8,480 research outputs found

    Energy-momentum tensor is nonsymmetric for spin-polarized photons

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    It has been assumed for a century that the energy-momentum tensor of the photon takes a symmetric form, with the renowned Poynting vector assigned as the same density for momentum and energy flow. Here we show that the symmetry of the photon energy-momentum tensor can actually be inferred from the known difference between the diffraction patterns of light with spin and orbital angular momentum, respectively. The conclusion is that the symmetric expression of energy-momentum tensor is denied, and the nonsymmetric canonical expression is favored.Comment: 3 pages, 1 figur

    Quantized electron transport by interference-induced quantum dots of two cross-travelling surface acoustic waves

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    In traditional approaches of obtaining quantized acoustoelectric current, a narrow channel is fabricated to form quantum dots, which hold a fixed number of electrons at a certain depth. We propose a natural way of forming quantum dots without the narrow channel, by the interference of two surface acoustic waves (SAWs) propagating across each other. A wide transportation area is defined by the usual (but widely separated) split-gate structure with another independent gate in between. This design can increase the quantized current by one to two orders of magnitude. The three-gate structure also allows separate control of the barrier height and the side-gate pinch-off voltage, thus avoids current leakage through the area beneath the side gates.Comment: 2 pages, 3 figure

    Co-existence of Gravity and Antigravity: The Unification of Dark Matter and Dark Energy

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    Massive gravity with second and fourth derivatives is shown to give both attractive and repulsive gravities. In contrast to the attractive gravity correlated with the energy-momentum tensor, the repulsive gravity is related to a fixed mass mxm_x, which equals a spin-dependent factor fσf_\sigma times the graviton mass. Therefore, particles with energy below mxm_x are both dark matter and dark energy: Their overall gravity is attractive with normal matter but repulsive among themselves. Detailed analyses reveal that this unified dark scenario can properly account for the observed dark matter/energy phenomena: galaxy rotation curves, transition from early cosmic deceleration to recent acceleration; and naturally overcome other dark scenarios' difficulties: the substructure and cuspy core problems, the difference of dark halo distributions in galaxies and clusters, and the cosmic coincidence. Very interestingly, Dirac particles have fσ=1/2f_\sigma=1/\sqrt 2, all bosonic matter particles have fσ=0f_\sigma=0, and the only exceptional boson is the graviton itself, which may have fσ>1f_\sigma>1.Comment: 3 pages, no figure; discussions added that low-energy gravitons can also serve as both dark matter and dark energy; references adde

    Poincar\'e subalgebra and gauge invariance in nucleon structure

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    By separating the gluon field into physical and pure-gauge components, the usual Poincar\'e subalgebra for an interacting system can be reconciled with gauge-invariance when decomposing the total rotation and translation generators of QCD into quark and gluon parts. The gauge-invariant quark/gluon parts act as the generators for the gauge-invariant physical component of the quark/gluon field, not the full quark/gluon field which also contains the gauge degrees of freedom. We clarify that the naive canonical decomposition of generators, while trivially respecting the Poincar\'e subalgebra, might not give a completely gauge-invariant quark-gluon structure of the nucleon momentum and spin, though limited invariance within a certain gauge class can be proven.Comment: 4 pages, no figure; presented at INT Workshop INT-12-49W "Orbital Angular Momentum in QCD", Seattle, February 6-17, 201

    Gauge invariance and hadron structure

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    We prove that the {\em gauge dependent} gluon spin, gluon and quark orbital angular momenta operators have {\em gauge invariant} expectation values on hadron states with {\em definite} momentum and polarization, therefore the conventional decomposition of nucleon spin into contributions from the spin and orbital angular momentum of quark and gluon is {\em gauge independent}. Similar conclusions apply to the {\em gauge dependent} quark momentum and kinetic energy operators, and accordingly nucleon momentum and mass structures.Comment: This revised version gives a complete, comprehensive proof of our conclusion, and explains why the specific calculations of Hoodbhoy, Ji and Lu (hep-ph/9808305) do not necessarily contradict our theore

    Spin content of the nucleon in a valence and sea quark mixing model

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    A dynamical valence and sea quark mixing model is shown to fit the baryon ground state properties as well as the spin content of the nucleon. The relativistic correction and the q3q3qqˉq^3{\leftrightarrow} q^3q\bar{q} transition terms induced by the quark axial vector current ψˉγγ5ψ\bar{\psi}\vec{\gamma}\gamma^5\psi in this model space is responsible for the quark spin reduction.Comment: 11 pages, Latex, one figure, to be published in Phys. Rev. C5

    Some Problems in Defining Functional Integration over the Gauge Group

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    We find that sometimes the usual definition of functional integration over the gauge group through limiting process may have internal difficulties.Comment: 2 pages revtex, no figur

    A Note on Functional Integral over the Local Gauge Group

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    We evaluated some particular type of functional integral over the local gauge group C^{\infty}({\bf R}^n, U(1)) by going to a discretized lattice. The results explicitly violates the property of the Haar measure. We also analysed the Faddeev-Popov method through a toy example. The results also violates the property of the Haar measure.Comment: 7 pages, Revte

    A Note on Invariant Measure on the Local Gauge Group

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    In this paper we investigated the problem of the existence of invariant meaures on the local gauge group. We prove that it is impossible to define a {\it finite} translationally invariant measure on the local gauge group C(Rn,G)C^{\infty}({\bf R}^n,G)(where GG is an arbitrary matrix Lie group).Comment: 4 pages, REVTE

    Generative Feature Replay with Orthogonal Weight Modification for Continual Learning

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    The ability of intelligent agents to learn and remember multiple tasks sequentially is crucial to achieving artificial general intelligence. Many continual learning (CL) methods have been proposed to overcome catastrophic forgetting which results from non i.i.d data in the sequential learning of neural networks. In this paper we focus on class incremental learning, a challenging CL scenario. For this scenario, generative replay is a promising strategy which generates and replays pseudo data for previous tasks to alleviate catastrophic forgetting. However, it is hard to train a generative model continually for relatively complex data. Based on recently proposed orthogonal weight modification (OWM) algorithm which can approximately keep previously learned feature invariant when learning new tasks, we propose to 1) replay penultimate layer feature with a generative model; 2) leverage a self-supervised auxiliary task to further enhance the stability of feature. Empirical results on several datasets show our method always achieves substantial improvement over powerful OWM while conventional generative replay always results in a negative effect. Meanwhile our method beats several strong baselines including one based on real data storage. In addition, we conduct experiments to study why our method is effective.Comment: Under Revie
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