Dispatches from Planet 3

by Marcia Bartusiak

  The eminent Henry Cavendish, discoverer of hydrogen and its connection to water, read Michell’s paper before the Royal Society over three successive meetings in December 1783 and January 1784. It was then published in the Royal Society’s Philosophical Transactions, taking up twenty-three pages in print. Michell was devoted to the Society and at least once a year traveled the arduous two hundred miles (three hundred kilometers) from Yorkshire to London to either attend its meetings or meet with Society friends. But for those December and January meetings, the reverend inexplicably stayed home. It could have been ill health, but some historians have speculated that Michell recognized the daring nature of his paper and thought it would be more readily accepted if his close friend and highly respected colleague presented it to the Society.

  The radical technique that Michell was proposing to apply to study the stars involved the speed of light. If astronomers closely monitored the two stars in a binary system moving around each other over the years, noted Michell, they could calculate the masses of the stars. It was a basic application of Newton’s laws of gravity. And if the motions of paired stars were affected by each other’s gravitation, suggested Michell, light should also be affected. This was an era when light was assumed to be made up of “corpuscles,” swarms of particles—largely because the great Newton had championed that idea.

  The eighteenth-century scientific paper in which John Michell first suggested the existence of the Newtonian version of a “black hole.”

  (Philosophical Transactions of the Royal Society of London, Michell 1784)

  Now imagine those particles journeying off a star and out into space. Michell assumed that they, too, would be attracted by gravity, just as matter is. The more sizable the star, the stronger the gravitational hold upon the light, slowing down its speed. There would be a “diminution of the velocity of [the stars’] light,” as the title of his paper announced. Measure the velocity of a beam of starlight entering a telescope and, voilà, you obtain a means of weighing the star.

  This is where the “black hole” possibility arises: Michell took this scenario to the extreme and estimated when the mass of the star would be so great that “all light . . . would be made to return towards it, by its own proper gravity”—like a spray of water shooting up from a fountain, reaching a maximum height, and then plunging back down to the bowl. With not one radiant corpuscle escaping from the star, it would remain forever invisible, like a dark pinpoint in the sky. According to Michell’s calculations, this transformation would occur when the star was about five hundred times wider than our Sun and just as dense throughout. In our solar system, such a star would extend past the orbit of Mars.

  In 1796, in the midst of the French Revolution, the mathematician Pierre-Simon Laplace independently arrived at a similar conclusion. He briefly mentioned these corps obscurs, or hidden bodies, in his famous Exposition du Système du Monde (The system of the world), essentially a handbook on the cosmology of his day. “A luminous star of the same density as the Earth, and whose diameter should be two hundred and fifty times larger than that of the Sun,” he wrote, “would not, in consequence of its attraction, allow any of its ray to arrive at us; it is therefore possible that the largest luminous bodies in the universe, may, through this cause, be invisible.” Laplace’s estimate for the width of the dark star differed from Michell’s because he assumed a greater density for sunlike bodies.

  But did it even make sense to predict the existence of stars that could never be seen? Laplace may have had second thoughts, or simply a loss of interest. In subsequent editions of Système du Monde, which he published up until his death in 1827, he expunged his invisible-star speculation and never referred to it again. Michell, on the other hand, displayed greater ingenuity by suggesting a way to “see” such invisible stars. If one of them revolved around a luminous star, he noted, its gravitational effect upon the bright star’s motions would be noticeable. It’s the very way that astronomers today track down black holes.

  In the end, though, Michell and Laplace were getting ahead of themselves, contemplating problems before the physics was in place to answer them. They didn’t yet realize that supergiant stars have far lower densities than the ones they envisioned. They also never considered that the same invisibility effect could happen if a star were smaller but very, very dense. They just assumed that all stars shared the same density as the Sun or Earth. Could anything be more dense than the elements found on Earth? It seemed unthinkable in the late eighteenth century.

