by Daniel Brouse
You can not really take a picture of a black hole. No light escapes from a black hole, so you won’t see anything. That is why it is called a black hole; however, theoretical astronomers have attempted to take the first picture. Instead, they try to piece together an image of a black hole’s “event horizon”. An event horizon is a term from general relativity that describes a boundary in spacetime. What is beyond the event horizon cannot affect an outside observer (the shell of “points of no return” at which the gravitational pull becomes so great as to make escape impossible including light.)
The event horizon allows you to see the shadow from a black hole. The Event Horizon Telescope links telescopes from around the world to create one Earth-sized telescope.
Vincent Fish, a research scientists at MIT, said, “What we expect to see is an asymmetric image where you have a circular dark region. That’s the black hole shadow. And there might be a bright ring at the edge of that—which is the photon ring [a spherical region of space where gravity is so strong photons are forced to travel in orbits]. Then around it you will see one side is bright and the other side is faint, so kind of like a crescent.
The reason for the crescent is that material near the black hole is moving at a few tenths of the speed of light. Special relativity tells you when particles emit photons—when they shine light at you—if the particles are moving towards you, it looks very bright, if they’re moving away from you, then it gets very dim. That produces this asymmetry.”
The Event Horizon Telescope will allow for the first time an image with high enough resolution to show the event horizon from a black hole. In early April 2017, astronomers took five days worth of images. It will take several months before the images can be pieced together to form “a picture of a black hole”.