Disappearing event horizons exist in the Kerr metric, which is a spinning black hole in a vacuum, if the angular momentum ( J ) is high enough. Transforming the Kerr metric to Boyer–Lindquist coordinates, it can be shown that the coordinate (which is not the radius) of the event horizon is, r ± ± = μ μ ± ± ( μ μ 2 − − a 2 ) 1 / 2 , where μ μ = G M / c 2 , and a = J / M c . In this case, "event horizons disappear" means when the solutions are complex for r ± ± , or μ μ 2 < a 2 . However, this corresponds to a case where J exceeds G M 2 / c (or in Planck units, J > M 2 ) ; i.e. the spin exceeds what is normally viewed as the upper limit of its physically possible values.