String-theoretic breakdown of effective field theory near black hole horizons

TL;DR

<3-5 sentence high-level summary>This work investigates whether the equivalence principle and low-energy EFT near black hole horizons can survive in a string-theoretic UV completion. It introduces the longitudinal spreading of strings in the near-horizon region and analyzes how a late-infalling detector could observe this spreading, potentially signaling EFT breakdown through a black-hole–driven boost that amplifies nonlocal string effects. The authors combine light-cone gauge calculations with gauge-invariant S-matrix results to show conditions under which this spreading leads to observable drama, discuss implications for the firewall paradox, and consider cosmological consistency and potential observational tests. They also explore symmetric and asymmetric trajectories, secondary probes, and the limits of perturbative control, outlining several avenues for future work in Kerr backgrounds, Hawking radiation, and AdS/CFT interpretations.

Abstract

We investigate the validity of the equivalence principle near horizons in string theory, analyzing the breakdown of effective field theory caused by longitudinal string spreading effects. An experiment is set up where a detector is thrown into a black hole a long time after an early infalling string. Light cone gauge calculations, taken at face value, indicate a detectable level of root-mean-square longitudinal spreading of the initial string as measured by the late infaller. This results from the large relative boost between the string and detector in the near horizon region, which develops automatically despite their modest initial energies outside the black hole and the weak curvature in the geometry. We subject this scenario to basic consistency checks, using these to obtain a relatively conservative criterion for its detectability. In a companion paper, we exhibit longitudinal nonlocality in well-defined gauge-invariant S-matrix calculations, obtaining results consistent with the predicted spreading albeit not in a direct analogue of the black hole process. We discuss applications of this effect to the firewall paradox, and estimate the time and distance scales it predicts for new physics near black hole and cosmological horizons.

Figures (6)

• Figure 1: Two configurations in which we assess the detectability of the spreading of the red string by the blue detector, as described in the text. In the left panel (a), the direction of relative motion is along the prescribed light cone directions $x^\pm$. In the right panel (b), the direction of relative motion is transverse to $x^\pm$.
• Figure 2: Two illustrative processes from the paper usSmatrix. They exhibit properties which fit with longitudinal spreading and are difficult to interpret purely in terms of transverse string spreading as described in the text.
• Figure 3:
• Figure 4: A cartoon of the proposed thought experiment. The early and late strings are displayed in red and blue respectively. The zigzag lines indicate the spreading of the early string as detectable by the late string, which only develops sufficient resolution to see it in the near horizon region. The dotted line is a constant $r$ locus from which both detector and string may be dropped at different times.
• Figure 5: Proportions in our setup. The separation between 1 and 2 is much larger than the intersection of String 1's worldsheet with the horizon, our condition (\ref{['separate']}).