Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos

by Scharf, Caleb

  Gendreau was helping to construct and calibrate the camera for a joint Japanese-American project called the Advanced Satellite for Cosmology and Astrophysics, or ASCA. Launched into orbit around the Earth in 1993 from the Uchinoura Space Center at the southern tip of Japan, ASCA spent the next eight years gathering images of the X-ray universe before burning up in the atmosphere over the Pacific Ocean. In the meantime, Gendreau moved to NASA’s Goddard Space Flight Center in Maryland, just outside Washington, D.C. An irrepressible creator and tinkerer, he was soon helping to lead a new NASA project in its infancy, a mission aiming to do what seemed to be the impossible. Instead of just studying the remote outward effects of black holes in the universe, NASA aimed to directly observe the event horizon itself.

  It might seem counterintuitive, as we think that the event horizon is effectively dark nothingness. In fact, it is—except that the space immediately outside the horizon around a feeding black hole is aglow with the final gasps of matter, and the brilliant disk of accreting material can highlight and pinpoint the location of its impending doom. For Gendreau and his fellow scientists, this is the key to seeing a black hole; all you have to do is to make an image of the intense X-ray light flooding from the disk and its immediate surroundings.

  The catch is that even for a supermassive black hole, that innermost disk is perhaps only a few light-days across, yet it may be tens of thousands of light-years away from us. If you want to look at the event horizon of the 4-million-solar-mass black hole at the center of the Milky Way, you need to be able to see with extraordinary resolution. It’s like taking a good image from Earth of a coin on the surface of the Moon, or seeing the individual pixels on a high-definition TV that is, rather inconveniently, more than three thousand miles away.

  Building a telescope to do this presents a phenomenal challenge. The physical properties of light itself create an unavoidable hurdle for any kind of astronomical telescope. Light behaves as electric and magnetic waves that are distorted—diffracted—as they pass into optical apertures and lenses. A perfectly clean wave front becomes a messy one, much like water sloshing in through a harbor entrance. This causes an inevitable blurring of the final image. The smaller the diameter of the instrument, the blurrier the image will be. That’s why astronomers love to build big telescopes—they’ll have a better chance of making a crisp, sharp picture. Understandably, to create an image of a distant event horizon is going to require an enormous telescope.

  With X-ray photons there is an additional obstacle. There’s a reason why we use X-rays to take images of the interior of our bodies: these small-wavelength photons penetrate better than visible-wavelength photons. Without using exotic materials or complex optical tricks, building an X-ray telescope the way you build an optical one is effectively impossible. Instead, astronomers rely on ingenious techniques to bring X-rays together to form an image. One way is to gently coerce the photons into focus, allowing them to skim like skipping stones across the highly polished surfaces of metal-coated silicon. By constructing a series of glassy cylinders within cylinders, like a set of nested Russian dolls, astronomers can coax the X-ray light toward a focal point sensor in a camera.

  It works, but it’s hard to make these telescopes big enough to form the sharp, high-resolution images we’d like. The clever solution is to make many little telescopes that act like they are all part of one giant telescope. To build an instrument capable of imaging the matter around an event horizon, we can place dozens of smaller telescopes in space, spread out in a great array many tens of miles across. As they gather up X-ray photons, they beam them across the vacuum to a single camera or detector. By merging these photons, and allowing the electromagnetic waves to combine, we can form an image of incredible resolution. Many telescopes become one.

  One design for such a system calls for two dozen small X-ray mirrors called “periscopes” to fly in a great swarm a mile wide. Each periscope is a set of perfectly flat surfaces positioned to gently channel the skittish X-ray photons and divert them toward a master “detector” spacecraft. The many-eyed swarm hovers in space, peering into the cosmos. The detector sits twelve thousand miles away and houses a sensitive digital camera that finally captures and measures the X-ray light. It’s a megalomaniac telescope, dozens of spacecraft exploded out into a huge flock-like formation deep in interplanetary space.

