Wheeler knew, Penrose knew—and current physics still contends—that given enough mass total collapse is inescapable. “The core like the Cheshire cat fades from view. One leaves behind only its grin, the other, only its gravitational attraction,” Wheeler has said. Everything we know about the strength and stability of matter against the unyielding force of gravity leads to that unavoidable end. The mass disappears from our view; only its gravitational attraction remains behind to affect us.
Wheeler tried to impart the insanity of the concept. Ride along on the collapsing ball of matter, he suggested, and its density would go up faster and faster until, in less than a second, it rose to infinity. “With this prediction of an infinite density,” wrote Wheeler, “classical theory has come to the end of the road. A prediction that is infinity is not a prediction. Something has gone wrong. … Infinity is a signal that an important physical effect has been left out of account.”
And there is something missing. Wheeler pointed out that answers will likely arrive with the successful merging of general relativity with quantum theory. Today we call it superstring theory or loop quantum gravity, the latest attempts to join the macrocosm ruled by gravity with the microcosm controlled by quantum forces. Quantum-gravity theorists don’t have as yet a definitive solution, but they are confident that something else happens inside the black hole, that quantum effects prevent a singularity from forming there.
In the early 1970s, one final escape hatch to prevent the creation of a black hole remained: pulsations. Carrying out computer simulations, researchers saw that a black hole can also vibrate—in a sense, ring like a bell if it’s disturbed. Could these pulsations get unstable and, by extracting energy from the hole, get stronger and stronger—so strong that the hole is violently torn apart? The answer in the end was an unequivocal no. The extra energy simply radiates away from the hole as gravitational waves, ripples in the very fabric of space-time. The black hole remains intact.
Tackling such relativistic problems was a brave, if not foolish, choice for a rising physics student at the time. Few then believed that a star underwent any kind of collapse. Thorne remembered being warned in the early 1960s that general relativity had “little relevance for the real Universe. … One should look elsewhere for interesting physics challenges.” Fortunately, Thorne ignored such skeptics. Those naysayers soon learned that general relativity was crucially needed in astrophysics—and in a big way. That’s because, while Wheeler, Zel’dovich, and others were grappling with the theory of gravitational collapsed objects and the revival of general relativity, astronomy was undergoing its own parallel revolution. It’s when observers started to gather an array of celestial radiations other than visible light, leading to some unexpected discoveries that cried out for explanation.
8
It Was the Weirdest Spectrum I’d Ever Seen
A new astronomy for the twentieth century arose in a very unusual spot. It took place amid central New Jersey’s potato fields. In the 1930s, Karl Jansky set up a unique radio receiver near the rural town of Holmdel and, in doing so, became the first person to snatch astronomy away from its dependence on the optical spectrum, beyond the narrow band of electromagnetic radiation visible to the human eye. His first, provisional step ultimately led to a new and golden age of astronomy that thrives to this day. But, as is often the case in astronomical history, Jansky began his investigations for a totally different reason.
In 1928, fresh out of college with a degree in physics and newly hired by Bell Telephone Laboratories, the twenty-two-year-old was assigned to investigate long-radio-wave static that was disrupting transatlantic radio-telephone communications. To track down the sources, he eventually built a steerable antenna—a spindly network of brass pipes hung over a wooden frame that rolled around, with a motorized push, on Model-T Ford wheels placed on a concrete track. It was known around the lab as “Jansky’s merry-go-round.”
Karl Jansky with the “merry-go-round,” his historic antenna that discovered radio waves emanating from the center of the Milky Way galaxy, initiating the field of radio astronomy. (Reprinted with permission of Alcatel-Lucent USA Inc.)
Setting up his antenna near Bell’s Holmdel station, Jansky soon learned that thunderstorms were a major cause of the disruptive clicks and pops during a radio phone call. But there was a steady yet weaker hiss that he also kept receiving. After a year of detective work, Jansky in early 1933 at last established that the disruptive 20-megahertz static (a frequency between the United States AM and FM bands) didn’t originate in the Earth’s atmosphere or on the Sun or from anywhere within our solar system. To his surprise, he saw that it was coming from the direction of the Sagittarius constellation, where the center of our home galaxy, the Milky Way, is located. Jansky affectionately dubbed the signal his “star noise.” For Jansky it hinted at processes going on in the galactic core, some twenty-seven thousand light-years distant, that visible light rays emanating from that region did not reveal. For unlike visible light, radio waves can cut through the intervening celestial gas and dust, in the manner of a radar signal passing through a fog.
Jansky’s unexpected discovery made front-page headlines in the New York Times on 5 May 1933, with readers being reassured that the galactic radio waves were not the “result of some form of intelligence striving for intra-galactic communication.” Ten days later NBC’s public affairs–oriented Blue Network broadcast the signal across the United States for the radio audience to hear. One reporter remarked that it “sounded like steam escaping from a radiator.”
