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It is almost midnight in New Mexico on the eighteenth of June in 1962. From a viewpoint at the altitude of low Earth orbit, 140 miles above the White Sands Missile Range near Las Cruces and the Rio Grande, the dark mass of our planet fills the sky. Here it is utterly silent in the vacuum of space, and the last appreciable wisps of atmosphere are more than sixty miles beneath us. Off to the southeast an almost full moon hangs brilliantly in the blackness of space. The tiny points of stars seem to litter the cosmos. All is completely still. Then an object appears below our vantage point, shining brightly in the lunar glare. It gradually moves closer. A slender white cylinder, a rocket almost thirty feet long, is climbing up through the Earth’s gravitational well. Its fuel is spent and it’s now coasting nearly vertically, slowing down as it rises. Very soon it will reach the apex of its flight right where we are perched, and will then begin to fall back to the Earth. Apart from a few yellowish lights scattered across the planetary surface below us, there is nothing to give away the bustling civilization living there. Up here is space, and this small vehicle is only making a brief visit to sound out the depths.
Packed into the side of its nose are three Geiger counters designed to measure radiation, particularly the presence of X-ray photons. This skinny arrow of a rocket is spinning around twice a second, and its crude detectors sweep across the face of the Moon, sniffing for radiation produced as the Sun’s light beats down on the lunar surface. There is not much. Then something happens. Away from the Moon, some 30 degrees of arc off in the seeming blackness of the sky, the counters begin to click faster and faster. Two, three, four times faster than they had before. X-ray photons are pouring out of a mysterious new place in the cosmos. The rocket catches this scent for only a brief few moments. Already it is beginning to slip back to Earth, starting a tumbling descent that will drop it onto the desert landscape of New Mexico. It is enough, though. For the first time, humans have seen evidence of a place in the universe that is aglow with the fire of something fierce and alien.
This arrow-like rocket lofted high above New Mexico in 1962 was one of a pioneering array of experiments to seek out forms of light from the cosmos usually blocked by the thick atmosphere of Earth. It was a part of a radically new type of astronomy, undertaken from space. One of the participants in this fledging scientific effort was a young Italian named Riccardo Giacconi. Born in Genoa in 1931, Giacconi had already led a life colored by great changes in the world. Through the turbulence of Fascist-led Italy during World War II and then through his own personal struggles in the face of conventional and stultifying science teaching, Giacconi nonetheless grew into a highly skilled scientist. By the late 1950s he had moved to the United States, where he helped develop and build a variety of experiments to detect exotic and fast-moving subatomic particles. Then, as the space age began in earnest, he found himself immersed in the design and launch of experiments on small rockets. These were not much more than modified missiles. Known as sounding rockets, they could not attain orbit around the Earth, but they could climb a few hundred miles before falling back down. During their five minutes or so in space it was possible to carry out all manner of experiments, from detecting radiation to measuring magnetic fields.
That is why, on a summer’s night in 1962, a set of radiation counters flew high above New Mexico. They were designed to probe the interaction of solar radiation with the Moon. As the intense stream of stellar particles hit the lunar surface, it was expected to scatter out X-rays and make the Moon the second-brightest source of this radiation in the sky compared to the Sun. The actual outcome was surprising and extraordinary. The brightest flood of X-ray photons that the rocket saw in space was not from the Moon at all. It came from an entirely different direction, toward the constellation of Scorpius, the Scorpion. What Giacconi and his colleagues had discovered was not generated in our solar system; it came from far beyond. This was the first sign of an entirely new universe, one unseen by human eyes. It was a regime of the most physically violent and energy-rich phenomena in the cosmos, the realm of what would come to be known as high-energy astrophysics. The mysterious new source of intense X-rays was given the name Scorpius X-1 and quickly prompted a flurry of effort to identify its origins.
A speculative picture emerged that astronomers have since confirmed. At the heart of Scorpius X-1, some nine thousand light-years distant from Earth, is an incredible system of two objects. One is a neutron star, a fearsomely dense core of nuclear matter. Its companion is a normal star, but that star is being eaten alive. The spacetime distortion around the neutron star siphons off stellar material from its companion, and as it plummets inward, kinetic energy is converted to other forms, including intense X-rays. It was obvious that if X-rays could be seen from this one corner of the universe, there must be myriad other places to look. In the years following the first detection of Scorpius X-1, increasingly sophisticated instruments were launched on sounding rockets in order to sniff for more sources of extraterrestrial X-rays. During one of these flights, in 1964, another new object showed up. This time it was an intense flood of X-rays from the direction of the constellation Cygnus, the Swan. Cygnus X-1, about six thousand light-years away from the Earth, was simply added to the growing atlas of the X-ray universe. But in 1970 a new type of X-ray experiment, an orbiting platform of radiation detectors, took a fresh look at Cygnus X-1 and found something incredible.
