If we ever saw evidence of a rapidly spinning black hole in the universe, how would we recognize it? University of Arizona physicist Sam Gralla is coming up with neat and clean answers to this question.
In an hour and twenty-one minutes, Sam Gralla has said the word “clean” eight times, “neat” one time, “simple” three times, “messy” four times and, (my personal favorite), “slop,” three times. The universe is a messy place and the physics of it is messy, too. But Gralla likes things neat and clean, so rapidly spinning black holes merging with other black holes is an appealing kind of physics. Neat and clean and simple. Just two things plus gravity. No slop.
Gralla is an assistant professor of physics at the University of Arizona. Thanks to him and his collaborators, black hole hunters will know they’ve witnessed a rapidly spinning black hole by its particular look and sound. Basically, he’s taken the saying, “If it looks like a duck and quacks like a duck…” and applied it to astrophysics.
“Now the only slop is about whether or not they exist,” Gralla says. “There’s no slop about whether they can and how we’d know.”
But first things first: What are black holes?
Black Holes: Stars, Reincarnated
Stars carry on being stars for billions of years, but even they don’t last forever. Eventually, a star runs out of energy in the form of hydrogen and its core begins to collapse into itself. As it does, the core becomes hotter and hotter, an unstable nuclear fire burning so wildly that the star throbs and huffs and puffs clouds of gas and dust.
A star that’s around three times the mass of our sun takes on a new life as a black hole, regions of the universe so dense and with gravity so strong that nothing—not even light—can escape. Black holes grow by swallowing up anything within reach. The most voracious of them are called supermassive black holes, and nobody quite knows how they get to be so big so quickly.
“Black holes are not easy to explain with words,” Gralla says. “But the universe is a fantastical place, and these objects exist.”
Even when describing the fantastical, Gralla’s pragmatism is front and center.
I ask Gralla, simply, “Why are black holes cool?”
“Uh, isn’t that self-evident? I mean, they’re just so different.”
“From the table we’re sitting at. Or from anything else in life.”
He’s not wrong, to be fair. The black holes Gralla describes are very different from the table in his office, a sparse-looking space with two blackboards (he’s a loyalist who requested they be installed for their aesthetic appeal and hand-washes them once a week), a $300 coffee maker (“Still far cheaper than feeding my two-a-day habit at a coffee place,” he insists) and a single paper fortune enshrined in acrylic glass that reads, “Either way you are right.” (“I just loved the notion of this cookie telling me I’m infallible”).
Black holes, indeed, are very different from any of this.
As black holes collect more and more cosmic mass, they spin faster and faster. And while nothing can go faster than light itself, so far, scientists have found evidence of black holes spinning at 99 percent the speed of light.
“But for what I’m interested in,” Gralla says, “you really need a couple more nines.”
Gralla’s interested in black holes spinning, theoretically, at 99.99 percent of the speed of light, and he’s spent years chipping away at one interesting, tidy question: How would we know it if we actually saw one?
We’d Know It if We Saw It
In 2014, Harvard University physicist Andrew Strominger, who Gralla calls “a visionary,” invited Gralla to do a postdoctoral fellowship where together they’d pursue an answer to that question.
Strominger, a preeminent string theorist whose circle of friends and collaborators include the late Stephen Hawking, says, “Sam was independently minded, and he had become an expert in the General Theory of Relativity. I’d heard about him through the grapevine and read his papers. I thought he was an interesting scientist with an interesting mind, and it’d be great to have him around.”
Using advanced mathematical techniques, Strominger wanted to make a prediction for what the Event Horizon Telescope would see if it were pointed at a rapidly spinning black hole.
The Event Horizon Telescope is attempting to take a picture of a black hole’s silhouette. It has just two targets as of now: Sagittarius A*, the supermassive black hole at the center of our Milky Way, and another supermassive one in the Messier 87 galaxy. Scientists working on the Event Horizon Telescope are now analyzing images captured last year, which will be released soon.
“I just thought it was ridiculous,” Gralla recalls. “I was like, ‘Look, there’s no way we’re going to be able to do this.’ It’s messy. The Event Horizon Telescope is looking at a black hole, but that black hole is surrounded by matter and stars and gas and dust and—”
I interrupt: “You say that so hatefully.”
“Yeah, because they’re all in the way of my black hole.”
He moved to Cambridge and got to work.
Gralla, Strominger, and Alexandru Lupsasca, a junior fellow in Harvard’s High Energy Theory Group, continued working together on their Event Horizon Telescope prediction for years after Gralla moved to Tucson to take up his post at the University of Arizona.
Galactic mess and all, what they discovered was a smoking gun.
If a source of light was orbiting around a normal black hole, one would expect the Event Horizon Telescope to observe it moving horizontally across the front of the black hole. In the case of a rapidly spinning black hole, they found, that light would be rotated 90 degrees. It would appear vertical.
The results of their work were published early this year in Monthly Notices of the Royal Astronomical Society.
Today, Strominger regards Gralla as “a wonderful human being with a brilliant mind and a fantastic future.”
