An illustration of a black hole. Despite how dark it is, all black holes are thought to have formed from normal matter alone, but illustrations like these are only partially accurate. Image credit: NASA / JPL-Caltech.

Ask Ethan: What should a black hole’s event horizon look like?

You might think that it should be all black, but then how would we see it?

“It is conceptually interesting, if not astrophysically very important, to calculate the precise apparent shape of the black hole… Unfortunately, there seems to be no hope of observing this effect.” -Jim Bardeen

Earlier this month, telescopes from all around the world took data, simultaneously, of the Milky Way’s central black hole. Of all the black holes that are known in the Universe, the one at our galactic center — Sagittarius A* — is special. From our point of view, its event horizon is the largest of all black holes. It’s so large that telescopes positioned at different locations on Earth should be able to directly image it, if they all viewed it simultaneously. While it will take months to combine and analyze the data from all the different telescopes, we should get our first image of an event horizon by the end of 2017. So what will it looks like? That’s the question of Dan Barrett, who’s seen some illustrations and is a bit puzzled:

Shouldn’t the event horizon completely surround the black hole like an egg shell? All the artist renderings of a black hole are like slicing a hard boiled egg in half and showing that image. How is it that the event horizon does…