The historic first black hole image debuted this month with much fanfare. But few news reports offered a complete description of what it actually showed.
The bright ring is the innermost part of the accretion disk (material spiraling inexorably inward), rendered in false color since the image was made with invisible radio waves. But the blackness within comprises more than just the event horizon (the sphere from within which light cannot escape).
A black hole’s immense gravity warps space, forcing light near it to curve strongly. To visualize this, imagine the event horizon covered with a map of Earth whose North Pole points directly at us. Without gravity, we’d only see the northern hemisphere. But with gravity, light from the back of the event horizon bends around to our eyes, so in this analogy we’d see the southern hemisphere as a highly-distorted ring encircling the equator. Its outermost edge is the South Pole, a single point warped into a circle.
Surrounding that is a second global map distorted into another, thinner, inside-out ring, from light that’s made a complete orbit before heading our way. Outside that is a still-thinner ring (from twice-orbiting light), and so on, ad infinitum. Of course, the event horizon is black, not an Earth map, so everything described to this point appears perfectly dark.
This inky circle is the black hole’s “shadow,” which is 2.6 times wider than the event horizon. The shadow reaches all the way to the inner edge of the glowing accretion disk.
Using his General Theory of Relativity, Albert Einstein predicted that starlight would bend (much more subtly) around the sun. When it was observed during a total solar eclipse (100 years ago next month), Einstein became a household name. It seems fitting that this latest breakthrough vindicates him yet again.
Next column: The largest constellation