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   <p class="main-text_H1-CHAPTER-HEAD" id="_idParaDest-2"><span class="main-text_H1-chapter-number">2 </span><br/>The Black Hole at the Center of the Milky Way</p> 
  
   <p class="main-text_text-chapter-opener-para"><span class="_idGenDropcap-1">A</span>top Mauna Kea, a dormant volcano on the island of Hawaii, the atmosphere is clear, calm, and dry for much of the year. There are no nearby mountain ranges to unsettle the atmosphere and few city lights to pollute the night skies. It’s an astronomer’s paradise.</p> 
   <p class="main-text_TEXT">This is the home of the Keck Observatory’s twin 33-foot (10 m) telescopes, each one roughly the size of a tennis court. They are the largest optical and infrared telescopes in the world. And it’s the place where Andrea Ghez and her team discovered the black hole at the center of the Milky Way galaxy.</p> 
   
   <p class="main-text_TEXT">When Ghez arrived at the University of California, Los Angeles, in 1994, she knew she wanted to study black holes. In the mid-1980s, scientists had discovered some galaxies that had very active centers. “I like to call them the prima donnas [leading ladies] of the galaxy world,” she said in a TED talk, “because they are kind of show offs.” </p> 
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   <p class="caption-B _idGenParaOverride-1">The Subaru and Keck 1 telescopes atop Mauna Kea in Hawaii are the largest optical and infrared telescopes in the world. </p> 
   <p class="main-text_TEXT">The center of these galaxies, called active galactic nuclei, is a lot brighter and more energetic than you’d expect if they were simply made of stars. (Unlike the nucleus of a cell, the nucleus of a galaxy is not a precisely defined structure. It is roughly the central 1 percent of the galaxy.) For this reason, astronomers believed that these galaxies had some voracious black holes. Black holes are notoriously sloppy eaters. And when they devour nearby materials, they belch jets of X-ray and radio wave energy.</p> 
   <p class="main-text_TEXT">Scientists like Ghez suspected that most—if not all—galaxies have giant black holes at their center. And many of them seemed to be quiet and not at all greedy. Astronomers had just the candidate for such a black hole at the center of the Milky Way: a mysterious source of radio waves called Sagittarius A* (Sagittarius A-star).</p> 
   <p class="main-text_TEXT">Ghez knew that if Sagittarius A* were indeed a black hole, even a quiet one, it would affect the orbits of nearby stars. Determining the mass of Sagittarius A*—critical to identifying it as a black hole—would be a matter of measuring the speed and radius of any stars orbiting Sagittarius A*. Ghez would use math developed in the early seventeenth century by astronomer Johannes Kepler. The third of his planetary laws describes a way to calculate the mass of the sun (or any other massive object) from measuring the orbital period (T) and orbit radius (R) of any orbiting satellite. It’s a straightforward calculation: R3/T2=M. R is the distance from Earth to the sun, about 93 million miles (150 million km). T is in years, and M is the mass of the central massive object (in this case, the black hole).</p> 
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   <p class="caption-A">This image shows X-rays (blue) from the Chandra telescope and infrared emission (red and yellow) from the Hubble telescope. The inset shows a closeup of Sagitarrius A* in X-rays only, covering a region half a light-year wide. (A light-year is the distance that light can travel in a vacuum in one year—about 5.9 trillion miles, or 9.5 trillion km.) Andrea Ghez and her team discovered this giant black hole at the center of our Milky Way galaxy.</p> 
   <p class="main-text_TEXT">Ghez knew that Keck telescopes would be powerful enough to allow her to see all the way to the center of our galaxy. But there was one problem. Even though the atmosphere at the summit of Mauna Kea, nearly 14,000 feet (4,267 m) above the surface of Earth, is fairly thin, “it’s like looking at a pebble at the bottom of a stream,” she explained. “The stream is continuously moving and turbulent, and that makes it very difficult to see the pebble on the bottom of the stream. Very much in the same way, it’s very difficult to see astronomical sources, because of the atmosphere that’s continuously moving by.” </p> 
   <p class="main-text_TEXT">To solve her problem, Ghez used a new technique called adaptive optics to correct for atmospheric distortions. Her technique uses lasers and mirrors in the telescope’s optics system to correct for the movement of the atmosphere.</p> 
   <p class="main-text_TEXT">The difference was amazing. Just as eyeglasses help people see things clearly, the new technique helped Ghez see and identify dozens of stars. She patiently tracked the orbits of those stars for sixteen years. She plotted their orbits. She determined their size and speed. One star, SO-2, took only fifteen years to orbit the mysterious dark object! Think about it. It takes our sun and our solar system about two hundred million years to travel around the center of the Milky Way galaxy. Stars closer to the center of the galaxy take five hundred years. The unusually speedy SO-2 is Ghez’s favorite star.</p> 
   <p class="main-text_TEXT">Using the data about SO-2’s orbit, Ghez calculated that the radius of the dark object it was orbiting (Sagittarius A*) was about the size of our solar system. Crammed into that area was the mass of four million suns. There was only one logical conclusion: Sagittarius A* was an enormous black hole.</p> 
   <p class="main-text_TEXT">Ghez had no aha moment with this discovery. It was the result of decades of painstaking observations. “I wish I had kept a diary,” she said, “because it’s been such a journey.” She had struggled to get funding. Reviewers of her project proposal had initially said, “Your technique won’t work, you won’t see anything.” Ghez proved the naysayers wrong, and that, she says, was pretty thrilling.</p> 
   <p class="main-text_TEXT">Some of Ghez’s findings are puzzling, even to her. Scientists know that a star is formed when gravity pulls together a cloud of gas and dust. The increased pressure inside the cloud causes the temperature at the core to rise to about 15 million°K, nuclear fusion begins, and voila! A star is born.</p> 
   <p class="main-text_TEXT">A black hole should be a terrible place for a stellar nursery. You would expect its strong gravity to pull matter away from a developing star. And yet Ghez finds lots of young stars orbiting Sagittarius A*. Scientists also expected to see a lot of older stars clustered around the black hole. They have been around the center of the galaxy for a long time, more than long enough to be pulled toward Sagittarius A*. But Ghez only saw a few. Why? Ghez doesn’t know, but she says that working to tease out the solution to the puzzle is “great fun.” </p> 
  
