10
Medieval Torture Rack
Black hole studies prospered under the tutelage of Wheeler, Zel’dovich, Thorne, and others. It was at this point that all of a black hole’s strange properties came to be identified and examined more deeply by theorists. Astrophysicists were catching “black-hole fever,” and the infection spread rapidly. “There is a Chinese proverb. Ten years ago the river flows east, and ten years later the river flows west. Things change unexpectedly. Suddenly black holes became the hottest subject in town,” says Hong-Yee Chiu today, who was once labeled a “crackpot” for being a supporter of neutron stars and black holes.
Fortune magazine, a purveyor of both business and research trends, noticed in 1969 “a notable migration of … scientists and graduate students into the fields of astronomy, astrophysics, cosmology, and relativity research.” And general relativity was growing faster than almost any other area. By this point centers specializing in relativistic physics had been set up in Moscow, Paris, Syracuse University, the University of Maryland, North Carolina, Princeton, Berkeley, Caltech, and Cambridge University. The best students in physics were flocking to them. “Particle physics was in a mess at this time,” recalls MIT physicist Alan Lightman, who under Thorne’s guidance received his PhD in black-hole physics during this era. “There were dozens of different theories of the strong force, hundreds of new kinds of elementary particles, and no clarity at all. General relativity was more attractive because it was not yet glutted with practitioners. And, with the discovery of neutron stars in 1967, people were beginning to believe in the validity of highly compact stars, including black holes.”
It was at this moment that magazines and newspapers began regularly publishing science stories filled with the frightening (yet strangely entertaining) consequences of being near a stellar-sized black hole. Black holes were proclaimed as “The Darkest Riddle of the Universe,” “The Dazzling Death Spasm of a Star,” “The Blob That Ate Physics,” “Vacuum Cleaner of the Cosmos,” and even the “Bermuda Triangles of Space.” “Of all the concepts conjured up by physicists,” wrote New York Times science editor Walter Sullivan in 1971, “none is more bizarre than that of the ‘black holes’ in outer space.”
They became a cultural phenomenon as soon as they arrived on the public’s radar, joked about on late-night television and in the press. A fake advertisement in the science fiction magazine Analog hawked “black-hole disposal units” in seven decorator colors that would suck up unlimited waste. T-shirts proclaimed that “black holes are out of sight.”
Theorists, meanwhile, found humor in the physics itself. They jokingly talked about how you would get “noodlized” as you passed, feet first, through an event horizon. From the classic general relativistic perspective (more on another scenario—the quantum view—in chapter 12), once the horizon is crossed, you can’t turn back. The only thing that lies ahead for you is the center of the black hole, whose gravitational pull increases as you plummet. In fact, the rise is so swift and sharp that the pull on your feet would be far greater than the pull on your head, so you’d get stretched out—just like a noodle. At the same time, you’d be crushed from side to side. It’s the same effect—the tidal force—in which the Moon pulls on the Earth’s oceans to generate the tides. But in the case of a black hole the tidal forces are gargantuan. In the blink of an eye, less than a millisecond for a stellar-size hole, your body would be broken up into cells, the cells into atoms, the atoms into elementary particles, the particles into quarks, and the quarks into entities yet to be figured out. Whatever the ultimate debris, all is swept into the infinitely dense singularity at the black hole’s center and crushed to oblivion. Wheeler liked to describe this final entity, situated deep in the black-hole well, as “mass without matter.”
Depending on the mass of the black hole, the timing of this scheme can change somewhat. Like a glutton’s expanding waistline, the event horizon stretches outward, wider and wider, as a black hole devours more mass. If the total mass of the black hole is sufficiently large, you would not even realize when you’ve passed the point of no return. A black hole’s event horizon is not a solid surface after all, but more like an invisible county or city line. Astronauts approaching the event horizon of a supermassive black hole, the kind that lurks in the hearts of most galaxies and contains the mass of millions or even billions of suns, would experience nothing but emptiness as they cross over. But eventually the tidal stretching will begin, as if in slow motion: head and feet pulled apart and the chest squeezed, as if he or she were on a medieval torture rack. For a black hole containing the mass of five billion suns, the fall from event horizon to final crunch would take the astronaut about twenty-one hours.
