# Superradiant scattering by a black hole binary

## TL;DR

This paper uncovers a new form of superradiance in black hole binaries, where waves can be amplified by extracting energy from the orbital motion, especially when black holes are non-spinning, with potential theoretical implications.

## Contribution

It introduces a novel superradiance mechanism in black hole binaries and demonstrates how EFT techniques can analytically compute the amplification factor under certain scale hierarchies.

## Key findings

- Amplification factor is extremely small, less than 1 part in 10^10.
- Superradiance is most pronounced when black holes are not spinning.
- The work provides a theoretical foundation for future studies on superradiance in binaries.

## Abstract

I present evidence of a novel guise of superradiance that arises in black hole binary spacetimes. Given the right initial conditions, a wave will be amplified as it scatters off the binary. This process, which extracts energy from the orbital motion, is driven by absorption across the horizons and is most pronounced when the individual black holes are not spinning. Focusing on real scalar fields, I demonstrate how modern effective field theory (EFT) techniques enable the computation of the superradiant amplification factor analytically when there exist large separations of scales. Although exploiting these hierarchies inevitably means that the amplification factor is always negligible (it is never larger than about one part in $10^{10}$) in the EFT's regime of validity, this work has interesting theoretical implications for our understanding of general relativity and lays the groundwork for future studies on superradiant phenomena in binary systems.