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Next week, physicists will pick up an old quest for new physics. A team of 190 researchers at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, will begin measuring the magnetism of a fleeting particle called the muon to exquisite precision, in an experiment known as Muon g-2 (pronounced "gee minus two"). They hope to firm up tantalizing hints from an earlier incarnation of the experiment that suggested that the particle is ever so slightly more magnetic than predicted by the prevailing standard model of particle physics. That would give researchers something they have desired for decades: proof of physics beyond the standard model. From 1997 to 2001, researchers on the original g-2 experiment at Brookhaven National Laboratory in Upton, New York, saw tantalizing, but inconclusive signs of an excess in the muon magnetism. In 2013, the team lugged the experiment to Fermilab, taking the 28-meter-wide ring by barge around the U.S. eastern seaboard and up the Mississippi River. Now, they have rebuilt, and over the next 3 years they hope to push it to its limits—and perhaps a thrilling new discovery.

The merger of two neutron stars captivated thousands of observers and fulfilled multiple astrophysical predictions

Last month, astronomers wowed the world when they announced that they had seen two neutron stars merge, apparently creating heavy elements such as gold and platinum and spewing them into space. Nuclear physicists at Michigan State University in East Lansing also cheered the find, as they are building an atom smasher, the $730 million Facility for Rare Isotope Beams (FRIB), that could decipher exactly how those elements were forged in the inferno. The elements presumably emerge from the so-called rapid neutron process, or r-process, in which a nucleus gains weight by gobbling up one neutron after another. At the same time, a nucleus can change its chemical identity through radioactive β decay, which turns neutrons into protons and bumps the nucleus up the periodic table of elements. Exactly how a nucleus evolves depends on the speed of the decay and the probability that it will soak up another neutron. Those parameters are poorly known. FRIB aims to change that by making as many of the neutron-laden nuclei as possible and measuring their masses and lifetimes. First proposed in 1999, the project didn9t get the greenlight for construction from the Department of Energy until 2014. Now, however, the project is ahead of schedule and on budget, and it should be up and running by 2021—if federal budget politics do not slow it down.

Four times in the past 2 years, physicists working with mammoth gravitational-wave detectors have sensed something go bump in the dark, sending invisible ripples through spacetime. This week, they announced the detection of a fifth such disturbance—but this time astronomers saw it, too, at every wavelength of light from gamma radiation to radio waves. Just as physicists had predicted, the unprecedented view of the cosmic cataclysm—in which two superdense neutron stars spiraled into each other—brought with it a cornucopia of insights, each one of them a major scientific advance. The combined observation with gravitational waves and light showed, as predicted, that so-called short gamma ray bursts, among the most powerful events in the cosmos, spring from neutron-star mergers. It demonstrated a hypothesized new type of stellar object called a kilonova because it shines thousands of times brighter than an ordinary nova. And it revealed that half the elements heavier than iron are produced, perhaps exclusively, in neutron-star death spirals.

The silvery vacuum chamber resembles a zeppelin, the vaguely Art Deco lines of the welds between its stainless steel panels looking at once futuristic and old-fashioned. Although earthbound, the blimp-sized chamber at the Karlsruhe Institute of Technology in Germany has an ethereal purpose: weighing the most elusive and mysterious of subatomic particles, the neutrino. Physicists dreamed up the Karlsruhe Tritium Neutrino (KATRIN) experiment in 2001. Now, the pieces of the €60 million project are finally coming together, and the roughly 140 members of the KATRIN team plan to start taking data early next year. It might seem absurd that physicists don9t know how much neutrinos weigh, given that the universe contains more of them than any other type of matter particle. However, it9s impossible to capture neutrinos, so there9s no simple way to measure their mass. Instead, for 70 years, physicists have tried to infer the neutrino9s mass by studying a particular nuclear decay from which the particle emerges—the beta decay of tritium. Time and again, these experiments have set only upper limits on the neutrino9s mass. In KATRIN, physicists are pushing that classic experiment to its ultimate limits, in what may be their last, best hope to weigh the neutrino.

Physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) have spotted a third merger of black holes, the ultraintense gravitational fields left behind when massive stars collapse. This time, the subtle tremor of spacetime that signaled the merger also revealed a key feature of the black holes: their spins, which were out of kilter. The observation could help reveal how the black holes paired up in the first place. A pair of black holes could be born from massive stars that collapse while orbiting each other. Or the black holes could form first and pair later—although either scenario is trickier than it sounds. If the black holes started out as paired stars, then they should spin in the same direction as their orbital axis. If the black holes formed before they paired, then they could spin in any direction. On 4 January, LIGO twin detectors in Livingston, Louisiana, and Hanford, Washington, spotted black holes of 31 and 19 solar masses spiraling together 3 billion light-years from Earth. By comparing the second-long ripple picked up by the detectors with previously calculated "waveforms," the LIGO team determined that the black hole spins were not aligned, and that there9s an 80% probability that at least one of them spun in generally the opposite sense of the orbital motion. In this case, at least, the dynamical pairing scenario seems more likely.