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<p class="sidebar_A-sidebar-head">Patrolling the Cosmic Highway, or How to Catch a Speeding Galaxy</p> 
   <p class="sidebar_A-sidebar-text-no-indent">How do astronomers measure the speed of distant objects such as stars and galaxies? After all, they’re light-years away! (Astronomical distances are so great that astronomers speak of them not in miles or kilometers but in light-years. A light-year is the distance that light can travel in a vacuum in one year—about 5.9 trillion miles, or 9.5 trillion km).</p> 
   <p class="sidebar_A-SIDEBAR-TEXT">To measure speed, astronomers rely on the Doppler shift. Depending on a star’s characteristics, including what it’s made of and its temperature, it will radiate light from a certain part of the electromagnetic spectrum. For example, the coolest stars in the universe, red dwarf stars, emit light at the red end of the visible spectrum. The hottest stars in the universe emit light at the blue end of the visible spectrum. The brightest visible star in our night sky is Sirius, a hot blue-white star. We can measure the temperature and chemical composition of Sirius and predict that it should radiate its light at a certain wavelength.</p> 
   <p class="sidebar_A-SIDEBAR-TEXT">If Sirius were to suddenly begin racing toward Earth (it won’t), the wavelengths would become even shorter and they would increase in frequency. The star would appear even bluer than it is now. Astronomers would say that the star had blue-shifted, and they could use that information to calculate how fast the star is speeding toward us. If Sirius began moving just as speedily away from us, the wavelengths that reach us from the star would lengthen and shift toward the cooler, red end of the spectrum. Astronomers would say that Sirius had red-shifted, and they could use that information to calculate how rapidly the star is receding. Blue- and red-shifting terminology comes from light in the visible spectrum. Astronomers use the terms, however, to describe a shifting in any part of the electromagnetic spectrum. If an object emitting gamma rays is moving away from us, for example, its wavelength might be stretched out into the X-ray part of the spectrum.</p> 
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   <p class="sidebar-caption-A">Astronomers can determine the speed and direction of a moving body by observing changes in the length and frequency of the wavelengths of that object. This infographic illustrates the changes associated with red-shifting (moving away from the observer) and blue-shifting (moving toward the observer).</p> 
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    <p class="main-text_H2-BHEAD">G2 Fireworks!</p> 
   <p class="main-text_text-no-indent _idGenParaOverride-1">And there are many other mysteries. In 2011 astronomers at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, discovered a strange object. </p> 
   <p class="main-text_TEXT">It appeared to be a cloud of gas, with a mass roughly three times that of Earth, in a wild orbit around Sagittarius A*. The object, dubbed G2, seemed to be making a beeline for the black hole, only to whip around it and shoot straight back into orbit again. Astronomers expected to see fireworks as G2 eventually met its end in the black hole.</p> 
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   <p class="caption-A">This view shows a simulation of how the gas cloud G2 approaching Sagitarrius A* may one day break apart. G2’s orbit is shown in red. The remains of the gas cloud are in red and yellow. The stars orbiting the black hole are shown with blue lines to mark their orbits.</p> 
   <p class="main-text_TEXT _idGenParaOverride-1">But G2 is still going strong. In 2014 Ghez and her team observed G2 continuing on its merry way around Sagittarius A*. The exact nature of G2 is still unclear. </p>  <p class="main-text_TEXT">But Ghez does not believe it’s a gas cloud. If it were, it would have been torn apart by the black hole. In a press release, Ghez confirmed, “G2 was basically unaffected by the black hole. There were no fireworks.” Instead, Ghez thinks that G2 is probably a star that was created when the black hole’s gravity drove two binary stars (two stars that revolve around each other) to merge into one.</p> 
  
