It may sound like science fiction, but it’s not: Scientists have created the first time crystal, using a chain of ions. Just as a standard crystal repeats in a regular spatial pattern, a time crystal repeats in time, returning to a similar configuration at regular intervals.

“This is a remarkable experiment,” says physicist Chetan Nayak of Microsoft Station Q at the University of California, Santa Barbara. “There is a ‘wow factor.’”

Scientists at the University of Maryland and the University of California, Berkeley created a chain of 10 ytterbium ions. These ions behave like particles with spin, a sort of quantum mechanical version of angular momentum, which can point either up or down. Using a laser, the physicists flipped the spins in a chain of ions halfway around, from up to down, and allowed the ions to interact so that the spin of each ion would influence the others. The researchers repeated this sequence at regular intervals, flipping the ions halfway each time and letting them interact. When scientists measured the ions’ spins, on average the ions went full circle, returning to their original states, in twice the time interval at which they were flipped halfway.
This behavior is sensible — if each flip turns something halfway around, it takes two flips to return to its original position. But scientists found that the ions’ spins would return to their original orientation at that same rate even if they were not flipped perfectly halfway. This result indicates that the system of ions prefers to respond at a certain regular period — the hallmark of a time crystal — just as atoms in a crystal prefer a perfectly spaced lattice. Such time crystals are “one of the first examples of a new phase of matter,” says physicist Norman Yao of UC Berkeley, a coauthor of the new result, posted online September 27 at arXiv.org.

Time crystals take an important unifying concept in physics — the idea of symmetry breaking — and extend it to time. Physical laws typically treat all points in space equally — no one location is different from any other. In a liquid, for example, atoms are equally likely to be found at any point in space. This is a continuous symmetry, as the conditions are the same at any point along the spatial continuum. If the liquid solidifies into a crystal, that symmetry is broken: Atoms are found only at certain regularly spaced positions, with voids in between. Likewise, if you rotate a crystal, on a microscopic level it would look different from different angles, but liquid will look the same however it’s rotated. In physics, such broken symmetries underlie topics ranging from magnets to superconductors to the Higgs mechanism, which imbues elementary particles with mass and gives rise to the Higgs boson.

In 2012, theoretical physicist Frank Wilczek of MIT proposed that symmetry breaking in time might produce time crystals (SN: 3/24/12, p. 8). But follow-up work indicated that time crystals couldn’t emerge in a system in a state of equilibrium, which is settled into a stable configuration. Instead, physicists realized, driven systems, which are periodically perturbed by an external force — like the laser flipping the ions — could create such crystals. “The original examples were either flawed or too simple,” says Wilczek. “This is much more interesting.”

Unlike the continuous symmetry that is broken in the transition from a liquid to a solid crystal, in the driven systems that the scientists used to create time crystals, the symmetry is discrete, appearing at time intervals corresponding to the time between perturbations. If the system repeats itself at a longer time interval than the one it’s driven at — as the scientists’ time crystal does — that symmetry is broken.

Time crystals are too new for scientists to have a handle on their potential practical applications. “It’s like a baby, you don’t know what it’s going to grow up to be,” Wilczek says. But, he says, “I don’t think we’ve heard the last of this by a long shot.”
There probably are related systems yet to be uncovered, says Nayak. “We’re just kind of scratching the surface of the kinds of amazing phenomena — such as time crystals — that we can have in nonequilibrium quantum systems. So I think it’s the first window into a whole new arena for us to explore.”

There’s much more to the universe than meets the eye, and a new web-based app lets you explore just how much our eyes are missing. Gleamoscope presents the night sky across a range of electromagnetic frequencies. Spots of gamma rays pinpoint distant feeding black holes. Tendrils of dust glow with infrared light throughout the Milky Way. A supernova remnant — the site of a star that exploded roughly 11,000 years ago — blasts out X-rays and radio waves.

Many of these phenomena are nearly imperceptible in visible light. So astronomers use equipment, such as specialized cameras and antennas, that can detect other frequencies of electromagnetic radiation. Computers turn the data into images, often assigning colors to certain frequencies to highlight specific details or physical processes.

In Gleamoscope, a slider smoothly transitions the scene from one frequency of light to another, turning the familiar star-filled night sky into a variety of psychedelic landscapes. Pan and magnification controls allow you to scan all around the night sky and zoom in for a closer look. The interactive map combines images from many observatories and includes new data from the Murchison Widefield Array, a network of radio antennas in Australia. Over 300,000 galaxies appear as dots in images of the new radio data, described in an upcoming issue of Monthly Notices of the Royal Astronomical Society. The radio map by itself can also be explored on mobile devices in a separate app called GLEAM, available on Google Play.