In lectures, Wheeler often liked to compare the distinction between the horizon and the crushing point to falling off a cliff onto some rocks below: “On first approach [the cliff] had sloped down safely, inviting the explorer to come closer. Unperceived in one’s eagerness to peer over the edge, the slope of the slippery grass increased. Then the shoes began to slide forward. Suddenly it became clear that disaster was inescapable, though it had not yet struck. That treacherous and unmarked point of no return symbolizes the equally treacherous and equally unmarked horizon of the black hole—as the rocks at the bottom symbolize the point of obliteration.”
And why can’t you escape? It’s because the event horizon marks the point at which an object would have to accelerate to the speed of light, 186,282 miles per second, to break free—far faster than the mere 7 miles (11 kilometers) per second we need to leave Earth. Once you pass through the event horizon, there is no way out. You’d have to turn around and flee at a speed faster than the speed of light—an impossibility, according to Einstein. It would take an infinite amount of energy to do so. As a consequence, the black hole holds on to you with an iron grip.
When Einstein declared that space and time are relative, nowhere is this more apparent than in the realm of a black hole. Time slows down within a strong gravitational field, a natural outcome of general relativity that’s been proven many times. You can think of each beat of a clock needing more time to climb out of its deep gravitational well. Indeed, as noted earlier, the clocks aboard the Global Positioning Satellites orbiting above us, whose signals help direct our driving and hiking, run just a tad faster than clocks down here on Earth, where the clench of gravity is more forceful. Black holes, the mightiest sinkholes in the universe, take this effect to the extreme.
An illustration of a stellar-sized black hole, as seen from a distance of about four hundred miles (644 kilometers), with the stars of the Milky Way in the background. The starlight is distorted and stretched as it gets bent by the black hole’s intense gravitational field before reaching our eyes. (Ute Kraus, Universität Hildesheim, courtesy of Wikimedia Commons)
Imagine you were miraculously able to sit on the surface of a collapsing star, just before it shrinks within its event horizon to become a black hole. Looking down at your watch, time is progressing normally. You see a second go by. Yet looking back out at the universe at large, billions of years are elapsing. The future history of the cosmos is rushing past at near-light speed. Anyone watching you from afar, however, sees something vastly different. Removed from the black hole’s immense gravitational field, the distant observer thinks you are taking an eternity to cross the event horizon. To you, of course, that’s not the case at all. In your time frame, you would instantly die. But to your observer, you appear to be motionless at the brink of the event horizon—forever young and forever saved from total destruction. From the perspective of your far-off observer, time near the black hole has practically ground to a halt. No wonder Russian theorists at first preferred to call the black hole a frozen star. In reality, though, this star would still look black to a distant onlooker, as the last light waves escaping from it are stretched silly to infinite lengths, rendering them invisible to our eyes.
The notion of the black hole as a frozen star affected astrophysicists’
beliefs for quite a while. It made them assume that a black hole would have no influence whatsoever on our present-day universe. From our reference frame in time, the star was essentially a petrified object, so why should it affect us? “As long as this viewpoint prevailed,” noted Richard Price and Kip Thorne in a book on black holes, “physicists failed to realize that black holes can be dynamical, evolving, energy-storing and energy-releasing objects.”
Astronomers were just starting to learn that: first with quasars, and later in discovering stellar black holes within our own home galaxy.
11
Whereas Stephen Hawking Has Such a Large Investment in General Relativity and Black Holes and Desires an Insurance Policy
Setting up an astronomical observation to look for a black hole within our galaxy took some time. Many had to be convinced they were not only worth seeking out but even possible to hunt down. For Oppenheimer back in the 1930s, black holes were strictly a theoretical problem. He wasn’t going to bother to look. Why should he? He’d proven that his gravitationally collapsed object would vanish from sight. Astronomers at the time weren’t interested for other reasons. They thought all this talk about dying stars collapsing was foolish. Stars simply didn’t behave like that.