   
   
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   <p class="sidebar-caption-A">Andrea Ghez is an astrophysicist at the University of California, Los Angeles. She has always loved puzzles—jigsaw puzzles, crossword puzzles, sudoku, and Kenken. She says that’s why she loves science. It’s another type of puzzle.</p>
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<p class="sidebar_A-sidebar-head">Profile: Andrea Ghez</p> 
   <p class="sidebar_A-sidebar-text-no-indent">When Andrea Ghez was a little girl in Chicago, Illinois, she dreamed of becoming a ballerina. Yet Ghez says she was meant to be a scientist. When she was four years old, she was fascinated by NASA’s 1969 <span class="sidebar_A-sidebar-CMS-ital _idGenCharOverride-5">Apollo</span> moon landing, the first time humans had ever stepped onto the lunar surface. She told her mother that she wanted to become NASA’s first female astronaut. Physicist Marie Curie and pilot Amelia Earhart captured her imagination too, and in school, she excelled in math and science. “The idea of the universe kept me up at night,” she said. </p> 
   <p class="sidebar_A-SIDEBAR-TEXT">She spent summers working on telescopes in Arizona and in the South American nation of Chile. She was hooked.</p> 
   <p class="sidebar_A-SIDEBAR-TEXT">In 1987 Ghez started graduate school at the California Institute of Technology in Pasadena. She wanted to study black holes, but the techniques for studying them were not very advanced. She ended up studying the birth of stars instead and became very skilled at producing images of stars from computer data. This skill was later key to her work on black holes.</p> 
   <p class="sidebar_A-SIDEBAR-TEXT _idGenParaOverride-1">In 1992 Ghez received her PhD in astronomy. Two years later, she joined the faculty at the University of California, Los Angeles, where she specializes in black holes. Ghez has received many awards, including a MacArthur Fellowship—often called a genius grant. She is sometimes referred to as a stargazing detective!</p> 
    
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   <p class="main-text_H2-BHEAD">Oops, You Got Too Close to a Black Hole. Now What?</p> 
   <p class="main-text_text-no-indent">In the 2014 movie <span class="main-text_CMS-ital _idGenCharOverride-5">Interstellar</span>, Earth is an ecological disaster. Former NASA astronaut Cooper (played by Matthew McConaughey) leads a mission to search for another habitable planet. He enters a black hole named Gargantuan and . . . no spoilers here!</p> 
   <p class="main-text_TEXT">Kip Thorne is a theoretical astrophysicist at the California Institute of Technology. He is an expert on black holes and was the script adviser and executive producer on the film. So the major plot points are based on real—or at least imaginable—science.</p> 
  
