Abstract
We show that the Newman-Janis shift property of the exact Kerr solution can be interpreted in terms of a worldsheet effective action. This holds both in gravity, and for the single-copy sqrt{mathrm{Kerr}} solution in electrodynamics. At the level of equations of motion, we show that the Newman-Janis shift holds also for the leading interactions of the Kerr black hole. These leading interactions are conveniently described using chiral classical equations of motion with the help of the spinor-helicity method familiar from scattering amplitudes.
Highlights
This connection between the NJ shift and scattering amplitudes suggests that the NJ shift should extend beyond the exact Kerr solution to the interactions of spinning black holes
We will only consider the effective operators in SEFT that involve one power of the electromagnetic field Aμ, which can be fixed by the three-point amplitudes
The Newman-Janis shift is often dismissed as a trick, without any underlying geometric justification
Summary
√ We begin our story concentrating on the slightly simpler example of the Kerr particle in electromagnetism. Where uμ and Ωμν are the linear and angular velocities, and SEFT contains additional operators coupling the spinning particle to the electromagnetic field. The effective action (2.1) can be written independently of the choice of SSC, at the expense of introducing an additional term from minimal coupling [94, 95, 107]. We will only consider the effective operators in SEFT that involve one power of the electromagnetic field Aμ, which can be fixed by the three-point amplitudes. Since these amplitudes are parity-even, the possible single-photon operators are. The unknown constant coefficients Bn and Cn are dimensionless
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