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  • richardmitnick 3:38 pm on May 27, 2016 Permalink | Reply
    Tags: , , , Symmetry Magazine   

    From Symmetry: “Low-mass particles that make high-mass stars go boom” 

    Symmetry Mag

    Symmetry

    05/26/16
    Matthew R. Francis

    Simulations are key to showing how neutrinos help stars go supernova.

    1
    http://www.nasa.gov

    When some stars much more massive than the sun reach the end of their lives, they explode in a supernova, fusing lighter atoms into heavier ones and dispersing the products across space—some of which became part of our bodies. As Joni Mitchell wrote and Crosby Stills Nash & Young famously sang, “We are stardust, we are golden, we are billion-year-old carbon.”

    2
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    However, knowing this and understanding all the physics involved are two different things. We can’t make a true supernova in the lab or study one up close, even if we wanted to. For that reason, computer simulations are the best tool scientists have. Researchers program equations that govern the behavior of the ingredients inside the core of a star to see how they behave and whether the outcomes reproduce behavior we see in real supernovae. There are many ingredients, which makes the simulations extraordinarily complicated—but one type of particle could ultimately drive supernova explosion: the humble neutrino.

    Neutrinos are well known for being hard to detect because they barely interact with other particles. However, the core of a dying star is a remarkably dense environment, and the nuclear reactions produce vast numbers of neutrinos. Both these things increase the likelihood of neutrinos hitting other particles and transferring energy.

    “We can estimate on a sheet of paper roughly how much energy neutrinos may deliver,” says Hans-Thomas Janka, a supernova researcher at the Max Planck Institute for Astrophysics in Garching, Germany. “The question still remains: Is that compatible with the detailed picture? What we need is to combine all the physics ingredients which play a role in the core of a collapsing star.”

    Things fall apart, the center cannot hold

    Typically, all the nuclear fusion in a star happens in its core: That’s the only place hot and dense enough. In turn, the nuclear fusion supplies enough energy to keep the core from compressing under its own gravity. But when a star heavier than eight times the mass of our sun exhausts its nuclear fuel and fusion halts, the core collapses catastrophically. The result is a core-collapse supernova: a shock wave from the collapse tears the star apart while the core shrinks into a neutron star or black hole. The explosion leads to more nuclear fusion and the spread of nuclei into interstellar space, where it can eventually be used in making new stars and planets. (The other major supernova type involves an exploding white dwarf, the source of many other common atoms.)

    Core-collapse supernovae are rare and extremely violent phenomena, sometimes outshining whole galaxies at their peak. The last relatively close-by supernova appeared in the sky in 1987, in the neighboring galaxy known as the Large Magellanic Cloud. Even if a supernova exploded close enough to observe in detail (while being far enough to be safe), we can’t see deep inside to where the action is.

    However, 24 neutrinos from the 1987 supernova showed up in particle detectors (built for studying proton decay) [I have seen this before, but no one ever says how they know this to be factual]. These neutrinos were likely born in nuclear reactions deep in the exploding star’s interior and confirmed theoretical predictions from the 1960s, when astrophysicists first began to study exploding stars.

    Supernova research really took off in the 1980s with growing computer power and the realization that a full understanding of core collapse would need to incorporate a lot of complicated physics.

    “Core-collapse supernovae involve a huge variety of effects involving all four fundamental forces,” says Joshua Dolence of the US Department of Energy’s Los Alamos National Laboratory. “The predicted outcome of collapse—even the most basic question of ‘Does this star explode?’—can depend on how these effects are incorporated into simulations.”

    In other words, if you don’t do the simulations right, the supernova never happens. While some stars may collapse directly into black holes instead of exploding, astronomers see both supernova explosions and their aftermaths (the most famous example being the Crab Nebula).

    Supernova remnant Crab nebula. NASA/ESA Hubble
    Supernova remnant Crab nebula. NASA/ESA Hubble

    Some simulations don’t ever show a kaboom, which is a problem: The energy released during the burst of neutrinos is enough to stall out the supernova before it explodes.

