Special Feature: Gravitational Waves Detected

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Northwestern Astrophysicists Part of Historic Discovery

LIGO-Virgo team is first to detect gravitational waves, observe colliding black holes

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February 11, 2016 | by Megan Fellman

On Sept. 14, 2015, LIGO detected gravitational waves for the first time. Northwestern's Vicky Kalogera and Shane Larson, both LIGO collaborators, describe the event. Produced by Erin Meyer and Kristin Samuelson

EVANSTON, Ill. --- Northwestern University’s Vicky Kalogera absolutely could not believe it. The data revealed that gravitational waves -- ripples in the fabric of spacetime first predicted by Albert Einstein in 1916 -- had been detected for the first time by an international scientific team to which she belongs. Even Einstein didn’t think such a discovery would ever happen.

The extremely difficult-to-detect gravitational waves arrived at the earth from a cataclysmic event in the distant universe: the merger of two black holes to produce a single, more massive spinning black hole. This event emitted more energy than anything directly observed before in the universe. Such a collision of two black holes had been predicted but never observed.

The gravitational waves were detected at 5:51 a.m. Eastern Daylight Time Sept. 14, 2015, by both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors. The surprising detection came only three days after the detectors were turned on again after a five-year renovation and upgrade.

The LIGO Scientific Collaboration (LSC), which carries out LIGO-related research, and the Virgo Collaboration in Europe made the discovery, confirming a major prediction of Einstein’s 1915 general theory of relativity.

Kalogera, an expert in black-hole formation in binary systems and in LIGO data analysis, had worked for nearly two decades for this moment. Soon after realizing there was a binary black hole represented by gravitational wave signals in the LIGO data, she emailed her Northwestern colleague Shane Larson -- both are members of the LIGO Scientific Collaboration -- to tell him the stunning news.

Since that day, Kalogera feels like she has been riding a roller coaster. The ride -- with 1,000 other scientists and engineers – has included thorough checking of data, multiple simulations, countless teleconferences and the writing of a flurry of scientific papers. Though the whirlwind permits little sleep, she and Larson are thrilled to be part of the discovery.

“To detect something in the first few days after turning on our new detectors and to have a detection of an unexpected source – ‘heavy’ binary black holes -- is just amazing,” Kalogera said.

An LSC member for more than 15 years, Kalogera is one of LIGO’s most senior astrophysicists and led the LSC’s astrophysics effort as the LIGO co-editor of the paper about the discovery’s implications. At Northwestern, she is director of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Kalogera also is the Erastus O. Haven Professor of Physics and Astronomy and associate chair of the physics and astronomy department in the Weinberg College of Arts and Sciences.

Larson is a research associate professor of physics and astronomy at Northwestern, a CIERA member and an astronomer at the Adler Planetarium in Chicago. He has been involved with LIGO for five years and with the gravitational-wave community for more than a decade.

Kalogera leads the LIGO research team at Northwestern, which currently includes Larson, two postdoctoral fellows, three graduate students and several undergraduate students. The team’s contributions to the Sept. 14 discovery include making predictions for anticipated detections, interpreting the astrophysics, analyzing the data and characterizing the detectors.

Northwestern alumnus David Reitze, executive director of the LIGO Laboratory at California Institute of Technology, knows Kalogera’s contributions well.

“Professor Kalogera’s group has had a very substantial and outsized impact within the LIGO Scientific Collaboration for the past 15 years, ranging from making astrophysical predictions for what LIGO will discover and how often detections will be made to the development of advanced statistical methods for extracting information from LIGO signals, particularly in the determination of black hole masses and spins,” Reitze said.

Read more about the historic discovery and Northwestern’s contributions in a Q&A with Kalogera and Larson, who recently spoke to Northwestern News.

What was the Sept. 14 event?

Larson: On Sept. 14, 2015, just before 5 a.m. here in Chicago, LIGO -- two detectors working together as a gravitational-wave observatory -- detected a significant signal in its data. This was the first time such a large signal had been detected. Upon analysis, it was discovered to be the gravitational-wave signature of two very heavy astrophysical black holes merging to form an even heavier black hole. The gravitational waves were produced during the merger’s final fraction of a second.

Each of the two black holes was about 30 times the mass of our sun. But they’re black holes, so they’re incredibly tiny compared to the sun. The sun is about 100 Earths across, but these black holes were less than 100 kilometers in radius, so one would cover the greater Chicago area.

