Suddenly any idea, no matter how far-fetched, was given consideration. “The discovery of quasars,” noted Schmidt, had “a profound impact on the conduct of those practicing astronomy. Before the 1960s, there was much authoritarianism in the field. New ideas expressed at meetings would be instantly judged by senior astronomers and rejected if too far out. … [But now] an attitude … evolved where even outlandish ideas in astronomy [were] taken seriously.”
Fred Hoyle and William Fowler, for example, dared to bring up general relativity, for so long ignored. A month before Schmidt even announced his identification of 3C 273 in Nature, Hoyle and Fowler published an article in the same journal pointing to gravity as a possible cosmic engine, in this case for the many active radio galaxies already found closer in. They imagined that up to one hundred million solar masses had accumulated in the center of these galaxies, behaving as one giant star. A sudden contraction of this mass “to the relativity limit”—that is, a catastrophic gravitational collapse—would then release the immense energies displayed by these galaxies. This extended an idea first published two years earlier by the Soviet physicist Vitaly L. Ginzburg, who had studied under Landau.
Schmidt’s discovery of quasars, coupled with Hoyle and Fowler’s intriguing theory, swiftly reverberated throughout the physics and astronomy communities. A special conference was quickly organized by a group of top relativists to bring astronomers, theorists, and physicists together to discuss the myriad questions on everyone’s mind concerning quasars. Funders for this large gathering included NASA, the US Navy, and the US Air Force. (Some in the military were interested because of that short-lived flurry of excitement in certain quarters, encouraged by Roger Babson’s enthusiasm, that the study of general relativity might lead to antigravity devices.) “For more than ten years,” said the conference’s invitation, “the nature of the strong extra-galactic radio sources has been one of the most fascinating problems of modern astronomy. … [The enormous] energy requirement has so far ruled out nearly all of the explanations and theories put forward to explain such extraordinary events. … It emerges that the many-sided and basic aspects of the question of gravitational collapse make it imperative to bring experts from many fields together for a thorough discussion.”
J. Robert Oppenheimer was among those invited. It looked as if his 1939 paper, neglected for nearly a quarter of a century, was finally going to get its day in the Sun. The science world was electrified. “I look forward to seeing you at the Dallas conference,” Penrose wrote Wheeler. “The subject is certainly a most intriguing and baffling one.”
9
Why Don’t You Call It a Black Hole?
The conference might never have happened were it not for strong martinis, coupled with a hot, lackluster Texas summer. The noted mathematician Ivor Robinson had just moved to Dallas at the start of 1963 to head the newly formed relativity group at the Southwest Center for Advanced Studies (what later became the University of Texas, Dallas), and he was bored. As one observer put it, he was pining “for people who would recognize … a null bivector when they saw one.” So, over the long Fourth of July weekend, he invited a number of friends to visit his locale, then far from the usual watering holes in the field of general relativity.
While everyone was lazily fanning themselves as they sat around a suburban Dallas swimming pool on July 6, drinks in hand, the center’s chief scientific officer, physicist Lauriston Marshall, suggested that a little conference, perhaps with some twenty-five participants, might be just the thing to put their new institute on the map. “Give a little spice to life,” he said. Robinson, along with relativists Alfred Schild and Engelbert Schücking, visiting from the University of Texas at Austin, jumped at the idea.
While the three mulled over potential topics in the ensuing days, Schücking happened to mention the newly discovered quasars. “Nobody knows quite what they are,” he pointed out. “Why don’t we hold a conference on the subject?” Everyone agreed, but they recognized that such a big topic required a far bigger platform than they originally planned. So, the initial idea to hold an intimate workshop was soon “Texanized … [into] a big bash in Dallas,” as Schücking put it. The Dallas money, coming from the city’s establishment to help build a Princeton in Texas, was “particularly valuable,” added Schücking, “since it could be spent on liquor, while the Lone Star State money from Austin could be used only for more sober expenses.”
But what to name the conference? Here was a small relativity group holding a conference on what was largely an astronomical subject. “We fixed that,” said Schücking. He, along with Schild and Robinson, invented the name for an entirely new field. The conference title, they decided, would be “The Texas Symposium on Relativistic Astrophysics,” and they invited nearly everyone they could think of with a connection to this fresh discipline. “Relativity was the sleeping beauty and quasars the prince that woke her up,” says German science historian Jürgen Renn. It was the moment when many top physicists first learned that general relativity might actually be significant in the physical world.
The meeting took place in December 1963, right before the Christmas holidays, at a hotel in downtown Dallas, just blocks from the street where President John F. Kennedy had been assassinated three weeks before. Conference organizers had been urged to cancel the event because of the tragedy, but they decided to stay the course. Texas governor John Connally, also injured in that terrible episode, welcomed the conferees during the opening address with his arm in a cast.
Three hundred scientists attended from around the world, with Oppenheimer chairing the first session. Schücking recalled how, just minutes before this session began, Oppenheimer “asked us to synchronize our watches. It was as if we were going to have another Alamogordo.” Also present was Karl Schwarzschild’s son Martin, who had grown up to become a Princeton University astronomer. All these relativists, astronomers, and astrophysicists felt a palpable excitement in the air. As one participant put it, “Many of those who attended felt that they [were] at a historic occasion where new ideas of paramount importance were presented that may profoundly influence all future thinking in the field.”