Lupsasca and Gralla have gone on to be each other’s most frequent coauthors, publishing seven papers together. “I think that speaks for itself,” Lupsasca says. “You don’t keep working with someone unless you enjoy it, and Sam is one of the very best to work with.”
“Physics, fundamentally, is a very human endeavor, and Sam is just a really funny guy. He’s full of these sassy quips,” Lupsasca adds. “Also, I didn’t drink fancy coffee until I met him.”
Physics is a human endeavor—yet the work of a physicist is to contemplate things like black holes, which, conceptually, are pretty far outside of the human experience.
An Interlude: The Basketball Court
I ask Gralla about this dichotomy: “Does thinking about black holes all the time not change the way you see the world? If you walked outside this building and past the basketball court, what would you be thinking?”
Gralla responds, “Well, definitely when I’m walking by the basketball court, I’m thinking about what I’m going to eat for lunch, because that’s the only reason I leave my office.”
Lunch. I refuse to accept what Gralla has told me so clearly here. I push. He’s annoyed, but amused.
“The tools we use for studying black holes are just utterly useless in real life. Yes, the physics of black holes are still true on a basketball court, but it’s just useless to me there. Nothing there is moving near the speed of light,” he insists. “If the basketball court suddenly started moving near the speed of light, I might have a chance of navigating things because I’ve studied the mathematics. You’d be horribly confused. But, like, given that it’s not, it’s just totally useless.”
He concludes with a verbal gavel strike: “Physics takes you so far afield from your everyday experience that you couldn’t survive if you tried to unite the two when you’re just walking around. If I need to think about black holes, I’ll use that framework. If I need to think about basketball, I’ll use a different framework.”
Later, as we’re staging a photo portrait of Gralla on the basketball court, he proves it.
In high school, Gralla was much more interested in sports than he was in black holes, so he played basketball. He can still dribble effortlessly between his legs while thinking only about the ball, his aching quads, the flash of a camera and how hot it is outside—not physics.
On the basketball court, physics is out of sight and out of mind, but only until Gralla’s back in his office.
Images are one way to see black holes. Another: gravitational waves, as predicted by Albert Einstein in 1916 and detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) 100 years later.
Like the ripples from a stone’s skip on a pond, any motion of matter disturbs the very fabric of spacetime. Coffee spilled. Page turned. The skipping stone. Although infinitesimal and imperceptible, all of these make ripples across the universe called gravitational waves.
Researchers may never have the technology to pick up on the gravitational waves produced by a spilled cup of coffee, but not all motion is created equal. The gravitational waves produced by two black holes merging are still infinitesimal (by the time they reach Earth, anyway), but—thanks to large-scale observatories like the American LIGO, the French-Italian collaboration Virgo, the German GEO600 and the Japanese KAGRA—they are no longer imperceptible.
LIGO was built to detect gravitational waves from neutron star collisions (even more powerful than black hole collisions), but when it turned on and detected gravitational waves two days later (on Gralla’s birthday), it was from two black holes merging 1.3 billion light years away.
When converted into sound at a frequency that’s easy for the human ear to interpret, gravitational waves created by two black holes merging make a “chirp.” Theoretical physicists predicted the sound decades before LIGO actually heard it, but until recently, nobody ever predicted that black holes merging could make anything but a chirp.
We’d Know It if We Heard It
While in Cambridge working on the Event Horizon Telescope prediction with Strominger, Gralla started meeting weekly with a group of astrophysicists from the Massachusetts Institute of Technology (MIT) that included professor Scott Hughes and then postdoc Niels Warburton.
“Sam’s a very no-BS guy, so it was really refreshing and a lot of fun having him around,” Hughes says. “From my perspective, the dude’s hilarious. He’s not shy about expressing his opinions and, more often than not, there’s validity to what he has to say, so I take him very seriously.”
When Gralla first started turning up at Hughes’ lab, Hughes was convinced that Gralla would end up collaborating exclusively with Warburton. Hughes was a senior faculty member, chairing multiple committees, teaching and advising graduate students. He had more than enough on his plate already.
“I wasn’t opposed to working with him by any stretch of the imagination, but the likelihood of me getting involved seemed low because of time restrictions,” Hughes recalls.
Then Gralla started talking about rapidly spinning black holes. He wondered if it might make a different sound—not a chirp—as it merged with another black hole.
“And as Sam was talking about it, I was going, ‘Yeah, if I have time, this might be a fun thing to do.’ I swear Sam could tell I was intrigued but not committed, so he gets this goofy grin on his face and goes, ‘Come on, Scott. We can write a paper and call it Inspiral into Gargantua.’ And at that point, I was just like, ‘Oh my God, we have to do this.’”
The film Interstellar, featuring a rapidly spinning black hole called Gargantua, had just hit theaters and was based largely on the work of Hughes’ doctoral advisor, Nobel laureate Kip Thorne.
“I had a lot going on, so it wasn’t trivial for me to find the time to do it, but Sam’s a charming fellow. He made a joke that cracked me up, so I decided to do a fair amount of work. What can I say?”