Mysterious flashes of radio waves from deep space keep coming, but they are just as mysterious as ever.

Gamma rays might have accompanied one of these eruptions, researchers report in the Nov. 20 Astrophysical Journal Letters. This is the first time high-energy photons have been associated with these blasts of radio energy, known as fast radio bursts. If the gamma rays did come from the same place as the radio waves, then the underlying source could be roughly 1 billion times as energetic as thought.
SUBSCRIBE
Another burst, meanwhile, takes the record for brightest blast. The signal was bright enough to reveal details about the magnetic field between galaxies, astronomers report online November 17 in Science.

Fast radio bursts, or FRBs, have intrigued astronomers since the first one was reported in 2007 (SN: 8/9/14, p. 22). Since then, astronomers have discovered 18 in total. In most cases, a blip of radio waves lasting just a few milliseconds appears in the sky and is never seen again. Only one so far is known to repeat (SN: 4/2/16, p. 12). Most seem to originate in remote galaxies, possibly billions of light-years away. Until now, no one has detected any other frequency of electromagnetic radiation besides radio waves coming from these cosmic beacons.

A flash of gamma rays appeared at about the same time and from the same direction as a radio burst detected in 2013, James DeLaunay, a physics graduate student at Penn State, and colleagues report. They pored over old data from the Swift observatory, a NASA satellite launched in 2004, to see if it recorded any surges of gamma rays that might coincide with known radio bursts.

“Gamma rays associated with an FRB would be an incredibly important thing to find,” says Sarah Burke Spolaor, an astrophysicist at the National Radio Astronomy Observatory in Socorro, N.M. But she urges caution. “We don’t have a good inkling of where a specific burst comes from.” That leaves room for other types of eruptions to occur in the vicinity just by chance. DeLaunay and collaborators calculate that the odds of that are low, about one in 800. But several researchers are taking a wait-and-see attitude before feeling more confident that the gamma rays and FRB are linked.

“It’s tantalizing, but a lot more would need to be found to be convincing,” says Jason Hessels, an astrophysicist at the Netherlands Institute for Radio Astronomy in Dwingeloo.
If the same source emits both the radio waves and gamma rays, that could rule out a couple of proposals for the causes of the eruptions. Powerful radio hiccups from pulsars, the rapidly spinning cores of dead stars, are one candidate that wouldn’t make the cut, because they aren’t known to generate gamma rays.

Collisions between two neutron stars, or between a neutron star and a black hole, look promising, says Derek Fox, an astrophysicist at Penn State and a coauthor of the study. The energy output and duration of the gamma-ray burst are a good match with what’s expected for these smashups, he says, though it’s not clear whether they happen often enough to account for the thousands of FRBs that astronomers suspect go off every day.

No one story neatly fits all the data. “I think there are at least two populations,” says Fox. Perhaps some FRBs repeat, while others do not; some belch out gamma rays, others do not. There might be no one type of event that creates all FRBs, but rather a multitude.

That idea is tentative as well. “It’s way too early to say if there are multiple populations,” says Laura Spitler, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn, Germany. A grab bag of cosmic calamities is plausible. But there are other astronomical events that exhibit enormous diversity, enough that all FRBs could also have just one type of trigger. “The data we have now isn’t sufficient to land on one side or the other,” Spitler says.

A more recent FRB, detected in 2015 at the Parkes radio telescope in Australia, shows off some of that diversity — and demonstrates how FRBs can be used as cosmological tools. A brief blast of radio waves from at least 1.6 billion light-years away is about four times as intense as the previous record holder. The signal’s vigor could be an intrinsic quirk of the underlying outburst, or could mean that this burst was unusually close to our galaxy — or both.

“What’s really exciting most about it is not just that it’s bright,” says Vikram Ravi, a Caltech astronomer and lead author of the study, “but really because of what we hope to use FRBs for.” This FRB was bright enough for Ravi and colleagues to deduce the magnetic field between galaxies. To do that, they measured the signal’s polarization, the alignment of radio waves imprinted by magnetized plasmas encountered en route to Earth. They found that, on average, the magnetic field is feeble, less than 21 nanogauss (or about one 10-millionth as strong as Earth’s magnetic field). That’s in line with astronomers’ theories about the strength of intergalactic magnetism.

“It’s not telling us anything that’s unexpected,” says Duncan Lorimer, an astrophysicist at West Virginia University in Morgantown who reported the first FRB in 2007. But it shows that FRBs can be used to learn more about intergalactic space, a region that is notoriously difficult to study. “It’s one thing to say we expect the magnetic field to be weak, but it’s another thing to actually measure it,” he adds. “It’s a signpost of things to come.”