By 1935, Jansky speculated that the cosmic static was coming either from the huge number of stars in that region or from “some sort of thermal agitation of charged particles,” which was closer to the truth. Years later, astronomers confirmed that the noise was being emitted by violent streams of electrons spiraling about in the magnetic fields of our galaxy. Just as an electric current, oscillating back and forth within an earthbound broadcast antenna, releases waves of radio energy into the air, so these energetic particles broadcast radio waves out into the cosmos, whose wavelengths are far longer than visible light. And Jansky was the first to detect them. He was Earth’s first eavesdropper on the universe.
Despite the worldwide publicity, however, few astronomers then appreciated Jansky’s new ear on the universe. Most were more comfortable with lenses and mirrors than with radio equipment. “The world of decibels and superheterodyne receivers … was far too removed from that of binary star orbits and stellar evolution for a connection to be forged,” explains science historian Woodruff Sullivan. And Bell Labs did no follow-up, since astronomy wasn’t its business at the time. The company put Jansky to work on more commercial problems. But one particular person was inspired by the Bell Labs employee’s innovation. An Illinois radio engineer and avid ham-radio operator, Grote Reber, erected a massive steel saucer—a thirty-foot-wide dish antenna—in his backyard and extended Jansky’s work. He showed that celestial radio waves were most intense along the plane of the Milky Way. In 1940 he sent his results to the Astrophysical Journal, which turned out to be the first paper on radio astronomy the publication had ever received. Only the intervention of a farsighted editor kept it from being rejected. Four years later Reber produced the first map of the entire “radio sky.” Along with the strong peak at the Milky Way’s center, there were secondary peaks in the directions of the Cygnus and Cassiopeia constellations.
Over this time World War II had intervened, essentially slowing any progress, but afterward the field of radio astronomy took off. The war itself was actually one of the reasons. Dozens of young physicists and engineers in Europe, Australia, and the United States had been introduced to the esoteric art of radio science while working on the development of radar during the conflict. After the war they were eager to apply their newfound skills to follow up on the work of radio astronomy’s two pioneers. They wanted to pinpoint the celestial objects putting out those mysterious radio signals. To this vanguard, the radio sky was a bla
nk page just waiting to be filled in. What happened next has been called “the most eventful era in the history of astronomy since the time of Galileo.”
Radio telescopes began cropping up around the globe, with England and Australia dominating the field at first. They found the nebulous remnants of ancient supernovae emitting loud radio squeals. And Cygnus A, one of the “brightest” objects in the radio sky, turned out to be a strange-looking galaxy located about six hundred million light-years away. Similar “radio galaxies” were found all over the heavens. By developing techniques to combine the signals from radio telescopes separated by a mile or more, which together then acted as one large telescope, they obtained enough resolution to see that the radio signals from these peculiar galaxies were emanating from giant lobes of gas, jutting out for a few hundred thousand light-years from the galaxy like the wings on a plane. How did they possibly originate?
The solution involved thinking about the universe in a whole new way. No longer was it just stars and galaxies floating in space but also particles like electrons racing within the electromagnetic fields filling interstellar and intergalactic space. It was these electrons that were releasing the radio waves as they spiraled about the magnetic field lines. By 1958 astrophysicist Geoffrey Burbidge figured that those gigantic lobes surrounding radio galaxies held as much magnetic and kinetic energy as if the matter of ten million suns had been completely converted into pure energy, according to E = mc2. Optical telescopes had been blind to this activity, making astronomers believe for centuries that the universe was fairly serene. But by reaching out into broader regions of the spectrum, astronomers now knew that something big was going on in distant space that traditional energy sources couldn’t account for. The universe was jam-packed with action.
Two giant radio lobes, each about six hundred thousand light-years across, encase the giant elliptical galaxy, Fornax A (seen in the center), situated some sixty million light-years from Earth. (Courtesy of NRAO/AUI and J. M. Uson)
Chemical power, like that from dynamite, was far too weak; even nuclear energy appeared dicey. “Nuclear fuel’s efficiency for mass-to-energy conversion is roughly 1 percent,” Kip Thorne once estimated. That means an active galaxy would need one billion solar masses of nuclear fuel to energize its radio-emitting lobes. Possible, but not likely. Energy drawn from matter annihilating antimatter was briefly considered but also discarded as a possible source. The universe just didn’t seem to harbor enough antimatter.
The mystery got even stranger. By the late 1950s, spurred on by these discoveries and not wanting to miss the boat, the United States built its own state-of-the-art radio observatories. One of them, a complex situated in California’s Owens Valley and run by Caltech, was able to narrow down the location of a radio source labeled 3C 48, for being the forty-eighth radio object in the Third Cambridge Catalogue of radio sources. Astronomer Allan Sandage soon used the grand 200-inch Hale telescope atop California’s Palomar Mountain to see what visible object might be situated at that spot. After carrying out a ninety-minute exposure and expecting to see another galaxy there within the Triangulum constellation, he instead found a pinpoint of light, a real surprise. To the eye, it was yellow in color, but also unusually bright in the ultraviolet region of the spectrum. At first, everyone just assumed it was a star in our own galaxy, making it the first known “radio star.” But there was a catch: “I took a spectrum … ,” said Sandage, “and it was the weirdest spectrum I’d ever seen.”