Kalogera: This one binary black hole merger emitted more energy, more power than anything in the universe we have directly observed so far. Where is all that energy going? It’s going into gravitational waves. The only way you can detect a pair of black holes is through gravitational waves.

That’s why this discovery is really two discoveries: It is the first time we have detected gravitational waves here on Earth, and it’s the first observation of a binary black hole. Two black holes, in orbit, spiraling together, and, at the end, they merge into one heavier black hole, creating the biggest explosion we have ever detected in the universe. This event outshines all the stars -- the billions and billions of stars in the billions and billions of galaxies in the universe.

How did you learn about the detection?

Kalogera: It was a very busy Monday. I saw some LIGO emails flying around that there was something weird in the data, but I didn’t have time to read them. The detector was not yet fully up and running -- it had only been three days since it went online. Nowhere in my mind was I thinking we’ve discovered gravitational waves.

That evening, while preparing dinner for my family, I saw a text message from a graduate student in my LIGO group: “Have you been keeping up with LIGO emails today? Loud trigger!” And that was the split second when I realized, those weird emails, they were about a potential discovery. I dropped everything, went to my laptop and started reading the emails. And there was a clear sense that we have a signal in our data. A gravitational wave signal that is very loud, very strong, and we definitely have to do something about it and figure out what exactly it is.

Larson: I was up late at night, working on my computer, and received Vicky’s email: “LIGO made a detection.” And I was astounded because we thought it might be years before we detected gravitational waves. Very early on after the detection, we had a guess as to how big the black holes were and how far away they were, and, as an astrophysicist, having those numbers is like having candy. You take those numbers [the LIGO data], and you can instantly start trying to figure out what they mean. I have a little section in my notebook, where you can see I scribbled out three pages trying to figure out how many black holes might be out there.

We knew a signal was coming, and it happened unexpectedly. Not, unreasonably, but it happened unexpectedly. We thought maybe we still had another year or two years before a discovery would happen. But the universe had a different plan for us.

Why is this so exciting?

Kalogera: Almost everything we currently know about the universe has been discovered with light of some kind, such as X-rays, infrared radiation and radio waves. Gravitational waves carry completely new information about black holes and other cosmic objects, and they will unlock a new part of the universe. These waves are very weak and challenging to detect here on Earth, but now we have detected our first burst of gravitational waves.

Einstein, throughout his lifetime, never imagined gravitational waves would have any significance for our search of understanding the universe. The idea that now, 100 years later, not only are we confirming his predictions, but we also are using these observations to learn about black holes, was unimaginable to Einstein, first of all, but even to us.

What is really surprising is that the detection happened only three days after Advanced LIGO began operation. During a five-year redesign and rebuild, the two detectors were significantly upgraded and made much more sensitive than the first-generation LIGO detectors. To detect something in the first few days after turning on our new detectors and to have a detection of an unexpected source -- binary black holes -- is just amazing. I can’t comprehend it.

Larson: This is a new kind of astronomy -- observing the universe using gravity itself. The information is very different from what we already get from light. We can’t ‘see’ black holes with telescopes. A black hole’s gravity is so strong, not even light can escape it. But black holes do radiate gravitational waves, which are produced by accelerating masses. This is the first time black holes have been directly detected by measuring them, through their gravity, as opposed to measuring the effect they have on other matter in the universe.

The detection of gravitational waves was the last big part of Einstein’s general theory of relativity we thought had to be confirmed. Probably the most important outcome of this discovery is that -- finally -- this thing we’ve always talked about has been confirmed. The last little screw that we were waiting to tighten has really been tightened now. The detection we’d all been waiting for -- we finally had it in our hands.

But that doesn’t mean there aren’t new things to discover. It’s like going to the edge of the Grand Canyon. You don’t go to just walk to the edge, you go to look over the edge. And, looking over the edge, you want to know what’s next. Now that we have gravitational waves, we want to know, what else can we now see? What else should we be expecting?

How will the discovery impact the average person?

Larson: It’s for the future to know. The question of whether or not the discovery of gravitational waves is going to change your life tomorrow is difficult to think about because turning discoveries into things that influence your life takes time. We don’t know, and we can’t know because it’s going to take a lot of clever people, thinking about this and trying to imagine: Why does this matter? What could I use this for? And we just haven’t had time to think about it. Right now, we’re all like, we’ve discovered black holes! That’s all we know right now.

