## Testing general relativity

The no-hair theorem of general relativity predicts that the spacetime
around a black hole can be expressed in terms of only three
parameters: the black hole mass, spin, and charge. Since it is hard
to see how a real astrophysical black hole could sustain a large
electric charge, the no-hair theorem predicts that the black hole can
be characterized by its mass and spin alone.

The strong curvature of spacetime near a black hole produces a dark
shadow surrounded by a bright photon ring. The shape of this shadow
is roughly circular. Detecting the shadow of a black hole and
establishing that it is indeed circular would constitute an
observational test of general relativity.

The diameter of the shadow is proportional to the mass of the black
hole and is mostly insensitive to the value of the black hole spin.
Detecting the shadow would also allow astronomers to obtain a direct
estimate of the ratio of the mass of a black hole to its distance from
the observer.

**Testing general relativity using the black hole shadow.**
General relativity predicts that the shadow of a black hole should
be circular (middle panel), but a black hole that violates the no-hair
theorem could have a prolate (left) or oblate (right) shadow. Future
EHT images of nearby supermassive black holes will be able to test
this prediction. (figures courtesy D. Psaltis and A. Broderick)