Tests of General Relativity with Black Hole Shadows

The imaging of black-hole shadows with the Event Horizon Telescope has opened a new window into the strong-field spacetimes of these extreme astrophysical objects. For the Kerr spacetime, the shadow of a black hole is nearly circular with a size that depends almost entirely on its mass. I will describe how this property of Kerr shadows allows us to perform null-hypothesis tests of General Relativity, when the mass of the black hole is known a priori. I will discuss metrics that deviate from Kerr and their signatures related to the shapes and sizes of black-hole shadows. I will conclude with a prognosis on what ground-based observations of shadows can tell us about black-hole metrics and the underlying theory of gravity.