   <p class="main-text_TEXT">What <span class="main-text_CMS-ital _idGenCharOverride-5">would</span> happen if you fell into a black hole? Believe it or not, scientists have been playing this what-if game for decades. It’s not just a party game. The answer depends upon our understanding of the laws of physics as they work in black holes.</p> 
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   <p class="caption-A">This diagram shows how things could hypothetically disappear into a black hole. </p> 
   <p class="main-text_TEXT"> Let’s say you are the captain of a crew of astronauts in a spaceship nearing a stellar black hole—ten to one hundred times the mass of our sun. Your ace first mate puts your spacecraft into orbit a safe distance from the black hole’s event horizon: the point of no return. It appears as a sharply defined, pure black disk. It is surrounded by light from nearby stars, but the black hole’s gravity has distorted the light so that it appears as a distorted smear.</p> 
   <p class="main-text_TEXT">You, being a brave astronaut, want to get a closer look at the event horizon. So you suit up in your space suit and climb into a small space capsule and slow your orbit. The black hole’s gravity pulls you in closer to the event horizon. Your bravery turns to foolhardiness when you decide to take a little space walk. You’re too close—you find yourself falling into the black hole, feetfirst! (Forget for a moment that you are being bombarded by a deadly shower of X-rays and gamma rays. They are the least of your worries!) As you get closer to the black hole, its gravitational pull grows enormously. You don’t feel it because, being in free fall, you’re weightless. But you’ll feel something soon enough. As you near the event horizon, the force of gravity is much stronger at your feet than at your head, so they accelerate faster. Scientists call this difference in gravity the tidal force—exactly like the moon’s pull of gravity that produces tides here on Earth.</p> 
   <p class="main-text_TEXT">As the black hole’s gravity continues its pull, your body becomes stretched out. You eventually snap into two pieces. Soon you are a mess of molecules and atoms, squeezed through the fabric of space and time into nothingness at the center of the black hole. Scientists call this process, no kidding, “spaghettification.” </p> 
   <p class="main-text_TEXT">As your crew watches your demise in horror, they observe something very odd. The closer you get to the event horizon, the more you appear to move in slow motion. The light coming from you takes on a redder and redder hue. The wavelengths get longer until the crew needs a radio telescope to see you. When you reach the event horizon, the black hole has warped the space-time fabric so much that to outside observers, time crawls to a near halt. If your crew were patient (and long-lived) enough, they would eventually see your image just fade away.</p> 
   <p class="main-text_TEXT">Conventional theories say that you’d continue falling—for hours, maybe—until the last second before you reached the singularity. Only then would you become crushed into the singularity.</p> 
   <p class="main-text_H2-BHEAD">Hawking Radiation</p> 
   <p class="main-text_text-no-indent">Recent theories have proposed that you might not actually disappear completely. These theories can be traced back to English cosmologist Stephen Hawking, of the University of Cambridge. In the mid-1970s, he tried to reconcile the general theory of relativity, which describes the universe on a large scale, and quantum mechanics, which explains the behavior of the universe on a very small scale. Experiments have confirmed predictions made by both theories, but in their current forms, they cannot both be right.</p> 
    <p class="main-text_TEXT">The general theory of relativity says that when stuff falls into a black hole, everything about its existence is erased—gone forever, just like that. But quantum mechanics tells us that information can never be lost. According to this theory, if you were able to reach into a black hole, you should be able to reconstruct what fell in. So which theory is right?</p> 
   <p class="main-text_TEXT">In the mid-1970s, Hawking predicted that based on quantum mechanics, black holes would leak particles and radiation. Over billions of years, they would eventually evaporate, or dry up. The Hawking radiation, as the evaporation is known, would be completely random. It wouldn’t carry any information at all about the stuff inside the black hole. Hawking’s predictions started a decades-long “black hole information problem.” Is information lost or not?</p> 
   
