As seen from the vantage point of an outside observer, the edge is extremely sharp. It is a mathematically perfect, spherical surface where light gets infinitely redshifted.

To an observer falling into the black hole, the boundary may be much more complicated than our 'axi-symmetric' mathematics would suggest. The horizon could be a turbulent surface rippling with gravitational radiation, or it might dissolve into a fuzzy quantum state at even finer scales of scrutiny. Someone falling into the horizon would experience NOTHING PECULIAR, except that once they cross this mathematical surface by even one millimeter, they can never turn back to escape the black hole. The 'Event Horizon' is a most peculiar concept in physics. Still, it is a theoretical idea which needs to be studied with real data before we can feel confident that we understand it. Getting the data, however, would be fatal! So, what's a physicist to do?

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