    If neutrinos cause the problem, they may also solve it. They carry energy away from one part of the dying star, but they may also transfer it to the stalled-out shockwave, breaking the stalemate and making the supernova happen. It’s not the only hypothesis, but currently it’s the best guess astrophysicists have, and most of the large computer simulations seem to support it so far. However, some of the most energetic supernovae—known as hypernovae—don’t seem to abide by the same rules, so it’s possible something other than neutrinos are responsible. What that something else might be is anyone’s guess.

    Explosions in the sky

    Core-collapse supernovae are natural laboratories for extreme physics. They involve particle physics, strong gravity as described by general relativity and nuclear physics, all mixed up with strong magnetic fields. All of those aspects must be implemented in computer code, which necessarily involves tough decisions about what details to include and what to leave out.

    “The major open questions revolve around understanding which physical effects are crucial to a quantitative understanding of supernova explosions,” Dolence says. His own work at Los Alamos involves testing the assumptions going into the various theoretical models for explosions and developing faster code to save on precious computer time. Janka’s work in Europe, by contrast, involves modeling the neutrino behavior as exactly as possible.

    Currently, both detailed and simplified approaches are needed, until researchers know exactly what physical processes are involved deep inside the dying star. Both methods use tens of millions of hours of computer time, distributed across multiple computers working in parallel. Even with certain simplifying assumptions, these simulations are some of the biggest around, meaning they require supercomputers at large research centers: the Leibniz Computing Center in Germany; the Barcelona Supercomputing Center in Spain; Los Alamos, Oak Ridge National Laboratory and Princeton University in the United States, and just a handful of others.

    “We have no proof so far except our calculations that neutrinos are the cause of the explosion,” Janka says. “We need to compare models with [astronomical] observations in the future.”

    The world’s current neutrino experiments are poised to catch neutrinos from the next event and are connected by the Supernova Early Warning System. But in the absence of a nearby supernova, massive supercomputer simulations are all we have. In the meantime, those simulations could still teach us about the extreme physics of dying stars and what role neutrinos play in their deaths.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 7:49 am on May 21, 2016 Permalink | Reply
    Tags: , , Symmetry Magazine, Planck scale   

    From Symmetry: “The Planck scale” 

    Symmetry Mag

    Symmetry

    05/19/16
    Rashmi Shivni

    The Planck scale sets the universe’s minimum limit, beyond which the laws of physics break.

    1
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    In the late 1890s, physicist Max Planck proposed a set of units to simplify the expression of physics laws. Using just five constants in nature (including the speed of light and the gravitational constant), you, me and even aliens from Alpha Centauri could arrive at these same Planck units.

    The basic Planck units are length, mass, temperature, time and charge.

    Let’s consider the unit of Planck length for a moment. The proton is about 100 million trillion times larger than the Planck length. To put this into perspective, if we scaled the proton up to the size of the observable universe, the Planck length would be a mere trip from Tokyo to Chicago. The 14-hour flight may seem long to you, but to the universe, it would go completely unnoticed.

    The Planck scale was invented as a set of universal units, so it was a shock when those limits also turned out to be the limits where the known laws of physics applied. For example, a distance smaller than the Planck length just doesn’t make sense—the physics breaks down.

    Physicists don’t know what actually goes on at the Planck scale, but they can speculate. Some theoretical particle physicists predict all four fundamental forces—gravity, the weak force, electromagnetism and the strong force—finally merge into one force at this energy. Quantum gravity and superstrings are also possible phenomena that might dominate at the Planck energy scale.

    The Planck scale is the universal limit, beyond which the currently known laws of physics break. In order to comprehend anything beyond it, we need new, unbreakable physics.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 12:21 pm on May 9, 2016 Permalink | Reply
    Tags: , , , , , , Symmetry Magazine   

    From FNAL: “Large Hadron Collider prepares to deliver six times the data” 

    FNAL II photo

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    May 9, 2016

    Media contact

    Andre Salles, Fermilab Office of Communication, asalles@fnal.gov, 630-840-6733
    Ivy F. Kupec, National Science Foundation, ikupec@nsf.gov, 703-292-8796
    Rick Borchelt, U.S. Department of Energy Office of Communications and Public Affairs, rick.borchelt@science.doe.gov, 202-586-4477
    Sarah Charley, US LHC, scharley@fnal.gov, 630-338-3034 (cell)

