Adapted from Neutrino Hunters: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe, by Ray Jayawardhana, by arrangement with Scientific American/Farrar, Straus and Giroux, LLC (US), HarperCollings (CA), Oneworld (UK). Copyright © 2013 by Ray Jayawardhana.
In the wee hours of February 24, 1987, atop Cerro Las Campanas in Chile, Ian Shelton decided to develop the final photographic plate of the night before heading to bed. Shelton, a resident observer employed by the University of Toronto, had been tinkering with a decades-old 10-inch telescope on the mountain, training the little instrument on one of the Milky Way's galactic sidekicks, the Large Magellanic Cloud (LMC). He lifted the photographic plate out of the developing tank and examined it to make sure the three-hour-long exposure had come out well. Then something caught his attention: a curious bright spot next to a familiar spider-shaped feature known as the Tarantula nebula. He wondered what the unusual spot might be and reasoned that it was likely a flaw in the plate itself. But just to be sure, he walked out of the telescope enclosure into the dry mountain air to look up at the sky with his own eyes. He saw a bright star in the LMC that had not been visible the night before. Shelton hurried over to one of the other telescope domes on the ridge to share the news.
As he discussed his puzzling find with astronomers Barry Madore and William Kunkel in the control room, Chilean telescope operator Oscar Duhalde piped in that he had seen the same star a few hours earlier, when he stepped out for a break. Together the four of them decided the “new” star had to be a supernova, an exploding star that could briefly outshine one billion suns. No other type of astronomical object was known to change in brightness so dramatically, from being too faint to register in photographs taken the night before to being easily spotted with the naked eye. That meant Shelton and Duhalde had discovered a supernova in a satellite galaxy of the Milky Way. A few hours later, working independently, an amateur astronomer in New Zealand saw the same thing.
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By midmorning, scientists around the world learned about the discovery, tipped off by phone calls from giddy colleagues and a telegram from the International Astronomical Union. Their delight had to do with the fact that “supernova 1987A” (as it came to be known) was the first one observed in our galactic neighborhood since the invention of the telescope nearly four centuries earlier.
Astronomers rushed to employ a mighty suite of optical, infrared and radio telescopes spread across the Southern Hemisphere, as well as x-ray and ultraviolet instruments onboard spacecraft, to watch the momentous event unfolding in the LMC. It was a period of frenzied activity that few scientists had ever experienced. As one ebullient astrophysicist declared, “It's like Christmas.”
These investigations of supernova 1987A provided broad support for the scenario that theorists had developed, with the help of complex simulations on supercomputers, for how an aging massive star self-destructs, with its core collapsing into a tightly packed ball of neutrons—called a neutron star—or into a black hole and with its expelled outer layers spreading outward to form a glowing cloud of debris. Yet the celebration was not limited to astronomers. For particle physicists, other observations of the supernova provided important clues to the nature of the ghostly subatomic particles known as neutrinos. Together the diverse studies of the 1987 supernova have built up anticipation of a similar stellar collapse right in our own galaxy—an event that could occur at any time and that should answer lingering questions about star death and the nature of neutrinos. This time neutrino hunters will probably be the first to detect the event.
Photon Forerunners
The late John Bahcall, then at the Institute for Advanced Study in Princeton, N.J., found supernova 1987A so exciting that he was losing sleep. There was a good reason for his excitement: Bahcall knew that the very first, and arguably the most important, harbingers of this cosmic cataclysm must have arrived hours before astronomers spotted the supernova using conventional telescopes. He was well aware that according to theoretical models of stellar evolution, the core collapse at the end of a massive star's life should result in a copious burst of neutrinos, which would flee the detonation site deep inside the star with little impediment. The visible fireworks would appear only later, when the star's outer mantle blew up. Minutes after he heard about supernova 1987A, Bahcall and two of his colleagues got to work to calculate how many neutrinos should have been recorded by the various neutrino detectors on Earth. They determined that the answer should be a few dozen neutrinos and submitted a paper with their conclusion to the journal Nature within a week so that their prediction could appear ahead of the actual measurement. [Editors' note: Scientific American is part of Nature Publishing Group.]
Meanwhile experimental physicists had begun to search through data recorded at several underground detectors around the world. Their best chance of registering supernova neutrinos was with the Kamiokande experiment in Japan, which consisted of a four-story-tall, cylindrical tank of purified water, surrounded by 1,000 phototubes to register flashes of light produced when neutrinos interacted with water atoms. A failure to measure neutrinos from supernova 1987A could imply a basic flaw in our understanding of how a supernova works.