This fully space-based orbiting observatory was named Uhuru, the Swahili word for “freedom,” in honor of the location of its launch platform in the Indian Ocean, just offshore from Kenya. This close to Earth’s equator, space launches can exploit the planet’s spin as an aid in getting to orbit, gaining an extra kick of speed for free. For the first time, Uhuru allowed astronomers to stare at X-ray sources in the sky to their hearts’ content, for far longer than the five minutes of the sounding rockets. They discovered that Cygnus X-1 flickered. The intensity of the X-ray photons changed rapidly several times a second, even as fast as a millisecond—a thousandth of a second. The only way this could happen was if the source itself was small—less than sixty thousand miles across. Otherwise, the finite travel time of photons from the near and far sides of any structure would blur out these variations. It’s like listening to music playing simultaneously from one speaker nearby and another speaker hundreds of feet away. This far apart, and the sound is out of sync and discordant. Put the speakers close enough together and it all comes into crisp harmony.
The clearly seen flickering was an indicator that Cygnus X-1 was a highly compact source of X-rays. At the same time, the amount of energy pouring out was terrific. X-ray photons take a lot of power to generate. The scientists looking at the new data knew that if this radiation was coming from ordinary matter, it was being heated to temperatures of millions of degrees. Maybe, they thought, it was matter falling onto yet another neutron star. At that point, however, new astronomical measurements of the spectrum of visible light in this system revealed that Cygnus X-1 consisted of two objects. These bodies swung about each other every six days, like children holding hands and waltzing furiously across a playground. One was a giant blue star. The other was an enigma, except that its mass was more than ten times that of the Sun. As we saw before, a neutron star cannot be that massive and support itself within the curved spacetime it generates. The only plausible answer was that this mysterious companion was a black hole. But with no surface for matter to crash into, how, precisely, was the black hole producing the energy that was pouring forth?
The groundwork for finding the answer had already been laid independently and simultaneously by two scientists living on opposite sides of the Iron Curtain during the Cold War between the Soviet Union in the east and the Western world. Yakov Zel’dovich was a brilliant physicist and a key architect of the USSR’s nuclear weapons program. Edwin Salpeter was a brilliant astrophysicist who had been born in Austria, obtained his education in Australia and England, and finally settled at Cornell University in upstate New York. I
n 1964, Zel’dovich and Salpeter had both realized that there were specific ways in which matter could become ensnared by black holes that would result in incredible violence. The idea came from the known behavior of gas subjected to great speeds and collisions. As matter fell inward or was accreted by a black hole, it would crash and pile up against itself. In technical terms, it would be “shocked,” much like the way a supersonic aircraft “shocks” the air around it to create a sonic boom. Temperatures in the gas would reach millions of degrees, and this would light up the black hole’s vicinity with X-ray photons. Armed with these ideas, astronomers came to the conclusion that Cygnus X-1 really does contain a black hole. It was the very first clear example of matter gurgling as it plunges through warped spacetime toward a singularity.
The 1960s and ’70s were remarkable decades for other branches of astrophysics as well. While Giacconi and his colleagues were busy launching their sounding rockets and satellites into space, scientists were also now exploring the cosmos at the opposite end of the electromagnetic spectrum. In this other realm, there were increasing signs of phenomena even more monstrous than Cygnus X-1 out in the distant universe. To appreciate this we need to follow a different astronomical story for a while, one that began somewhat earlier.
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The field of radio astronomy was born serendipitously in the 1930s. While working for the Bell Telephone Laboratories in New Jersey, the physicist Karl Jansky noticed a curious signal in a new type of shortwave antenna he had built. Jansky had been set the task of trying to understand sources of radio emission that might interfere with Bell Labs’ plans for a transatlantic shortwave radiotelephone service. His antenna was a large box-like construction of wood and stiff metal cable about a hundred feet long and twenty feet in cross section that sat on four Model T Ford car wheels. With some muscle power, the whole thing could be rotated on a circular track, and the antenna could be repositioned. By moving the antenna, Jansky could crudely locate where sources of radio noise were coming from.
The antenna picked up nearby thunderstorms with huge electrical arcs that sent out shortwave radio waves. It could pick up distant thunderstorms as well. And it also picked up a faint hiss of something else. Jansky carefully waited, monitoring this noisy static and watching it vary in intensity over the following weeks and months. Finally he was able to figure out where it originated. It wasn’t coming from Earth at all. It came from a direction toward the constellation of Sagittarius, which was also the direction of the center of the galaxy. He realized that whatever was generating these radio waves was an utter mystery. Something was lurking out there among the stars.
When he published his results on “extraterrestrial electrical disturbances,” it caused a bit of excitement—even The New York Times ran a story on it in May 1933. Jansky was eager to build a better antenna to find out what was going on, but Bell Labs wasn’t having any of it. Having ascertained that this source of radio noise was something they would just have to live with, they sent poor Jansky back to work on other projects. No one knew what the origin of this radio emission was, but it acted as the seed for an entire field of discovery that would unfold over the following decades.
