Where does Kerr collide?

In optics the Kerr effect is the change a medium's refractive index undergoes in response to an applied electric field, where the induced birefringence grows with the square of the field strength, so the material splits light into components that travel at different speeds. John Kerr's 1875 A new relation between electricity and light: Dielectrified media birefringent reported that a glass held in a strong electrostatic field acts on transmitted light like a weak uniaxial crystal, and the same quadratic response reappears as the optical (AC) Kerr effect, in which an intense light beam modulates the index it propagates through. The referent is a nonlinear electro-optic coupling in a material, exploited in Kerr cells, ultrafast shutters, and Kerr-lens mode-locked lasers, and it has nothing to do with the spacetime of a spinning mass beyond sharing the name of its discoverer.[1]

Unit

The field-induced change in refractive index, set by the Kerr constant of the medium

Recovery means

Not applicable

Measured by

Quantified by the Kerr constant from the induced phase retardation versus the square of the applied field, or by the nonlinear index in the optical case

In general relativity the Kerr metric is the exact solution of Einstein's field equations describing the curved, stationary spacetime outside an uncharged, rotating mass, and it is the geometry believed to govern astrophysical black holes once they settle down. Roy Kerr's 1963 Gravitational field of a spinning mass as an example of algebraically special metrics found this solution, which adds angular momentum to the static Schwarzschild case and introduces frame dragging, an ergosphere, and a ring-shaped singularity. The referent is a spacetime geometry fixed by just two numbers, mass and spin, named for a different scientist entirely, and it has no connection to a field-induced change in a material's refractive index save the shared surname.[2]

Unit

A spacetime geometry parameterised by the mass and angular momentum of the rotating body

Recovery means

Not applicable

Measured by

Tested through black-hole spin estimates from X-ray spectra, ringdown gravitational-wave modes, and black-hole imaging