Sure enough, to the utter relief of scientists the world over, the neutrino signal stood out clearly in the data, leaving no doubt as to its provenance. The phototubes at the Kamiokande detector picked up 11 flashes in a burst lasting several seconds, nearly three hours before the first optical sighting of the supernova by astronomers in Chile and New Zealand. Halfway around the world, a similar neutrino detector located in a shallow salt mine under Lake Erie, not far from Cleveland, registered eight flashes at exactly the same time as Kamiokande. Later, scientists learned that a third, oil-based detector, at the Baksan Neutrino Observatory in the Caucasus Mountains in Russia, had also recorded five neutrinos.
The two dozen neutrinos detected were just a few of the billions on billions of neutrinos sweeping past our planet in a burst that originated in the heart of the exploding star in the LMC. Because all three of these neutrino “observatories” are located in the Northern Hemisphere, whereas the LMC is in the south, the neutrinos had to traverse from one side of Earth to the other, through our planet's interior, and enter the detectors from below.
Detecting a grand total of two dozen particles may not sound like much to crow about. But the significance of these neutrino events is underlined by the fact that they have been the subject of hundreds of scientific papers over the years. Supernova 1987A was the first occasion in which we have observed neutrinos coming from an astronomical source other than the sun. As John Beacom, a theoretical physicist at Ohio State University, says, “Neutrinos allow us to see the interior of a massive star at the end of its life, so we can do astrophysics that astronomers could otherwise never do.”
The supernova neutrino detections, as sparse as they were, validated some important details of how a massive star blows up. Astrophysicists were pleased to find that the number and energies of the neutrinos they measured agreed with their expectations based on theoretical calculations of the explosion. Thanks to the excellent agreement between theory and observation, researchers concluded that the supernova did not lose energy through some mysterious process—by, say, neutrinos emitting hypothetical particles called axions or leaking into enigmatic extra dimensions. The arrival of neutrinos over several seconds, rather than all in a single burst, confirmed that they took some time to make their way out of the extremely dense shrunken core, as predicted.
Besides, the measurements revealed clues to the nature of neutrinos themselves. Because the neutrinos reached Earth no more than three hours before the supernova was captured in an optical photograph, they must have traveled pretty close to light speed. Lighter particles travel faster than heavier ones, so the scientists reasoned that the mass of a neutrino must be quite small. In fact, based on the particles' arrival time from supernova 1987A, scientists were able to show that despite their prodigious numbers, neutrinos would be unlikely to account for the mysterious “dark matter” permeating the universe. What is more, when a media frenzy broke out in 2011 about neutrinos traveling faster than light, one strong counterargument was based on observations of the 1987 supernova. If these particles indeed travel as fast as the experiment initially reported, the neutrino burst from supernova 1987A should have reached Earth years earlier than the optical light, not mere hours before.
Gearing Up
Supernova 1987A has whet the appetite of astrophysicists who want to learn about the inner workings of dying stars. “Imagine what we could learn if we were to detect 1,000 neutrinos from a nearby supernova,” muses Alex Friedland of Los Alamos National Laboratory. Such a prodigious event would not only allow us to pinpoint the sequence of events as the explosion proceeds but would definitively tell us what has become of the ill-fated star. Particle physicists are also interested in neutrinos from supernovae because they provide a rare opportunity to understand how these elusive particles behave under extreme conditions that cannot be replicated in a laboratory.
What both sets of scientists need to achieve their goals is a core-collapse supernova in our own galaxy. Surprisingly, no supernova has been seen in the Milky Way since 1604, when stargazers, including German mathematician Johannes Kepler, noticed a “new star” in the constellation Ophiuchus. At its peak, this supernova was so bright that it was visible during the daytime. Just three decades earlier, in 1572, observers in Europe, including the legendary Danish astronomer Tycho Brahe, had seen another one. Current evidence suggests that both those supernovae resulted from the explosion of a stellar cinder known as a white dwarf, which either gobbled material from a companion star or merged with another white dwarf, rather than from the core collapse of a massive star at the end of its life.
Based on their observations of other galaxies, today's astronomers estimate that at least a few massive stars must explode each century in the Milky Way. Even if interstellar material blocks light from a supernova in the distant realms of our galaxy, it does not hinder the passage of neutrinos, so detecting a burst of neutrinos would reveal the death of a massive star anywhere in the Milky Way. We have had sensitive neutrino detectors operating for about a quarter of a century, and if our estimates are correct, we are due for a galactic supernova any day now. “It would be a once-in-a-lifetime opportunity, so we better be prepared,” says Georg Raffelt of the Max Planck Institute for Physics in Munich.
