Suppose you want to see something so far away that it looks really small in the sky. Think of standing in New York and trying to read the date on a quarter in Los Angelesthat small. Any tiny distortion can ruin the image, but after you correct for these, ultimately, what gets you is refraction = the bending of light at the edge of your telescope. So the bigger your telescope, the higher quality image.
How big a telescope is practical? The remarkable answer is that there is now a radio telescope "as big as the earth". The trick is done by combining data from 3 telescopes at widely dispersed places on the earth, and waiting for the earth to rotate so that these are in different positions relative to the astronomical target.
Think of a CAT scan. X-rays are sent through the body in all different directions, and received at the opposite side of the body. Each X-ray detector contains information about the entire line of flight of the X-rays, and no single detector can tell you about any single point. By combining the data from all the detectors in the right way, an image can be formed. There's a mathematical way to disentangle the data"deconvolution" is the word they useand with the wide availability of number crunching computers, this has become routine.
The same kind of math is used to deconvolve data from the 3 radio telescopes. The result is the world's first image of a black hole, and it looks just like what the theorists predict it should look like! There's a dark circle at the center, where light goes in but not out, and then there's a bright ring around that circle, where matter falling into the BH has become superhot.
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