  Talk to Gendreau and other astrophysicists who are just as passionate about pushing the boundaries of technology and knowledge, and you’ll come away with the sense that we could really do this. That’s despite some enormous technical hurdles. For example, once we launch a spacecraft like this, we need to position it with a precision of a few ten billionths of a meter in order for the combined light to come into correct focus. Even the ethereal force of solar radiation, the beating pressure of stellar photons, is enough to disturb such a delicate ballet. The tiny gravitational pull of other solar system planets, like Jupiter, hundreds of millions of miles away can also throw things out of alignment. All that has to be accounted for and corrected for in order for this armada to hold its position in interplanetary space.

  Impossible? It’s not. Spacecraft engineering and technology for measuring position and orientation have come a very long way since the early days of crude rockets flung skyward. Standing in his laboratory, Gendreau and I sip our steaming-hot coffee and talk of superfluid helium gyroscopes and other tricks that exploit quantum physics to make the necessary measurements to hold a spacecraft steady. Even the engineering required for a ship to reorient itself fractions of a micrometer at a time is under consideration. It’s breathtaking that this is within our reach should we choose to place resources into such an enterprise. A mission like this, now given the official and immodest name of Black Hole Imager, or BHI, would let us see through the dust and gas cloaking the core of our own galaxy, or other places like it. It would allow us to peer into the very workings of a black hole.

  We could watch as matter spirals inward, observing its textures and behaviors. We might see how a spinning hole launches its great jets of racing particles, how it reaches out into the universe. We would see the precise mechanism of gravity’s engines, the origins of the vast outpourings of energy. The BHI would track material as it sweeps around the innermost parts of the accreting disk and as it is caught up in the whirling spacetime itself. It’s the ultimate experiment at bath time, peering into the drain as the water and suds vanish with a slurp. The black hole will bend and distort the light we see, a perfect test of our skills in applying the theories physicists first drafted on paper and chalkboard almost a century ago.

  Let’s suppose that we do it. We build such a remarkable extension of our human senses, and we peer into the pinholes of spacetime that puncture the universe. We will discover surprises that we could never have anticipated. Whatever they are, they will be wonderful. Finally, thousands of generations after our hominid ancestors loped across the plains of Earth, we would be witness to the endpoints of matter in the universe. We have already found matter’s starting point. In the mottled haze of cosmic background photons and the faintest recesses of electromagnetic radiation, we see the imprints of the primordial cosmos, the first steps of normal matter into a nearly 14-billion-year history. And in our great particle accelerators we are re-creating the conditions of the universe mere instants after the Big Bang, letting us peer into the exotic fields and particles that are our distant progenitors.

  But now, as we stare into the twisted chasms the universe has made in itself, we see the same matter leaving us behind. For all intents and purposes it is sinking into, but also out of, this cosmos. With a final impossibly dim and reddened glimmer, these particles are releasing themselves to eternity as they pass across the event horizon. Yet just before the end of this remarkably long journey, from Big Bang to oblivion within the sheath of an event horizon, matter plays one final role: it gives up what energy it can, and that energy surges back out into the universe to sculpt and color the very environment we occupy.

/>   In a fit of enthusiastic optimism I tell Gendreau that when the BHI sends back its first picture of an event horizon, he and I will take a trip. We’ll fly across one of Earth’s great oceans and make our way to a small town surrounded by green hills. There we’ll go for a walk on the grounds of Thornhill Rectory, looking for a spot where centuries earlier John Michell might have paused to breathe in the fresh air and gaze upward. When we think we’ve found it, we’ll make a little monument, an image in a frame planted in the ground with a spike. Here, at last, the dark stars will have come home.

  NOTES

  1. DARK STAR

  Chandra: One of NASA’s “Great Observatories,” ranked as the Hubble Space Telescope’s equal in ambition and cost. Launched on July 23, 1999. Information is available at the NASA/Chandra Science Center/Harvard site: http://chandra.harvard.edu.