Kate Scholberg of Duke University agrees. She and her colleagues have set up the SuperNova Early Warning System—SNEWS for short—a coordinated network to provide rapid notifications of core-collapse explosions in the galaxy. The plan is that detectors around the globe that are sensitive to supernova neutrinos—such as IceCube in Antarctica, the Large Volume Detector and Borexino in Italy, and a bigger, more sensitive version of Kamiokande, called Super-Kamiokande, in Japan—will report candidate bursts to a central computer at Brookhaven National Laboratory on Long Island. “If several neutrino detectors light up at once, there's a very good chance a supernova has gone off nearby,” Scholberg explains.
If the SNEWS computer finds a coincidence within 10 seconds between signals from two detectors, it sends out an alert to observatories worldwide. Scholberg and her colleagues hope that telescopes on the ground and in space will be able to record electromagnetic radiation, including visible light, radio waves and x-rays from the explosion, sooner rather than later and to watch its early stages unravel. “The idea is to have as many people looking as possible, everywhere, to have the best chance possible of pinpointing early light,” she says.
“Measuring neutrinos from a galactic supernova will tell us an enormous amount,” Scholberg says. “It's an unbelievably rich mine of information.” The detectors will record how the number and energy of the arriving neutrinos evolve over time, which will give us insight into how the explosion unfolds. Among other things, scientists will be able to determine whether the star's core collapses all the way into a black hole, from which nothing—not even neutrinos—can escape, or whether it stops short, forging a neutron star instead. If a black hole were to form, the stream of neutrinos racing outward from the supernova would come to a sudden halt. If the end product were a neutron star, on the other hand, the stellar cinder would continue emitting neutrinos for about 10 seconds while it cools down, so the neutrino stream should dwindle slowly instead of cutting off abruptly.
A galactic supernova could also shed light on the nature of the neutrinos themselves. For example, physicists have been struggling to determine what they call the “mass hierarchy” of neutrinos. In effect, they want to know if there are two heavy mass states plus one light state or one heavy and two light states, and they believe that measuring supernova neutrinos would nail down the answer. What is more, in a supernova core the density of neutrinos is so high that interactions among neutrinos, which are otherwise oblivious to one another's presence, could alter their behavior. “We might see some exotic collective oscillations of neutrinos,” Scholberg says. “If there are any anomalies in their behavior, they could point to new physics beyond the Standard Model,” the well-tested framework of fundamental forces and elementary particles.
Fortunately, several existing detectors are sensitive enough to register neutrinos from a supernova that occurs anywhere in the Milky Way. Super-Kamiokande, for example, would register several thousand hits from a supernova near the galactic center, more than 25,000 light-years away. It could even pinpoint the direction the neutrinos come from to within a few degrees, corresponding to a patch of the sky several times bigger than the full moon. IceCube, which would record a million events, is best for tracking how the neutrino stream evolves with time. “We will be able to see the entire 10-second story of the explosion unfold in snapshots taken every few milliseconds,” says IceCube principal investigator Francis Halzen of the University of Wisconsin–Madison. “We will be able to pin down the exact moment that the neutron star forms.”
The current detectors, however, are sensitive only to one variety of neutrinos, namely electron antineutrinos. (Neutrinos and their antimatter counterparts each come in three so-called flavors: electron, muon and tau.) “Observing only one flavor is like taking a photograph through a single-color filter,” Scholberg observes. She would rather have the full-color view. As a first step toward developing multicolor vision, Scholberg and her Canadian colleagues are building a dedicated apparatus, called the Helium and Lead Observatory (HALO), at the SNOLAB in Ontario. Using 80 tons of lead as the detector material, HALO is sensitive to electron neutrinos, so it will complement other existing detectors that register their antimatter twins. HALO is fairly small as neutrino detectors go, so a supernova would have to explode within the nearer half of the galaxy to be detectable.
Waiting Game
As exciting as the prospects are, realizing them will have to wait until a core-collapse supernova goes off in the galactic neighborhood. The long wait is frustrating. As Ohio State's Beacom says, it is “a matter of holding your breath.” The problem is that current observatories are not sensitive enough to detect many neutrinos from supernovae in other galaxies. For example, Super-Kamiokande would register a single paltry event from an explosion in the Andromeda galaxy, the Milky Way's nearest comparably sized neighbor, 2.5 million light-years away.
Although all the evidence suggests that aging, giant stars such as Betelgeuse and Eta Carinae will meet fiery ends in the near future, we do not know when their demise will come. In cosmic terms, “near future” could well be several hundred thousand years from now. That said, the odds are pretty good that a massive star somewhere in the galaxy will explode in the next few decades. As Los Alamos National Laboratory's Friedland has told me, “If I had to bet on what would happen first, the next galactic supernova or building the next big particle collider in the U.S., my money would be on the supernova.” Even if the supernova is so far away from Earth that we cannot observe its light through the dusty veil of the Milky Way, it will shine brightly in the neutrino detectors around the world. It will be a sensational event, a watershed moment that neutrino hunters will celebrate like none other.