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Kaboom! When Black Holes Collide

Scientific breakthroughs don’t always come when you expect them. In the case of Vicky Kalogera, an astrophysicist at Northwestern University in Evanston, Illinois, the news came on September 14, 2015, when she was getting dinner on the table for her family. Her busy day hadn’t allowed her to pay much attention to the flurry of e-mails from her colleagues on the Laser Interferometer Gravitational-Wave Observatory (LIGO) project. This project to find and observe gravitational waves in space is funded by the US National Science Foundation (NSF). Two instruments operating in unison—one in Louisiana and the other in Washington—offer astronomers a whole new way of studying the universe. Unlike astronomical telescopes, LIGO cannot detect electromagnetic radiation at any wavelength or frequency. Instead, it “feels” invisible gravitational waves. Decades in the making, LIGO had just begun collecting data when the detectors hit the jackpot.

That fall day, a text message from one of Kalogera’s graduate students spelled it out: “Have you been keeping up with LIGO emails today? Loud trigger!” That’s when she knew something big had happened.

That “something big” was a signal from outer space, picked up by the LIGO detectors. It confirmed the existence of gravitational waves, predicted by Einstein a century ago. Kalogera, along with one thousand other scientists and engineers, had been working toward this moment for nearly two decades.

On September 14, 2015, LIGO instruments detected gravitational waves produced by two merging black holes. The waves in this image show their approximate location on a sky map of the southern hemisphere.

Kalogera and the other members of the LIGO team—more than one thousand scientists from ninety different institutions—were sworn to secrecy until the data could be thoroughly analyzed to make sure the signal was real. It was.

On February 11, 2016, LIGO laboratory executive director David Reitze held a press conference. “Ladies and gentlemen, we have detected gravitational waves. We did it. I am so pleased to be able to tell you that. . . . These gravitational waves were produced by two colliding black holes that came together, merged to form a single black hole about 1.3 billion years ago.”

The announcement stunned the scientific community. Even US president Barack Obama tweeted: “Einstein was right! Congrats to @NSF and @LIGO on detecting gravitational waves—a huge breakthrough in how we understand the universe.”

Why was the discovery so exciting? Remember that Einstein’s general theory of relativity states that space and time curve in the presence of mass. This curvature is responsible for the effect we call gravity. When two objects interact with each other, they stretch and squeeze space-time. If those two objects are not very massive, like a couple of pool balls or you and your friend, the effect is so small as to be unnoticeable. If they are very massive, like orbiting or colliding black holes, they send out gravitational waves in space-time, much as a stone tossed into a pond creates ripples in the water.

The LIGO discovery was not only the first time physicists were able to prove the existence of gravitational waves. It was also the first experimental evidence for binary black holes. As the black holes orbited around each other, gravitational forces pulled them closer and closer together until they finally merged in a colossal collision that LIGO was able to capture.

These plots show the signals of gravitational waves detected by the twin LIGO observatories at Livingston, Louisiana, and Hanford, Washington. The signals came from two merging black holes, each about thirty times the mass of the sun, lying 1.3 billion light-years away. These waveforms show what two merging black holes should look like according to the equations of Albert Einstein’s general theory of relativity. The LIGO data very closely match Einstein’s predictions.

Vicky (Vassiliki) Kalogera was born and raised in a small town in Greece, where young people, especially young women, were not generally expected to study math and science. But her parents encouraged her to work hard and pursue her interests. She had a female teacher in middle school who showed her that math was fun and a science teacher who opened up the wide world of physics.

In Greece, as in much of Europe, students are expected to choose their area of study before they go to college. Kalogera chose physics. “Once I got into college for physics,” she said, “I got more and more exposed to astronomy and astrophysics, and that became a true passion. And I also discovered that people could be paid to actually do research; this wasn’t something that my family was aware of, or that I was aware of! So this was when I realized I could go to graduate school and pursue research.”