  Both Michell and Laplace were working with an inadequate law of gravity and the wrong theory of light. They didn’t yet know that light never slows down in empty space. Proving the existence of such dark stars required more advanced theories of light, gravity, and matter. The modern conception of the black hole would not emerge for nearly a century. It had to wait for the entrance of the twentieth century’s most inventive natural philosopher, Albert Einstein.

  CHAPTER NINE

  As Though No Other Name Ever Existed

  How the term “black hole” entered the scientific

  literature

  THE term “black hole” has a deep, dark past and a notorious reputation. In June 1756, on the banks of the Hooghly River in Calcutta, India, at the British garrison of Fort William, 144 British men and two women were taken prisoner by the troops of the new Nawab of Bengal, Siraj-ud-daula. Siraj’s men incarcerated at least sixty-four of the hostages for a night in a tiny cell known as the “black hole.”

  Only twenty-one survived the hot night, which was suffocating—literally. Ever since that horrific event, the words “black hole” have referred to a place of confinement, a locked cell, where it was anticipated that once you went in, you never came out. How did the term come to signify objects in outer space?

  Toward the end of the 1960s, when astronomers were coming to recognize that massive stars upon running out of fuel might actually collapse to a singular point (with, theoretically, infinite density and zero volume), they had a problem. For many years, theorists had been calling such an entity a “gravitationally collapsed object,” a real mouthful to pronounce over and over again in a lecture. Soon, some shortened the awkward phrase to “collapsar,” while others preferred “dark star.” In short, the terminology kept shifting. That all changed in 1967, when the noted Princeton University physicist John Archibald Wheeler supposedly linked the term “black hole” to the cosmos. The attribution of that lexical connection, however, has recently been challenged.

  Physicist John Wheeler was often credited

  with assigning the phrase “black hole” to

  gravitationally collapsed stellar objects.

  (Photograph by Roy Bishop, Acadia University,

  courtesy AIP Emilio Segrè Visual Archives)

  Wheeler usually told his side of the story in the following fashion. It was the fall of 1967, and he was attending an impromptu conference at the NASA Goddard Institute for Space Studies in New York City. Pulsars had just been detected for the first time, and astronomers were asking whether those mysterious, pulsed radio waves were coming from red giant stars, white dwarfs, or neutron stars. According to Wheeler, he told the assembly that his “gravitationally collapsed objects” might be responsible. “Well, after I used that phrase four or five times, somebody in the audience said, ‘Why don’t you call it a black hole?’ So I adopted that,” said Wheeler.

  While pulsars were first detected in 1967, however, their existence remained a well-kept secret until 1968; the public announcement of the discovery was made in February 1968, when a paper on the topic was finally published in the journal Nature. Did Wheeler misremember the nature of his fall conference? There was a meeting on supernovae at the Goddard Institute in November 1967, but Wheeler’s name is missing from the official conference proceedings.

  What Wheeler did do, without dispute, is use the phrase “black hole” during an after-dinner talk at the annual meeting of the American Association for the Advancement of Science in N
ew York City on December 29, 1967. The term then made it into print when an article based on his talk, titled “Our Universe: The Known and the Unknown,” was published in American Scientist in 1968. Wheeler’s enduring credit for introducing the phrase is due to that popular paper.

  Yet firm evidence exists that the term actually arose much earlier, even in print. It was casually bandied about at the 1963 Texas Symposium for Relativistic Astrophysics. Reporting on the Texas conference, the science editor for Life magazine at the time, Albert Rosenfeld, used the term “black hole” in an article on the newly discovered quasars. He noted how astrophysicists Fred Hoyle and William Fowler suggested that the gravitational collapse of a star might explain the quasar’s energy. “Gravitational collapse would result in an invisible ‘black hole’ in the universe,” wrote Rosenfeld. Rosenfeld today is sure he didn’t invent the term but overheard it at the meeting, although he cannot recall the source.