This burst encountered different environments than a burst reported last year in Nature, which suggested an FRB origin in a highly magnetized environment, possibly near young stars in a remote galaxy (SN Online: 12/2/15). There’s no hint that the latest burst originated in a similar locale.

“I don’t think we contradict each other at all,” Ravi says. “Some FRBs originate in very magnetic environments and some don’t. Given that these are the only two FRBs where these measurements have been made, it’s hard to tell.”

Lucy didn’t let an upright stance ground her. This 3.2-million-year-old Australopithecus afarensis, hominid evolution’s best-known fossil individual, strong-armed her way up trees, a new study finds.

Her lower body was built for walking. But exceptional upper-body strength, approaching that of chimpanzees, enabled Lucy to hoist herself into trees or onto tree branches, paleoanthropologist Christopher Ruff of Johns Hopkins University School of Medicine and his colleagues report November 30 in PLOS ONE.

Lucy, and presumably other members of her species, “combined walking on two legs with a significant amount of tree climbing,” says coauthor John Kappelman, a paleoanthropologist at the University of Texas at Austin. A Kappelman-led team concluded earlier this year that, based on numerous bone breaks, Lucy fell to her death from a tree, either while climbing or sleeping (SN: 9/17/16, p. 16). That’s a controversial claim, dismissed by some researchers as a misreading of bone damage caused by the fossilization process.
Debate about whether A. afarensis spent much time in trees goes back to shortly after the discovery of Lucy’s partial skeleton in 1974. Additional discoveries of A. afarensis fossils have only intensified disputes between those who regard the ancient species as primarily designed for walking and others convinced that Lucy’s crowd split time between walking and tree climbing (SN: 12/1/12, p. 16; SN: 7/17/10, p. 5).

Ruff’s team measured the internal structure and strength of Lucy’s two surviving upper arm bones and one upper leg bone, including the knob at the top of the upper leg that forms the hip joint. Data came from high-resolution X-ray CT scans taken in 2008 while her remains were in the United States for a museum tour.

These scans were compared with those of present-day people, chimps and bonobos, as well as 26 fossil hominids. These hominids — including both australopithecines like Lucy, as well as early members of the human genus, Homo — date to between 2.6 million and 600,000 years ago.

Lucy’s long, weight-bearing upper arms most closely resemble the anatomy of chimps, the scientists say. Studies of various living animals, including humans and chimps, indicate that daily behaviors during growth influence the development of limb bones. Thus, it’s plausible that Lucy pulled herself into trees from an early age, adding to the strength and length of her upper arms, the team proposes.

Although Lucy walked upright, she had a less efficient gait than that of people today and Homo erectus individuals dating to between 1.6 million and 700,000 years ago, the researchers say. The stress of supporting a robust upper body with a slighter lower body would have interfered with Lucy’s two-legged stride, they hold. Traits such as a relatively small hip joint and short legs limited Lucy’s ability to walk long distances, the investigators add.
Ruff’s study supports proposals over the last few decades that, for her size, Lucy had longer, stronger arms and smaller hip joints than people now do, says paleoanthropologist Carol Ward of the University of Missouri School of Medicine in Columbia. It’s plausible that a small hip joint slightly undermined Lucy’s stride, but that hasn’t been conclusively demonstrated, Ward adds.

Biological anthropologist Philip Reno of Penn State takes a harder line. “This new analysis does not resolve any of the debates regarding the use of tree climbing or the effectiveness of upright walking in australopithecines.” Radical, humanlike changes to the pelvis and foot in Lucy’s species suggest that her large upper arms were simply evolutionary holdovers from hominids’ tree-dwelling ancestors, not the consequences of extensive tree climbing, Reno argues. It’s hard to say how Lucy’s relatively small hip joint interacted with many other skeletal and muscular forces affecting an upright gait, he adds.

The big question, in Ward’s view, is whether skeletal changes in early Homo conducive to walking and running arose as a result of largely abandoning tree climbing or for other reasons, such as an increasing emphasis on arms and hands capable of manipulating objects in precise ways.

There’s no bow or festive wrap, but I hope that you will consider this issue a gift of sorts. That is how the staff of Science News thinks of it, our year-end recap of the top science stories. In these pages, you’ll find the stories that continued to resonate well after we first covered them and many that we expect will resonate for years to come — all collected in one easy-to-read, extremely portable, no-batteries-required package (unless you are reading this on a smartphone or tablet, that is).
Gravitational waves, of course, occupy the top spot on our list this year. The “of course” reflects the fundamental importance of the detection of this elusive form of energy, announced in February. The finding confirmed key theories in physics, sure, but even more exciting is what it promises for the future. Gravitational waves are powerful tools for probing the universe. Just as the Hubble Space Telescope revealed cosmic beauty in electromagnetic radiation, gravitational wave detectors may show scientists an unprecedented view of far-off
Closer to home, the Zika virus became one of our most closely watched stories this year, as the extent of human suffering caused by the mosquito-borne virus became clear. But it’s also a tale of progress: Scientists have responded swiftly, creating a robust literature on the virus in a short time. We still don’t have all the answers, but we’ve come a long way in terms of creating the knowledge urgently needed to inform health recommendations.

