16 décembre 2016 ~ 0 Commentaire

Wandering stones’ of Death Valley explained Wandering stones’ of Death Valley explained Antimatter detector

The Alpha Magnetic Spectrometer will seek antimatter in deep space, and measure cosmic rays closer to home. MIT A couple of days before lift-off, Mark Sistilli went down to the space-shuttle launch pad in Cape Canaveral, Florida, to meet researchers working on the Alpha Magnetic Spectrometer (AMS) and to sneak a last nervy glimpse of their 7-tonne cosmic-ray detector before the shuttle’s cargo doors closed. « The AMS was tucked in and ready to go, » says Sistilli, NASA programme manager for the mission. On Friday 29 April, if all goes to plan, the AMS will leave on board the Endeavour shuttle, bound for the International Space Station (ISS). It is arguably the station’s most important scientific payload so far. The AMS is a cylindrical magnet, which has already flown on a 1998 shuttle flight, surrounded by a suite of new instruments for detecting cosmic rays. It is the result of former NASA administrator Dan Goldin’s quest to find meaningful science projects for the ISS, and of Nobel-prize-winning physicist Samuel Ting’s unorthodox ideas about antimatter. The US$2-billion experiment has been sold partly as a search for regions of the Universe containing gas, stars and planets made exclusively of antimatter. This has raised eyebrows among those high-energy and particle physicists who doubt that such regions exist. Ting, the mission’s spokesman, acknowledges the scepticism, but says that there’s no such thing as mainstream in physics. « Science doesn’t depend on a vote, » he says. Leading up to the launch, he and other scientists have also emphasized the unprecedented sensitivity of the AMS to the cosmic rays that rain down on Earth, which should yield more dependable science than the mission’s headline might suggest. Detecting cosmic rays Alongside detecting any heavy antimatter nuclei – which would be a smoking gun for regions of antimatter in the Universe – the AMS will produce definitive data on the energy, charge and composition of cosmic rays from the Sun and from astrophysical sources such as supernovae and gamma-ray bursts. « We’ll be able to measure cosmic-ray fluxes very precisely, » says AMS team member Fernando Barao, at the Laboratory of Instrumentation and Experimental Particle Physics in Lisbon. « The best place to be is space because you don’t have Earth’s atmosphere that destroys cosmic rays. » Theoretical physicist John Ellis at CERN, Europe’s particle-physics lab near Geneva, Switzerland, who is not working on the mission, agrees, adding that AMS will be a big advance over previous space-based cosmic-ray detectors, such as the Russian–Italian Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), launched in 2006. PAMELA detected positrons from the halo of the Milky Way that may have been a signal of dark-matter particles annihilating there. With 200 times the collecting area of PAMELA, the AMS should be vastly more sensitive to the same signal, if it’s real. « It’ll be far and away the most detailed measurement of cosmic-ray flux we’ve been able to get, » Ellis says, « it has way more scientific interest than any other experiment on the space station. » That is a crucial part of the experiment’s politics. Astronauts’ role in scientific discovery helps to maintain public support for the US space programme, says NASA astronaut Michael Massimino. He points to the Hubble Space Telescope, serviced five times by astronauts on shuttles, as a prime example. « The AMS has that potential as well, » he says. Fast-track into space Astronauts on Endeavour , which will be captained by Mark Kelly – husband of Gabrielle Giffords, the Arizona congresswoman shot in the head at a constituency event in January – will use robotic arms on the shuttle and the ISS to ease the AMS into position. Once operational, the AMS will be controlled from Earth. Compared to an independent instrument, being on board the space station will increase the AMS’s power and data-transfer rates. It will also be easier to fix if something goes wrong. « NASA is very proud that this is on the International Space Station, » says Sistilli. « We always hoped we would have science payloads. » Apart from its mission, AMS is also controversial for bypassing the peer review that NASA normally requires of science missions. But Sistilli emphasizes that the project was endorsed by committees convened by the US Department of Energy, which is supplying $50 million of the funding. NASA is providing $85 million, and the remainder comes from a consortium of 16 countries including France, Portugal, China, Taiwan and Spain. Go Advanced search __UL__ HomeNews & Comment;Research;Careers & Jobs;Current Issue;Archive;Audio & Video;For Authors;News & Comment;News;2014;November;Article; Nature News Sharing__LI____LI____UL____LI____LI____LI____LI____LI____LI____LI__’Wandering stones’ of Death Valley explained; Scientists spot ice shoving rocks on Racetrack Playa in California, resolving a longstanding geological enigma.Alexandra Witze; 27 August 2014 Article tools Rights & Permissions Secrets of sliding stones A combination of ice and wind pushes rocks across Racetrack Playa. Richard Norris & Jim Norris Ending a half-century of geological speculation, scientists have finally seen the process that causes rocks to move atop Racetrack Playa, a desert lake bed in the mountains above Death Valley, California. Researchers watched a pond freeze atop the playa, then break apart into sheets of ice that – blown by wind – shoved rocks across the lake bed.A quantum world arising from many ordinary ones;Russian summer tops ‘universal’ heatwave index;Environmental degradation boosts jellyfish blooms; Until now, no one has been able to explain why hundreds of rocks scoot unseen across the playa surface, creating trails behind them like children dragging sticks through the mud. « It’s a delight to be involved in sorting out this kind of public mystery, » says Richard Norris, an oceanographer at the