For decades, particle physicists have yearned for physics beyond their tried-and-true standard model. Now, they are finding signs of something unexpected at the Large Hadron Collider (LHC), the world's biggest atom smasher at CERN, the European particle physics laboratory near Geneva, Switzerland. The hints come not from the LHC's two large detectors, which have yielded no new particles since they bagged the last missing piece of the standard model, the Higgs boson, in 2012, but from a smaller detector, called LHCb. It precisely measures the decays of familiar particles, especially those called B mesons, each of which contains a massive particle called a bottom quark bound to a lighter type of antiquark. LHCb physicists reported their latest result 18 April in a talk at CERN, and the faint signal jibes with a half-dozen related clues that LHCb physicists have found that all seem to point to some new physics taking place within the roiling innards of a B meson. If it's not a mirage of statistical fluctuations, that new physics could include a new force-carrying particle called a Z' or exotic particles called leptoquarks.

Scientists hunting unseen dark matter are looking deeper into the shadows. With searches for a favored dark matter candidate—weakly interacting massive particles (WIMPs)—coming up empty, physicists are now turning to the hypothetical "dark sector": an entire shadow realm of hidden particles. This week, physicists will meet at the University of Maryland in College Park for a workshop, sponsored by the U.S. Department of Energy (DOE), to mull ideas for a possible $10 million short-term experiment that would complement the agency9s current WIMP search and other dark-matter efforts. And many researchers believe DOE should focus on the dark sector. Whereas WIMPs would be a single massive particle tacked onto the standard model of known particles, the dark sector would consist of a slew of lighter particles and forces—such as a dark version of electromagnetism—with tenuous connections to known particles. To spot dark-sector particles, physicists will have to rethink their detection techniques, but the new experiment could be small and cheap, physicists say. Still, DOE officials warn that the $10 million isn9t a sure thing.

A costly and controversial space-based cosmic ray detector has found possible signs of dark matter, the invisible stuff thought to supply most of the universe9s mass. Or so says Samuel Ting, a Nobel Prize–winning particle physicist at the Massachusetts Institute of Technology in Cambridge and leader of the Alpha Magnetic Spectrometer (AMS), which is perched on the International Space Station (ISS). In 2014, AMS researchers reported an unexpected flux of positrons above a certain energy, which could come from dark matter particles colliding and annihilating one another to produce electron-positron pairs. Now, with three times as many data, AMS researchers have shown that the excess falls sharply at slightly higher energy. That cutoff could be further evidence of dark matter collisions, as the dark matter particles9 mass would limit the positrons9 energies. However, the positrons could also come from a pulsar or, some cosmic ray experts say, even the interactions of cosmic rays themselves. And time may be running out for the AMS. It narrowly avoided the budget ax a year ago, but this October astronauts are scheduled to perform a spacewalk to repair the 8500-kilogram detector. Even if the AMS runs until the ISS shuts down, perhaps in 2024, it may never collect enough data to resolve the issue.

No instrument has revealed more about the heavens than NASA9s Hubble Space Telescope, which cost nearly $5 billion to build and launch. Yet a team of researchers thinks it can match some of Hubble9s capabilities with a telescope costing millions of dollars, hoisted to the edge of space on a balloon. During an 18-hour flight over Canada in September 2019, the $2 million Superpressure Balloon-borne Imaging Telescope (SuperBIT) held steady enough to image distant the stars with a resolution of 260 milliarcseconds, only a factor of five less sharp than Hubble and at the theoretical limit for its 0.5-meter mirror. To compete with space telescopes, however, a balloon-borne telescope must also stay aloft for many nights. To achieve that, the team hopes to fly SuperBIT on one of NASA9s new superpressure balloons, which can stay aloft for many weeks. SuperBIT researchers hope to do an observing run in 2021 and are working on a larger, 1.5-meter telescope that they hope will reproduce some of Hubble9s capabilities for a fraction of the cost.

For 130 years, Le Grand K, a gleaming cylinder of platinum-iridium alloy at the International Bureau of Weights and Measures in Sevres, France, has served as the world9s standard for mass. Now, in a revolution far less bloody than the one that cost King Louis XVI his head, it will cede its throne as the one, true kilogram. When the 26th General Conference on Weights and Measures convenes next week in Versailles, France, representatives of the 60 member nations are expected to vote to redefine the International System of Units so that four of its base units—the kilogram, ampere, kelvin, and mole—are defined indirectly, in terms of physical constants that will be fixed by fiat. They9ll join the other three base units—the second, meter, and candela (a measure of a light9s perceived brightness)—that are already defined that way. To take effect on 20 May 2019, the rewrite aims to make the units more stable and allow investigators to develop ever more precise and flexible techniques to mete them out. But even proponents of the arcane changes acknowledge they may bewilder nonexperts.