Other stories made this year’s list with a more mixed pedigree. The discovery of a (relatively) nearby exoplanet energized many of our science fiction–fueled fantasies of other worlds, for instance. Research moved ahead on what some call “three-parent babies” — using mitochondrial donors to replace a woman’s own disease-prone mitochondria in egg cells — despite a lack of clarity on the procedure’s efficacy. Melting Arctic sea ice has led to a historically significant opening of passageways between the Pacific and the Atlantic oceans. New hope for the battle against Alzheimer’s disease seemed worthy of mention. All these developments and more were regarded by Science News reporters and editors as milestones of discovery or news of importance to society.

We also decided to add some other elements to our year-in-review coverage for 2016. Guided by the deft hand of Beth Quill, our enterprise editor, we augmented our Top 10 list with an essay by managing editor Tom Siegfried about two of physics’s noteworthy recent failures and how the two are related. Science journalist and author Sonia Shah offers a roundup of 2016 in public health, reminding us of the thorny problems associated with infectious diseases, from antibiotic resistance to the resurgence of yellow fever. Other pieces illustrate some of the challenges facing the driverless car revolution, as well as what Science News reporters see on the horizon for the coming year.

We have tried to pack as much science as possible into this issue, from the biggest stories to the more obscure nuggets of discovery and surprise. I can’t think of a gift I’d more like to receive.

Forget honking Vs of geese or gathering herds of wildebeests. The biggest yearly mass movements of land animals may be the largely overlooked flights of aphids, moths, beetles, flies, spiders and their kin.

About 3.5 trillion arthropods fly or windsurf over the southern United Kingdom annually, researchers say after analyzing a decade of data from special entomological radar and net sweeps. The larger species in the study tended to flow in a consistent direction, suggesting that more species may have specialized biology for seasonal migrations than scientists realized, says study coauthor Jason Chapman, now at the University of Exeter in Penryn, England.
The creatures detected in the study may be little, but they add up to roughly 3,200 metric tons of animal weight, Chapman and colleagues report in the Dec. 23 Science. That’s 7.7 times the tonnage of U.K. songbirds migrating to Africa and equivalent to about 20,000 (flying) reindeer.

These are “huge flows of biomass and nutrients,” Chapman says. “One of the things we hope to achieve in this work is to convince people who are studying terrestrial ecosystems that they cannot ignore what’s happening in the skies above them.”
Biologist Martin Wikelski of the Max Planck Institute for Ornithology in Radolfzell, Germany, who wasn’t part of the study, calls these migrants “aerial plankton.” It’s a reference to the much-studied tiny sea creatures whose movements and blooms power oceanic food webs. Understanding insect migrations and abundances is crucial for figuring out food webs on land, including those that link insects and birds. That’s “particularly important nowadays as we are starting to lose many of our songbirds,” he says.
The word migration applied to arthropod movements doesn’t mean one animal’s roundtrip, Chapman says. Instead, the term describes leaving the home range and undertaking a sustained journey, maybe cued by seasons changing or food dwindling. A return trip, if there is one, could be the job of a future generation.

The migrants he studied, traveling at least 150 meters aboveground, aren’t just accidentally blowing in the wind, he says. Many of the tiniest — aphids and such that weigh less than 10 milligrams — take specific measures to start their journey, such as trekking to the top of a plant to catch a gust. Juvenile spiders stand on tiptoe reeling out silk until a breeze tugs a strand, and them, into the air. “They only do this when wind conditions will enable them to be caught and taken up; otherwise, it’s a terrible waste of silk,” Chapman says. Some caterpillars also spin silk to travel, and mites, with neither wings nor silk, can surf themselves into a good breeze.

The basic idea that a lot of arthropods migrate overhead is “absolutely not” a surprise to behavioral and evolutionary biologist Hugh Dingle of the University of California, Davis. He says so not dismissively, but joyously: “Now we have really good data.”

This smallest class of migrants, sampled with nets suspended from a big balloon, makes up more than 99 percent of the individual arthropods and about 80 percent of the total mass. They didn’t show an overall trend in flight direction. But radar techniques refined at Rothamsted Research in Harpenden, England, showed distinct seasonal patterns in direction for medium-sized and larger insects.

