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 the 21st is here and black holes are as enigmatic as ever. Only recently could researchers pinpoint them. Yet to truly understand, they must see – something which black holes do not immediately support.
The Challenge of Trying to "See" a Black Hole
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.
Collaborating Telescopes and Very Long Baseline Inferometry
This is the intuition behind Very Long Baseline Inferometry (VLBI), a technique in radio astronomy that allows telescopes around the world to combine simultaneous observations of the same object and produce an image as if it came from one big telescope. It’s essential to observing black holes such as Sagittarius A* that are 26,000 light years away. Its enormous event horizon makes up for its distance, and VLBI allows telescopes to widen their baseline to completely capture the largest observable black holes in the sky. A June 2009 paper by MIT Haystock Observatory astronomers Vincent Fish and Sherman Doeleman ("Observing a Black Hole Event Horizon: (Sub)Millimeter VLBI of Sgr A*", MIT Press/Cornell University Library, arXiv:0906.4040v1 [astro-ph.GA]) modeled a VLBI system that, when collecting Sagittarius A* radiation emissions, can see with great detail the black hole’s shadow, produced by its bright background, the Milky Way.
The resolution of the Fish and Doeleman images improves significantly when more observatories are involved, suggesting that an “Event Horizon” telescope, directly observing Sagittarius A* and other black holes, could be realistically created. Such a telescope could even allow observers to witness relativistic occurrences along the event horizon, including the bending of time (known as “gravitational redshift”). But considering an under-enthused investment climate and public for all space and aeronautics programs (of which there exist 69 worldwide), its realization in the near future is unlikely.
2009 marks the 400 year anniversary of Galileo first observing the sky through his great invention, the telescope. But where he went blind, observers today have the chance to see beyond the instinctual capabilities of the eye. VLBI – when, or if, it comes into common practice – can offer professionals and amateurs alike to learn more about black holes, the formation of galaxies, and the history of the universe.
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