    1
    Collisions recorded on May 7, 2016, by the CMS detector on the Large Hadron Collider. After a winter break, the LHC is now taking data again at extraordinary energies. Image: CERN

    Experiments at the LHC are once again recording collisions at extraordinary energies

    Editor’s note: The following news release about the restart of the Large Hadron Collider is being issued by the U.S. Department of Energy’s Fermi National Accelerator Laboratory on behalf of the U.S. scientists working on the LHC. Fermilab serves as the U.S. hub for the CMS experiment at the LHC and the roughly 1,000 U.S. scientists who work on that experiment, including about 100 Fermilab employees. Fermilab is a Tier 1 computing center for LHC data and hosts a Remote Operations Center to process and analyze that data. Read more information about Fermilab’s role in the CMS experiment and the LHC. Fermilab scientists are available for interviews upon request, including Joel Butler, recently elected next spokesperson of the CMS experiment.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    After months of winter hibernation, the Large Hadron Collider is once again smashing protons and taking data. The LHC will run around the clock for the next six months and produce roughly 2 quadrillion high-quality proton collisions, six times more than in 2015 and just shy of the total number of collisions recorded during the nearly three years of the collider’s first run.

    “2015 was a recommissioning year. 2016 will be a year of full data production during which we will focus on delivering the maximum number of data to the experiments,” said Fabiola Gianotti, CERN director general.

    CERN Fabiola Gianotti
    Fabiola Gianotti

    The LHC is the world’s most powerful particle accelerator. Its collisions produce subatomic fireballs of energy, which morph into the fundamental building blocks of matter. The four particle detectors located on the LHC’s ring allow scientists to record and study the properties of these building blocks and look for new fundamental particles and forces.

    “We’re proud to support more than a thousand U.S. scientists and engineers who play integral parts in operating the detectors, analyzing the data and developing tools and technologies to upgrade the LHC’s performance in this international endeavor,” said Jim Siegrist, associate director of science for high-energy physics in the U.S. Department of Energy’s Office of Science. “The LHC is the only place in the world where this kind of research can be performed, and we are a fully committed partner on the LHC experiments and the future development of the collider itself.”

    [Never should it be forgotten that this work could have proceeded i the US had the US Congress followed through with funding for the Superconducting Super Collider which had begun construction in Texas. In 1993, our congress decided to stop this project and leave this research to others.]

    Between 2010 and 2013 the LHC produced proton-proton collisions with 8 teraelectronvolts of energy. In the spring of 2015, after a two-year shutdown, LHC operators ramped up the collision energy to 13 TeV. This increase in energy enables scientists to explore a new realm of physics that was previously inaccessible. Run II collisions also produce Higgs bosons — the groundbreaking particle discovered in LHC Run I — 25 percent faster than Run I collisions and increase the chances of finding new massive particles by more than 40 percent.

    Almost everything we know about matter is summed up in the Standard Model of particle physics, an elegant map of the subatomic world.

    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    During the first run of the LHC, scientists on the ATLAS and CMS experiments discovered the Higgs boson, the cornerstone of the Standard Model that helps explain the origins of mass.

    CERN/ATLAS
    CERN ATLAS Higgs Event
    ATLAS

    CERN/CMS Detector
    CERN CMS Higgs Event
    CMS

    The LHCb experiment also discovered never-before-seen five-quark particles, and the ALICE experiment studied the near-perfect liquid that existed immediately after the Big Bang. All these observations are in line with the predictions of the Standard Model.

    CERN/LHCb
    LHCb

    AliceDetectorLarge
    ALICE

    “So far the Standard Model seems to explain matter, but we know there has to be something beyond the Standard Model,” said Denise Caldwell, director of the Physics Division of the National Science Foundation. “This potential new physics can only be uncovered with more data that will come with the next LHC run.”

