They are the most powerful objects in the universe. Nothing, not even light, can escape the gravitational pull of a black hole. Astronomers now believe there are billions of them out in the cosmos, swallowing up planets, even entire stars in violent feeding frenzies. New theoretical research into the twisted reality of black holes suggests that three-dimensional space could be an illusion. That reality actually takes place on a two-dimensional hologram at the edge of the universe.
What is a Black Hole?
By William R. Harris, HowStuffWorks.com
Black Holes? Absurd!
Black holes are almost as difficult to imagine as they are to detect, but a few scientists have been up for the task over the centuries. Cambridge scholar John Michell wrote a paper in 1783 in which he hypothesized the existence of “dark stars” — stars so large and with so much gravity that light wouldn’t escape their surfaces. Most astronomers of the day thought it was an absurd notion.
Then, in 1915, Einstein published his general theory of relativity, providing a framework that allowed for a reinterpretation of Michell’s hypothesis. An Indian graduate student by the name of Subrahmanyan Chandrasekhar piggybacked on Einstein’s theories to suggest that stars of a certain size — much larger than our sun — would experience a catastrophic collapse at the end of their lives, thereby transforming the bodies into cosmic vacuum cleaners whose powerful gravity could suck all light and matter into their black maws.
The thought experiments of these scientists and many others have produced our modern conception of black holes. We now believe they are the end products of enormous stars, which often explode in spectacular supernovas before shriveling up into tiny, cold, superdense balls. At the core rests a singularity, a point where all of the object’s matter is compressed into a region of infinite density. Enveloping the singularity is a sphere of extraordinary gravitational pull directed toward the singularity. The outer edge of the sphere forms the event horizon. As long as an object remains beyond the event horizon, it can escape. But if it falls within this point of no return, it can’t escape gravity and disappears down the black hole’s gullet.
Since the late 1920s, astronomers have been on the hunt for black holes, trying to prove their existence with empirical data. Although it’s difficult to observe an object that doesn’t transmit light, scientists have found other ways to “see” black holes indirectly. One tried-and-true method involves looking for a visible star whose orbit is disturbed in some unexpected way. In many cases, the star’s motion can only be explained if one assumes it’s locked in the gravitational grip of an unseen object. Astronomers have found numerous stars fitting this description and now believe several of these so-called binary star systems, such as Cygnus X-1, may harbor black holes. In fact, sometimes, binary systems can contain two black holes.
Astronomers have other tricks for detection up their sleeves. They know, for example, that black holes emit other forms of electromagnetic energy, so a search for strong sources of radio waves and X-rays could reveal potential targets. X-ray emissions have proven to be particularly telling because all matter sucked into a black hole produces a blast of X-rays just before it gets swallowed.
In 2007, NASA’s Chandra Observatory spotted X-ray “echoes” coming from Sagittarius A* (Sgr A*), the black hole thought to be at the center of our own Milky Way. Scientists speculate that the original burst of energy came roaring through space when a planet-sized object slid over Sgr A*’s event horizon and disappeared forever.
Still, astronomers would love to observe a black hole directly, and they’ve set their sights on Sgr A*. Although Sgr A* resides in our home galaxy, it’s 26,000 light-years away from Earth — much too far away for optical telescopes to see clearly. In fact, astronomers would need an optical telescope 3,107 miles (5,000 kilometers) in diameter just to get a good look at the black hole, so scientists are turning to a technique known as very long baseline interferometry (VLBI). This technique involves linking a global network of radio telescopes — in essence creating a single, enormous device — to produce images of far-away objects.
In 2008, MIT scientists studied the event horizon of Sgr A* using a three-telescope array. Although the team wasn’t able to produce images of the event horizon’s full silhouette, it was able to see bright spots believed to be either super-heated matter swirling around the black hole or a high-speed jet of matter being ejected by the black hole. Future VLBI studies using more radio telescopes should provide a fully resolved image of Sgr A*’s event horizon. The same technique may also be able to see the black hole at the center of M87, a giant elliptical galaxy lying 60 million light-years away, even more clearly. When they obtain these images, astronomers will finally have the evidence they need to say black holes are a reality and not a figment of our imagination.
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