“That’s the big surprise for us,” Chapman says. “We assumed that those flows would just be determined by the wind.” But medium-sized and large insects such as lacewings and moths overall tended to head northward from May through June regardless of typical wind direction. And in August and September, they tended southward. “Lots of insects we didn’t think capable of this are clearly doing it,” he says.

Managing such a feat takes specialized biology for directed, seasonal migrations. Many of these arthropods must have some form of built-in compass plus a preferred direction and the genetics that change that preference as they or their offspring make the return migration. Entomologists have known some migratory details of monarch butterflies in North America and a handful of other such insects, many of them pest moths. But speculating about specialized migrants, Chapman says, “there must be thousands of these.”

The moon formed at least 4.51 billion years ago, no more than 60 million years after the formation of the solar system, researchers report online January 11 in Science Advances. This update to the moon’s age is in line with some previous estimates (SN Online: 4/17/15), although some argue the moon formed 150 million to 200 million years after the solar system’s birth.

A precise age is important for understanding how Earth evolved and how the solar system behaved in its formative years, says study coauthor Melanie Barboni, a geologist at UCLA. “If we want to understand other solar systems,” she says, “the first thing we have to do is understand ours.”

A run-in between Earth and something roughly the size of Mars is thought to be responsible for the creation of the moon. To nail down when this happened, Barboni and colleagues examined fragments of the mineral zircon brought back from the moon by the Apollo 14 astronauts. Relative amounts of uranium and lead as well as abundances of hafnium isotopes and the element lutetium provided radioactive decay clocks that record when the early moon’s global ocean of magma solidified. Hafnium and lutetium help determine when a crust formed over the moon’s liquid mantle while the radioactive decay of uranium to lead pinpoints when the zircon crystallized.

Previous analysis of the same zircon fragments revealed a similar age (within 68 million years after the formation of the solar system), but came with larger uncertainties. New techniques for uranium-lead dating and for understanding how the bombardment of the lunar surface by cosmic rays alters hafnium led to the improved age estimate.

Fossils of a giant otter have emerged from the depths of an open-pit mine in China.

The crushed cranium, jaw bone and partial skeletons of at least three animals belong to a now-extinct species of otter that lived some 6.2 million years ago, scientists report January 23 in the Journal of Systematic Palaeontology.

At roughly 50 kilograms in weight, the otter would have outclassed today’s giant otter, a river-dwelling South American mammal weighing in at around 34 kilograms. Scientists named the new species Siamogale melilutra, a nod to its unusual mix of badger and otter features. Melilutra is a mash-up of the Latin words for both creatures.

Badgers and otters both belong to a group of carnivorous animals called Mustelidae, but scientists have had trouble figuring out where to place extinct members in the mammalian family tree. (European badgers and modern otters share similar-looking teeth and skulls.) Still, Siamogale melilutra, however badgerlike, is indeed an otter, researchers concluded after CT scanning, reconstructing and analyzing the fossil skull.

Based on plant and animal fossils found near the collection site, scientists believe that the ancient otter probably lived in the shallow lake of a warm and humid swamp, lush with broad-leaved evergreens and grasses.

WASHINGTON — Lone neutrons quickly decay, but scientists don’t agree on how long the particles stick around before their demise. New experiments could resolve the dispute — or deepen the mystery.

Outside of a nucleus, neutrons survive only about 15 minutes on average. They quickly decay into a proton, an electron and an antineutrino. Two methods used for measuring the neutron lifetime disagree, leaving scientists uncertain about the subatomic particle’s true longevity (SN: 5/19/12, p. 20).
One technique involves containing chilled neutrons in a trap, or “bottle,” waiting awhile, and counting the remaining neutrons to determine how many decayed. Other experiments monitor beams of neutrons and count the number of decays by detecting the protons emitted. Bottle measurements come up with lifetimes about 9 seconds shorter than beam measurements.

“This is actually fairly important for a number of things,” physicist Robert Pattie said January 29 at a meeting of the American Physical Society. In particular, pinning down the neutron’s lifetime is necessary for understanding how atomic nuclei began forming after the Big Bang. Scientists’ befuddlement makes it harder to calculate the properties of the early universe.

One drawback of typical bottle experiments is that neutrons can be absorbed or otherwise lost when they hit the wall of the bottle. So Pattie, of the Los Alamos National Laboratory in New Mexico, and colleagues are working on an updated bottle-style measurement using a magnetic field to keep neutrons from hitting the bottom of the trap, while gravity keeps them from flying out of the top.

