A monster lurks at the center of our Milky Way galaxy. You won't see this monster directly, and you won't even see the stars residing anywhere near the center, but you can gaze in the center's direction by finding the constellation Sagittarius. The constellation's brighter stars form a "teapot" poking into the Milky Way from below. Some 25,000 light years -- the distance light travels in 25,000 years -- in this direction lies our galaxy's center. Dust obscures our visual view, but we can detect infrared and radio radiation coming from the center.
Our galaxy is typical of the hundred billion galaxies in the observable part of the universe. It's shaped like a giant pancake, with our sun halfway out from the center. Stars are packed tightly nearer the center, with thousands of stars lying within three light years of the center. By contrast, the nearest other star to our sun is Alpha Centauri, four light years away.
Astronomers have confirmed this black hole's existence by using infrared telescopes to observe the motion of stars near the center. Stars throughout our galaxy move in ellipses around the center; our sun completes such an orbit once every 225 million years. But stars near the center orbit at much higher speeds, because the center's gravity pulls slower stars toward the middle, causing them to speed up while continuing to orbit, or perhaps to fall into the central black hole itself. Observations show stars near the center moving at orbital speeds as high as 5000 kilometers per second--two percent of light speed and equivalent to circling Earth in eight seconds.
These observations of the speeds of stars near the center enable scientists to infer the mass of the central black hole, which turns out to be the mass of 4 million suns crammed into a sphere whose radius is smaller than the distance between the sun and our innermost planet Mercury.
The film "Interstellar" showed images of the distortion of dust and gas around the central black hole as calculated by astrophysicist Kip Thorne, the film's scientific advisor. These calculations are based on Einstein's general theory of relativity, the theory of distortions of space and time caused by gravity. These distortions are far stronger near the black hole than elsewhere in our galaxy. By studying these distortions, astrophysicists can get new information about our two most fundamental theories of nature, namely general relativity and quantum physics.
So scientists are rigging up the largest telescope in history to observe this region. The edge, or "event horizon," of a black hole is the imaginary spherical shell within which you'd better not fall if you are to have any hope of getting back out. Once inside, even light itself cannot escape, which is of course why they call this thing "black."
Unusual things happen right at the event horizon, things that might exceed the predicted bounds of general relativity or quantum physics or both. For example, Stephen Hawking predicts that quantum fluctuations (related to Heisenberg's "uncertainty principle") near the event horizon could create electron-positron pairs. A positron is an anti-electron, identical to an electron but with a positive instead of negative electric charge. One member of the pair would then fall into the black hole while the other escapes. In this way, a black hole could emit electrons and other particles, a phenomenon called "Hawking radiation." You learned something about this if you saw the recent film "The Theory of Everything." There's been some good science from Hollywood recently!
The "Event Horizon Telescope" would observe such phenomena. It's not a single telescope, but rather an Earth-spanning array of radio telescope arrays. You've probably seen photographs of large arrays of many dish-shaped radio telescopes that detect radio waves from space. Such an array combines the radio signals received by each of its dishes, and electronically combines these signals in a technique called "interferometry" to obtain images that are in many ways equivalent to what could be obtained by a single giant radio receiver the size of the entire array.
In an extension of this technique, the Event Horizon Telescope will combine the signals from six separate arrays spread around the globe at Hawaii, California, Arizona, Mexico, Chile, and Antarctica. The data will be in many ways equivalent to the output of a single Earth-sized dish: A telescope whose aperture is the size of the planet!
These arrays are being connected as you read this. For more details, search for "The Dark Lab," Science, 6 March 2015, pages 1089-1093. Stay tuned.
Art Hobson is a professor emeritus of physics at the University of Arkansas. Email him at [email protected].
Commentary on 07/14/2015