Similarly, disappearing event horizons can also be seen with the Reissner–Nordström geometry of a charged black hole if the charge ( Q ) is high enough. In this metric, it can be shown that the singularities occur at r ± ± = μ μ ± ± ( μ μ 2 − − q 2 ) 1 / 2 , where μ μ = G M / c 2 , and q 2 = G Q 2 / ( 4 π π ϵ ϵ 0 c 4 ) . Of the three possible cases for the relative values of μ μ and q , the case where μ μ 2 < q 2 causes both r ± ± to be complex. This means the metric is regular for all positive values of r , or in other words, the singularity has no event horizon. However, this corresponds to a case where Q / 4 π π ϵ ϵ 0 exceeds M G (or in Planck units, Q > M ) ; i.e. the charge exceeds what is normally viewed as the upper limit of its physically possible values. Also, actual astrophysical black holes are not expected to possess any appreciable charge.