Physicist Craig Huffer of North Carolina State University in Raleigh and colleagues are working on a bottle experiment that uses a magnetic field to trap neutrons. Rather than counting neutrons at the end, the researchers detect flashes of light produced as neutrons inside the bottle decay away.

In beam experiments, accuracy depends on making a careful count of the neutrons beamed in and the protons produced in the decays, physicist Shannon Fogwell Hoogerheide of the National Institute of Standards and Technology in Gaithersburg, Md., explained at the meeting on January 30. She and colleagues are refining their beam measurement to better enumerate protons and neutrons.
Some scientists have suggested the discrepancy could have deeper meaning. The short lifetime in bottle measurements could indicate that neutrons are somehow disappearing unexpectedly, making the lifetime appear shorter than it really is. “Kind of an out-there mechanism is that they’ve gone into some kind of alternative reality, which we call the mirror world,” says physicist Ben Rybolt of the University of Tennessee. In such a world, all the particles we know of would be duplicated — mirror protons, neutrons and electrons could exist, which would interact only very slightly with the particles we know.

Jumping to such an explanation for the neutron lifetime discrepancy is “a little bit of a leap,” Rybolt acknowledges, but such mirror particles could also explain the conundrum of dark matter, an unseen substance indicated by the motions of stars inside galaxies. To test the idea, Rybolt and colleagues are proposing to shoot beams of neutrons at a barrier and check if any make it through, which could indicate the particles had briefly become mirror neutrons.

The problem of measuring the neutron’s lifetime is complex enough that a variety of new techniques are under preparation to unravel the issue. “I don’t think one additional experiment can resolve the discrepancy,” says Huffer. Instead, multiple new measurements with different techniques should eventually converge on the correct value.

Almost every night that the constellation Orion is visible, physicist Mark Vagins steps outside to peer at a reddish star at the right shoulder of the mythical figure. “You can see the color of Betelgeuse with the naked eye. It’s very striking, this red, red star,” he says. “It may not be in my lifetime, but one of these days, that star is going to explode.”

With a radius about 900 times that of the sun, Betelgeuse is a monstrous star that is nearing its end. Eventually, it will no longer be able to support its own weight, and its core will collapse. A shock wave from that collapse will speed outward, violently expelling the star’s outer layers in a massive explosion known as a supernova. When Betelgeuse detonates, its cosmic kaboom will create a light show brighter than the full moon, visible even during the daytime. It could happen tomorrow, or a million years from now.
Countless stars like Betelgeuse — any of which could soon explode — litter the Milky Way. Scientists estimate that a supernova occurs in our galaxy a few times a century. While brilliantly gleaming supernovas in far-flung galaxies are regularly spotted with telescopes trained on the heavens, scientists eagerly hope to capture two elusive signatures that are detectable only from a supernova closer to home. These signatures are neutrinos (subatomic particles that stream out of a collapsing star’s center) and gravitational waves (subtle vibrations of spacetime that will also ripple out from the convulsing star).
“These two signals, directly from the interior of the supernova, are the ones we are really longing for,” says Hans-Thomas Janka, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany. Unlike light, which is released from the star’s surface, stealthy neutrinos and gravitational waves would give scientists a glimpse of the processes that occur deep inside a collapsing star.

Supernovas offer more than awe-inspiring explosions. When they erupt, the stars spew out their guts, seeding the cosmos with chemical elements necessary for life to exist. “We clearly wouldn’t be here without supernovas,” says Vagins, of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo. But the processes that occur within the churning mess are still not fully understood. Computer simulations have revealed much of the physics of how stars explode, but models are no substitute for a real-life nearby blast.

One inspiration for scientists is supernova 1987A, which appeared in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, 30 years ago (SN: 2/18/17, p. 20). That flare-up hinted at the unparalleled information nearby supernovas could provide. With the detectors available at the time, scientists managed to nab just two dozen of 1987A’s neutrinos (SN: 3/21/87, p. 180). Hundreds of papers have been written analyzing that precious handful of particles. Calculations based on those detections confirmed scientists’ hunch that unfathomably large numbers of neutrinos are released after a star’s core collapses in a supernova. In total, 1987A emitted about 1058 neutrinos. To put that in perspective, there are about 1024 stars in the observable universe — a vastly smaller number.

Since 1987, neutrino detectors have proliferated, installed in exotic locales that are ideal for neutrino snagging, from the Antarctic ice sheet to deep mines across the globe. If a supernova went off in the Milky Way today, scientists could potentially nab thousands or even a million neutrinos. Gravitational wave detection has likewise come on the scene, ready to pick up a slight shift in spacetime stirred up by the blast. Detecting such gravitational waves or a surfeit of supernova neutrinos would lead to a distinct leap in scientists’ knowledge, and provide new windows into supernovas. All that’s needed now is the explosion.

