2009/12/31 by Alberto Nicolis, Riccardo Rattazzi, Enrico Trincherini · 1 citation
Physics and Astronomy · #hep-th
paper · pdf · doi:10.1007/jhep05(2010)095
published as JHEP 1005:095,2010 · 21 pages. v2: Typos in eq. (2.41) and (2.41) corrected; discussion of section 2.3 modified accordingly. Other sections and conclusions unchanged. Matches the Erratum published in JHEP
arxiv created 2012/09/17 · arxiv updated 2012/09/18
In QFT, the null energy condition (NEC) for a classical field configuration is usually associated with that configuration's stability against small perturbations, and with the sub-luminality of these. Here, we exhibit an effective field theory that allows for stable NEC-violating solutions with exactly luminal excitations only. The model is the recently introduced `galileon', or more precisely its conformally invariant version. We show that the theory's low-energy S-matrix obeys standard positivity as implied by dispersion relations. However we also show that if the relevant NEC-violating solution is inside the effective theory, then other (generic) solutions allow for superluminal signal propagation. While the usual association between sub-luminality and positivity is not obeyed by our example, that between NEC and sub-luminality is, albeit in a less direct way than usual.