (or index) can increase the risk of crashes in other stocks (or indices). Our paper explicitly takes this contagion risk into account and studies its impact on the portfolio decision of a CRRA investor both in complete and in incomplete market settings. We find that the investor significantly adjusts his portfolio when contagion is more likely to occur. Capturing the time dimension of contagion, i.e. the time span between jumps in two stocks or stock indices, is thus of first-order importance when analyzing portfolio decisions. Investors ignoring contagion completely or accounting for contagion while ignoring its time dimension suffer large and economically significant utility losses. These losses are larger in complete than in incomplete markets, and the investor might be better off if he does not trade derivatives. Furthermore, we emphasize that the risk of contagion has a crucial impact on investors' security demands, since it reduces their ability to diversify their portfolios
What is a “Just” System for Financing Schools: An Evaluation of Alternative Reforms
Principles for public school finance are outlined with respect to an equitable allocation of educational resources by the state. The argument is advanced that equal dollars per pupil is a practical, reasonable, acceptable and attainable initial basis for school financing. Objections to the equal dollars scheme are considered, leading to an analysis which suggests that the appropriate policy choice for school finance reformers is enactment of full state financing of education
What is Fair Pay for Executives? An Information Theoretic Analysis of Wage Distributions
The high pay packages of U.S. CEOs have raised serious concerns about what
would constitute a fair pay.Comment: 16 page
What is the fate of a black hole embedded in an expanding universe?
Within a large class of exact solutions of the Einstein equations describing
a black hole embedded in a Friedmann universe it is shown that, under certain
assumptions, only those with comoving Hawking-Hayward quasi-local mass are
generic, in the sense that they are late-time attractors.Comment: 9 pages, LaTex, to appear in Phys. Lett.
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