Early warning
Despite estimates that a few stars explode in the Milky Way every century, no one has glimpsed one since the early 1600s. It’s possible the explosions have simply gone unnoticed, says physicist John Beacom of Ohio State University in Columbus; light can be lost in the mess of gas and dust in the galaxy. A burst of neutrinos from a supernova would provide a surefire signal.

These hermitlike elementary particles shun most interactions with matter. Produced in stars, radioactive decay and other reactions, neutrinos are so intangible that trillions of neutrinos from 1987A’s explosion passed through the body of every human on Earth, yet nobody felt a thing. For supernovas like 1987A, known as type 2 or core-collapse supernovas, about 99 percent of the explosion’s energy goes into the tiny particles. Another, less common kind of supernova, type 1a, occurs when a remnant of a star called a white dwarf steals matter from a companion star until the white dwarf explodes (SN: 4/30/16, p. 20). In type 1a supernovas, there’s no core collapse, so neutrinos from these explosions are much less numerous and are less likely to be detectable on Earth.

For scientists studying supernovas, neutrinos’ reluctance to interact is an advantage. The particles don’t get bogged down in their exit from the star, so they arrive at Earth several hours or even more than a day before light from the explosion, which is released only after the shock wave travels from the star’s core up to its surface. That means the particles can tip off astronomers that a light burst is imminent, and potentially where it is going to occur, so they can have their telescopes ready.

Most neutrino experiments (there are more than a dozen) weren’t built for the purpose of taking snapshots of unpredictable supernovas; they were built to study neutrinos from reliable sources, like the sun, nuclear reactors or particle accelerators. Nevertheless, seven neutrino experiments have joined forces to create the SuperNova Early Warning System, SNEWS. If neutrino detectors in at least two locations report an unexpectedly large burst of neutrinos, SNEWS will send an e-mail alert to the world’s astronomers. The experiments involved are a weirdly diverse bunch, including IceCube, a detector composed of light sensors frozen deep in the ice of Antarctica (SN: 12/27/14, p. 27); Super-Kamiokande, which boasts a tank filled with 50,000 tons of water stationed in a mine in Hida, Japan; and the Helium and Lead Observatory, or HALO — with the motto “astronomically patient” — made of salvaged lead blocks in a mine in Sudbury, Canada. Their common thread: The experiments are big to provide a lot of material for neutrinos to interact with — such as lead, water or ice.

With light sensors sunk kilometers deep into the ice sheet, IceCube’s detector is so huge that it could pick up traces of a million neutrinos from a Milky Way supernova. Because it was designed to capture only the highest energy neutrinos that are rocketing through space, it’s not sensitive enough to detect individual neutrinos emitted during a supernova. Instead, IceCube’s focus is on the big picture: It catalogs an increase of light in its detectors produced by neutrinos interacting in the ice in time slices of two billionths of a second, says IceCube leader Francis Halzen of the University of Wisconsin–Madison. “We make a movie of the supernova.”

Super-Kamiokande is the neutrino detector that can pinpoint the location of the impending stellar paroxysm. It is a successor to Kamiokande-II, one of a few detectors to spot a handful of neutrinos from 1987A. Shortly after a burst of neutrinos from a nearby supernova, the detector could direct astronomers to zero in on a few degrees of sky. If that happens, says neutrino physicist Kate Scholberg of Duke University, “I expect anybody with the capability will be zooming in.”
On the lookout
Various neutrino detectors await signals emitted from a supernova, including Super-Kamiokande in Japan, IceCube in Antarctica and HALO in Canada. They are joined by a gravitational wave observatory, LIGO, with detectors in Louisiana (shown) and Washington state.

Languages of a supernova
Light and neutrinos are two of several languages that a supernova speaks. In that sense, supernova 1987A was a “Rosetta stone,” Beacom says. By scrutinizing 1987A’s light and its handful of neutrinos, scientists began piecing together the theoretical physics that explains what happened inside the star. In a future supernova, another language, gravitational waves, could add nuance to the tale. But the explosion has got to be close.

If neutrinos are elusive, gravitational waves border on undetectable. Minute tremors in space itself, predicted by Einstein’s general theory of relativity, are generated when massive objects accelerate. In 2016, scientists with the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, announced the first direct detection of gravitational waves, produced by two merging black holes (SN: 3/5/16, p. 6). That milestone required a pair of detectors so precise that they can sense quivers that squish the detectors’ 4-kilometer-long arms by just a tiny fraction of the diameter of a proton.