  Twelve billion years: Travel time for photons from this distant location, corresponding to a cosmological redshift of 3.8 (the ratio between the apparent recession velocity and the speed of light) and a co-moving distance (used in Hubble’s law) of about 23 billion light-years for a flat, vacuum energy–dominated cosmological model. In other words: a very long, long way away.

  superclusters: Collections of galaxy clusters and galaxies spanning hundreds of millions of light-years.

  Gondwana: Southern supercontinent believed to have existed from approximately 510 to 200 million years ago, which subsequently broke apart to form Africa, South America, Antarctica, Australia, and India. See, for example, Peter Cattermole, Building Planet Earth: Five Billion Years of Earth History (London: Cambridge University Press, 2000).

  Bayeux Tapestry: The sixty-eight-meter-long tapestry narrating the Norman invasion of England in 1066. Halley’s comet is depicted, and was likely seen four months prior to the invasion. The seventy-five-to-seventy-six-year orbital period of the comet was first determined correctly by the English astronomer Edmond Halley in 1705.

  past few decades: Two excellent further sources are Kip Thorne’s Black Holes and Time Warps: Einstein’s Outrageous Legacy (New York: W. W. Norton & Company, 1994), and the book by Mitchell Begelman and Martin Rees, Gravity’s Fatal Attraction: Black Holes in the Universe (Cambridge: Cambridge University Press, 2nd ed., 2010).

  Thornhill: The Church of St. Michael and All Angels, Thornhill Parish, has both a memorial to John Michell in its tower and a more modern plaque to commemorate his accomplishments. The memorial is notable for its effusive descriptions of his tender and affectionate traits.

  personal detail: More material is now coming to light about Michell, but I have drawn on numerous scraps of information from many different (often online) sources to produce a very modestly detailed portrait. Another source is Sir Archibald Geikie’s Memoir of John Michell, M.A., B.D., F.R.S., fellow of Queens’ college, Cambridge, 1749, Woodwardian professor of geology in the university 1762 (Nabu Press, 2010, reprint of original written in 1918), also available online at the University of California Libraries Digital Archives.

  works on navigation and astronomy: One of Michell’s best-known contributions to physical science was his role in the invention of the torsion balance with Henry Cavendish. This wonderful device allows the gravitational force between two ball-like masses to be measured (an extraordinary accomplishment given the weakness of the forces involved), and hence for the gravitational force law to be calibrated by measuring the gravitational constant. Cavendish and Michell have each occasionally and interchangeably been referred to as “the man who weighed the world,” although Michell died in 1793, some four years before Cavendish made the actual measurement of the Earth’s density.

  title of his paper: Michell’s presentation is published as a “Letter to Henry Cavendish” by John Michell, Philosophical Transactions of the Royal Society of London 74 (1784): 35–57.

  Maxwell’s work: These four relationships were published in toto by Maxwell in 1865, following two earlier papers that had laid the groundwork. James Clerk Maxwell, “A Dynamical Theory of the Electromagnetic Field,” Philosophical Transactions of the Royal Society of London 155 (1865): 459.

  Einstein would later write: This comment was made by Einstein in 1940 in the article “Considerations Concerning the Fundaments of Theoretical Physics” Science 91 (1940): 487.

  Michelson-Morley experiment: Often cited (as it is here) as the best failed experiment ever. But of course it didn’t really fail, it was simply so well executed that it revealed the truth. Michelson and Morley’s original paper is quite excellent: Albert A. Michelson and Edward W. Morley, “On the Relative Motion of the Earth and the Luminiferous Ether,” American Journal of Science 34 (1887): 333–45.

  special theory of relativity in 1905: Published by Albert Einstein as “Zur Elektrodynamik bewegter Körper,” which translates into English as “On the Electrodynamics of Moving Bodies,” Annalen der Physik 322, no. 10 (1905): 891.