Wheeler, in contrast, did come to imagine neutron stars and black holes as real denizens of the universe. But even he was shortsighted at first about the possibilities of finding them. In 1964, he published an article for the book Gravitation and Relativity on the neutron star, what he called the “superdense star,” in which he said, “There is about as little hope of seeing such a faint object as there is of seeing a planet belonging to another star.” (Of course, both are now observed regularly by astronomers.) But this was 1964, when such technological abilities were hardly even fantasies. And no one had yet conceived that a neutron star could emit intensive radiation from its poles as it wildly spins—radiation detected by us across the electromagnetic spectrum, from radio to X-rays.
Zel’dovich and his team, however, were already thinking about this problem deeply. How do you make the invisible, well, visible? How could you possibly detect a pitch-black object journeying through the darkness of space? At first, they borrowed an idea that John Michell had mentioned back in the eighteenth century: look for a luminous star that is wiggling back and forth because a dark companion is tugging on it as it orbits. If that companion emits no light and gravitational measurements show it weighs a few solar masses or more, then it could very well be a black hole. Zel’dovich drafted an astronomy graduate student, Oktay Guseynov, to comb the binary-star catalogs to come up with candidates. By 1966 they found five. (In their report to the Astrophysical Journal, they inserted a little jab against those astronomers who continued to insist that stars lose enough mass to avoid collapse. Yes, the Soviet researchers conceded, stars can shed matter but “not because they ‘wish’ to be white dwarfs or ‘fear’ to be collapsed.”) Working with astronomer Virginia Trimble, Kip Thorne later came up with eight more black-hole candidates. But in the end, none of these turned out to be viable contenders. There are many reasons that a companion could be very faint and still not be a black hole. What was needed was a new tool altogether to conduct a search.
Fortunately, Zel’dovich and his colleague Igor Novikov realized within a few years that there was another way to unmask a black hole, a variation of the accretion process they had already mentioned for quasars. Imagine a black hole in orbit around a brightly glowing star, whose surface is releasing streams of gas in a stellar wind. The black hole might even be tugging away its companion’s outer atmosphere. Eventually, some of that gas will reach the black hole and be captured by its powerful gravitational field. As this gas plummets toward the hole, with its atoms churning and colliding, it is heated to millions of degrees. And in the process, it emits copious radiation, not as visible rays but as X-rays. Though black holes are invisible against the dark backdrop of space, they would give themselves away by how they affect their surroundings. A black hole would announce itself by the glow of fiercely energetic X-rays enveloping it (just as with quasars). “[This] proposed method of searching for ‘black holes’ in binaries,” Zel’dovich and Novikov later wrote, “reminds one of the well-known case of searching for a lost key under a lamppost: the key is sought where it is easier to find.” Seeking out bright X-ray sources is less painful than searching through piles of stellar catalogs for wiggling stars. Fortunately, around the same time the two Soviet researchers arrived at this realization, X-ray astronomy was fast maturing as a new means of studying the universe.
X-ray astronomy was almost doomed from the start. Soon after World War II, researchers from the US Naval Research Laboratory used surplus German V-2 rockets to loft instruments high above our atmosphere to capture X-rays emanating from the Sun. Such rays are impossible to detect on the ground. Though they can easily penetrate matter over short spans, they are completely absorbed in our broad atmosphere, being very short electromagnetic waves that span the width of an atom. Although the solar X-rays these pioneers detected were very intense from a layperson’s point of view, the output was relatively meager by cosmic standards. So, making estimates based on that 1948 discovery, theorists figured that X-ray emissions from faraway stars would be minuscule by the time they reached Earth, a billion times fainter than the Sun’s X-ray output. Since such a signal was impossible to detect with 1950s technology, it didn’t seem worth pursuing.
But American desires to better monitor Soviet nuclear bomb tests from space in the early 1960s accelerated efforts in improving X-ray detectors. Such bombs release appreciable X-rays. A twenty-eight-year-old named Riccardo Giacconi, who arrived in the United States as a Fulbright scholar from Italy and stayed to serve as a physicist with the private research firm American Science and Engineering, headed up the effort. His team’s newly built instruments worked nearly perfectly when measuring a series of American bomb tests. It wasn’t long before they turned those same detectors to the heavens.
