Physics – High Energy Physics – High Energy Physics - Theory
Scientific paper
2007-10-18
Class.Quant.Grav.25:125014,2008
Physics
High Energy Physics
High Energy Physics - Theory
v2: 18 pages, streamlined version to appear in Class. Quantum Grav
Scientific paper
10.1088/0264-9381/25/12/125014
We analyse the relation between two a priori quite different expansions of the string equations of motion and constraints in a general curved background, namely one based on the covariant Penrose-Fermi expansion of the metric G_{\mu\nu} around a Penrose limit plane wave associated to a null geodesic \gamma, and the other on the Riemann coordinate expansion in the exact metric G_{\mu\nu} of the string embedding variables around the null geodesic \gamma. Starting with the observation that there is a formal analogy between the exact string equations in a plane wave and the first order string equations in a general background, we show that this analogy becomes exact provided that one chooses the background string configuration to be the null geodesic \gamma itself. We then explore the higher-order correspondence between these two expansions and find that for a general curved background they agree to all orders provided that one works in Fermi coordinates and in the lightcone gauge. Requiring moreover the conformal gauge restricts one to the usual class of (Brinkmann) backgrounds admitting simultaneously the lightcone and the conformal gauge, without further restrictions.
Blau Matthias
Weiss Sebastian
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