  This simple fact: These have been discussed over the past century in many excellent accounts beyond those of Einstein himself. To my mind, the flexibility of time is still perhaps the most amazing and bewildering aspect. Little wonder that relativity has also been a topic for philosophers to mull over.

  general theory of relativity: I explore this topic in significantly more detail in chapter 3. To note here, however: there is some confusion in popular accounts about when Einstein published this theory. Although he presented the correct theoretical ideas in 1915, they were in a series of papers that included retractions and corrections of his earlier efforts. In 1916 he finally presented his better-known and more complete discussion and review article, “The Foundation of the General Theory of Relativity,” Annalen der Physik 49 (1916).

  In 1927, Heisenberg: His paper was “Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik,” translated roughly into English as “On the visualizable [or “intuitive”] content of quantum theoretical kinematics and mechanics,” Zeitschrift für Physik 43 (1927): 172. The word anschaulichen apparently defies easy translation into English.

  In the 1920s astronomers: Throughout this period the great English physicist Sir Arthur Eddington played numerous roles. Not least was his devastating (and ultimately incorrect) critique later on of Chandrasekhar’s 1935 presentation on white dwarfs.

  in 1935 Chandrasekhar presented: Key papers were “The Highly Collapsed Configurations of a Stellar Mass,” Monthly Notices of the Royal Astronomical Society 95 (1935): 207, and “Stellar Configurations with Degenerate Cores,” ibid., 226.

  extreme states of matter: Both nuclear bombs and stellar interiors involve environments where nuclear constituents (protons and neutrons) can become dissociated from their usual place within atomic nuclei. For objects like neutron stars, the need to use general relativity to understand their structure presents an additional challenge.

  John Wheeler: John Archibald Wheeler (1911–2008) was one of the great American theoretical physicists who worked on general relativity. He also worked on the Manhattan Project and mentored many extraordinary scientists, including Richard Feynman and Kip Thorne. Generations of students know him as one of the authors of the seminal textbook titled Gravitation with Charles Misner and Kip Thorne (San Francisco: W. H. Freeman, 1973)—all 1,200 pages of it.

  NASA Goddard Institute for Space Studies: One of the least-known NASA outposts, home to some of the best planetary and climate science research going on today. Tom’s (the restaurant) is still there, and tourists are often lined up taking pictures of it, because its exterior is used in the opening sequence for the TV show Seinfeld.

  2. A MAP OF FOREVER

  oldest recognizable: I have taken a small liberty here in making this a statement of fact. It seems that not everyone agrees on the celestial interpretation of these carvings and paintings from tens of thousands of years ago (during the Paleolithic). One interesting reference is the discussion by Amelia Sparavigna, “The Pleiades: The Celestial Herd of Ancient Timekeepers” (online in the physics preprint archives at http://arxiv.org/a
bs/0810.1592).

  range of photon wavelengths: Sources differ a little on the actual sensitivity range of human eyes, but between about 380 and 750 nanometers is typical. Sensitivity is not uniform, but peaks at around 550 nanometers (green light) and is a combination of the different cone and rod receptor sensitivities in the human retina. For comparison, bees have orange to blue sensitivity along with some ultraviolet sensitivity—spanning wavelengths of approximately 300 to 600 nanometers.

  Harlow Shapley: Many sources exist on Shapley’s life and long career. A detailed obituary was published in Nature: Z. Kopal, “Great Debate,” Nature 240 (1972): 429. The American Institute of Physics holds the transcript of an interview with Shapley in 1966 (www.aip.org/history/ohilist/4888_1.html). The Franklin Institute holds details of his life and family (www.fi.edu/learn/case—files/shapley/index.html).

  “The Sun is…” Shapley quote is taken from the published results of his globular cluster survey, “Globular Clusters and the Structure of the Galactic System,” Publications of the Astronomical Society of the Pacific 30 (1918): 42.

  modern mapping of the cosmos: Edwin Hubble, also using the Mount Wilson Observatory, found in the early 1920s that many nebulae were actually galaxies in their own right, separate and very distant from ours—something that Harlow Shapley did not initially agree with. By 1929, Hubble had also shown that they are all moving away from one another—the first direct evidence of the expansion of the universe. Edwin Hubble, “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae,” Proceedings of the National Academy of Sciences of the United States of America 15 (1929): 168.