After Kalogera graduated from the Aristotle University of Thessaloniki (Greece) in 1992, she went to graduate school at the University of Illinois at Urbana-Champaign, where she studied compact objects. After receiving her PhD in 1997, she continued her research at the Harvard-Smithsonian Center for Astrophysics, where she learned of LIGO.

At the time, the project was a physics experiment to detect gravitational waves as a way of confirming Einstein’s general theory of relativity. “It was not thought widely as a new way of doing astronomy,” she said. “Most people were doubtful and dismissive, and thought that it would never really happen. . . . So at the time I decided to join the LIGO project, I was not encouraged by my mentors. But I was really curious about it . . . so I just dipped my toe into the LIGO collaboration.” Kalogera has become one of LIGO’s most senior astrophysicists, riding gravitational waves to new frontiers in astronomy.

A Signal Delivers a Lot of Bang for the Buck

The LIGO scientists gained an astounding amount of information from the signal. They could tell that the two colliding black holes had twenty-nine times and thirty-six times the mass of the sun, respectively. In the final, furious moments before the collision, the black holes orbited each other 35 times per second. As they moved closer, their orbital speed shot up to 250 times per second before finally merging. The combined black hole had a mass of about sixty-two times the sun’s mass, a little less than the combined mass of both black holes. The “missing” mass was released as a result of the force of the collision, and it took the form of gravitational wave energy. So about three times the mass of the sun (13 million trillion trillion pounds, or 6 million trillion trillion kg) was converted into gravitational wave energy. That is more than ten times the combined light energy of every star and galaxy in the observable universe!

The LIGO scientists didn’t have to wait long to find a second black hole collision. On December 26, 2015, just as many scientists were home enjoying the holidays with their families, the detectors picked up the strong signal of yet another black hole collision, 1.4 billion light-years away. These black holes were smaller—just 7.5 and 14.2 times the mass of the sun. While the first signal in September had lasted 0.2 seconds, the second one in December lasted a full second. LIGO had also recorded the final twenty-seven orbits of the black holes before they crashed into each other. The merger of the two black holes released only one solar mass of energy. The fact that these two events happened so close in time is a strong indication that black hole collisions are much more common than scientists had originally thought.

This illustration shows a simulation of two galaxies—each with supermassive black holes in the middle—in the process of merging. The black holes orbit each other for hundreds of millions of years before they merge to form a single supermassive black hole that sends out intense gravitational waves.

Kalogera is excited about the LIGO discoveries. They make her even more passionate about learning how binary black hole systems form. Astrophysicists have two models to describe the formation of these systems. One model says that two similar stars are formed from the same cloud of gas, like identical twins splitting from the same egg in their mother’s womb. The twin stars grow up together, orbiting each other throughout their lives in relative peace. When they exhaust their nuclear fuel, they explode into supernovas and collapse into black holes. The binary stars become binary black holes.

The second model says that the two stars formed independently but inside the same cluster of stars. Inside that cluster, the more massive stars move toward the center and become black holes. They eventually dance around each other before finally colliding and merging into binary black holes. Kalogera hopes that LIGO data will help determine which model is correct.

One of the great scientific mysteries is the nature of the matter and energy that make up our universe. Less than 5 percent of the universe is made of ordinary matter—the stuff we can actually detect, such as the protons, neutrons, and electrons that make up atoms. The rest of the universe is made of dark energy (68 percent) and dark matter (27 percent).

Dark matter is nearly impossible to detect because it does not absorb, reflect, or give off light in any part of the electromagnetic spectrum. But scientists know that dark matter is out there because things happen in the universe that can’t be explained without the existence of something like dark matter. For example, the galaxies and galaxy clusters in our universe are rotating at great speed. The gravity generated by the matter we can observe in those galaxies—matter such as protons and neutrons—is simply not enough to hold them together. According to scientific theory, the matter we can see should fly apart, scattering stars and gas. But it doesn’t. So some type of matter must be exerting enough gravitational pull to keep the matter together.

Dark energy is even more mysterious. Scientists think this form of energy is embedded in the space-time fabric. In theory, the force of gravity should be slowing the expansion of the universe. But the redshifts in distant galaxies indicate that the universe is actually expanding faster and faster. Scientists think that dark energy may be behind the increasingly fast expansion of the universe.