By the late 1950s, this field had developed to the point where astronomers had custom-built radio telescopes in the form of huge metal dishes and antennae arrays scanning and surveying the sky. They had found many sources of radio emissions in the universe, including the bright one toward the center of the Milky Way that Jansky had managed to catch a whiff of. From electrically charged gas to planets, stars, and other galaxies, the cosmos hums with natural radio noise. Now scientists were hunting for new exotica, and hundreds of distant objects were being detected. In among those was a peculiar category of incredibly small but bright sources. Astronomers went to their visible light telescopes and took photographic images to try to figure out what these were. All they could see were tiny star-like blue dots where the radio waves were coming from. But splitting and dispersing the visible light out into a spectrum made things even more confusing: spikes and bands of light showed up at wavelengths that were hard to match up with the signatures of familiar phenomena. These mysterious objects looked like stars, but their fingerprints were all wrong.
Finally, in 1962, a series of careful astronomical measurements by a number of scientists in Australia pinpointed a particularly bright example of this mysterious class of object. The precise location soon enabled the Dutch-born astronomer Maarten Schmidt to target it with the giant two-hundred-inch-wide telescope on Mount Palomar in Southern California, successfully capturing a clean and precise spectrum of the object’s visible light. Schmidt had immigrated to the United States a few years earlier to work at the California Institute of Technology and was already known for his studies of how stars formed from interstellar gas. As he pored over the photographs of dispersed light that had been taken of the unknown point of brightness, Schmidt began to realize what he was looking at. He knew that atoms of hydrogen emit and absorb light at very specific wavelengths. It’s like a set of unique keys. The spectrum in front of him had these key marks, but they were all shifted downward toward redder colors, making them difficult to recognize. The simplest answer, in his view, was that this object was moving away from us at an incredible rate of speed: about thirty thousand miles per second.
Schmidt knew that according to special relativity the photons of a fast-receding source would be shifted to lower energies, a Doppler effect that would move the wavelengths of the spectral keys. That implied two things. First, to appear to be moving this fast, the object had to be caught up in the expansion of the universe and was literally about 2 billion light-years away. Second, if it was that far away, then for us to see it at all it had to be pumping out energy at an enormous rate. Schmidt ran the numbers by hand and arrived at an astonishing answer. This object was a trillion times as luminous as the Sun. It was pumping out as much energy as all the stars contained in a hundred normal galaxies. An object like this was beyond belief. It seemed impossible, and was so shocking that Schmidt recalls telling his wife that evening that “something terrible” had happened at work. The foundations of astrophysics had been well and truly shaken.
In the following years there was raging debate about what these “quasi-stellar radio objects” could possibly be. People eventually shortened that awkward handle to “quasars,” but the mystery remained. By this time astronomers had found that many recognizable galaxies were also strong sources of radio waves. In these systems there was often no obvious single bright source; instead there were pairs of vast cloud-like “lobes” of radio glow. These were like nearly symmetrical dumbbells interleaved with the stars of the galaxies. They were invisible to normal telescopes of glass and brass, but strikingly obvious with radio dishes. Extending as far as a few hundred thousand light-years, through and beyond the galaxies containing them, these extraordinary glowing structures also required enormous amounts of energy to produce. When people first computed just how much, it seemed implausible for any known physical process. It was comparable to directly converting a million or more times the mass of the Sun into pure energy. The theory of relativity showed that energy and mass were equivalent through the famous expression E = mc2. But there were seemingly few ways for nature to make this conversion with the efficiency required to explain what the radio astronomers were seeing. A source like nuclear fusion was woefully inadequate.
Figure 8. A modern image of the radio-wave light emitted from a galaxy that is 600 million light-years away. This remarkable object, known as Cygnus A, was discovered by radio astronomy in 1939. Early measurements simply showed a basic dumbbell structure extending across six hundred thousand light-years. With time, better and better images revealed the incredible thread-like structures between these huge clouds of what were eventually recognized as seething-hot electrons. The galaxy of stars is invisible in this image but lies between the clouds.
Even earlier in the 1930s, astronomers using optical telescopes had also recognized tha
t many otherwise unremarkable galaxies showed evidence of extremely bright and hot spots toward their centers. In a few, there were even signs of curiously straight rays or “jets” of light emanating for thousands of light-years from these peculiar nodes. Puzzles abounded. The universe was once again full of strange and contrasting phenomena. Many demanded energy sources that far exceeded anything possible from chemical or even nuclear reactions. Could there possibly be a link between all these things?
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The pieces were indeed all falling into place. Someone just needed to connect them. A number of incredibly compact and energetic environments seemed to exist in our own galaxy, with Cygnus X-1 emerging as the prototype. Mysterious radio emission was emanating from the center of the Milky Way. Radio astronomy had also found not just hundreds of distant sources across the sky, but also signs of the strangest forms and structures, great zones of radio emission spanning thousands upon thousands of light-years and containing colossal amounts of energy. Visible light revealed bright cores in many galaxies, together with some remarkable glowing ray-like prominences. It also seemed that these features might have a common point of origin with the mysterious and incredibly luminous quasars. But the power source for these phenomena was a huge puzzle. The generation of energy by matter falling toward extremely compact objects certainly met the efficiency requirements. But the kinds of examples starting to be discovered in the Milky Way were on a tiny scale compared to what was being seen in other galaxies and from the mysterious quasars. This couldn’t be the answer, unless the black holes were enormous, millions to billions of times more massive than the Sun.
Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos Page 9
