Stars spin. Every star in the sky rotates. So, there was still the possibility that a stellar collapse is avoided when rotation is taken into account. That was on the minds of many. They continued to believe that the infamous “singularity” was imaginary, simply an artifact of the way Einstein’s equations were solved in that special case of a motionless star collapsing in a completely symmetric manner. Total collapse to “zero volume” still appeared too fantastical. But to prove that, relativists had to conquer their biggest unsolved problem: setting up the equations of general relativity in such a way that they could handle a rotating star. It was the field’s holy grail. The solution eluded theorists for decades. That is, until Roy Kerr, a mathematical physicist and New Zealander, tackled it.
Roy Kerr in 2013. (Courtesy of the University of Canterbury)
Right after World War II, Kerr completed his bachelor’s and master’s degrees at what is now the University of Canterbury in New Zealand, at a time when the library “was so bad that its modern physics books talked about ether theory,” he recalled. Kerr became interested in general relativity after he moved to Cambridge University in England for his doctoral studies, where for his dissertation he considered how particles move under its rules, such as two stars closely orbiting one another.
By the start of the 1960s relativists were being reenergized with the introduction of new mathematical approaches from differential geometry to solving Einstein’s field equations, which opened the door for more physics getting done. This new development created great excitement among relativists, shaking them out of their doldrums. Kerr got caught up in the fervor and began working on some of his own solutions, continuing as he settled into a job in 1962 at the University of Texas in Austin, where a new Center for Relativity was getting established.
It was tough going for Kerr, and after a few months of toil, a colleague in Austin showed him a paper about to be published that seemed to indicate that almost no solutions were possible for the problem he was working on. But skimming through the article, Kerr noticed a mistake in one of the equations, which indicated that wasn’t true. “The next few weeks turned into a furious cocktail of adrenaline, trancelike bouts of distraction, and the smoke from seventy cigarettes per day,” recounted physicist Fulvio Melia, Kerr’s biographer. Kerr went on to reduce the problem to a set of “fourth order” differential equations, which agreed with the results made by another team of relativists, Ivor Robinson and Andrzej Trautman. Whereas those men were attempting to calculate the most general cases, Kerr at that point decided on a different strategy. He threw out any result that had no connection to the physical world. “I wanted to find a solution that could represent something we find in the universe,” he says. He got rid of awkward terms by taking advantage of certain symmetries, a move that some thought inelegant. Most important, he chose a coordinate system that was axially symmetric. In other words, it had the potential to handle a rotation.
Kerr knew he was closing in when his solution matched Newton’s law of gravity, in the case of an observer being far from the source of gravity. But from that perspective, the rotation was not obvious. The next day in his office, with his boss Alfred Schild sitting nearby in an old armchair in anticipation, the young mathematician sat at his desk with pencil and paper to verify that the object he had placed in his virtual space-time did indeed have angular momentum. After half an hour, chain-smoking cigarettes as he calculated, Kerr turned to his companion and uttered, “Alfred, it’s spinning.” More than that, the rotating object was dragging space-time around with it, like the cake batter that circulates in the bowl around a whirling beater.
Known as “frame dragging,” this was a relativistic effect that two Austrian physicists, Josef Lense and Hans Thirring, first predicted using approximation methods in 1918. Kerr at last had the full solution. Schild was overjoyed by the result. “Cutting through the billowing smoke from his pipe,” reported Melia, “[Schild] rushed to the desk and looked over Kerr’s shoulder at the scratchings on the table.” He immediately recognized that Kerr had at last found the way to refashion Einstein’s equations to handle rotation. “I do not remember how we celebrated,” recalled Kerr years later, “but celebrate we did!”
In the parlance of general relativists, Kerr had devised a new “metric” to describe the space-time around a spinning object with mass. He had conquered general relativity’s Mount Everest of problems. The magnitude of this accomplishment was colossal—so much so that Kerr was quickly offered a tenured professorship at the university. It was quality—not quantity—that led to this outcome. His final paper, published in 1963 within a month of his submission to Physical Review Letters, was a mere one-and-a-half pages long. Schild was so excited by Kerr’s accomplishment that he wanted the university to bathe its campus tower in a celebratory orange glow, just the way the college always did when its football team won a game. (It didn’t happen.)