It was the great convergence. At last general relativity and astrophysics were being directly linked. By the time of the conference, astronomers had identified nine quasars. Had Oppenheimer’s gravitationally collapsed objects at last been sighted? As Cornell University astrophysicist Thomas Gold wittily suggested in an after-dinner talk at the meeting, relativists were now more than “magnificent cultural ornaments but might actually be useful to science! Everyone is pleased: the relativists who feel they are being appreciated, who are suddenly experts in a field they hardly knew existed; the astrophysicists for having enlarged their domain, their empire, by the annexation of another subject—general relativity. … So let us all hope that it is right. What a shame it would be if we had to go and dismiss all the relativists again.”
The invitation to the conference outlined a clear and concise agenda. “Among the problems raised are the following,” said the notice:
(a)The astronomers observed some unusual objects connected with radio sources. Are these the debris of a gravitational implosion?
(b)By what machinery is gravitational energy converted into radio waves?
(c)Does gravitational collapse lead … to indefinite contraction and a singularity in space-time?
(d)If so, how must we change our theoretical assumptions in order to avoid this catastrophe?
That last statement was particularly interesting. It suggests that physicists were still hoping to sweep the ugly singularity under the rug. Despite the work of Oppenheimer and Snyder, Wheeler and Zel’dovich, gravitational collapse remained a hard pill to swallow. Some attendees didn’t even know the possibility existed until they attended the conference. It was the first they heard of such a stellar outcome.
One question was on the top of everyone’s minds: What was the energy source for those objects putting out such intense radio a
nd optical radiation? The object 3C 273 was emitting the energy of a trillion suns. How long had this been going on? How long would it last? Scientists already knew that nuclear reactions were simply too inefficient to produce such a flood of continuous energy. That was the reason the spotlight was on gravity—gravitational collapse, to be exact. As matter accelerates in its fall toward a black hole, immense energy is released, far more than if that matter was used as fuel in nuclear burning.
On the first morning of the conference, Fred Hoyle and William Fowler discussed their idea of a giant body of matter contracting. In its outer region, thousands of pockets of condensed matter, each some one hundred solar masses, go through nuclear burning, not over billions of years, as in the case of our Sun, but in a swift “whoosh,” over the matter of a week. At the same time, in the innermost region of this gargantuan body of matter, contraction continues. A “superstar,” weighing some one hundred million solar masses or more, emits a tremendous blast of energy as it catastrophically shrinks down to a point. How did such a mammoth star get there in the first place? “For the moment,” wrote Hoyle and Fowler in their published remarks, “we ignore the question of how such an object might be formed—the observational evidence would seem to give strong support to the postulate of the existence of massive objects, and it is therefore reasonable to inquire into their properties without further ado. We turn a blind eye, a deaf ear, and a cold shoulder to written, oral, and implied criticism, respectively.” Hoyle and Fowler went on to suggest that a kind of “antigravity” field (something, of course, not seen in physics) kept the superstar from total collapse to a singularity; pushing outward, they wrote, the field causes the crushed star to bounce back, releasing the observed radiation. These oscillations would continue but slowly subside over time, until the object shrinks entirely behind its event horizon and vanishes from sight. Maarten Schmidt was right; astronomers had entered an era where, as the noted music composer Cole Porter put it, “anything goes.”
From left to right, Fred Hoyle, Ivor Robinson, Engelbert Schücking, Alfred Schild, and Edwin Salpeter at the 1963 Texas Symposium on Relativistic Astrophysics. (American Institute of Physics Emilio Segrè Visual Archives, E. E. Salpeter Collection)
Some held open the possibility that the extremely intense gravitational field of this collapsed superstar—not light waves being stretched over long distances in an expanding universe—generated the quasar’s tremendous redshift. This meant that the quasar could be massive yet small, and relatively close by. But it was soon settled that this couldn’t be the case; if it were true, the motions of stars in our Milky Way, situated near such a massive object, would be greatly altered by its strong gravity field. Such deviations were not observed at all in our local galactic neighborhood. By some calculations, if 3C 273 were such a galactic star, it couldn’t be more than a third of a light-year distant from us, practically inside our solar system, which would have greatly affected planetary motions. And if such a star were receding from us at thirty thousand miles (forty-eight thousand kilometers) per second, our galaxy couldn’t gravitationally hold onto it for very long.
Other mechanisms were also considered. Perhaps the quasar phenomenon arose when matter and antimatter annihilated one another in a galaxy’s center? When a bit of matter meets up with its antimatter counterpoint, the pair obliterate each other, leaving nothing behind but a burst of pure radiation. But how could these disparate materials be kept apart for so long before that?
Conference participants excitedly argued over these various ideas, one after the other. Could it be that a quasar was a massive chorus of supernovae exploding all at once? Not really. To produce such energy, you’d need a hundred million going off. Why (and how) could so many stars be exploding simultaneously? Moreover, each million or more stars would have to be stuffed into a volume only a few light-years wide. Is that even possible?