Kalogera: Had you asked Einstein 100 years ago, “Why are you curious about understanding gravity? Why do you want to develop a mathematical theory that only 10 people on Earth can understand?” his answer probably would not have been very informative about how it would affect people’s lives. And yet today, general relativity plays a role in your everyday life. GPS technology -- the technology that allows our phones to show us a map and say, this is where we are -- it doesn’t work without general relativity. That knowledge affects how satellites move and informs minuscule corrections of their motion. If we couldn’t make those corrections, GPS would not work. We would be getting our location wrong by hundreds and hundreds of feet.

That’s the beauty of basic science. Basic science pursued by us humans, by those who have an innate, almost inexplicable curiosity about figuring out how nature works. The technology of today is rooted in the basic science discoveries of the past.

What is Northwestern’s contribution to the discovery?

Kalogera: Northwestern, through my involvement, has been a LIGO institutional member for 15 years. I have had a good number of graduate students, undergraduate students and postdoctoral fellows who have worked in my group on LIGO research, with a focus on astrophysics and data analysis. It’s very satisfying now that the young students I trained in gravitational-wave astrophysics and data analysis are now themselves professors at other universities, and they are building up their own research groups.

The long-term goal for the LIGO detectors and its observations is to do astrophysics. We want to use the gravitational-wave observations to learn about our universe for decades and centuries to come. In this context, Northwestern has played a major role within the LIGO collaboration. When I first joined LIGO as a young researcher in 2000, Rainer Weiss of MIT, one of the fathers of the gravitational-wave field, would say, “Vicky is the first ‘card-carrying’ astrophysicist in our collaboration.” I was the first, and I was the only one for quite some time. This discovery of binary black holes connects to our broad-based understanding of how black holes form and evolve in the universe. And binary black hole formation as well as gravitational-wave data analysis are areas of research strength at Northwestern.

Making this discovery required the work of scientists and engineers across disciplines over many decades, starting as early as 1957. The LIGO Scientific Collaboration is inherently an interdisciplinary collaboration. In LIGO and at Northwestern, we enable and emphasize interdisciplinary work whenever it is appropriate for us to make progress. I work on astrophysics and data analysis, Shane [Larson] works on astrophysics and data analysis across the gravitational wave spectrum, and another Northwestern colleague, Selim Shahriar, who is not an astrophysicist but an electrical engineer, focuses on the laser physics of this enterprise. He is looking for ways to improve the sensitivity of our LIGO detectors and broaden the spectrum that the detectors are sensitive to, so we can discover more gravitational wave sources.

What exactly is a gravitational wave?

Larson: We just celebrated the 100th anniversary [in 2015] of Einstein’s understanding of what we call general relativity, which is the modern way that physicists and astrophysicists think about gravity. The big thing about general relativity is that we think about gravity in terms of what it does to the structure of the universe. The fabric the universe is made of is what is called “spacetime.” We’re used to thinking about space and time separately. I measure space with rulers; I measure time on my watch. But what Einstein taught us is that space and time are actually manifestations of the same thing. When we think about gravity, what we’re actually thinking about is the fact that the structure of the universe has a shape to it.

LIGO doesn’t look at the universe in light; LIGO looks at the universe in gravity. Black holes, by virtue of the way they’re constructed, are pure gravity. If a black hole moves toward you, for example, the gravitational strength you feel from that black hole goes up. But the information that the gravitational strength has increased has to get to you. The way the universe gets it to you is that it propagates a gravitational wave through the structure of space and time. Gravitational waves are the idea that the shape of spacetime can move from one place in the universe to another, much like a ripple in a pond.

In a pond, ripples can originate from a stone falling into the water. But in the case of what LIGO just saw, gravitational waves emanated from where the two black holes, slowly swirling around each other in an orbit, merged to form a new, bigger black hole. An enormous amount of energy from the merger went into the gravitational waves, and that’s why we can see them so far across the universe. The waves are ripples in the fabric of spacetime.

Describe a black hole.

Kalogera: A black hole is a dead star. It’s the remnant of a very massive star. A black hole is predicted by Einstein’s theory of general relativity for what happens when gravity is the ultimate winner.