This all occurred just around the time that the Texas Symposium was being planned. Hearing that the symposium’s organizers were scheduling someone else to discuss his solution at one of the sessions, Kerr made sure he’d be the one at the podium (though he may have regretted that decision). “It went over like a lead balloon,” Kerr now recalls. In the few months between the publication of his paper and the Texas Symposium, Kerr had adapted his approach to handle an object collapsed to a Schwarzschild sphere. Given that quasars were the symposium’s main focus, he wanted to show how his solution might “explain the large energies emitted by quasi-stellar sources in terms of the gravitational collapse of large masses.” Rotation made a difference, he told the audience.
But the astronomers at the conference did not appreciate at all what the relativist had accomplished. They hardly listened while Kerr gave his ten-minute talk. Many slipped out of the auditorium to take a break; others snoozed in their chairs; a few ignored the speaker entirely and talked among themselves. The astronomers didn’t think space-time metrics or Schwarzschild surfaces had anything to do with quasars.
The relativists in the room, however, were riveted. At the end of the talk Achilles Papapetrou, a noted Greek relativist, got up and declared that Kerr had arrived at the rotation solution that he and others had been trying to find for some three decades. He shook his fist and scolded the audience for not listening. In the greatest of ironies, the astronomers in the room effectively yawned at the news. Kerr that December day was handing astronomers on a silver platter what came to be understood as the first model of a spinning black hole. If clever astrophysicists at the meeting had listened and made the leap, they would have found a bit sooner another possible source for powering those quasars they were so excited about, the specific raison d’être of the meeting—tapping into a black hole’s rotational energy.
A black hole’s spin is one key to its power. Think of an ice skater, arms spread out, who then brings them in to spin faster and faster. It’s the simple result of conservation of angular momentum: as the width of a rotating object decreases, its spin increases. A big rotating star that suddenly collapses to a small black hole takes this to the extreme; the black hole ends up spinning at humongous velocities. So fast, Kerr recognized, that the black hole develops two surfaces. The inner boundary is the standard event horizon, where any matter or light that crosses can never leave. But there’s an outer boundary as well, spherical in shape but somewhat flattened so that it touches the black hole at its poles. Any light or matter entering the region between the two boundaries is whirled around at high speed and, if positioned in just the right way, has a chance to escape. The way out is along magnetic field lines, which direct the matter straight out of the black hole’s north and south poles.
To be fair, Kerr didn’t offer these details at the Texas Symposium. He didn’t think of the region between the two boundaries in this way. He didn’t even have his two surfaces defined correctly, due to his rush to get a result before the meeting began. But it was a start. And by 1969 Roger Penrose fully demonstrated how this special plac
e between a black hole’s inner and outer boundaries, which came to be called the ergosphere, can act as an energy amplifier. “Erg” is derived from the Greek word for work or energy, and that’s exactly what the ergosphere provides. Penrose showed how any matter and light that enters into this special realm and then escapes actually gains energy—a lot of energy—from the black hole’s rapid rotation. The loser here is the black hole, whose spin gets reduced a smidgen in the ergosphere process.
There was another outcome from Kerr’s solution. For the staunchest opponents to gravitational collapse, rotation had always stood out as a last hope, a star’s potential savior from total oblivion. But that was proven not to be true. Though rotation added some exciting new properties to a black hole, it didn’t prevent the black hole from forming at all. Moreover, Kerr’s solution was later proven by others, including Stephen Hawking, Brandon Carter, and David Robinson, to be the only type of black hole possible. Chandrasekhar called that discovery “the most shattering experience” of his scientific life, the realization that Kerr’s solution “provides the absolutely exact representation of untold numbers of massive black holes that populate the universe … that a discovery motivated by a search after the beautiful in mathematics should find its exact replica in Nature.”
A rotating black hole’s two surfaces: the event horizon, from which nothing can escape once it enters, and the ergosphere, an outer region where it is possible to extract energy from the hole. (Messer Woland, courtesy of Wikimedia Commons)
By the latter half of the 1960s, science fiction writers were fully alert to this new celestial kid on the block. In an episode called “Tomorrow Is Yesterday,” which initially aired on 26 January 1967, during the first season of Star Trek, the starship Enterprise encounters an invisible “black star,” as the narration put it, whose immense gravitational attraction dragged the spaceship dangerously close. Gravitationally collapsed objects were also routinely referred to as “dark stars,” along with frozen stars and “collapsars.” The expression we all know and love—black hole—was not made official until the end of 1967.
The term black hole, of course, has had a dark and 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 2 women were taken prisoner by the troops of the nawab of Bengal, Siraj ud-Daulah. According to one historian, Siraj’s men incarcerated at least 64 of the hostages for a night in a tiny, cramped cell known as the “black hole.” Few more than 20 reportedly survived the hot, suffocating night. 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.
