Celestial amplitudes on electromagnetic backgrounds: T-duality from S-duality

What is the boundary holographic dual of S-duality for gauge theories in asymptotically flat space-times? Celestial amplitudes, by virtue of exhibiting holographic properties of the S-matrix, appear well-suited for studying this question. We scatter electrically and magnetically charged massless scalars off non-trivial electromagnetic potentials such as shockwaves, spin-one conformal primary waves, conformally soft modes and their magnetic duals which we construct. This reveals an intricate relation between conformally soft solutions, descendant CFT three-point functions and, by means of the two-dimensional shadow transform, CFT two-point functions. By comparing celestial amplitudes on electric and magnetic dual backgrounds, we provide evidence that the four-dimensional flat-space holographic dual of S-duality in Abelian gauge theory is two-dimensional T-duality. Moreover, we demonstrate that for two-dimensional boundary actions describing low-energy sectors of the bulk gauge theory, S-duality can be explicitly implemented as a T-duality transformation. We show that the Dirac quantisation condition guarantees gauge invariance in the eikonal re-summation for scattering from potentials which for magnetic scalars can be expressed in terms of 't Hooft loops.