Various theories try to explain dark matter and dark energy. One theory says that our understanding of gravity may be wrong. Another theory suggests that dark energy may be a previously unidentified force in nature. Most theories predict that dark matter is made of some as-yet-undetected particle. Several experiments are under way at detectors to either create or look for these particles.

LIGO’s results have at least one scientist thinking that a type of black hole known as a primordial (very first) black hole might be the elusive dark matter. Scientists haven’t proven that primordial black holes are actually a real thing. But if they do exist, they formed in the first thousandths of a second after the big bang. They weren’t created from stars, like stellar black holes, because stars didn’t exist yet. Scientists think that primordial black holes formed from the collapse of great big balls of gas.

The first two black holes LIGO discovered are too large to fit the predictions of the mass of most stellar black holes. But they’re too small to fit the predictions for supermassive black holes such as Sagittarius A*, at the center of our galaxy. These “Mama Bear” black holes—neither too big nor too small—might be primordial black holes.

Alexander Kashlinsky, an astrophysicist at NASA, has suggested that primordial black holes could account for the “missing” dark matter. He estimates that dark matter could be entirely accounted for with three trillion solar masses’ worth of black holes.

How Did LIGO Detect Gravitational Waves?

It’s very likely that gravitational waves pass through our bodies all the time. But since gravity is a weak force in the universe, we don’t feel a thing. A passing gravitational wave might change the distance between you and the person sitting next to you by about a millionth of the diameter of a proton. It would take an awfully sensitive instrument to detect a gravitational wave. In fact, Einstein doubted scientists would ever be able to detect them.

The LIGO scientists decided to give it a go. They spent decades planning and constructing two observatories, each one functioning like an exquisitely sensitive pair of rulers. One is in Hanford, Washington, and the other is in Livingston, Louisiana, 1,865 miles (3,002 km) away. The LIGO team put the two detectors far apart on purpose. This way, they could tell whether a little jiggle was due to local vibrations only or to a gravitational wave from outer space. It would also allow them to get a general sense of the direction from which the wave came. Each observatory has an L-shaped vacuum tube, from which a vacuum pump has removed all air and other gases. Each arm of the tube is of equal length, a little more than 3.2 feet (1 m) wide and 2.5 miles (4 km) long. The arms are so long that project engineers had to raise them by 3.2 feet at each end so that they lay flat above Earth’s curvature below.

A computer-controlled laser beam at the crook of the two arms is split into two. Each beam shoots down and hits a mirror at the end of the arm. The mirror sends the light bouncing back to the source at the crook of the arms. The speed of light in a vacuum is constant, so the beams should return to the source at the same time. Unless, that is, a gravitational wave ripples through LIGO’s arms. Then one arm of the L would be stretched out while the other would be squeezed short. Since the beam in the stretched arm would have to travel a longer distance than the one in the squeezed arm, the two beams would arrive back at the source at different times. The strength of the gravitational wave would dictate this difference in time. So a mild gravitational wave would lead to only a slight difference in the time it took both beams of light to return. A stronger wave would create a greater difference in the timing.

Each arm of the LIGO observatory is 2.5 miles (4 km) long. This infographic shows how mirrors and a beam splitter direct the light beams in the arms. Changes in the behavior of the light waves indicate an interaction with gravitational waves.

The LIGO instruments are supersensitive. They detect all sorts of sound vibrations from nearby traffic, earthquakes, and even distant ocean waves. To filter out all that noise, the team outfitted each detector with complex suspension systems, like sophisticated versions of those in a car, to insulate against outside vibrations.

“Gravity’s Music”

In September 2015, after years of hard work, the LIGO scientists finally declared that the observatory was ready to detect gravitational waves. It didn’t take long for the first signal to arrive, first at the Louisiana detector and seven milliseconds later at the Washington site. The “chirp” announced the passage of the gravitational wave across the continent.