Scripps Institution of Oceanography in La Jolla, California, who led the research with his cousin James Norris, an engineer at Interwoof in Santa Barbara, California. The work was published on 27 August in PLoS ONE 1 . Geologists previously speculated that some combination of wind, rain and ice would have a role. But few expected that the answer would involve ice as thin as windowpanes, pushed by light breezes rather than strong gales. Visitors to Death Valley have to go out of their way to visit Racetrack Playa, which sits 1,130 metres above sea level and is a bumpy three-hour drive from the nearest town. The researchers began studying the region in 2011, setting up a weather station and time-lapse cameras and dropping off rocks loaded with Global Positioning System (GPS) trackers. The rocks were designed to start recording their position and speed as soon as something made them move. Dennis Flaherty/Alamy Rocks at Racetrack Playa seem to move on their own, leaving mysterious trails behind. What was not clear was how long the Norrises would have to wait. Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, has been studying the playa since 2007 as an analogy to lake beds on other planets. He had little faith that the GPS-equipped rocks would move in a time frame that anyone would capture. « I thought it was going to be the most boring experiment in the history of science, » he says. But when the researchers travelled to the playa in December 2013 to check instruments and change batteries, they found a huge ice-encrusted pond covering about one-third of the 4.5-kilometre-long playa. After several days of camping, they decided to sit above the southern end of the playa on the morning of 20 December. « It was a beautiful sunny day, and there began to be rippled melt pools in front of us, » Richard Norris says. « At 11:37 a.m., very abruptly, there was a pop-pop-crackle all over the place in front of us – and I said to my cousin, ‘This is it.’  » They watched as the ice began moving past the rocks, mostly breaking apart but also shoving them gently. The rocks began to inch along, but their pace was too slow to spot by eye. « A baby can get going a lot faster than your average rock, » Richard Norris says. But when the ice melted away that afternoon, they saw freshly formed trails left behind by more than 60 moving rocks. And on 9 January, James Norris returned to the playa with Lorenz and was able to record video of the roving rocks. « This is transformative, » says Lorenz. « It’s not just an anecdotal report, but we have before and after pictures, and meteorological information simultaneous with the event. » By the end of the winter, the farthest-moving rock had travelled 224 metres. Racetrack Playa rocks move rarely – « maybe a few minutes out of a million, » Lorenz says. And the two events the scientists saw, with thin ice panes shoving the stones across a wet playa, do not necessarily explain every instance of rocks moving there. « But this breaks the back of the problem scientifically, » Lorenz says. « It is ice shove. » Solving the Racetrack Playa mystery is not exactly a major scientific breakthrough, Lorenz says, but the work does show the rare combination of conditions that allow rocks to move seemingly on their own. And ice shove can have notable effects – in 1952, it uprooted enough telephone poles at a lake in Nevada to break a transcontinental telephone line. One person who is happy to see the latest results is Dwight Carey. As a university student in the 1970s, he helped with an experiment in which two rocks were placed in a corral on the playa. Over the course of a winter, one stone moved out of the corral, unobserved, and the other did not 2 . The new explanation « makes sense to me », says Carey, who is now an environmental regulatory consultant in Brea, California. « Eventually you’re going to get enough force on the pile of ice behind the rocks to be able to move them. Two sightings in Minnesota have set physicists buzzing about whether the first direct detection of dark matter has been made. If confirmed, it would mark the end of a decades-long search for the mysterious particles thought to make up as much as 85% of matter in the Universe. But most agree that the signals are not statistically significant enough to be attributed to dark matter rather than to conventional particles. The two events were caught in 2007 in super-cooled crystals of germanium and silicon in the underground Cryogenic Dark Matter Search II (CDMSII) experiment in the Soudan Mine in Minnesota. Last week, CDMSII scientists announced that they have seen candidates for the dark-matter particles known as weakly interacting massive particles (WIMPs), each with a mass of 30–60 gigaelectronvolts – roughly 30–60 times that of a proton. But from the analysis, team scientists think that there is a 25% chance that both events might be false-positives caused by background radiation. Those odds are not good enough to claim a definitive detection of WIMPs, says Timothy Sumner, a physicist at Imperial College London. « Statistically, it’s not compelling, » he says. « The best we could call it is a hint, » adds John Ellis, a theoretical physicist at CERN, Europe’s high-energy physics lab near Geneva, Switzerland. « An interesting hint. » The possible detection is the latest in a series of potential dark-matter sightings. In August 2008, an Italian-led satellite-based experiment known as PAMELA reported an excess of antielectrons (positrons) that could have stemmed from the annihilation of dark-matter particles. And in October 2009, NASA’s Fermi Gamma-ray Space Telescope saw a haze of high-energy light in the centre of our Galaxy that could be a dark-matter signature. The CDMSII result will now spur physicists at the Large Hadron Collider (LHC) at CERN to try to generate WIMPs in their collisions. « The LHC would see this very easily and relatively quickly, » says Ellis – and could potentially produce a detectable WIMP signal by the end of next year.

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