Composite Mediators and Lorentz Violation

We briefly review the history and current status of models of particle interactions in which massless mediators are given, not by fundamental gauge fields as in the Standard Model, but by composite degrees of freedom of fermionic systems. Such models generally require the breaking of Lorentz invariance. We describe schemes in which the photon and the graviton emerge as Goldstone bosons from the breaking of Lorentz invariance, as well as generalizations of the quantum Hall effect in which composite excitations yield massless particles of all integer spins. While these schemes are of limited interest for the photon (spin 1), in the case of the graviton (spin 2) they offer a possible solution to the long-standing UV problem in quantum linear gravity.Comment: 6 pages, no figures, revtex4. To appear in the proceedings of the Third Meeting on CPT and Lorentz Symmetry, University of Indiana at Bloomington, August 4-7, 2004. v2: Some typos fixed, explanation of why GR evades the Weinberg-Witten theorem clarifie