No abstract available

[en] The common belief of equivalence between the manifestly covariant calculation and the LF (light-front) calculation linked by the LF energy integration of the Feynman amplitude is not always realized. Our example of a light-front calculation with a fermion loop explicitly shows that the persistent end-point singularity in the nonvalence contribution to the bad component of the current J^- leads to an infinitely different result from that obtained by the covariant Feynman calculation unless the divergence is properly subtracted. Ensuring the equivalence to the Feynman amplitude, we have identified the divergent term that needs to be removed from J^-. Only after this term is subtracted is the result covariant and then it satisfies current conservation. The same calculation with the boson loop, however, does not exhibit such a singular behavior and without any adjustment yields a result identical to the Feynman amplitude. Numerical estimates of the nonvalence contributions are presented for the cases of both fermion and boson constituents. (c) 2000 The American Physical Society

[en] We consider pure gravity in light-cone gauge and derive the complete quintic interaction vertex. Up to quartic order, the Kawai-Lewellen-Tye (KLT) relations can be made manifest at the level of the Einstein-Hilbert Lagrangian. The quintic interaction vertex represents an essential first step in further extending the off-shell validity of the KLT relations to higher order vertices

[en] As complement to Chruściel and Paetz (2013 Class. Quantum Grav. 30 235036) we analyse Killing Initial Data (KID) on characteristic Cauchy surfaces in conformally rescaled vacuum space-times satisfying Friedrich’s conformal field equations. As an application, we derive and discuss the KID equations on a light-cone with vertex at past timelike infinity. (paper)

[en] We give an elementary proof of positivity of total gravitational energy in space-times containing complete smooth light-cones. (fast track communications)

[en] The possibility of studying non-perturbative effects in string theory using a world sheet lattice is discussed. The light-cone lattice string model of Giles and Thorn is studied numerically to assess the accuracy of ''coarse lattice'' approximations. For free strings a 5 by 15 lattice seems sufficient to obtain better than 10% accuracy for the bosonic string tachyon mass squared. In addition a crude lattice model simulating string like interactions is studied to find out how easily a coarse lattice calculation can pick out effects such as bound states which would qualitatively alter the spectrum of the free theory. The role of the critical dimension in obtaining a finite continuum limit is discussed. Instead of the ''gaussian'' lattice model one could use one of the vertex models, whose continuum limit is the same as a gaussian model on a torus of any radius. Indeed, any critical 2 dimensional statistical system will have a stringy continuum limit in the absence of string interactions. 8 refs., 1 fig. , 9 tabs

No abstract available

[en] Reminiscences of my intellectual and personal interactions with Professor Nambu, and discussion of his contributions to string theory. Switching to my own research, by expressing the string coordinates as bispinors, I suggest a kinematical framework for the interacting (2,0) theory on the light-cone by using chiral constrained “Viking” superfields

[en] The light-front (LF) canonical quantization of quantum chromodynamics in covariant gauges is discussed. The Dirac procedure is used to eliminate the constraints in the gauge-fixed front form theory quantum action and to construct the LF Hamiltonian formulation. The physical degrees of freedom emerge naturally. The propagator of the dynamical ψ₊ part of the free fermionic propagator in the LF quantized field theory is shown to be causal and not to contain instantaneous terms. Since the relevant propagators in the covariant gauge formulation are causal, rotational invariance--including the Coulomb potential in the static limit--can be recovered, avoiding the difficulties encountered in light-cone gauge. Wick rotation may also be performed allowing the conversion of momentum space integrals into Euclidean space forms. Some explicit computations are done in quantum electrodynamics to illustrate the equivalence of front form theory with the conventional covariant formulation. LF quantization thus provides a consistent formulation of gauge theory, despite the fact that the hyperplanes x^±=0 used to impose boundary conditions constitute characteristic surfaces of a hyperbolic partial differential equation. (c) 1999 The American Physical Society

[en] In analogy with the invariance of mechanical laws in systems moving at differing uniform velocities, the isotropy of space can be extended to the propagation of light. Experimental evidence, as for instance that by Michelson, supports Einstein's theory that it is impossible to measure time differences by means of interference displacements. The paper analyses 3 possible theoretical assumptions for 2 coordinate systems, S and S', all satisfying the principle of constant light velocity in at least one of these systems. It is then shown that by replacing the basis of c = c' by the time constancy principle, (t = t'), transformation formulae for x, y, z and c, respectively for x', y', Z' and c' can be derived which also satisfy relativity criteria. (A.L.)