Together, Hughes, Warburton and Gralla developed the theoretical tools to experiment with the idea. They used a supercomputer to calculate the gravitational radiation from a supermassive black hole spinning at 99.99 percent the speed of light merging with a smaller black hole.
In his last week at Harvard, just before coming to the University of Arizona, they found it: another smoking gun.
“I was sitting there with [Hughes and Warburton], and we were all looking at the signal, realizing at the same time,” Gralla says.
Warburton, Gralla recounted, looked up from the graph and asked, “Wait a minute—is it getting quieter?” It was. The chirp, on the other hand, gets louder. It couldn’t be a chirp. It wasn’t a chirp. It was a sound entirely different from what any theoretical physicist had ever predicted: As it merges with another one, a rapidly spinning black hole sits on one note and slowly fades out.
It is a song.
According to Hughes, both he and Warburton were “a little surprised and thought it was super cool. We were excited.” Gralla, on the other hand, “was kind of like, ‘Yeah, this is basically what I expected.’”
“I mean, we didn’t party. We didn’t think we were going to win a Nobel Prize or anything,” Gralla says. “It just gave us all renewed vigor to actually figure out the details, which we did after I came to the University of Arizona.”
Gralla’s blasé response to their truly novel finding, says Hughes, was “classic Sam.”
“If black holes really do rotate that fast, this smoking gun signature—the song—would let you know that they do,” Gralla says.
The team published their results in a paper indeed titled Inspiral into Gargantua.
As for what forces send a black hole spinning at 99.99 percent the speed of light? Gralla smiles. “That’s a messier question than I like to work on.”
Excellent Taste in Problems
It’s his obsession with interesting, tidy questions, his collaborators agree, that makes Gralla a scientist to watch.
“The hardest part about physics is figuring out what to work on,” Lupsasca says. “And if you look at Sam’s record, he’s demonstrated excellent taste in problems.”
“He’s very methodical, and yet at the same time, he’s really open to new ideas, ready to learn new things, and he thinks outside the box,” Strominger says.
“If you read a book by your favorite author, you’ll recognize their style. It’s the same with physicists,” Lupsasca says. “Sam is a leader in the younger generation of relativists because he comes up with ideas that others wouldn’t.”
“He’s very careful and principled in his approach. He’s like a machine. He can look at a thorny, complicated problem, distill it to its essence, state it clearly, break it into pieces, and tackle it one chunk at a time. Methodical. That’s the word for Sam,” Lupsasca says. “His approach sets him apart.”
“He does amazing work where he can identify clean problems with simple equations,” Hughes says. “A lot of people who have strong mathematical talent like Sam are good at finding solutions to problems that are nicely formulated but not necessarily relevant. You might call it ‘unicorn husbandry.’ Sam really is good at saying ‘Actually, that’s not a unicorn. It’s just a horse with a little bit of a bump on its head. Let’s dive in and pull out an insight that’s clean, applicable and really close to situations we see in nature.’”
And pulling out a clean, applicable insight is exactly what Hughes, Warburton and Gralla did when they uncovered the song of a rapidly spinning black hole collision.
A Great Movie
I ask Gralla how he’d describe the song. It seems this question, like many others, is too messy for his sensibility. He’s reticent.
“I think I’m taking your questions too literally. Maybe I’m too analytical. I don’t have too much of a poetic association with it,” says Gralla. “I mean, it’s a note that rises in frequency and amplitude for a while and then it saturates and fades away.”
After a short silence, Gralla asks, “It’s a little like that THX sound, isn’t it?”
Gralla and I make the familiar sound in unison.
The “THX sound” is the audio trademark of the company George Lucas and Tomlinson Holman cofounded to ensure quality sound reproduction at movie theatres. In the 1980s, 1990s and 2000s, “THX” would appear on screen in silver letters while the deep note played throughout the theater.
He says he’ll have to play it again to confirm that it’s similar, so we listen to the THX sound on YouTube. The resemblance is undeniable.
Gralla grins. Waxing poetic, he says, “You could say it’s the sound of the start of a great movie to be played out over the next 50 years.”
Sarcasm aside, what he’s suggesting is that, maybe someday—someday in the next 50 years even—scientists will actually hear the song of a rapidly spinning black hole crashing into another black hole.
I ask him, “Can you imagine what it’d feel like if the Event Horizon Telescope or LIGO confirmed either of your predictions in your lifetime?”
It Can Happen
Gralla writhes in his chair, mulling it over.
“I’m not holding my breath,” he says. “The Event Horizon Telescope only has two targets, so we’d have to get incredibly lucky for one of them to be rapidly rotating and have the orbiting light source we need. LIGO might be less of a long shot, but we’d still need some fortuitous circumstances. You need black holes, you need them spinning fast and you need them to form in binaries.”
Gralla doesn’t like the sound of his own practicality. “I don’t want to seem too negative,” he says. “I mean, that’s the dream of every theoretical physicist: Make a precise prediction that has only one interpretation and then someone sees it.”
In his first and only earnest display of warm, unfiltered sentimentality over the course of our entire conversation, Gralla concludes, “But for me, just knowing that it can happen is more than enough.”