Gravitational waves are not actually sound or light waves. You can’t hear or see them, so there is no literal chirp. So the LIGO scientists devised a way of converting gravitational waves to sound waves. The technology is something like the way a mechanical wave you create by plucking a string on an electric guitar can be converted to sound with an amplifier. When the LIGO team first converted the gravitational wave to a sound wave, it sounded like the thump of a heartbeat. If the team shifted the frequency to make the wave easier to hear, it sounded a bit like a raindrop falling into a bucket of water. LIGO scientist Gabriela González, speaking at a press conference in San Diego, called it “gravity’s music.” You can go online and hear the sounds of the collision at LIGO’s website at https://www.ligo.caltech.edu/video/ligo20160211v2.

Ultimately, the LIGO collaboration would like to team up with other astronomers to use X-ray, radio, and visible light telescopes to further study colliding black holes. If telescope astronomy creates a silent movie version of colliding black holes, pairing it with LIGO’s soundtrack will give scientists a more complete understanding of the event. The main challenge is that the two LIGO detectors aren’t very good yet at pinpointing the source of the gravitational waves they pick up. That the September signal came first to the Louisiana site and then to the Washington site indicated only that the gravitational wave came from the Southern Hemisphere. It is difficult for astronomers to figure out exactly where to aim their telescopes if they are to coordinate their efforts.

“The [LIGO] detectors act more like our ears, and less like our eyes,” Kalogera says. “Our eyes are like regular telescopes; we point with our eyes. In contrast, our ears can hear sounds that come from in back of us, from under the floor, in all directions.” A larger network of detectors, Kalogera says, could work together to help scientists zero in on the source of the gravitational waves, much as a collection of satellites helps GPS systems pinpoint a location on Earth.

LIGO can only detect gravitational waves within a certain range of frequencies. But different sources in the cosmos emit gravitational waves with different frequencies. Other detectors can be designed to pick up waves with higher and lower frequencies. A collection of gravitational wave detectors could “hear” the entire orchestra of music from the cosmos, from the high-frequency waves of binary neutron star systems to the low-frequency rumble of a supermassive black hole.

Scientists plan to build a third LIGO observatory in India, and Japan is building an underground one. Italy and Germany have smaller detectors similar to LIGO. These may become part of a larger network of gravitational wave detectors. Scientists from six nations are working to launch, in 2034, three networked space-based observatories that could detect very low-frequency gravitational waves.

After LIGO detected gravitational waves from two colliding black holes, scientists from Johns Hopkins University’s Department of Physics and Astronomy published a hypothesis that the LIGO discovery could be a signature of dark matter. This image, using data from the Chandra and Hubble telescopes in 2012, shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520. The cluster was formed from a violent collision of massive galaxy clusters about 2.4 billion light-years from Earth.

For many of us, the Global Positioning System (GPS) is part of our everyday lives. We use it on our smartphones to find our friends, the nearest pizza parlor, and to play games. Originally developed for military navigation, GPS is based on an array of approximately thirty-two satellites orbiting Earth. No matter where you are on Earth, you are reachable by at least four of those satellites. At frequent intervals, each one sends radio signals about its position and its current time. Once your GPS device picks up signals from at least three satellites, it can pinpoint your location on Earth, to within about 49 feet (15 m).

On board, each GPS satellite has an atomic clock (a superaccurate clock that runs on atomic vibrations). Einstein’s general theory of relativity predicts that as gravitational pull becomes stronger, the more time will appear to slow down. So the satellite clocks in space, where the gravitational pull is weaker, will seem to run faster than those on Earth. Yet special relativity also says that the satellite’s clocks are moving relative to a clock on Earth, so they will actually seem to run slower. Taken together—the effects of gravity plus the relative relationship between space clocks and Earth clocks—the overall result is that time on a GPS satellite clock runs faster than a clock on Earth by about thirty-eight microseconds each day. That may not sound like much, but without making the proper adjustments for time, the GPS coordinates for where you are on Earth would be off by at least 6 miles (10 km) each day. We’d be lost without general relativity!