    For example, the Standard Model contains no explanation of gravity, which is one of the four fundamental forces in the universe. It also does not explain astronomical observations of dark matter, a type of matter that interacts with our visible universe only through gravity, nor does it explain why matter prevailed over antimatter during the formation of the early universe. The small mass of the Higgs boson also suggests that matter is fundamentally unstable.

    The new LHC data will help scientists verify the Standard Model’s predictions and push beyond its boundaries. Many predicted and theoretical subatomic processes are so rare that scientists need billions of collisions to find just a small handful of events that are clean and scientifically interesting. Scientists also need an enormous amount of data to precisely measure well-known Standard Model processes. Any significant deviations from the Standard Model’s predictions could be the first step towards new physics.

    The United States is the largest national contributor to both the ATLAS and CMS experiments, with 45 U.S. universities and laboratories working on ATLAS and 49 working on CMS.

    CERN, the European Organization for Nuclear Research, is the world’s leading laboratory for particle physics. It has its headquarters in Geneva. At present, its member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, the Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Romania is a candidate for accession. Cyprus and Serbia are associate members in the pre-stage to membership. Turkey and Pakistan are associate members. India, Japan, the Russian Federation, the United States of America, Turkey, the European Union, JINR and UNESCO have observer status.

    See the full from FNAL article here .
    See the Symmetry article here .

    Please help promote STEM in your local schools.

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 4:53 pm on May 5, 2016 Permalink | Reply
    Tags: , , Symmetry Magazine   

    From Symmetry: “Following LIGO’s treasure maps” 

    Symmetry Mag

    Symmetry

    05/05/16
    Andre Salles

    Astronomers around the world are looking for visible sources of gravitational waves.

    On the morning of September 16, 2015, an email appeared in 63 inboxes scattered around the globe. The message contained a map of the cosmos and some instructions, and everyone who received it knew the most important thing was to keep it secret.

    It wasn’t until five months later that the world found out what the owners of those inboxes knew: that two days earlier, on September 14, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves for the first time.

    Caltech/MIT Advanced aLigo detector in Livingston, Louisiana
    Caltech/MIT Advanced aLigo detector in Livingston, Louisiana

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    That secret was shared with 63 astronomy collaborations, and it sparked the start of a worldwide treasure hunt. Astronomers searched the skies for rare and faint objects that might be the source of the detected ripples in space-time.

    Searching for the optical counterparts to those waves is a crucial step after the initial detection. The additional information can provide interesting scientific results for both gravitational-wave scientists and astronomers. Gravitational waves may be caused by several different phenomena such as neutron stars colliding or, in the case of the first signal, a pair of black holes merging.

    Black holes merging Swinburne Astronomy Productions
    Black holes merging Swinburne Astronomy Productions

    Cornell SXS team. Two merging black holes simulation
    Cornell SXS team. Two merging black holes simulation

    Studying these objects can be its own reward for astronomers, so they prepare for months or even years to drop everything at a moment’s notice to follow up signals whenever they appear.

    But in September, the email from LIGO took most of those astronomers by surprise. In fact, according to LIGO collaboration member Daniel Holz of the University of Chicago, the clear, crisp signal caught just about everyone off guard. Advanced LIGO, the most recent upgrade that had quadrupled their sensitivity, had just begun its engineering run—they had barely turned the machine on when they hit pay dirt.

    “It was insane, incredible,” Holz says. “We all worked very hard, and to have what you hope for and dream about land in your lap so fast, so early and so emphatically was like my wildest dreams coming true.”

    The signal was detected loud and clear at 4:50 a.m. Chicago time, so Holz was able to see it when he checked email at 7 a.m. His initial thought was that it might have been a mistake, but by the time he’d bicycled to work and had his morning tea, many of the obvious ways the signal could have been an error had been eliminated. By the end of that day, it was likely that the LIGO team had the real thing on their hands.

    “We were prepared to do a lot of analysis, and that work can take months,” Holz says. “But this case was so emphatic that within hours we were quite confident that we had something incredibly interesting.”

    The collaboration still analyzed the signal for two days before sending it out to astronomy teams. Marica Branchesi, an astronomer who has been part of LIGO and its sister experiment Virgo since 2009, was part of the small group that sent the September 16 email. She says extra care was taken with this first signal.

