Next step in black hole science: Making a 'movie'
When researchers unveiled the first image of what had been considered unobservable, scientists gasped and people around the world stared at the photo, and then into the sky, in awe.
Capturing the first image—an almost spooky ring of light—around a black hole some 55 million light-years away from Earth, in the galaxy known as Messier 87, mesmerized people and inspired curiosity about science and the seemingly endless power of nature.
So, if a single image can inspire this much global interest, what kind of furor would a movie of a black hole—the undulating, brightening, and dimming of circling plasma—bring?
It seems it won’t be too long before we find out.
Lia Medeiros, physicist, astrophysicist, and National Science Foundation fellow
Credit: The University of Arizona
Lia Medeiros, a physicist, astrophysicist, and National Science Foundation fellow, is working to put together a movie of sorts of a black hole, using data from the Event Horizon Telescope, a global telescope array that gave scientists the data needed to capture that first black hole image. Think of it as an extremely high-tech, and a little sci-fi-sounding version, of a flip-book you made as a child. Medeiros will create a series of images that vary gradually from one to the next and then put them together so we can see the motion of the black hole—a black hole a lot closer to home.
And she’s going to do it using machine learning.
Medeiros was one of the many scientists who worked on the Event Horizon Telescope project to produce that first black hole image from Messier 87, or M87, a giant elliptical galaxy in the constellation Virgo. That massive black hole—more than 6 billion times bigger than the sun—exerts a tremendous gravitational pull on anything within or even close to the event horizon, the boundary in which nothing escapes its monstrous gravitational force.
Now, though, the scientist is focusing her research on the super-massive black hole at the center of our own galaxy, Sagittarius A*, also known as Sgr A*. This black hole, which is about 26,000 light-years from Earth, has an event horizon with a radius of about 7.9 million miles, making it about 18 times the diameter of the sun. It’s big. And hundreds of scientists have spent their entire careers studying it. Now, we actually could get something akin to a movie of it. Taking all of this together, Sgr A* is likely to be critical for the tests scientists are looking forward to performing on the theory of gravity.
Medeiros, who defended her black hole-focused dissertation for her PhD at the University of California, Santa Barbara, about a month before the first image of a black hole was announced in early April, has been working on the Event Horizon team for about four years, researching M87 as well as Sgr A*. Now, her focus is largely on creating a “movie” of Sgr A*.
The Event Horizon Telescope array has collected data from 12 hours of observations on both M87 and Sgr A*. Because M87 is so much larger than the other black hole being observed, the timescale over which its images vary is much longer, so scientists did not see much variations in the data or the images rendered during that 12 hours. Sgr A* is another matter. Its images do vary significantly because during that 12-hour time span, the region that emits the light we observe was fluctuating—changing brightness and shape—because the plasma swirling around the black hole is swirling.
“The image of Sgr A* changes faster than the time it takes us to collect enough data to reconstruct the image,” says Medeiros.
That movement is what she wants to capture.
“A time series of images would be incredibly exciting, since it can help us understand what causes the variability that has been observed for Sgr A* as well as many other black holes,” says Medeiros. “Sgr A* has been observed to flare very significantly. There's a lot we still don't understand about what causes these events. By studying how the image of the black hole varies as a function of time throughout the observations, we hope to be able to understand what causes these flares.”
Chi-Kwan Chan, assistant astronomer at the University of Arizona Department of Astronomy and Steward Observatory
Credit: The University of Arizona
Chi-Kwan Chan, assistant astronomer at the University of Arizona Department of Astronomy and Steward Observatory, says many scientists are eager to understand the variability—the light becoming brighter and dimmer—of black holes, and he’s hoping a movie will help them do that. “Because black hole accretion disks, formed by the in-falling plasma, are so variable, a static image won't give us the full understanding,” he says. “Getting a moving image will help us understand what is happening.”
A movie could test our best theories
A movie of a black hole could help scientists understand far more than black holes themselves.
Medeiros’ work actually could help scientists better understand the workings of the universe. It even could change the way we think about gravity. That’s right. We might have to rethink everything we learned about gravity in grade school by analyzing a time series of images of these gravitational monsters. Think of black holes as the perfect test bed for everything we know about Einstein’s theory of relativity, one of the towering achievements of 20th-century physics.
Scientists basically will be pitting Einstein's theory of general relativity, which tells us what we know, or think we know, about gravity, against the most powerful gravitational forces in the universe. It’s about comparing these new black hole observations with predictions based on our mathematical models of them. And if general relativity doesn’t fully hold up at the event horizon, then the theory may need to be rethought.
Her work also could tell us more about how quantum mechanics, which is still quite mysterious to the best physicists in the work, interacts with the theory of gravity.
