X-rays appear to be trickling away from Pluto, even though the dwarf planet has no obvious way of making the high-energy photons, a new study reports.

Given what researchers have learned about Pluto since the New Horizons spacecraft flew by in 2015 (SN: 8/8/15, p. 6), the discovery is surprising. For many planets and comets, X-rays are generated when the solar wind, a stream of charged particles from the sun, runs into neutral gas atoms or magnetic fields from these bodies. But the environment around Pluto isn’t conducive to producing X-rays: the dwarf planet has no measurable magnetic field, its atmosphere is very thin, and it’s losing that atmosphere at rates much lower than expected.
“We naively thought Pluto might be losing its atmosphere at the same rate as [some] comets,” says Carey Lisse, a planetary astronomer at the Applied Physics Laboratory in Laurel, Md. “We knew comets make X-rays, so we hoped that Pluto did, too.” Instead, interactions between the solar wind and a tenuous tail of methane gas hundreds of times longer than Pluto’s diameter might be the culprit, Lisse and colleagues suggest online October 25 on arXiv.org.

Lisse’s team used the Chandra X-ray telescope, once in 2014 and three more times in 2015, to look for Pluto X-rays. Chandra detected just seven photons streaming from Pluto in a total of about two days’ worth of observing time. Though the signal isn’t strong, that’s about six or seven more photons than expected based on New Horizons’ measurements of Pluto’s atmosphere and the solar wind.

“It’s a very puzzling finding,” says Konrad Dennerl, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. “I’m not fully convinced,” he adds. “It’s a very low signal.”

Lisse and collaborators note that the signal appears to follow Pluto across the sky. They detected X-ray photons on four separate occasions. The energy of the photons doesn’t appear to match that of the spurious X-ray noise that peppers the telescope, so the signal appears genuine. Still, Lisse and Dennerl are teaming up to get some Pluto time with another X-ray observatory, the European Space Agency’s XMM-Newton satellite.

“We understand that there’s a bit of skepticism,” Lisse says. “We’re going to do some follow-up with a totally different instrument to verify this.”

X-rays from Pluto aren’t just a quirky detail about this specific dwarf planet. If other bodies in the Kuiper belt, the ring of icy debris just past Neptune’s orbit, have atmospheres, then X-ray observations could help detect them.

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.
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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.

Observations of the dense remnant of an exploded star have provided the first sign of a quantum effect on light passing through empty space.

Light from the stellar remnant, a neutron star located about 400 light-years away, is polarized, meaning that its electromagnetic waves are oriented preferentially in a particular direction like light that reflects off the surface of water (SN: 7/8/06, p. 24). That polarization is evidence of “vacuum birefringence,” a quantum effect first predicted 80 years ago caused by light interacting with the vacuum of space in a strong magnetic field. Scientists report the result in a paper to be published in the Feb. 11, 2017 issue of Monthly Notices of the Royal Astronomical Society.
“It’s the most natural explanation,” says astrophysicist Jeremy Heyl of the University of British Columbia in Vancouver, who was not involved with the new result. But he cautions, other sources of polarization could mimic the effect, and additional observations are necessary.

According to quantum electrodynamics, the theory describing how light interacts with charged particles such as electrons, empty space isn’t really empty. It is filled with a roiling soup of ethereal particles, constantly blipping into and out of existence (SN: 11/26/16, p. 28). As light passes through the void, its wiggling electromagnetic waves interact with those particles. Under strong magnetic fields, light waves that wiggle along the direction of the magnetic field will travel slightly slower than light oscillating perpendicular to the direction of the magnetic field, which rotates the overall polarization of light coming from the star.

A similar effect commonly occurs in a more familiar situation, in what are known as birefringent materials. The liquid crystals in computer monitors similarly rotate the polarization of light. Horizontally polarized light, for example, is sent to each pixel, but a filter lets only vertically polarized light escape. To switch on a pixel, the liquid crystals twist the light waves 90 degrees so the waves will pass through.

But evidence for the quantum version of the effect was not easy to come by. Observing it requires a magnetic field stronger than those that can be produced in the laboratory, says astrophysicist Roberto Mignani of the National Institute for Astrophysics in Milan, coauthor of the new study. The magnetic field around the neutron star that Mignani and colleagues studied is about 10 trillion times the strength of Earth’s. But the star is incredibly faint, making measurements of its polarization difficult. “A neutron star of this kind is about as bright as a candle halfway between the Earth and the moon,” Mignani says.

Using the Very Large Telescope in Chile, the scientists found that visible light from the neutron star was about 16 percent polarized, a result consistent with scientists’ theories of vacuum birefringence. But, says Heyl, the polarization could also occur as a result of an unexpectedly large amount of plasma surrounding the star.
For airtight evidence of the effect, scientists could study X-rays from neutron stars, where the polarization effect should be even stronger. Although no telescope currently exists that can make such measurements, there are several proposed X-ray satellites that may soon be able to clinch the case for vacuum birefringence.

Scientists might want to keep their fingers crossed. If future measurements overturned the evidence for vacuum birefringence, the effect’s absence would be difficult to reconcile with the theory of quantum electrodynamics, Heyl says. “It’s essentially one of the basic predictions of the theory, so to fix it you’d really have to rip the theory all the way back down to the foundations and rebuild it.”

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.