After the star runs out of fuel, what is left is a lot of mass. The gravity is very intense and wants to pull the mass down toward the center. Not having any nuclear fuel means the ball of mass can’t be heated up, and hence, gravity cannot be resisted anymore. At the end stages of massive stars’ lives, gravity for the most massive stars wins altogether. It’s an ultimate win for gravity. It pulls the mass together into an unimaginable object, a singularity that basically does not allow anything -- not even light -- to escape. Matter goes in, and it gets trapped and captured forever. This is what a black hole represents.

For decades after Einstein’s prediction, we had no proof that black holes exist in nature. In the late 1960s and going into the early 1970s, we got the first evidence for the existence of black holes. This indirect evidence was quite robust, and there was widespread acceptance in the scientific community that black holes exist. However, we never had detected an emission of any type directly from black holes. By the nature of black holes, the only possible emission is gravitational waves.

Up until now, we didn’t have detectors that were sensitive enough to detect gravitational waves. The big discovery we are experiencing has to do partly with this direct observation for the first time of gravitational waves emitted directly from a black hole and being detected directly by instruments on Earth. The other major discovery is realizing black holes don’t just exist with gas around them but actually can exist in pairs. Black holes do come in pairs, the way astrophysics theory had predicted.

And it is amazing to think that the two main ways of forming binary black holes in the universe are being studied -- and we are known for this in our community -- at Northwestern. One formation path in my research group, and the other path in Professor Fred Rasio’s group.

What is LIGO?

Larson: LIGO is two things. It’s two detectors working together, an instrument we built specifically for looking at the universe using gravitational waves. One detector is in Livingston, Louisiana, and the other is in Hanford, Washington. LIGO’s laser interferometers detect gravitational waves from minute oscillations between pairs of suspended mirrors set into motion as the waves pass through the earth.

But LIGO also is a collaboration of more than 1,000 scientists and engineers from universities around the United States and in 14 other countries. When we talk about LIGO making a discovery, what we mean is all these people, the LIGO Scientific Collaboration, the ones who conduct the research and have been working on the project for two decades or more.

It’s a very big project, lots of work, lots of people involved all in one place, for one purpose, to make this discovery. In science, the biggest resource you have is your people. They are the ones who have to think this is possible, they’re the ones who have to imagine how we’re going to build the machine, they’re the ones who have to sit and write the computer programs and sift through the data. You can’t do this without the investment of people.

How does the detector work?

Larson: The LIGO detector is not a telescope. We don’t look through LIGO. In reality, LIGO is much more like your ears than it is like your eyes. We often talk about LIGO listening to the universe because the waves arrive from every possible direction. Similar to the way people can hear someone talking to the side, behind or in front of them, LIGO can detect gravitational waves coming from any direction to it from space.

Gravity stretches and warps spacetime, and we want to measure that shape somehow. The way LIGO makes detections is by measuring small changes in distances between two points in space and time.

If you look at an aerial picture of LIGO, it’s this gigantic L-shape, where each of the two arms is 4 kilometers long, and at its corner is a building. We are constantly trying to measure the distance from that corner out to the ends of both of those arms.

To do that, we shine lasers back and forth, up and down the arms. We time how long it takes the laser to go one way and back and compare it with how long it takes another laser to go down the other arm and come back. If we shine the lasers out and they come back at the same time, we know the arms are exactly the same length. But if a gravitational wave comes by, it stretches one arm a bit longer, and simultaneously squeezes the other arm a bit shorter. If you wait a little bit, the arms switch which one is stretching and which one is squeezing, back and forth as the waves go by. The laser that went down the stretched arm takes a little bit longer to get back, while its buddy gets back first. Then we know a gravitational wave went by. Something knocked the laser off course.

Kalogera: The LIGO detectors collect data about where the suspended mirrors at the end of the two arms actually are in space. By tracking the motion of these mirrors, we ask the question, are they being disturbed in a way that is characteristic of what we’d expect of a gravitational wave signal? It’s all done through applied mathematics, algorithms and computer simulations.

The algorithm tells us if there is something consistent with what we expect as gravitational wave sources in a tiny segment of data. And that triggers further analysis, where we ask, what is the source of the gravitational waves? Black holes? How many black holes?

This is what happened Sept. 14 -- a very loud, very strong signal appeared in the LIGO data. That initial news was amazingly exciting, but we immediately knew we had a lot of work to do before we could accurately announce a gravitational-wave detection. We all have worked hard to reach the conclusion stage, the point where we can talk to the whole world about this.

And, finally, the moment comes!

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