Quantum governs the seemingly unpredictable world of subatomic particles, while general relativity is thought to explain gravity up to the enormous scale of black holes. They both are great theories that so far have passed every experiment scientists have executed around them. However, they don’t always work when they are considered together. Scientists need to test these theories in more extreme conditions. Enter the black hole as the perfect test bed.
Having a black hole movie could be a scientific game changer because they are one of the only types of objects in the universe that scientists need both theories to explain. Black holes, simply put, live at the intersection of quantum and gravity. Movies of a black hole could give scientists the information they need to see if they behave the way we expect them to, helping them figure out the complicated intersection of two major scientific theories.
“We have not been able to put [the theory of relativity and quantum mechanics] together,” says Medeiros. “A black hole is one of the few things in the universe that requires both theories to understand it because it’s very, very massive and it’s very, very small. They are at the extreme of both theories. If we see something unexpected, it could be because both or one of the theories is breaking down. It’s not that Newton or Einstein were wrong, but those equations were simplifications, and if you go to an extreme situation, the theories might not work. The equation might not be complete."
She adds, “If we see something unexpected, it could change everything. If we find something that doesn’t behave the way we expect, it’s an incredibly exciting opportunity to better explain the universe.”
Creating the first movie of a black hole is exciting for Medeiros but not quite as exciting as getting answers to these scientific questions. She decided to become a physicist during her junior year in high school when her teacher started talking about black holes, gravity, and space-time. Medeiros’ imagination was afire and she asked more questions than her teacher could answer.
“Black holes, gravity, and how they interact with time fascinated me as a teenager. I asked my teacher what I could do in college that would let me study these questions,” she says. “He said, 'Physics or astrophysics,' and I said, “Great! I’ll do both.”
The Submillimeter Telescope, Mt. Graham, Arizona.
Credit: The University of Arizona
How she’s going to do it
So, if Sgr A* is 26,000 light-years from Earth, how exactly is Medeiros going to make a movie of it?
Well, that’s where machine learning and a whole lot of data from telescopes positioned around the world come in.
To put together the first movie, or actually the time sequence of images, she is using the massive amount of data collected by the Event Horizon Telescope array—the same kind of data that was used to create the initial black hole image. The array doesn’t capture actual images of the black hole. It captures petabytes of raw data about the radio waves emanating from the ring-shaped silhouette of a black hole.
Medeiros is using the El Gato (Extremely LarGe Advanced TechnOlogy) computer cluster, which is a high-performance system jointly funded by the National Science Foundation and the University of Arizona. Using specially designed hardware, like Nvidia K20X GPUs and Intel Xeon Phi 5110p coprocessors, the system takes the massive amounts of data and mathematically transforms it into an image—or for Medeiros, images.
“We’re creating simulations and then we’re using those simulations as a training set for the algorithm so we can create a time series of images,” she explains. “The movie is a time series of images. We want to see how the image of the black hole changes as a function of time.”
One problem is that the data she is getting is not complete. There are a lot of missing pieces. Medeiros needs to fill in the missing data to create a complete image.
What she needs is a smart algorithm that she can feed datasets and simulations into to train it so it can fill in the missing pieces and help her create the time series of images.
What she needs is machine learning.
Medeiros is writing an algorithm based on principle component analysis (PCA), which is a method of analyzing or representing complex data to reduce a large set of variables to a small set that still contains most of the information in the large set. It also can be said that PCA, an algebra operation that underlies a lot of machine learning applications, is reducing the data into its principal components. As a data analysis and predictive tool, it’s often used in genetics and finance.
For her black hole movie production, Medeiros is taking that PCA algebra operation and adding it to the algorithm she’s creating, making it a smart algorithm that will learn from the data she’s feeding into it.
“The idea is that I will apply it to my training set and I will get 10 to 20 images,” she explains. “I can use the training set to identify images that will act like building blocks of what a black hole could look like. With these building blocks, I can make an image with the algorithm filling in the parts of what we don’t have data for.”
To create that first image of a black hole, scientists needed 12 hours of observational data from the telescopes to feed through their algorithms, which did not use machine learning. With machine learning, Medeiros can create a time sequence of images from the same amount of data.
The actual math behind Medeiros' algorithm that could give the world its first movie of a black hole is only several lines of code. Because of an embargo, she could not say if she is running any black hole datasets yet.
“I don’t know how I would go about it if I wasn’t using PCA,” she says. “I probably wouldn’t even be trying to work on creating this movie without this algorithm.”
For the researcher, her work is bringing together years of her own curiosity and devotion to math, science, and understanding the massive and tiny parts of our universe. “As humans, we’re tiny little dots on another tiny dot floating through space, and we’ve come up with a language—math—to help us understand the universe we live in,” Medeiros says. “This realization has always inspired me.”
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