By the 1950s, with particle physicists constructing big accelerators to search for new particles, cosmic-ray physicists began to focus more on the origin of the “rays.” How and where are cosmic rays being created in the vastness of the universe? they asked. Millikan was wrong on all counts. But other ideas were already in the wind. As early as 1934, Walter Baade of the Mount Wilson Observatory and Caltech physicist Fritz Zwicky suggested that the rays came from spectacular stellar blasts, explosions earlier dubbed “supernovae.” It’s an idea that holds up to this day. More recently, active galactic nuclei have also come to be suspected as rich sources of the rays: a spinning supermassive black hole at a galaxy’s center spews out blazing jets of particles into space, acting like an electrical generator as it rotates.
All of the above may be contributing to the signal gleaned by the Utah array. The Milky Way lives in the outskirts of the Virgo Supercluster of galaxies, and the hotspot resides in the very direction of that vast supercluster, home to tens of thousands of galaxies. The cosmic rays arriving on Earth from that bearing could then be the collective shout from the myriad supernovae and active galaxies occupying the supercluster. Here we are, they are saying, here we are.
CHAPTER TWENTY
Einstein’s Symphony
Finding Einstein’s long-sought ripples in space-time
HEADING in from the southern sky at the speed of light, a gravitational wave passed through the Earth on September 14, 2015, in less than a second. Such events have occurred ever since our solar system coalesced out of a nebulous cloud more than four billion years ago. But this time was different. This time researchers finally snared that faint swell in space-time, ushering in a new age of astronomy as game-changing as the telescopic era introduced by Galileo.
Einstein first mentioned the possibility of gravitational waves (or gravity waves, as they’re more popularly known) more than one hundred years ago. He predicted that a pair of masses, such as two stars moving around each other, would undulate the very fabric of space-time. These waves then would move outward, much like the ripples generated when a stone is dropped into a pond, getting weaker and weaker as they spread. This pattern is far different from the way electromagnetic waves propagate. Light travels through space; gravity waves, by contrast, are vibrations in the very framework of space-time—compressing and stretching space-time (and any object caught within it) as they pass by.
Ever since the 1960s, scientists had been attempting to capture a gravity wave. At the University of Maryland, physicist Joseph Weber constructed the first detectors, large cylinders of metal, called Weber bars, surrounded with sensors that he configured to “ring” like a bell whenever a gravity wave passed through them. He claimed to have observed such ringing a number of times, starting in 1969, but the detections were never confirmed. His effort, however, founded a new field of study, stimulating others to come up with new schemes.
In 1972, at MIT, physicist Rainer Weiss wrote a landmark report, the first complete examination of an approach known as “laser interferometry.” He suggested arranging a set of mirrors in the form of an L—one in the corner, the others at each end; continually bouncing laser beams up and down each arm to keep an accurate tab on the distance between them; and then having the beams recombine (optically “interfere” with one another) to check if a gravity wave had wiggled the mirrors.
Weiss and others built small laboratory prototypes, but the MIT physicist knew that no cosmic waves would ever be found unless the mirrors were separated by miles. The longer the distance, the greater the sensitivity of the measurement. By the 1980s, tired of his slow progress, Weiss joined forces with Caltech theorist Kip Thorne, then the world’s top expert on gravity waves, and experimentalist Ronald Drever, also at Caltech, to take a giant leap and seek National Science Foundation (NSF) funds to construct a pair of large detectors with arms two-and-a-half miles (four kilometers) long, set geographically apart to rule out local noise.
Upon hearing of this proposal, the physics community quickly protested; it was aghast at the idea that the NSF might spend money on such a gamble when so much of the technology still needed to be invented. It was only after a decade of campaigning and politicking that the funds were finally approved and ground was broken for the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 1994. One detector resides in Livingston, Louisiana, the other 1,900 miles (around 3,000 kilometers) northwest in Hanford, Washington. Turned on in 2001, and advanced and improved over the years, both instruments at last found their quarry that fateful September night.
