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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.

black hole, Sagittarius A*

The Hungry Black Hole

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G2 Is Entering the Galactic House

Simulations of the dust and gas cloud G2 on its orbit around the Milky Way central black hole SgrA*.
Photo courtesy of M. Schartmann and L. Calcada/ European Southern Observatory and Max-Planck-Institut fur Extraterrestrische Physik.

The heart of our Milky Way Galaxy is home to a black hole named Sagittarius A* (known as Sag A* for short).  It contains the equivalent of three to four million times the mass of the Sun, and as black holes, it’s kind of a quiet, friendly one. By that, I mean that it’s not constantly gulping down massive amounts of material as some other galaxies’ black holes do.  Sure, it does eat, but it seems to do so only sporadically.

Astronomers and lab physicists at the Lawrence Livermore Labs have created a supercomputer simulation showing a gas cloud called G2 which is on its way to being Sag A*’s latest snack.

Three-dimensional, volume visualization spanning the period 2010 to 2020, of the gas and dust cloud as it approaches the Sgr A* black hole near the center of the Milky Way galaxy. (Courtesy Dr. P. Chris Fragile)

They came up with six simulations showing how the event might unfold as the cloud approaches the black hole. The cloud’s makeup is still not well-understood and astronomers are still trying to figure out where it came from in the first place. It first showed up in 2002, and observers rushed to chart its size and other characteristics. The cloud appears to have a fair amount of dust that is quite warm — 500 degrees Kelvin, which is about twice as hot as Earth’s surface. The gas entrained in the cloud is superheated hydrogen and is about twice as hot as the surface of the Sun.

This cloud could be a burp of gas from an old star that was starting to lose some of its mass. Or, as some astronomers have suggested, it might have been material that didn’t quite form a planet. And, now it’s heading toward the region of the black hole, poised for a close brush with the gravity well of the black hole sometime in September 2013. As it gets nearer, the cloud will get heated to incredibly high temperatures which will make it easy to spot using radio and x-ray telescopes on Earth.

It’s likely that not all of the cloud will get sucked into the realm of the black hole. A great deal of it may pass well outside of the “point of no return” near the black hole. But, it will get shredded apart by the close encounter with the black hole’s incredibly strong gravitational pull.

Astronomers will be able to watch the black hole dally with the cloud (like a cat dallies with a mouse) for a few years. The closest approach of the cloud will take a few months, and they should be able to track as the cloud is affected by its encounter and breaks apart.