Over the following two years, a handful of similar objects were discovered. On first look they appeared to be simply faint stars within the Milky Way, just like 3C 48. But again, after viewing the light waves emanating from these radio stars more closely, optical astronomers found that they all displayed spectral features unlike any star ever observed. The spectra of these stars didn’t match any known chemical element. Could there be other chemicals out there, as yet undiscovered? It was like riding down a familiar turnpike and finding that all the road signs were written in gibberish. Astronomers couldn’t even find evidence that hydrogen—the main component of all stars—was present. Yet, everyone kept assuming they were stars because, well, they looked like stars through an optical telescope. For one, they flickered. If this strange object were a far-off galaxy, it was considered “utterly ludicrous” to believe that some one hundred billion stars could turn their brightness on and off in sync so swiftly. Not until February 1963 was the identity of these peculiar radio beacons finally unmasked.
On the fifth day of that month, thirty-three-year-old Maarten Schmidt, who had arrived a few years earlier at Caltech from the Netherlands, was sitting at his desk attempting to write an article on the radio star known as 3C 273 for the British journal Nature. Australian radio astronomers had just gone to extraordinary lengths to view this source. They cut down trees and tipped a weighty radio dish beyond its safety limits to better pinpoint the position of this radio star caught low on the horizon. With the improved coordinates, Schmidt was able to use the Palomar telescope to find the star in visible light and obtain an optical spectrum. With the spectrum spread before him, Schmidt came at last to recognize a familiar pattern of spectral lines that had eluded him for weeks. The pattern resembled the specific wavelengths of light typically emitted by simple hydrogen when energized—but they were in the wrong place! That’s why hydrogen had appeared to be missing. The hydrogen lines were there, but shifted waaaay over, toward the red end of the spectrum. That meant this starlike object was moving away from us at a tremendous speed. Just as the pitch of an ambulance siren gets lower as it races away from us, a light wave is stretched to longer lengths (gets “redder”) when its source recedes—a type of Doppler shift. Consequently, this “redshift” lets astronomers gauge both how fast a celestial object is moving and also its distance.
In this way, Schmidt swiftly grasped what that redshift meant. It turns out that 3C 273 was not an unusual star situated within the Milky Way but rather a bizarre object located some two billion light-years away (one of the farthest cosmic distances ever recorded at that time). It was rushing through space at nearly thirty thousand miles (forty-eight thousand kilometers) per second, carried outward with the swift expansion of the universe. Schmidt knew that only an incredibly bright source could be visible from such a distance; he figured that 3C 273 was radiating the power of trillions of stars and suspected it was the brilliant and very disturbed nucleus of a distant galaxy. This galaxy appeared starlike only because it was so far away.
With that revelation, all fell into place. The spectra of other mystifying radio stars were quickly deciphered. These blue, extragalactic specks were soon referred to as quasi-stellar radio sources (QSRS) or quasi-stellar objects (QSO) by the California astronomers. Before long, they were simply called quasars, a term at first scorned by old-school astronomers. Not until 1970 did Chandrasekhar, by then the Astrophysical Journal’s editor-in-chief, permit its official use and that was only after Schmidt convinced him that the name could no longer be ignored. “The Astrophysical Journal has up till now not recognized the term ‘quasar,’” Chandra wrote in a footnote to one of Schmidt’s papers, “and it regrets that it must now concede.”
The first known quasar, 3C 273, as imaged by the Hubble Space Telescope’s Wide Field Planetary Camera 2. The diffraction spikes demonstrate the quasar is truly a point-source of light, like a star, and hence very compact. (Courtesy of NASA/Space Telescope Science Institute)
The quasar 3C 273 is now considered relatively close to us, as quasars go. Its distance is small potatoes compared to those of later finds. Over the past five decades, astronomers have now identified quasars out to a distance of some thirteen billion light-years, which means they were bright and active less than a billion years after the Big Bang. The fact that earthbound observers are able to see such quasars across the vastness of the universe means that these objects are the most powerful denizens of the heavens.
But what could possibly be the source of such monstrous energy, eve
ryone asked upon the quasar’s discovery? “The insult was not that they radiate so much energy,” said Schmidt, “but that this energy was coming from a region probably no more than a light-week across.” Astronomers came to know this by seeing the quasars dim and brighten over a matter of weeks or days. In the case of 3C 273, they checked old photographic plates of the thirteenth magnitude object (roughly four hundred thousand times fainter than the star Sirius), going back some seventy years. In one picture it was faint, a month later it was brighter. Such relatively swift fluctuations meant that the quasar’s power source was small, perhaps less than the diameter of our solar system. That’s because any quick luminosity changes in a vastly larger object would get lost in the noise. Yet from such a cosmically tiny region spewed the energy of billions of suns. Tapping into such a cosmic dynamo for just one second would power the world for a billion billion years. What cosmic process could conceivably generate such energies?
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