    “Because it was the first candidate, we took the time to do more analysis and be sure it was an event,” she says. “This is something we had dreamed of for a long time.”

    While LIGO’s extraordinary sensitivity allows it to detect gravitational waves, which result from warped space-time, pinpointing the source of those waves is another matter. LIGO uses a pair of massive laser interferometers, one located in Washington state, the other in Louisiana. With two detectors, LIGO can figure out which direction the waves are coming from, but a third detector (the Advanced VIRGO detector, located in Italy and coming online later this year) will enable them to triangulate the signal.

    What Branchesi and the LIGO/Virgo team sent to astronomers on September 16 was a sky map that covered 600 square degrees, an area 6000 times larger than the full moon, with probabilities assigned to pixels.

    The maps:
    5
    Comparison of different GW sky maps, showing the 90% credible level contours for each algorithm. This orthographic projection centered on the centroid of the LIB localization. The inset shows the distribution of the polar angle θHL (equivalently, the arrival time difference ∆tHL).

    6
    Sky at the time of the event, with the LALInference skymap, contoured in deciles of probability. View is from the South Atlantic Ocean, North at the top, with the Sun rising and the Milky Way diagonally from NW to SE.

    “The region of sky is huge,” Branchesi says. “It’s a challenge to cover. With such a large region, you can find many objects that look like they might be the counterpart, but aren’t.”

    The LIGO team also did not know at the time what we know now—that this particular gravitational wave was caused by a pair of black holes, which are unlikely to be visible with telescopes (though the Fermi Gamma-ray Space Telescope did pick up a burst of gamma rays in the same area).

    NASA/Fermi Telescope
    NASA/Fermi Telescope

    But Marcelle Soares-Santos, an astrophysicist who works on the Dark Energy Survey at the US Department of Energy’s Fermilab, says she would have followed up on the LIGO email regardless.

    Dark Energy Icon

    “There may be something,” she says, “but we don’t know unless we look. We don’t expect a pair of black holes to be visible, but if the area near the black holes is full of matter, maybe we can detect that.”

    Soares-Santos is part of a roughly 25-member team within the Dark Energy Survey called DES-GW, dedicated to following up signals from LIGO. The effort began in 2013, when LIGO put out an open invitation to astronomers to search for optical counterparts.

    “It seemed like a challenging thing to do, to find a transient object in a huge area of sky,” she says. “But then I realized that the Dark Energy Camera is a perfect tool for a discovery like this.”

    Dark Energy Camera,  built at FNAL
    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile
    CTIO Victor M Blanco 4m Telescope interior
    Dark Energy Camera, built at FNAL; NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile

    That camera, the main instrument of the survey, has several advantages, Soares-Santos says: It has a wide field of view, it’s on a large telescope (the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile), and it has a particular sensitivity to the red end of the spectrum, which helps astronomers chase down the faint objects they’re looking for.

    DES-GW has an agreement with the main Dark Energy Survey: If a signal from LIGO comes in, astronomers drop everything and use the camera to chase it. That’s because the objects that are likeliest to be found are neutron stars, the smallest and densest types of stars known to exist.

    Neutron star merger depicted Goddard
    Neutron star merger depicted, Goddard

    They are thought to form when a massive star collapses, creating a supernova, and they fade quickly, rapidly rendering them undetectable.

    When two neutron stars are formed side by side, the theory goes, these stars create detectable gravitational waves. Spotting two neutron stars (or a neutron star paired with a black hole) would be like finding buried treasure. And it would be just as difficult, according to Stephen Smartt of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) collaboration, which also followed up this first signal.

    U Hawaii Pann-STARRS1 Telescope
    U Hawaii Pann-STARRS1 interior
    U Hawaii Pann-STARRS1 Telescope

    “The sky maps we receive are 500 to 1000 square degrees, which is a big chunk of sky,” he says. “Only a small number of facilities are able to map out that area to faint limits.”

    Most of the teams working on these optical follow-ups expect the counterparts to be faint, Smartt says, because if they were bright and common, then the currently running surveys would probably have spotted them already.

