Black holes are changing our understanding of everything

One thing upon which my friend Jeremy Clarke and I always agreed is the value of seeing the world from different points of view. In that sense we partially agreed on everything. This essential skill needs to be learned, and I assert that nature is a wonderful teacher. Perhaps the most surprising and bamboozling example can be found in the study of black holes.  

Here are two apparently contradictory properties of black holes. When viewed from the outside, time stops on the event horizon – the boundary that marks the black hole’s edge, beyond which light cannot escape – and anyone attempting to cross into the interior would be seen frozen there forever. From the perspective of an astronaut falling into a supermassive black hole, however, nothing unusual happens as she travels across the horizon and into the interior, although she would be trapped in a flowing river of space carrying her to oblivion.