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   <p class="caption-A">Professor Stephen Hawking attends a screening of the 2013 documentary <span class="main-text_caption-A-CMS-ital _idGenCharOverride-6">Hawking</span>. The film uses his own words and interviews with people who know him well to tell the life story of this iconic physicist. </p> 
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<p class="sidebar_A-sidebar-head">Quantum Theory</p> 
   <p class="sidebar_A-sidebar-text-no-indent">Quantum theory is the science of understanding the behavior of very small things such as atoms and subatomic particles, which are even smaller than atoms. Danish physicist Niels Bohr was one of the pioneers of quantum theory. He won a Nobel Prize in 1922 for his groundbreaking theories. Quantum theory is mind-boggling. In fact, Bohr himself once said, “Anyone who is not shocked by quantum theory has not understood it.” </p> 
   <p class="sidebar_A-SIDEBAR-TEXT">Quantum theory is based on some radical principles:</p> 
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    <li class="sidebar_A-sidebar-ul-first">Both energy and matter are made of individual units, or quanta. (The word comes from the Latin <span class="sidebar_A-sidebar-CMS-ital _idGenCharOverride-5">quantum</span>, “a share or portion of something.”)</li> 
    <li class="sidebar_A-SIDEBAR-UL-MIDDLE">On a large scale, energy and matter behave as waves. But on the atomic and subatomic level, energy and matter may behave either as particles or as waves. This theory is known as the wave-particle duality.</li> 
    <li class="sidebar_A-SIDEBAR-UL-MIDDLE">It is impossible to accurately measure both the position and the momentum of a particle at the same moment in time.</li> 
    <li class="sidebar_A-sidebar-ul-last">Any measuring device will affect the behavior of subatomic particles. So quantum mechanics can only help determine the <span class="sidebar_A-sidebar-CMS-ital _idGenCharOverride-5">probability</span> of the location or momentum of subatomic particles. This is known as the uncertainty principle.</li> 
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   <p class="sidebar-caption-A">Dutch physicist Paul Ehrenfest took this photo of Niels Bohr <span class="sidebar_A-sidebar-caption-directional">(right) </span>and Albert Einstein <span class="sidebar_A-sidebar-caption-directional">(left)</span> at the 1930 Solvay Conference on physics in Brussels, Belgium. The famous conference is still held every year.</p> 
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   <p class="sidebar_A-sidebar-H2head">Entanglement</p> 
   <p class="sidebar_A-sidebar-text-no-indent">Quantum theory makes some pretty wild predictions. Matter can be in more than one place or state at a time. An electron, for example, can be in two places at once, or it can simultaneously spin in opposite directions. It is only by measuring an object that it is forced into a well-defined place or state. According to quantum theory, it is possible that we live in a multiverse, a universe with many parallel worlds. Things can disappear and reappear somewhere else.</p> 
   <p class="sidebar_A-SIDEBAR-TEXT">The theory even predicts that when two particles interact, they become entangled. Two entangled electrons, for example, will always have opposite spins. One will spin in a clockwise direction and the other will spin counterclockwise, for a net spin of zero. But in the tiny quantum world, particles such as electrons do not have a defined spin until they are measured. Once a scientist measures the spin of one electron, the spin of its entangled partner immediately takes the opposite spin, even if they are physically separated. Einstein was skeptical of much of quantum theory, calling it “spooky action at a distance.” </p> 
   <p class="sidebar_A-SIDEBAR-TEXT">In 2015 scientists at Delft University of Technology in the Netherlands proved quantum entanglement. They trapped two electrons inside two diamonds—one per diamond—in different labs on the Delft campus, 0.8 miles (1.3 km) apart. They zapped each electron with a laser beam to entangle each with a photon. They sent those photons down an optical fiber to a third location, leaving the electrons behind. If the two photons arrived at precisely the same time (and they did), they would interact with each other and become entangled—and entangle their distant electron buddies as well. And whenever the scientists measured the spin of one electron in its diamond, the one across campus instantly took on the opposite spin. The two electrons were truly entangled.</p> 
    
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   <p class="main-text_H2-BHEAD">A WAY OUT</p> 
   <p class="main-text_text-no-indent">In 2012 a team of scientists from the University of California, Santa Barbara, suggested a possible answer. They proposed that an energetic “firewall” exists just inside a black hole’s event horizon. It would fry any entering astronaut or star to a crisp immediately. But based on the theory of quantum entanglement—which says that two particles separated by a great distance can still communicate with each other—there is another possibility. There could be two versions of the astronaut or star: one inside the event horizon of the black hole, incinerated, and another, whole, outside the event horizon.</p> 
   <p class="main-text_TEXT">Hawking and two colleagues at the University of Cambridge and Harvard University have come up with what may be at least a partial solution to the problem they first identified in the 1970s. In 2016 they wrote a paper “Soft Hair on Black Holes.” The paper suggests that very low or even zero-energy quantum particles (soft hair) sit on the edge of the black hole. These hairs capture and store information from the particles falling into the black hole. Hawking told the <span class="main-text_CMS-ital _idGenCharOverride-5">New York Times</span>, “If you feel you are trapped in a black hole, don’t give up. There is a way out.” </p> 
   <p class="main-text_TEXT">According to Hawking, the soft hairs would store whatever fell into the black hole as something like a holographic image (a 3-D representation of a two-dimensional image). The image would remain at the event horizon in a coded form.</p> 
   <p class="main-text_TEXT">This ongoing discussion among physicists won’t be resolved anytime soon. Meanwhile, it’s probably best to steer clear of black holes.</p> 
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