Black holes might be the most generally popular subject in astrophysics, and with good reason: mystery is the essential foundation of popularity. They were proposed to exist as early as the 18th century, sought after intensely starting in the mid-20th, but only recently could researchers pinpoint them. Yet to truly understand, they must see – something which black holes do not immediately support.
Stephen Hawking once said that searching for a black hole is like “looking for a black cat in a coal cellar.” To the amateur observer it is perhaps even more abstract. Trying to “see” a black hole is like trying to “see” sound. Ears and eyes are detecting instruments that rely on the brain to interpret the information they gather. But no instrument can visualize what it cannot detect – one reason why humans cannot see in the dark.
So if the cat in the coal cellar meows, how can the listener be sure he is hearing a cat? He might imagine a cat based on his memory of sounds, but he has no scientific way to judge its size, shape, or color. Thus is the struggle between black holes and telescopes – they may indicate the black holes are there through gravity lensing or x-ray emission, but any given telescope can detect them only by blocking out all other senses.
But what if multiple telescopes of different designs are focused on the same area – for example, Sagittarius A*, the super massive black hole at the center of the Milky Way Galaxy? If the cat meows in a lighted room, the brain can confirm that it is a cat when the ear collects the sound and the eye focuses on its source. By likewise acting as the ears, eyes, and brain, multiple telescopes can view a black hole and produce a true real-time image. Quite literally, seeing the image is seeing the black hole.