# Event Horizon of a Black Hole

For a non-spinning (stationary) black hole , the event horizon radius, $R_{h}$, is the location (radius) from the center of the black hole for which light originating at radii less than the horizon radius cannot escape to infinity because it would be subject to an infinite redshift. The event horizon radius in this case is synonymous with the Schwarzschild radius. The event horizon formula for a spinning black hole is more complicated and in the equatorial plane, it is
$\frac{R_{h}}{R_{g}} = 1 + \sqrt{(1 – a^{2})}$
where $a$ is the dimensionless angular momentum of the black hole, lying between $0$ and $1$, and $R_{g}$ is the gravitational radius.