This illustration shows the generation of gravitational waves, ripples in space-time, as two black holes spiral into one another, heading toward eventual collision.
(LIGO / T. Pyle)
The wave first arrived at Livingston two hours before dawn, at exactly 4:50:45 a.m. Central Daylight Time. Seven-thousandths of a second later, Hanford also sensed the wave. But the operators in the main control room at each site didn’t notice. LIGO was then conducting an engineering run, a check on some newly installed equipment. Data was being collected, but the sound alert, which goes off whenever a candidate signal passes a certain threshold, was not on. That awaited the official scientific run of the new, “advanced” detectors, which was set to occur a few days later.
Instead, the data silently streamed into the automatic analysis pipeline, where, within a few minutes, the waveform popped up on the computer monitor of LIGO collaborator Marco Drago at the Albert Einstein Institute in Hannover, Germany. A member of LIGO’s coherent wave burst group, the young postdoc was among the first to see the signal. It was beautiful, clear, and strong. In fact, it was so picture-perfect that Drago and his colleagues, who soon gathered together, just assumed it was a “blind injection,” someone from LIGO secretly sending out a fake signal to test the system. But they soon learned that wasn’t the case. Could it have been a hacker? That, too, was a concern and, therefore, was thoroughly checked out. In the end, LIGO scientists finally realized they had their Cinderella scenario or “golden event,” as Drago put it—a gravity wave always hoped for but never expected as a first detection. It stood high above the noise.
As LIGO continued to gather data, teams of theorists deciphered the inaugural wave’s message according to Einstein’s general theory of relativity. In less than a second, the signal had swept upward in frequency from about 30 hertz (cycles per second) to around nearly 300 hertz. Because that’s the same frequency range as sound, it can be heard as a musical glissando that starts as a deep bass and swiftly ends near middle C. Gravity-wave astronomy is adding sound to our cosmic senses. This “chirp” was just the type of signal that would be expected to occur when two black holes, long orbiting one another, swirled together ever faster until they merged to form a single black hole. Such a collision had never before been demonstrated; the LIGO observations not only confirmed that it had occurred, but also indicated the sizes of the black holes. One of the holes weighed thirty-six solar masses, the other twenty-nine solar masses. The resulting combined black hole, at sixty-two solar masses, was less massive than the sum of the two because some of the mass was instantly converted into pure gravitational-wave energy—fifty times more energy than all the stars in the universe were radiating at that moment. At the collision site, such a spacequake would be deadly, but by the time the waves reached Earth some 1.3 billion years later, they moved the LIGO mirrors a mere fraction of the width of a proton. That’s why only gravity waves from the universe’s most violent events are currently measurable.
And this detection was just the start. Other signals were soon spotted in the ensuing months and years. The LIGO instrumentation is continually being improved, so that it will eventually be able to register waves arriving from even farther regions of the universe. A similar detector called Virgo is now operating in Italy in coordination with LIGO. Gravity-wave astronomers expect someday to see events weekly, possibly even daily. Black-hole collisions are their big game, but other types of events are als
o expected to turn up. Kip Thorne describes them as “the warped side of the universe.”
The Laser Interferometer Gravitational-Wave Observatory in
Livingston, Louisiana.
(Caltech/MIT/LIGO Lab)
It was almost guaranteed that researchers would hear the resounding crash of two city-sized neutron stars (paired together in a binary system) spiraling into each other as their orbital dance decays. And in due course, they did. Both LIGO and Virgo detected their first neutron-star collision on August 17, 2017. Such events may turn out to be the bread-and-butter of these detectors’ trade—and the most entertaining. Less dense than black holes, a pair of neutron stars takes longer to merge, so the final recordable signal can last a minute or more instead of fractions of a second. The gravity-wave “telescopes” register a sinusoidal tune that sweeps to higher and higher frequencies as the two balls of pure neutrons spiral into one another. As soon as they touch, the two stars are shredded to pieces, releasing a burst of electromagnetic radiation across the spectrum, from radio waves to gamma rays. What happens after the collision depends on the situation: The remnants might coalesce into a new, more massive neutron star, if it’s rotating particularly fast. Or if heavy enough, they might condense to utter invisibility, forging a black hole.