For particle physicists eager to explore new frontiers, spotting the Higgs boson has become a bittersweet triumph. Detected in 2012 at the world9s biggest atom smasher, the Large Hadron Collider (LHC), the long-sought particle filled the last gap in the standard model of fundamental particles and forces. But since then, the standard model has stood up to every test, yielding no hints of new physics. Now, the Higgs itself may offer a way out of the impasse. Experimenters at the LHC, located at CERN, the European particle physics laboratory near Geneva, Switzerland, plan to hunt for collisions that produce not just one Higgs boson, but two. Finding more of these rare double-Higgs events than expected could point to particles or forces beyond the standard model and might even help explain the imbalance of matter and antimatter in the universe. Last week, more than 100 physicists met at a workshop at the Fermi National Accelerator Laboratory in Batavia, Illinois, to hone the conceptual tools needed for the search. Projections suggest the LHC will have to run until the 2030s to spot such events, but experimenters think they can beat that estimate as their Higgs-finding methods continue to improve.

The next dream machine for U.S. nuclear physicists received a ringing endorsement this week in a report from the National Academies of Sciences, Engineering, and Medicine. The report committee hailed science that could be done by the proposed Electron-Ion Collider (EIC), a billion-dollar accelerator that would probe the incredibly messy innards of protons and neutrons. The glowing assessment should help the Department of Energy justify building the EIC at one of two national laboratories competing to host it. Physicists at Brookhaven National Laboratory in Upton, New York, use their Relativistic Heavy Ion Collider (RHIC) to smash heavy nuclei such as gold and re-create the soup of subatomic particles that filled the infant universe. They eventually want to add an electron accelerator to convert RHIC into the EIC. However, the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, has recently upgraded its Continuous Electron Beam Accelerator Facility, a machine used to fire electrons into stationary targets to study protons, neutrons, and nuclei. They hope to add an ion accelerator to it to make the EIC. For now, Brookhaven and Jefferson Lab scientists are collaborating on the R&D, as the project probably won9t get the go-ahead for several years.

When Albert Einstein died in 1955, he had spent lonely decades trying in vain to unify the theories of gravity and electromagnetism. Stephen Hawking, the great British physicist who died last week at age 76, also worked until the end. But he focused on perhaps the most important problem in his area of physics, one his own work had posed: How do black holes preserve information encoded in the material that falls into them? Hawking realized in 1974 that through a subtle quantum effect a black hole can radiate energy and evaporate. But then a black hole should destroy any infalling information, which cannot come back out in the random radiation. Such information loss would wreck quantum mechanics, and Hawking spent much of his later years trying to figure out how a black hole could preserve information after all, even as the degenerative nerve disease amyotrophic lateral sclerosis rendered him immobile and able to speak only through a computerized voice synthesizer. Ironically, Hawking9s disability may have helped him avoid the isolation that enveloped Einstein, as Hawking had to rely on collaborators to flesh out his ideas and so remained connected to his peers.

The budding discipline of quantum acoustics could shake up embryonic quantum computers. Such machines run by flipping quantum bits, or qubits, that can be set not only to zero or one, but, bizarrely, to zero and one at the same time. The most advanced qubits are circuits made of superconducting metal, and to control or read out a qubit, researchers make it interact with a microwave resonator—typically a strip of metal on the qubit chip or a finger-size cavity surrounding it—which rings with microwave photons like an organ pipe rings with sound. But some physicists see advantages to replacing the microwave resonator with a mechanical one that rings with quantized vibrations, or phonons. A well-designed acoustic resonator could ring longer than a microwave one does and could be far smaller, enabling researchers to produce more compact technologies. But first scientists must gain quantum control over vibrations. And several groups are on the cusp of doing that, as they reported at a recent meeting.

This year9s Nobel Prize in Physics honors the human desire to understand both the fundamental nature of the universe and its details. Half of the $900,000 prize goes to Princeton University cosmologist James Peebles, for laying the foundations of modern-day cosmology. The other half is split between astronomers Michel Mayor of the University of Geneva in Switzerland and Didier Queloz, now at the University of Cambridge in the United Kingdom, who discovered the first planet around another sunlike star. In 1965, Peebles predicted that the big bang should have left an afterglow. That cosmic microwave background (CMB) was discovered the same year. Peebles also deduced that tiny fluctuations in the temperature of the CMB across the sky could be used determine the amounts of ordinary matter, dark matter, and space-stretching dark energy that make up the universe. Mayor and Queloz changed astronomy in a stroke in 1995, when they found a gas giant half the mass of Jupiter in an orbit much tighter than Mercury9s. Since then the floodgates of discovery have opened and astronomers have found thousands more exoplanets of every description.

