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Abstract We point out that in many eternal black holes, including a Schwarzschild eternal black hole and an eternal black hole in AdS 5, instant folded strings are created in the past wedge and render the region just outside the horizon singular. We also make a conjecture regarding instant folded D-branes and discuss their possible implications for eternal black holes. In particular, we argue that the bulk modes responsible for Poincare recurrence, when it occurs in the dual quantum field theory, are either instant folded strings or instant folded D-branes.

Abstract We consider the backreaction of the winding condensate on the cigar background. We focus on the case of the SL(2, ℝ) k /U(1) cigar associated with, e.g., the near-horizon limit of k NS5 black-branes. We solve the equations of motion numerically in the large k limit as a function of the amplitude, A, of the winding mode at infinity. We find that there is a critical amplitude, A c = exp(−γ/2), that admits a critical solution. In string theory, the exact SL(2, ℝ) k /U(1) cigar CFT fixes completely the winding amplitude, A s , at infinity. We find that in the large k limit there is an exact agreement, A c = A s . The critical solution is a cigar with a puncture at its tip; consequently, the black-hole entropy is carried entirely by the winding condensate. We argue that, in the Lorentzian case, the information escapes the black hole through this puncture.

Abstract We study the recent claim that black NS5-branes are filled with folded strings. We calculate, in the near-extremal case, the number of folded strings at the black fivebranes interior, using different approaches, and get the exact same answer. The backreaction of the folded strings leads us to argue that the interior of the black fivebrane is AdS 2 (times a compact manifold) and that infalling matter cannot reach the interior, due to a shock wave at the horizon. These considerations also suggest a novel insight into the black fivebranes entropy.

Abstract We show that in string theory, due to non-perturbative effects, there are cases in which two states that semi-classically are completely different, are in fact the same. One state cannot be excited without exciting the other; they are two components of the same state in the exact theory. As a result, in some situations that include black holes, the nature of information in string theory is dramatically different than in field theory. In particular, each general-relativity state, that lives in the atmosphere of black fivebranes, is accompanied with an excitation that lives on folded strings, which fill the black-hole interior. This is likely related to the way that information is extracted from black holes in string theory, and we refer to it as stringy information.

Abstract It is well known that non-perturbative α ′ corrections to the SL(2, ℝ)/U(1) cigar geometry are described via a condensation of a Sine-Liouville operator that schematically can be written as W + + W − , where W ± describe a string with winding number ±1. This condensation leads to interesting effects in the cigar geometry that take place already at the classical level in string theory. Condensation of the analytically continued Sine-Liouville operator in the Lorentzian SL(2, ℝ)/U(1) black hole is problematic. Here, we propose that in the black hole case, the non-perturbative α ′ corrections are described in terms of an operator that can be viewed as the analytic continuation of the fusion of W + and W − . We show that this operator does not suffer from the same problem as the analytically continued Sine-Liouville operator and argue that it describes folded strings that fill the entire black hole and, in a sense, replace the black hole interior. We estimate the folded strings radiation, and show that they radiate at the Hawking temperature.

Abstract We show that negative (∇Φ)2, where Φ is the dilaton, leads to a rapid creation of folded strings. Consequently it appears that the interior of the SL(2, ℝ) k /U(1) black hole is not empty, but is filled with folded strings.

Abstract We elaborate on the recent claim [1] that non-perturbative effects in α′, which are at the core of the FZZ duality, render the region just behind the horizon of the SL(2, ℝ) k /U(1) black hole singular already at the classical level (g s = 0). We argue that the 2D classical SL(2, ℝ) k /U(1) black hole could shed some light on quantum black holes in higher dimensions including large black holes in AdS5 × S 5.

Abstract The potential behind the horizon of an eternal black hole in classical theories is described in terms of data that is available to an external observer — the reflection coefficient of a wave that scatters on the black hole. In GR and perturbative string theory (in α′), the potential is regular at the horizon and it blows up at the singularity. The exact reflection coefficient, that is known for the SL(2, ℝ) k /U(1) black hole and includes non-perturbative α′ effects, seems however to imply that there is a highly non-trivial structure just behind the horizon.