Gravitational waves from a supernova should be even harder to tease out than those from merging black holes. The pattern of ripples is less predictable. Surveys of the properties of the many supernovas detected in other galaxies indicate that the explosions vary significantly from one to the next, says astroparticle physicist Shunsaku Horiuchi of Virginia Tech in Blacksburg. “We ask, ‘Is there a standard supernova?’ The answer is ‘No.’ ”

Despite the challenges, finding gravitational waves from supernovas is a possibility because the explosions are chaotic and asymmetrical, producing lumpy, lopsided bursts. An explosion that expands perfectly symmetrically, like an inflating balloon, would produce no gravitational waves. The gravitational wave signature thus can tell scientists how cockeyed the detonation was and how fast the star was spinning.

Gravitational waves might also reveal some of the physics of the strange stew of neutrons that makes up a protoneutron star — the beginnings of an incredibly dense star formed in a supernova. Scientists would like to catalog the compressibility of the neutron-rich material — how it gets squeezed and rebounds in the collapse. “The gravitational wave signature would have an imprint in it of this stiffness or softness,” says computational astrophysicist Tony Mezzacappa of the University of Tennessee.

There’s a chance the supernova would collapse into an even more enigmatic state — a black hole, which has a gravitational field so strong that not even light can escape. When a black hole forms, the flow of neutrinos would abruptly drop, as their exit route is cut off. Detectors would notice. “Seeing the moment that a black hole is born,” says Vagins, “would be a tremendously exciting thing.”

While neutrinos can be oracles of supernovas, a stellar explosion could reveal a lot about neutrinos themselves. There are three types of neutrinos: electron, muon and tau. All are extremely light, with masses less than a millionth that of an electron (SN: 1/26/13, p. 18). But scientists don’t know which of the three neutrinos is the lightest; a nearby supernova could answer that question.

Supernovas, with all the obscure physics at their hearts, have a direct connection to Earth. They are a source of many of the elements from which planets eventually form. As stars age, they fuse together heavier and heavier elements, forging helium from hydrogen, carbon from helium and so on up the periodic table to iron. Those elements, including some considered essential to life, such as carbon and oxygen, spew out from a star’s innards in the explosion.

“All the elements that exist — that are here on Earth — that are heavier than iron were either made in supernovas or other cataclysmic events in astronomy,” says physicist Clarence Virtue of Laurentian University in Sudbury, Canada. Gold, platinum and many other elements heavier than iron are produced in a chain of reactions in which neutrons are rapidly absorbed, known as the r-process (SN: 5/14/16, p. 9). But scientists still argue about whether the r-process occurs in supernovas or when neutron stars merge with one another. Pulling back the curtain on supernovas could help scientists resolve the dispute.

Even the reason supernovas explode and sow their chemical seeds has been vigorously debated. Until recently, computer simulations of supernovas have often fizzled, indicating that something happens in a real explosion that scientists are missing. The shock wave seems to need an extra kick to make it out of the star and produce the luminous explosion. The most recent simulations indicate that the additional oomph is most likely imparted by neutrinos streaming outward. But, says Mezzacappa, “At the end of the day, we’re going to need some observations against which we can check our models.”

Hurry up and wait
Supernovas’ potential to answer such big questions means that scientists are under pressure not to miss a big break. “You better be ready. If it happens and you’re not ready, then you will be sad,” Scholberg says. “We have to be as prepared as possible to gather as much information as we possibly can.”
If a detector isn’t operating at the crucial moment, there’s no second chance. So neutrino experiments are designed to run with little downtime and to skirt potential failure modes — a sudden flood of data from a supernova could crash electronics systems in an ultrasensitive detector, for example. Gravitational wave detectors are so finicky that interference as subtle as waves crashing on a nearby beach can throw them out of whack. And in upcoming years, LIGO is scheduled to have detectors off for months at a time for upgrades. Scientists can only hope that, when a supernova comes, everything is up and running.

Some even hope that neighboring stars hold off a little longer. “It seems like I’m always telling people that I’d like Betelgeuse to go off one year from now,” jokes Bronson Messer, a physicist who works on supernova simulations at Oak Ridge National Laboratory in Tennessee. With each improvement of the simulations, he’s eager for a bit more time to study them.

Messer keeps getting his wish, but he doesn’t want to wait too long. He’d like to see a supernova in the Milky Way during his lifetime. But it could be many decades. Just in case, Vagins, who’s been taking those nightly peeks at Betelgeuse, is doing his part to prepare the next generation. He no longer scans the skies alone. “I’ve already taught my 6-year-old son how to find that star in the sky,” he says. “Maybe I won’t get to see it go, but maybe he’ll get to see it go.”