# Observing Black Holes Spin

## TL;DR

Black hole spin measurements, derived from X-ray, gravitational wave, and radio observations, are crucial for understanding black hole growth, jet formation, and testing gravity theories, with future observatories promising enhanced precision.

## Contribution

This paper reviews current observational methods for measuring black hole spin and discusses their astrophysical implications and future prospects.

## Key findings

- Current measurements constrain black hole growth models.
- Spin observations provide insights into jet physics.
- Future observatories will improve measurement precision.

## Abstract

The spin of a black hole retains the memory of how the black hole grew, and can be a potent source of energy for powering relativistic jets. To understand the diagnostic power and astrophysical significance of black hole spin, however, we must first devise observational methods for measuring spin. Here, I describe the current state of black hole spin measurements, highlighting the progress made by X-ray astronomers, as well as the current excitement of gravitational wave and radio astronomy based techniques. Today's spin measurements are already constraining models for the growth of supermassive black holes and giving new insights into the dynamics of stellar core-collapse, as well as hinting at the physics of relativistic jet production. Future X-ray, radio, and gravitational wave observatories will transform black hole spin into a precision tool for astrophysics and test fundamental theories of gravity.