There will be another type of signal in the gravity-wave sky, although far less frequent. A solitary tsunami of a wave may hit our shores whenever a star within our local galactic neighborhood explodes as a brilliant supernova. This happens when the star’s nuclear core runs out of fuel, collapses, and sends out a shock wave and a flood of neutrinos that blows the rest of the star apart. Examining the gravitational waveforms from such a spectacular event will allow astronomers to see, for the first time, the birth of a neutron star or black hole at the end of a star’s life.
All the while, playing in the background amid these chirps and pops, could be ongoing rhythms—a steady hum. When a neutron star forms, for instance, it might briefly vibrate and develop a bump on its surface, an inch-high “mountain” that freezes into place for a while. This deformation, jutting out like a finger, would send out a continual set of gravity waves as it continually “scrapes” the space around it.
And beneath all those varied gravity-wave songs, astronomers expect an underlying murmur—constant, unvarying, and as delicate as a whisper. This buzz would be the faint reverberation of our universe’s creation, its remnant thunder echoing down the passages of time. “That is the prize,” says MIT physicist Nergis Mavalvala. That’s because these primordial waves would bring us the closest ever to our origins, perhaps verifying that the universe emerged as a sort of quantum fluctuation out of nothingness. Future laser interferometers in space may be the first to see this gravitational-wave background.
Finally, there is the tantalizing prospect of encountering the unanticipated. Some theorists already wonder whether there might be relics from the early universe, highly energetic “defects” that were generated as the cosmos cooled down over its first second of existence. These include one-dimensional cosmic strings, extremely thin tubes of space-time in which the energetic conditions of the primeval fireball still prevail. Wiggling around like rubber bands, they would produce plenty of gravity waves. Not until astronomers scanned the heavens with radio telescopes did they discover pulsars and quasars. What else might be skulking about in the darkness of space, as yet unseen?
CHAPTER TWENTY-ONE
Underground Astronomy
Learning about the cosmos with detectors
buried in the Earth
DEEP beneath the South Pole, thousands of detectors, set within a cubic kilometer of ice, lie in wait. While looking up toward the surface, they also peer downward, hoping to catch certain elementary particles from the northern sky that travel through the Earth daily. Nearly all of these elusive particles—called neutrinos—blithely pass through our dense planet like ghosts on the run. Most of the time no signal is registered by the instruments. But on rare occasions a neutrino and a detector collide.
Between 2010 and 2013, this frigid array of detectors, known as the IceCube Neutrino Observatory, recorded some 35,000 neutrinos journeying through our entire planet to the Antarctic ice—a minuscule number compared with the trillions that traversed the Earth over that time. Most of the recorded neutrinos were generated locally, when cosmic rays impacted the northern atmosphere. But a tiny fraction of them appeared to have arrived from events far outside the Milky Way—either from massive stars exploding in distant reaches of the universe, or from the active cores of blazing galaxies. The ultrahigh energy of this special set of particles, far beyond the levels of the other neutrinos, revealed them for what they were.
An image of one of the highest-energy neutrino events registered by the IceCube Neutrino Observatory, shown at the bottom, superimposed on a view of the laboratory at the South Pole. When the neutrinos cross the underground detectors, they leave these tracks of light.
(IceCube Collaboration)
With this success at identification, the IceCube detectors offer an entirely new way to survey the cosmos, an endeavor that couldn’t have been imagined less than a century ago. Indeed, the very idea of the neutrino was first thought too crazy to be true, the physics equivalent of unicorns or elves. Even more peculiar was where the neutrino’s story began: in a German prisoner-of-war camp during World War I.