    “They could have already been discovered and we haven’t recognized them,” he says, “but most astronomers think that is unlikely.”

    Essentially, Smartt says, astronomers are looking for something bright, fleeting and newly formed—something that hasn’t shown up on previous sweeps of the survey area. Soares-Santos notes that astronomers are essentially looking for an object like a supernova, but fainter, redder and decaying faster.

    “A supernova lasts about a month,” she says. “These last about 10 days. That’s why we want to be quick.”

    The initial sky map sent to astronomers showed two areas of high probability, one in the northern part of the region and one in the southern part. Pan-STARRS, based in Hawaii, concentrated on the northern one, finding roughly 60 transient objects and analyzing them. They discovered nothing unusual and, as more analysis was done on LIGO’s end, learned that they were looking in an area less likely to be the source. But Smartt’s very happy to keep following up these signals.

    “It was an amazing discovery,” he says. “These follow-ups are a high-risk project, and we don’t know if we will hit gold or find nothing.”

    But finding the sought-for objects would open up doors to new science, from probing the origin of heavy elements to high-energy physics and even constraining theories of modified gravity.

    “The payoff is so great, it’s worth pursuing,” he says.

    DES scanned the southern area and similarly found nothing unusual. More detailed maps were provided later, showing that they too were off the mark somewhat, but as the system improves, this should be less of an issue. And there will be plenty of opportunity to put it through its paces in the future.

    “At first [DES-GW] was seen as high-risk,” Soares-Santos says. “Now the perception is that there is still a risk involved, but there will not be a lack of events. Everybody is very happy we did this.”

    And the results of following these signals will be beneficial to astronomy as well. DES scientists will learn more about objects they rarely observe, like binary neutron stars, but they could also potentially use that information to aid in their main mission to learn more about dark energy. Soares-Santos explained that they could use neutron stars the same way they are using supernovae now, to study how the universe has expanded over time.

    “In principle, if the rates are as high as we think they could be, we could have another probe for DES,” she says.

    Branchesi agreed that the system, though currently working well, will improve. In particular, the LIGO/Virgo team wants to get the alerts to astronomers sent out no more than a few minutes after gravitational waves are detected. And with the Advanced VIRGO detector coming online soon, the probability maps will get much more exact.

    But she says she was happy with how well such a vast and diverse group of physicists and astronomers worked together not only to detect gravitational waves for the first time, but also to follow up that detection with solid observation. That, she says, will only get better as well.

    “There’s a lot of us, and it’s important that we work together,” she says.

    LIGO is still holding an open call for astronomy collaborations that would like to look for optical counterparts to gravitational wave signals. It’s a chance, Holz says, to be part of something that has captivated the world.

    “Our community is very excited, the broader scientific community is excited and the public is excited,” he says. “It’s similar to the Higgs discovery, but different, because it’s opening up an entirely new window. It’s enabling the first step in a whole new way to probe the universe, and the excitement is about where we’re headed. It’s revolutionary.”

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:40 pm on May 3, 2016 Permalink | Reply
    Tags: , , EXO-200 experiment, , Symmetry Magazine   

    From Symmetry: “EXO-200 resumes its underground quest” 

    Symmetry Mag
    Symmetry

    05/03/16
    Matthew R. Francis

    EXO-200 Enriched Xenon Observatory near Carlsbad, New Mexico
    SLAC EXO-200 Enriched Xenon Observatory near Carlsbad, New Mexico

    The upgraded experiment aims to discover if neutrinos are their own antiparticles.

    Science is often about serendipity: being open to new results, looking for the unexpected.

    The dark side of serendipity is sheer bad luck, which is what put the Enriched Xenon Observatory experiment, or EXO-200, on hiatus for almost two years.

    Accidents at the Department of Energy’s underground Waste Isolation Pilot Project (WIPP) facility near Carlsbad, New Mexico, kept researchers from continuing their search for signs of neutrinos and their antimatter pairs. Designed as storage for nuclear waste, the site had both a fire and a release of radiation in early 2014 in a distant part of the facility from where the experiment is housed. No one at the site was injured. Nonetheless, the accidents, and the subsequent efforts of repair and remediation, resulted in a nearly two-year suspension of the EXO-200 effort.

