Regge behavior saves String Theory from causality violations

TL;DR

The paper tests whether Regge behavior in bosonic string theory rescues causality in the presence of higher-derivative gravity corrections by analyzing high-energy string-brane collisions. It shows that a field-theory truncation to massless modes can exhibit causality-violating time delays for certain polarizations, but when the full Reggeized string amplitude is included, these violations are suppressed by powers of Y = log(α' s) and no net causality violation remains. The results support the view that an infinite tower of higher-spin exchanges, encoded in Reggeization, is essential to maintain causal propagation, and they connect string dynamics to potential intra-string processes at small impact parameters via branch-cut phenomena. Overall, Regge behavior provides a robust mechanism to preserve causality in gravity-like setups within string theory, aligning with the broader CEMZ perspective on higher-derivative corrections.

Abstract

Higher-derivative corrections to the Einstein-Hilbert action are present in bosonic string theory leading to the potential causality violations recently pointed out by Camanho et al. We analyze in detail this question by considering high-energy string-brane collisions at impact parameters $b \le l_s$ (the string-length parameter) with $l_s \gg R_p$ (the characteristic scale of the D$p$-brane geometry). If we keep only the contribution of the massless states causality is violated for a set of initial states whose polarization is suitably chosen with respect to the impact parameter vector. Such violations are instead neatly avoided when the full structure of string theory - and in particular its Regge behavior - is taken into account.