The British physicist James Chadwick had been studying the phenomenon of radioactivity in Berlin under Hans Geiger (of Geiger counter fame) when the war broke out. Chadwick was soon sent to an internment camp set up at a racecourse just outside the city. To while away the hours of confinement, he began teaching physics to his prison-mate Charles Ellis, a young and sociable cadet from Great Britain’s Royal Military Academy who had arrived in Germany on holiday just before the war’s unexpected eruption. Together, the two compatriots organized a small research lab in one of the horse stables, an endeavor that was surprisingly tolerated by the camp’s senior officials and generously supported by Chadwick’s former German scientific colleagues.
The experience hooked Ellis. After the war, he committed to a career in physics instead of the army and ended up conducting experiments at the famous Cavendish Laboratory in Great Britain, where he studied a troubling anomaly. Whenever a radioactive nucleus decayed by ejecting an electron, something went awry. Ellis and a colleague noticed that the energy of the nucleus before it radioactively decayed was more than the total energy of the system afterward (that is, the combined energy of the depleted nucleus and the fleeing electron). It looked as if energy were disappearing, which violated one of the most sacred rules of physics—conservation of energy. Energy can be neither created nor destroyed.
But Wolfgang Pauli, a Viennese physicist, had an abiding faith that atoms were obeying the physical laws of the land, which led him to a radical proposition. In 1930, he suggested that an entirely new particle, invisible to ordinary instruments, could explain the energy discrepancy. Every time a nucleus spewed out an electron, it also emitted a neutral, phantom-like particle that seemed to vanish, carrying away that extra bit of energy and balancing the books.
Wolfgang Pauli in 1930, around the time
he first hypothesized a new neutral particle,
later dubbed the neutrino.
(Photograph by Francis Simon, courtesy AIP Emilio
Segrè Visual Archives, Francis Simon Collection)
Usually undaunted by new concepts, Pauli this time was intimidated by the outrageousness of his idea. “Dear radioactive ladies and gentlemen,” he teasingly wrote his friends, then attending a physics conference in Germany. “For the time being, I dare not publish anything about this idea and address myself confidentially first to you, dear radioactive ones, with the question of how it would be with the experimental proof of such a [particle].” He thought of his remedy as “desperate.” It wasn’t traditionally acceptable for theorists to conjure up particles out of whole cloth, especially particles that seemed impossible to catch
.
That all changed in 1932. That year, Chadwick discovered the first known electrically chargeless particle—the neutron—which at last gave Pauli the courage to officially publish his idea that another neutral particle might exist. Soon after, physicist Enrico Fermi dubbed Pauli’s hypothetical particle the neutrino, Italian for “little neutral one.” The name was apt, for at the time the neutrino was thought to have no mass. According to Pauli’s theory, it was nothing more than a spot of energy that flew off at the speed of light.
Despite Chadwick’s discovery of the neutron, it took years to prove that neutrinos were more than figments of Pauli’s imagination—so long, in fact, that some physicists began to call his particle “the little one who was not there.” Pauli had reason to be apprehensive. The neutrino is so oblivious to ordinary matter that it would take a stack of lead, thousands of light-years in length, to stop one in its tracks. Neutrinos bolt through the Earth as if it’s no more substantial than a cloudy mist.
But the odds of catching one are considerably increased if there is a flood of such particles coming at you. Indeed, that’s how they were finally cornered. In the mid-1950s, physicists Clyde Cowan and Frederick Reines set up a detector outside a South Carolina nuclear power plant and each hour caught a few neutrinos out of the trillions generated by the reactor’s core. Receiving news of the verification while attending a conference in Zurich, Pauli celebrated with colleagues by climbing the town’s local mountain and enjoying several wine toasts at the top. With a friend on each arm helping him on the way down, Pauli turned to one and remarked, “All good things come to the man who is patient.”
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