The age of quantum computing may have begun not with a flashy press conference, but with an internet leak. According to a paper posted briefly—and presumably mistakenly—to the website of a NASA laboratory, physicists at Google have used a quantum computer to perform a calculation that would overwhelm the world9s best conventional supercomputer. Although the abstruse computation has no known use, the result means scientists have passed a long-anticipated milestone known as "quantum supremacy." The Google computer lacks the ability to correct errors, which may be key to making a full-fledged quantum computer. And many researchers worry that after all the hype surrounding quantum supremacy, quantum computing may experience a letdown like the one that plagued artificial intelligence from the 1970s until the current decade, when technology finally caught up with aspirations. In the leaked paper, however, the 76 authors conclude: "We are only one creative algorithm away from valuable near-term applications."

How does our predictable everyday world emerge from the hazy, probabilistic rules of quantum mechanics? That puzzle has vexed physicists since quantum theory emerged in the 1920s to explain the behavior of atoms and other infinitesimal things. Now, researchers fiddling with an electron trapped in a diamond have confirmed an essential prediction of a theory that strives to explain this transition from the quantum to classical realm. Called "quantum Darwinism," the theory argues that classical states are simply the quantum ones that are most fit to survive interactions with their environment to imprint themselves over and over on their surroundings. In the experiment, researchers observe the electron imprinting its quantum state on surrounding atomic nuclei, as predicted by the theory. However, physicists caution that the experiment is far from the final word on the matter.

MRI scanners can map a person9s innards in exquisite detail, but they say little about composition. Now, physicists are pushing MRI to a new realm of sensitivity to trace specific biomolecules in tissues, a capability that could aid in diagnosing Alzheimer9s and other diseases. The advance springs not from improved scanners, but from better methods to solve a notoriously difficult math problem and extract information already latent in MRI data. Researchers are already using the new techniques to trace a fatty molecule called myelin in the brains of people with multiple sclerosis, mild cognitive impairment, and Alzheimer9s disease. They also show that the scans can trace a molecule called proteoglycan in knee cartilage. Before the new scans can be used to diagnose specific diseases, they must be validated against clinical data and tissue samples. But some researchers think the techniques will appear in the clinic within 5 years.

Particle physicists at Europe's CERN laboratory in Switzerland say they have observed bizarre new cousins of the protons and neutrons that make up the atomic nucleus. Protons and neutrons consist of other particles called quarks, bound by the strong nuclear force. By smashing particles at high energies, physicists have blasted into fleeting existence hundreds of other quark-containing particles. Until recently, all contained either two or three quarks. But since 2014, researchers working with CERN's Large Hadron Collider have also spotted four- and five-quark particles. Such tetraquarks and pentaquarks could require physicists to rethink their understanding of quantum chromodynamics, or they could have less revolutionary implications. Researchers hope that computer simulations and more collider studies will reveal how the oddball newcomers are put together, but some wonder whether experiments will ever provide a definitive answer.

For more than a century, studies of liquid helium, the only substance that won't freeze at absolute zero temperature, have proved a gold mine for physicists. Liquid helium bagged five Nobel Prizes—one more than superconductivity, the other star of condensed matter physics. It displays bizarre quantum mechanical properties such as "superfluidity"—the strange ability to flow without resistance—and has been the nexus between fundamental theory and condensed matter physics. Now, however, the field is ailing. Especially in the United States, the field is contracting, as older physicists retire and few younger ones enter the field. Even submissions to the field's main journal are falling. Liquid helium's advocates think it still has much to offer, and researchers recently convened a special workshop at the State University of New York at Buffalo to map out the field's grand challenges. But with much already known, breakthroughs are hard to come by. And, some physicists say, the field has become insular, defined by narrow questions and exotic, challenging experimental techniques. The survival of a storied field may depend in making liquid helium relevant to the rest of physics again.

The Axion Dark Matter Experiment (ADMX) doesn9t look like much. Yet the modest 4-meter-long metal cylinder, housed at the University of Washington9s Center for Experimental Nuclear Physics and Astrophysics, is key to one of the more important and promising investigations in particle physics. Starting late next year, ADMX will search for elusive, superlight particles called axions, which might make up dark matter whose gravity holds the galaxies together. Axions are a dark horse for dark matter; most theorists prefer a different hypothesis, so-called weakly interacting massive particles, or WIMPs. But decades of searching have turned up no proof that WIMPs exist. Now ADMX researchers are about to put axions to the test. In the next few years, their data ought to provide a clear-cut yes-or-no answer about whether axions constitute the universe9s dark matter.

In a year of triumphs for their reigning models, cosmologists and particle physicists yearn for something new that they can't explain.