One particular rocket flight dramatically set the stage, initiating the birth of X-ray astronomy. Under the light of a full Moon, the launch took place one minute before midnight on 18 June 1962. Giacconi, along with his colleagues Herbert Gursky, Frank Paolini, and Bruno Rossi, had mounted their payload, three large Geiger counters, onto a small Aerobee rocket and launched it from the White Sands Missile Range in southern New Mexico. After reaching an altitude of 140 miles (225 kilometers), the rocket plunged back to Earth. Two of the detectors, sweeping the sky as the rocket spun on its long axis twice a second, recorded 350 seconds of useful data. They were six of the most fruitful minutes in astronomical history.
This was when the United States was gearing up for its longtime goal to land a man on the Moon by the end of the decade. Giacconi and his colleagues were trying to detect X-rays from the Moon. They figured the radiation would be generated as the energetic solar wind struck the lunar surface and were hoping that the X-ray spectrum would help them determine the Moon’s composition. But Giacconi and Rossi had also long suspected that extrasolar X-rays would be emanating from such celestial objects as supernova remnants. We were “trying to get support … frankly wherever we could,” said Giacconi. Unable to garner a NASA grant to look for X-rays from deep space, the two cunningly used the US Air Force–funded Moon test as an opportunity to take a look around the celestial sky while the rocket was in space.
No lunar X-rays were detected during the brief flight, but the rocket team hardly despaired. They found something far more enticing, the kind of novel signal they had been hoping for all along. The rocketborne detectors noticed a huge flux of X-ray radiation arriving from a region of the sky in the direction of the constellation Scorpius. Hence, the name it eventually received: Sco X-1, as it was the first X-ray source in that sector of the sky. Sco X-1, located some nine thousand light-years from Earth, blazed with an X-ray intensity beyond anyone’s imagination, tremendously more powerful than our Sun’s meager output. It was millions of times stronger tha
n the X-ray radiation emitted by normal stellar sources. This was such a leap in strength that at first the team worried their instrument had been damaged, producing a false signal. Others in the scientific community were wary as well and demanded confirmation.
Proof arrived when further rocket flights found additional sources similar to Sco X-1. And starting in 1970, as soon as astronomers placed X-ray-detecting satellites into orbit, many more were found. With these advanced tools, astronomers learned that many of these energetic sources are neutron stars in binary systems. The X-rays are released as matter from the normal, visible star is drawn away and funneled onto the surface of its superdense neutron star companion. Many of these neutron stars appear to regularly pulse in X-rays, as the neutron star rapidly spins. The X-ray “hotspots,” formed at the poles where the matter is magnetically drawn, go in and out of view much like the rotating lamp of a lighthouse. This was an important turning point. With the mounting evidence that neutron stars populated the galaxy—both as radio pulsars and as X-ray-emitting sources—it became easier for astronomers to take the plunge and accept the possibility that black holes existed as well. “The discovery of pulsars,” noted Kip Thorne at the time, “opened the floodgates.” Astronomers were at last willing, he said, “to take seriously some of the wildest meanderings of theoreticians’ minds, including speculations about the role of black holes in the Universe.”
This played out when one of the X-ray sources appeared to be in a class by itself. Fully revealed in 1964 during one of the continuing series of rocket flights, it was labeled Cygnus X-1, signifying its location in that constellation (also known as the Northern Cross). By then, other research groups were joining in the hunt with their own rocket launches, and each proceeded to measure a different X-ray brightness for this source, which puzzled everyone. A long look by the first X-ray satellite, Uhuru, in 1971 finally disclosed that this luminous patch in Cygnus was atypical. Rather than emitting regular pulses of X-rays like the other sources, it underwent sporadic variations. There was no discernible pattern to its flare-ups. Sometimes its signal flickered over periods as short as millionths of a second, a sign that whatever was giving off those X-rays had to be fairly compact. If the source was as big as a normal star, the radiation pulse would have lasted much longer.
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