Carroll, SeanJohnson, MatthewRandall, Lisa2012-01-242009Carroll, Sean, Matthew C. Johnson, and Lisa Randall. 2009. Extremal limits and black hole entropy. Journal of High Energy Physics 2009(11): 109.1029-8479http://nrs.harvard.edu/urn-3:HUL.InstRepos:8015828Taking the extremal limit of a non-extremal Reissner-Nordstrom black hole (by externally varying the mass or charge), the region between the inner and outer event horizons experiences an interesting fate--while this region is absent in the extremal case, it does not disappear in the extremal limit but rather approaches a patch of \(AdS_2 \times S^2\). In other words, the approach to extremality is not continuous, as the non-extremal Reissner-Nordstrom solution splits into two spacetimes at extremality: an extremal black hole and a disconnected \(AdS\) space. We suggest that the unusual nature of this limit may help in understanding the entropy of extremal black holes.en-USblack holesblack holes in string theoryhigh energy physics theorygravitation and cosmologyastrophysics and astroparticlesJournal Article2012-01-2410.1088/1126-6708/2009/11/109