A singular and sudden gravitational collapse, the kind that Hoyle and Fowler discussed, had a problem as well. Physicist Freeman Dyson emphasized this point. A collapse would generate lots of energy, he pointed out, but only for a short time—a day at most. Yet quasars shine on and on and on. Gravitational collapse is over quickly, but quasars blaze fiercely for a million years or more.
The Soviet physicists Yakov Zel’dovich and Igor Novikov would soon point out that great energies can be released as nearby dust and gas is drawn toward a massive collapsed object, accumulating into a disk that surrounds the object. This circulating matter radiates and glows for many years as it spirals in, they said, until it ultimately reaches the point of no return and disappears behind the curtain of the event horizon. But such an idea didn’t come up at any open session at the conference. (The Soviets were not given permission to travel to Texas.) In the end, no single scheme came out on top by the conference’s last day. Rather than counting on new physics to explain a quasar’s energy, many held out for more ordinary astrophysical processes, something like gas clouds raining down on a central cluster of stars. Energy would be gained as the gas plunged in toward the galactic nucleus, possibly converted to an explosive form due to shocks and collisions along the way.
It would take a few years for all the varied hypotheses, both bizarre and mundane, to settle down—and bizarre won. It’s now generally accepted that a quasar’s power source is a supermassive black hole, spewing energy as it feeds on an accretion disk of matter swirling around it, just as Zel’dovich and Novikov early on surmised and Cornell University physicist Edwin Salpeter also independently suggested in 1964. (More on this process in chapter 11.)
Today the First Texas Symposium is also remembered for a brief talk that heralded a major breakthrough in black-hole physics. It came from an up-and-coming relativist named Roy Kerr, but at the time the astrophysicists in the audience hardly noticed. Kerr’s presentation wasn’t even mentioned at the end of the conference, when three participants presented a summary of the meeting’s highlights. But, as you’ll see, that gaffe was eventually amended.
While the early 1960s were watershed years for astronomy, what with the discovery of quasars, it was also a time of innovation for relativity, which was on the brink of entering its golden age. For decades, remarked physicist George Gamow, general relativity stood “in majestic isolation, a Taj Mahal of science, having little if anything to do with the rapid developments in other branches of physics.” But now with advances in instrumentation, some spurred by the needs of World War II, experimental physicists started rechecking Einstein’s predictions with exquisite precision. More than that, they also began carrying out new experiments. “A new and very able younger generation has come on the scene,” wrote Wheeler to a colleague at MIT. “Improvement in experimental techniques and a new aggressiveness on the part of the experimenters have broken the subject out of the strait jacket of the old traditional … tests of general relativity.”
In 1960, for example, Robert Pound and Glen Rebka finally measured the “gravitational redshift,” another effect long predicted by Einstein but one that had been highly elusive due to the great precision required. It took four decades of waiting, but Einstein’s third prediction of gravity’s behavior (along with light bending and orbit shifting) was finally proven. Simply put, a light wave will stretch out (get longer and hence “redder”) as it flees from a strong gravitational field. Pound and Rebka measured this very effect on the campus of Harvard University as they directed gamma rays, ejected by radioactive atoms, up a tower several stories high within the physics building. By the time the radiation traveled some seventy-four feet (twenty-two meters) to reach the top, the gamma rays had lengthened just a smidgen—by an amount that matched Einstein’s expectation.
It’s because of the gravitational redshift that clocks run slower on Earth than in space. You can think of the light waves as springs—coils that get stretched as they climb out the Earth’s gravitational well. And as the waves get longer, their frequency—the number of waves passing by us each second—is reduced. If the frequency of those gamm
a rays were being used as a clock, then the “tick of the clock” is slowed down by Earth’s gravitational field. We don’t notice this change ourselves, since the very atoms in our body are slowed down as well. We recognize the effect only by comparison. A clock freely floating in space feels no such gravitational effect and so runs faster by comparison. The high-stability clocks aboard the Global Positioning System (GPS) satellites, perched high above the Earth, run a bit faster. As a result, periodic corrections for general relativity must be programmed in to make sure the navigation of our cars, boats, and planes down here on Earth doesn’t go awry (perhaps the first time that general relativity was needed to help us in our everyday lives).
How much a clock slows down depends on the strength of the gravitational field. If a person could miraculously survive on the surface of a neutron star, where gravity is a trillion times stronger than Earth’s, they would age noticeably slower than a person more loosely grounded on terra firma. As a decade passes by on Earth, the Neutronian would experience about eight years of time going by.
What with astronomers talking about gravitational collapse and experimentalists once again testing Einstein’s predictions, it’s as if the field of general relativity were waking up from a deep sleep. Theorists were renewing their interest as well.
The biggest hurdle for everyone at that time was describing a real star. Up to this point, all the work on “gravitationally collapsed objects” started with a ball of matter that was completely still. It was the only way that scientists, such as the groups at Princeton and in the Soviet Union, could solve the equations. But this was a very unrealistic simulation.
Black Hole Page 12