Astronomers used a perhaps unlikely tool to understand what they saw in their first historic image of the Milky Way’s supermassive black hole: a library of computer simulations that mimic the black hole with millions of variations.
Take a picture of a black holeas Sagittarius A* at the center of our galaxy, with the Event Horizon Telescope (EHT) is one of the most complicated feats an astronomer can undertake. The new picture combines petabytes of data from eight radio telescopes around the world; advanced computer algorithms then go to work transforming all this disparate data into a false-color image.
But, “Taking a picture is just the beginning,” said Chi-Kwan Chan at a University of Arizona. statement. Chan is an astronomer at the Steward Observatory in Arizona and leads the EHT Collaboration’s project to build a library of millions of black hole simulations, which can then be compared to an image of reality.
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These artificial images are vital. Astronomers have only ever been able to image two black holes, and there is so much about these black holes that is uncertain or unknown, and some features of the images are open to interpretation or completely confusing. Instead of a catalog of actual black hole images to compare against, astronomers instead use a library of simulated images.
Creating these simulations was almost as complex as taking the real images themselves. To assemble the library, the astronomers used the Texas Advanced Computing Center’s Frontera supercomputer and other similar instruments. In total, the project relied on 80 million CPU hours of processing time – the equivalent of 2,000 laptops running at full speed without interruption for an entire year – to produce 5 million simulated images.
Each of these simulations is unique, changing the values of different properties: the spin of the black hole, the strength of its magnetic field, the amount of gas falling on it, the amount of spacetime distorted by gravity, the level of turbulence in the disc, and many others. The scientists then performed 11 tests on each simulation to see which matched the real image best.
“It’s remarkable that we understand Sagittarius A* so well that some models pass 10 out of 11 tests,” Chan said. However, none passed all 11 – there was always something the researchers couldn’t explain. In particular, most models seemed to stumble over the amount of turbulence-induced variability seen in the hot plasma disk surrounding Sagittarius A*. The real black hole appeared to be much quieter than the simulations predicted.
It’s a characteristic of Sagittarius A* that scientists are still trying to figure out, though Chan doesn’t think a little mystery is a bad thing.
“I think it’s more exciting than if everything just worked out,” he said. “Now we can learn new physics and better understand our own black hole.”
It would perhaps be prideful to expect to be able to understand everything about our local supermassive black hole after photographing it just once. Yet astronomers have been able to learn a lot about it in part thanks to these valuable simulations.
We now know that the black hole’s magnetic field is as strong as a fridge magnet, but it’s still enough to push the falling gas away from its mouth. We know that the amount of matter passing past the Sagittarius A* event horizon and feeding the black hole is relatively tiny — for a press conference Revealing the image of Sagittarius A* held in Washington, DC, Thursday, May 12, EHT scientists drew an analogy to a human eating just one grain of rice every million years. And we also know that Sagittarius A* is probably spinning – although there’s no direct measurement of this, all simulations where it didn’t spin have been ruled out.
Now scientists need to go back to the drawing board to figure out why the disk around Sagittarius A* appears less active than expected. It’s a complex problem, involving the constant feedback between the black hole’s magnetic field and falling plasma, and how the strong gravitational field affects this relationship.
Once scientists solve this mystery, the results will in turn populate their library of possible black holes, and maybe one day help us better understand other black holes when they too are imaged.
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