  The phrase was mentioned again a week later at an American Association for the Advancement of Science meeting held in Cleveland. Ann Ewing of Science News Letter reported that astronomers and physicists at the conference were suggesting that “space may be peppered with ‘black holes.’” The person who used the term there was Goddard Institute physicist Hong-Yee Chiu, who had originated the term quasar in 1964 in Physics Today and had also attended the Texas Symposium. Was he introducing another fun term to the public? No, answers Chiu; he borrowed it from the man who may have coined the phrase from the start.

  From 1959 until 1961 Chiu was a member of the Institute for Advanced Study in Princeton, and during that time Prince-ton physicist Robert H. Dicke, both an experimental and a theoretical contributor to the study of gravitation, spoke at a colloquium about how general relativity predicted the complete collapse of certain stars, creating an environment where gravity was so strong that neither matter nor light could escape. “To the astonished audience, he jokingly added it was like the ‘Black Hole of Calcutta,’” recalls Chiu. A couple of years later, when Chiu started working at the Goddard Institute, he heard Dicke there casually use the phrase once again during a series of visiting lectures. In this way, Dicke may have released the term into the scientific atmosphere.

  Physicist Robert Dicke was likely the first to

  introduce the term black hole, during a lecture at

  Princeton in the early 1960s.

  (Courtesy AIP Emilio Segrè Visual Archives, Physics Today

  Collection)

  Loyola University physicist Martin P. McHugh, while working on a biography of Dicke, discovered it was one of Dicke’s favorite expressions. He often used it with his family in an entirely different context. His sons told McHugh that their father exclaimed, “Black Hole of Calcutta!” whenever a household item appeared to have been swallowed up and gone missing.

  Yet, Wheeler still deserves a large portion of the credit for placing the phrase into the scientific lexicon. Given Wheeler’s status in the field, his decision to adopt the moniker bestowed a gravitas upon it, giving the science community permission to embrace the term without embarrassment. “He simply started to use the name as though no other name had ever existed, as though everyone had already agreed that this was the right name,” said his former student, Caltech physicist Kip S. Thorne.

  Wheeler’s strategy worked splendidly. Within a year of his New York talk, the idiom gradually began to be used in both newspapers and the scientific literature—although for a while at first it was written down as “the black hole,” an expression so exotic it needed to be held at a distance within quotation marks. Some, like Richard Feynman, thought the term was obscene. “He accused me of being naughty,” Wheeler recalled in his autobiography, Geons, Black Holes, and Quantum Foam: A Life in Physics. But Wheeler was attracted to its link to other physics terms, such as “black body,” an ideal body that absorbs all the radiation that falls upon it and emits all the energy it absorbs. A black hole does the former but not the latter. It seemingly emits nothing . . . zip . . . nada (more on this in Chapter 30). We look in and see only a dark emptiness. “Thus black hole seems the ideal name,” concluded Wheeler. Moreover, it fit the very physics of the situation. The collapsed stellar remnant, with its infinite density, was literally digging a hole—a bottomless pit—into the flexible fabric of space-time.

  “The advent of the term black hole in 1967 was terminologically trivial but psychologically powerful,” said Wheeler. “After the name was introduced, more and more astronomers and astrophysicists came to appreciate that black holes might not be a figment of the imagination but astronomical objects worth spending time and money to seek.” The black hole had finally made it into the big time.

  CHAPTER TEN

  Like This World of Ours

  Confirming that extrasolar planets do exist

  IN 2017 an international team of astronomers thrillingly revealed, after examining a collection of data gathered by both NASA’s Spitzer Space Telescope and an array of telescopes around the world, that they had found an extrasolar planetary system with at least seven members—all roughly the size of the Earth. These newfound celestial bodies were closely circling a red, Jupiter-sized star known as TRAPPIST-1. The star had been named after the TRAnsiting Planets and PlanetesImals Small Telescope network in Chile and Morocco, which first encountered this extrasolar system. At least three of TRAPPIST-1’s rocky planets are likely to harbor liquid water, but so could all seven.

