The great irony of black holes is that, in all the decades that we astrophysicists have talked about them, we never had any direct observational evidence for them. When astronomers said they had “found black holes” in this or that location in a faraway location in the universe, what this really meant was a very compact object—an enormous concentration of mass, far greater than that of any conventional star or planet.On Wednesday all that changed. The key characteristic of a black hole, what makes it black and a hole, is the existence of an event horizon—a one-way membrane or, if you like, a boundary in spacetime. Because of it, black holes gulp everything but do not allow any matter within, or even light, to escape. The Event Horizon Telescope (EHT) has now precisely offered that evidence in the form of a shadow that the black hole necessarily casts.Strictly speaking, the system did not see an event horizon, which cannot be seen by definition. Furthermore, although an event horizon necessarily implies a shadow and silhouette, the converse is not true. Nonetheless the observations are still so precise that whatever is casting the shadow must be exotic. No ordinary body could be…Read More…
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