Wheeler repeatedly told the tale that he first used the term black hole in the fall of 1967 at a conference quickly set up at the NASA Goddard Institute for Space Studies in New York City after radio pulsars were discovered. Were the mysterious beeps coming from red giant stars, white dwarfs, neutron stars? According to Wheeler, he told the assembled astronomers they might be his “gravitationally collapsed objects.” “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.
But, while pulsars were discovered in 1967, their existence remained a well-kept secret until 1968, the announcement held off until that February, when the discovery paper was finally published by Nature. The pulsar conference at the Goddard Institute did not take place until May. Perhaps Wheeler misremembered the conference taking place in 1967. There was a meeting on supernovae at Goddard in November 1967, but Wheeler’s name is not found in the conference’s proceedings. What is indisputable is that Wheeler used the phrase during an after-dinner talk at the annual meeting of the American Association for the Advancement of Science (AAAS) in New York City on 29 December 1967. It then made it into print when an article based on that talk, titled “Our Universe: The Known and the Unknown,” was published in American Scientist in 1968. It was from this publication that Wheeler is traditionally credited for the origin of the term black hole.
Yet there’s firm evidence that the term actually arose much earlier. For one, it was casually bandied about at the 1963 Texas Symposium, four years before Wheeler adopted it. The science editor for Life magazine at the time, Albert Rosenfeld, used the term black hole in an article on the newly discovered quasars. Reporting on the Texas conference, he noted how 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 could not recall the source. Could Hoyle have used the term in his discussion? More than a decade earlier, the British astrophysicist had wryly dubbed the explosive theory of the universe’s origin as the “Big Bang.” Was he using his talent for evocative astrophysical nicknames once again? Or were young graduate students and post-docs playfully using the term in the hallways of the conference?
The phrase was mentioned again a week later at an AAAS 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 organized the session that Ewing covered and had also attended the Texas Symposium. Chiu had originated the term quasar; 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 Princeton physicist Robert Dicke, both an experimentalist and theorist on gravitation, spoke at a colloquium and mentioned how general relativity predicted the complete collapse of certain stars, creating an environment where gravity was so strong that no light or matter 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 casually use the phrase again there during a series of visiting lectures. In this way, Dicke may have released the term into the scientific atmosphere. It was one of Dicke’s favorite expressions, for he often used it with his family in an entirely different context. His sons recall their father exclaiming, “Black Hole of Calcutta!” whenever a household item appeared to have been swallowed up and gone missing.
If Wheeler was unaware of these earlier uses, though, could he have been influenced by a poem titled “Music of the Spheres” written by A. M. Sullivan, which focused on the eighteenth-century astronomer William Herschel? The poem was published in the New York Times on 26 August 1967, just a few months before Wheeler’s talk at the AAAS meeting in the city.
When the long eye of Herschel
Burrowed the heavens
Near the belt of Orion
He trembled in awe
At the black hole of Chaos.
Whoever inspired the phrase, Wheeler still deserves much of the credit for its placement 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 Kip Thorne.
Wheeler’s strategy worked splendidly. Within a year of his 1967 New York talk, the idiom was gradually being used in both newspapers and the scientific literature—although for a while it was first 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,” said Wheeler. But Wheeler was attracte
d to its link to other physics terms, such as blackbody, an ideal body that absorbs all the radiation that falls on it and is also the perfect emitter. A black hole does the former but not the latter. It emits nothing … zip. … nada. 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 singularity, with its infinite density, was literally digging a hole—a bottomless pit—into the flexible fabric of space-time. And like some cosmic karma, the name also pays homage to the very man who started it all, Karl Schwarzschild. Schwarz means “black” in German.
“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. Its intriguing name provided the object with a beguiling personality it had lacked before.
Even Chandrasekhar returned to the subject, figuring it was now safe to get back into the game without ridicule. He had been away for nearly forty years, starting with his infamous kerfuffle with Arthur Eddington. And by the mid-1970s black holes were no longer the static objects that Chandra first encountered. Now they were active, twirling cosmic entities. After gulping down a large helping of matter, a black hole’s event horizon can shake, rattle, and roll. Within eight years of his coming back to the topic, Chandra wrote one of the definitive books on the subject, The Mathematical Theory of Black Holes, which neatly packaged all the various techniques required to study a black hole’s behavior. It remains a classic in physics departments to this day.
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