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Black Hole: From Mathematics to Reality

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A Mystery Solved

More than fifty years ago a mathematician named Roy Kerr began looking at weird theoretical beasts called “black holes”.  The idea of massive objects whose gravity is so high that nothing, not even light, can escape them, was not a new one in his time. But, they were considered theoretical, and nobody had observed one.  There were a lot of people who though they didn’t really exist.  Yet, there they were, theoretical objects that Albert Einstein and others suggested would have strange effects on space-time.

A NASA artist’s concept of a black hole surrounded by an accretion disk. The blue lines are magnetic field lines. Courtesy NASA.

Einstein had written equations that described the properties of static masses (which black holes were considered at the time).  He and others apparently thought they just hung there in space (if they existed at all). Some years later, astronomer Karl Schwarzschild found solutions to Einstein’s equations, but that didn’t bring black holes any closer to being discovered. That had to wait until we understood more about them.

We now know that black holes aren’t static. And, this is because Roy Kerr took another look at Einstein’s equations and worked with them under the assumption that black holes were actually rotating. He clarified that the size and shape of black holes could be described with two numbers: their masses and their rates of rotation. He worked with another scientist, Alfred Schild, and the two of them introduced the Kerr-Schild spacetime concept in 1965 that made it possible to understand (and look for) black holes. Not only did their work cover black holes, but it also applied to other massive (and weird) objects like neutron stars and compact binary star systems.

The gravitational lensing effect a black hole has on light that passes by from a more distant object (in this case, the simulation shows the Milky Way as a black hole passes between us and the more distant plane of our galaxy. (From UrbanLegend, Wikimedia Commons at: http://commons.wikimedia.org/
wiki/File:Black_hole_lensing_web.gif)

Today, we find black holes all over the place. They’re lurking in the hearts of galaxies as supermassive black holes. Some of these SMBHs (as they’re called) have the equivalent mass of millions or billions of Suns. Stellar-mass black holes (which form when very massive stars die in supernova explosions) exist throughout our galaxy. Understanding the effects of massive, rotating objects on nearby material allows astronomers to look for the radiation given off in the immediate vicinity of a black hole as interstellar material (gas, dust, stars) funnels in.  A black hole also warps space-time, producing a lensing effect on light that passes near the cosmic beast.

Our own Milky Way Galaxy has a black hole at its heart. It’s called Sagittarius A*, and it contains about the same mass as four million Suns. Astronomers are still studying this black hole and others to understand how they form at the hearts of galaxies. There is evidence of a smaller black hole in the vicinity of Sagittarius A*. Could it be heading toward a union with the larger black hole?  No one’s sure yet, but it’s an intriguing idea.

Today, some 50 years after Kerr’s work opened the door to black hole understanding, astronomers are honoring him at a conference in Warsaw, Poland called the 20th International Conference on General Relativity Theory and Gravitation. It’s a fitting honor for a man whose work solved, literally and figuratively, the mystery behind the once-theoretical black holes.

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Black Holes: They’re Everywhere!

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Cosmic Zombies

This image shows the location of black hole candidates found around the central region of the Andromeda Galaxy (M31). Most are stellar-mass black holes, while the central black holes in M31 are supermassive behemoths. Courtesy NASA/Chandra/

I like to read about black holes. I like to write about them, too—they’re fascinating. When I was a kid, black holes were one of those “weird cosmic things” that we’d read about in science fiction. In scientific circles, people considered them a theoretical possibility, but nobody had actually observed one. Turns out you really don’t observe a black hole. You look for the effect of the black hole on material (or space) around it. A black hole’s gravity  warps space/time, which causes light to “bend” as it travels past, sort of like looking at a straw in a glass of water. That same gravity also sucks in nearby material, which coalesces into a wide disk of material called an “accretion disk”.  The “stuff” in the disk gets funneled into the black hole, and on the way in it gets superheated. The highly hot material gives off radiation, and astronomers can spot THAT around the area of the black hole.

The black hole itself is a tightly compacted bundle of materials packed together so well and has a gravity so strong that  nothing that gets inside it—not even light—can escape it. That’s the part about black holes I think fascinates us. What’s it LIKE in there? Nobody who goes in can ever tell you because they, and their messages, would never get out.

Black holes are a continuing source of study for astronomers. They find them all over the place, scattered throughout our galaxy. Most of them are stellar black holes, meaning they were created when old, massive stars died. However, at the center of our galaxy, there is at least one (and probably two) supermassive black holes.

It turns out that other galaxies have black holes. That makes sense. They have stars, including massive ones,l so when those stars die, depending on their masses, they too will make black holes. And, the centers of other galaxies have supermassive black holes, too. Astronomers are still figuring out how those behemoth black holes get into the central regions during a galaxy’s evolutionary history.

The Chandra X-Ray Observatory can easily spot the x-rays coming from regions around black holes in our galaxy and in others, too. Its data has revealed a large number of these cosmic zombies in the Andromeda Galaxy, our nearest spiral neighbor in space. Chandra has spied out 35 black hole candidates seemingly swarming around the center of the galaxy. Seven of those candidates lie dangerously near the core, about a thousand light-years away. Eight black hole candidates are associated with globular clusters, which is an interesting result. The Milky Way’s globulars have not, so far, shown us that they have any black holes.

Now, the interesting thing about Andromeda’s black holes is that in a few billion years, they’ll belong to the Milky Way.  Or, to be more accurate, they’ll be part of a new galaxy that forms when the Milky Way and Andromeda merge. That process will begin in perhaps five billion years and take around 10 billion years to complete. At the end, there’ll be a massive elliptical galaxy containing the stars, black holes, and planets that used to exist in the two separate galaxies.

Want to read more about Andromeda’s newly found black holes? Check out the Chandra X-Ray Observatory Web page for details!