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Friday
Feb222013

Black hole super(computer)models

by Sarah Scoles

This is a black hole's self-portrait.

Black holes are the strangest objects in the universe. Spaghettification, singularities, the speed of light's embarrassing status as "lower than the speed required to escape gravitation's pull." Black holes allow us to investigate the kinds of extreme science that we can't replicate here on Earth. Extreme mass, extreme spacetime curvature, extreme energy, extreme density, extreme gravitation.

Scientists like to test their ideas about mass, spacetime, energy, density, and gravitation on black holes and see how they hold up in this fringe case.

But before they can determine whether their theories produce results that hold up in the real world, they have to come up with what "holding up in the real world" looks like.

That's where models come in.

Yeah, sure. Those models. They come in. To a supercomputing facility. Sure.

 

Scientists have these theories (well-established explanations of how/why things happen in nature), and scientific theories provide predictions about how objects and forces behave in the real world. Sometimes, those predictions are simple to determine.

Newton's law of universal gravitation (okay, it's a law and not a theory, but you get my point), for instance, predicts that if I throw this banana toward the hallway, it will eventually fall to the ground. If the banana instead floated into my coworker's office, the real-world evidence would not match the prediction, we would all have to stop throwing bananas and take a second look at gravitation.

But somebody already did take another look at gravitation: Einstein, who made the theory of general relativity. While Newton's conception of gravity works for middleweights, it doesn't work for the extremes. It also has no idea about spacetime, though if Newton had somehow come up with the idea of some "fabric" of the universe warping all over the place, he probably would have freaked out.

To test Einstein, though, you can't just throw a banana around. It's not massive enough. It is, after all, just a fruit. In fact, we don't have anything lying around Earth that can test the edges of general relativity. Which is why we turn to black holes.

And models. Black holes and models.

Before scientists can find out whether black holes obey Einstein's theory, they have to figure out how a black hole obeying Einstein's theory would look. This is an example of a theory's implications being un-simple to sort out. 

To determine how a perfectly generally relativistic black hole would look, scientists have to use supercomputers like the ones at the Texas Advanced Computing Center, which, wouldn't you know, produced black-hole models published in a January Science paper.

The article "Alignment of Magnetized Accretion Disks and Relativistic Jets with Spinning Black Holes" has warped ideas about ...

... jets and accretion disks. 

Where is the black hole? In the middle of all that stuff. (Credit: NASA).

Jets and disks are made of fast-moving materials that is falling into (disks) or being shot away from (jets) the black hole. Because of the material's high energy and temperature and magnetic interactions, it emits radiation--X-rays, radio waves, gamma rays...the usual. The disk swirls around the black hole (which we can't see, as it's black), and the jets are perpendicular to it.

Or are they?

As a press release from the computing center says, "For decades, a simplistic view of the accretion disks and polar jets reigned. It was widely believed that accretion disks sat like flat plates along the outer edges of black holes and that jets shot straight out perpendicularly."

Not true, says this new study by J. McKinney, et al. They found the picture is not consistent at different distances from the black hole, and also that it is actually more like a movie than a picture.

Specifically, like this movie.

Where the jet is close to the black hole, it is aligned with the black hole's spin. For the parts of the jet that are farther away, though, the jet is parallel to the disk's rotation axis, an interaction that causes a warp in the disk itself. 

The scientists also turned up the magnetism, so that the field lines "threading" the black hole were as strong as the black hole's gravitation (read: curiously strong). When the field is this strong, it powers the jet. Which is potentially an answer to the age-old question, "What powers black hole jets?" The powerful jet then is so powerful that it can influence the disk, instead of only the other way around.

The paper's lead author provided a multi-metaphor quote that is potentially more confusing than enlightening but is funny nonetheless:

"People had thought that the disk was the dominant aspect," McKinney said. "It was the dog and the jet was the wagging tail. But we found that the magnetic field builds up to become stronger than gravity, and then the jet becomes the dog and the disk becomes the wagging tail. Or, one can say the dog is chasing its own tail, because the disk and jet are quite balanced, with the disk following the jet — it's the inverse situation to what people thought."

Yes, like that. 

So we have a theory, and we have a model of how that theory looks when applied in a specific situation.

Now what?

Test time! Time to look at a real black hole and see if it really looks like the supercomputed model. If it does, 1 point for the model and the theory. If it doesn't, perhaps the model is not complex enough or has a flaw, or maybe the theory has flaws. But using the Event Horizon Telescope, scientists will be able to look at actual gas around actual black holes and determine how model-like it is.

The only minor issue left to figure out is how the modeled black hole would actually look if you saw it through a telescope from billions of light-years away. It's the difference between seeing the paths of fast-moving matter, as we do in the simulation, and seeing the resultant photons, as we do through telescopes. As McKinney said, "We're in the process of making our simulations shine."

It puts a new meaning on the old song "Black Hole Sun."

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