An Invitation to Complex Spacetime I: Newman–Janis from Amplitudes

Joonhwi Kim, Physics / Caltech

The Kerr-Newman black hole solution was historically obtained by performing an “imaginary displacement” to the charged non-spinning black hole. This is called the “Newman-Janis shift.” Yet, no one has succeeded to provide a clear explanation of this trick ever since its introduction in 1965. In 2020, however, an interesting “derivation” of the trick appeared from the study of quantum scattering amplitudes. I will explain how our perceived classical world emerges from quantum particles and “derive” the geodesic equation from graviton amplitudes. Then, by tweaking the amplitudes we will explore the “magic cube of black holes,” including the Newman-Janis shift. This interplay between classical relativity and quantum physics will eventually enable us to answer the ultimate question, “Why does prof. Teukolsky recommend QFT courses to astro students?”

Host: Rui