  More exciting is that these terrestrial-like worlds are located a relatively scant thirty-nine light-years away in the direction of the Aquarius constellation. In cosmic terms, that’s practically next door. Such proximity will allow astronomers to achieve one of their fondest dreams: eventually using current and future telescopes to study the planets’ atmospheres in search of gases conducive to life, such as oxygen, ozone, and carbon dioxide.

  An artist’s concept of TRAPPIST-1’s seven planets based on data about their diameter, mass, and distance from the host star.

  (NASA/JPL-Caltech)

  According to the Extrasolar Planets Encyclopedia, the number of extrasolar planets so far revealed in our galaxy now totals in the thousands. The TRAPPIST system was only one of the latest finds in the burgeoning field of exoplanetary astronomy.

  Although this is a rather new scientific field, speculation that planetary systems circle other stars started long, long ago—in ancient times. In the fourth century BCE, the Greek philosopher Epicurus, in a letter to his student Herodotus, surmised that there are “infinite worlds both like and unlike this world of ours.” As he believed in an infinite number of atoms careening through the cosmos, it only seemed logical that they’d ultimately construct limitless other worlds.

  The noted eighteenth-century astronomer William Herschel, too, conjectured that every star might be accompanied by its own band of planets but figured they could “never be perceived by us on account of the faintness of light.” He knew that a planet, visible only by reflected light, would be lost in the glare of its sun when viewed from afar.

  But astronomers eventually realized that a planet might be detected by its gravitational pull on a star, causing the star to systematically wobble like an unbalanced tire as it moves through the galaxy. Starting in 1938, Peter van de Kamp at Swarthmore College spent decades regularly photographing Barnard’s star, a faint red dwarf star located six light-years away that shifts its position in the celestial sky by the width of the Moon every 180 years, faster than any other star. By the 1960s, van de Kamp got worldwide attention when he announced that he did detect a wobble, which seemed to indicate that at least one planet was tagging along in the star’s journey. But by 1973, once Allegheny Observatory astronomer George Gatewood and Heinrich Eichhorn of the University of Florida failed to confirm the Barnard-star finding with their own, more sensitive photographic survey, van de Kamp’s celebrated claim of detecting the first extrasolar planet disappeared from the history books.

  The wobble technique lived on, however, in another fashion. Astro
nomers began focusing on how a stellar wobble would affect the star’s light. When a star is tugged radially toward the Earth by a planetary companion, the stellar light waves get compressed—that is, made shorter—and thus shifted toward the blue end of the electromagnetic spectrum. When pulled away by a gravitational tug, the waves are extended and shifted the other way, toward the red end of the spectrum. Over time, these periodic changes in the star’s light can become discernible, revealing how fast the star is moving back and forth due to planetary tugs.

  In 1979, University of British Columbia astronomers Bruce Campbell and Gordon Walker pioneered a way to detect velocity changes as small as a dozen meters a second, sensitive enough for extrasolar planet hunting to begin in earnest. Constantly improving their equipment, planet hunters were even more encouraged in 1983 and 1984 by two momentous events: the Infrared Astronomical Satellite (IRAS) began seeing circumstellar material surrounding several stars in our galaxy; and optical astronomers, taking a special image of the dwarf star Beta Pictoris, revealed an edge-on disk that extends from the star for some 37 billion miles (60 billion kilometers). It was the first striking evidence of planetary systems in the making, suggesting that such systems might be common after all.

  The first indication of an actual planet orbiting another star arrived unexpectedly and within an unusual environment. In 1991, radio astronomers Alex Wolszczan and Dale Frail, while searching for millisecond pulsars at the Arecibo Observatory in Puerto Rico, saw systematic variations in the beeping of pulsar B1257+12, which suggested that three bodies were orbiting the neutron star. Rotating extremely fast, millisecond pulsars are spun up by accreting matter from a stellar companion. So, this system, reported Wolszczan and Frail, “probably consists of ‘second generation’ planets created at or after the end of the pulsar’s binary history.”