    Things are looking up now, though: Repairs to the affected area of the site are complete, new safety measures are in place, and scientists are back at work in their separate area of the site, where the experiment is once again collecting data. That’s good news, since EXO-200 is one of a handful of projects looking to answer a fundamental question in particle physics: Are neutrinos and antineutrinos the same thing?

    The neutrino that wasn’t there

    Each type of particle has its own nemesis: its antimatter partner. Electrons have positrons—which have the same mass but opposite electric charge—quarks have antiquarks and protons have antiprotons. When a particle meets its antimatter version, the result is often mutual annihilation. Neutrinos may also have antimatter counterparts, known as antineutrinos. However, unlike electrons and quarks, neutrinos are electrically neutral, so antineutrinos look a lot like neutrinos in many circumstances.

    In fact, one hypothesis is that they are one and the same. To test this, EXO-200 uses 110 kilograms of liquid xenon (of its 200kg total) as both a particle source and particle detector. The experiment hinges on a process called double beta decay, in which an isotope of xenon has two simultaneous decays, spitting out two electrons and two antineutrinos. (“Beta particle” is a nuclear physics term for electrons and positrons.)

    If neutrinos and antineutrinos are the same thing, sometimes the result will be neutrinoless double beta decay. In that case, the antineutrino from one decay is absorbed by the second decay, canceling out what would normally be another antineutrino emission. The challenge is to determine if neutrinos are there or not, without being able to detect them directly.

    “Neutrinoless double beta decay is kind of a nuclear physics trick to answer a particle physics problem,” says Michelle Dolinski, one of the spokespeople for EXO-200 and a physicist at Drexel University. It’s not an easy experiment to do.

    EXO-200 and similar experiments look for indirect signs of neutrinoless double beta decay. Most of the xenon atoms in EXO-200 are a special isotope containing 82 neutrons, four more than the most common version found in nature. The isotope decays by emitting two electrons, changing the atom from xenon into barium. Detectors in the EXO-200 experiment collect the electrons and measure the light produced when the beta particles are stopped in the xenon. These measurements together are what determine whether double beta decay happened, and whether the decay was likely to be neutrinoless.

    EXO-200 isn’t the only neutrinoless double beta decay experiment, but many of the others use solid detectors instead of liquid xenon. Dolinski got her start on the CUORE experiment, a large solid-state detector, but later changed directions in her research.

    CUORE experiment UC Berkeley
    CUORE experiment UC Berkeley

    “I joined EXO-200 as a postdoc in 2008 because I thought that the large liquid detectors were a more scalable solution,” she says. “If you want a more sensitive liquid-state experiment, you can build a bigger tank and fill it with more xenon.”

    Neutrinoless or not, double beta decay is very rare. A given xenon atom decays randomly, with an average lifetime of a quadrillion times the age of the universe. However, if you use a sufficient number of atoms, a few of them will decay while your experiment is running.

    “We need to sample enough nuclei so that you would detect these putative decays before the researcher retires,” says Martin Breidenbach, one of the EXO-200 project leaders and a physicist at the Department of Energy’s SLAC National Accelerator Laboratory.

    But the experiment is not just detecting neutrinoless events. Heavier neutrinos mean more frequent decays, so measuring the rate reveals the neutrino mass — something very hard to measure otherwise.

    Prior runs of EXO-200 and other experiments failed to see neutrinoless double beta decay, so either neutrinos and antineutrinos aren’t the same particle after all, or the neutrino mass is small enough to make decays too rare to be seen during the experiment’s lifetime. The current limit for the neutrino mass is less than 0.38 electronvolts—for comparison, electrons are about 500,000 electronvolts in mass.

    2
    SLAC National Accelerator Laboratory’s Jon Davis checks the enriched xenon storage bottles before the refilling of the TPC. Brian Dozier, Los Alamos National Laboratory

    Working in the salt mines

    Cindy Lin is a Drexel University graduate student who spends part of her time working on the EXO-200 detector at the mine. Getting to work is fairly involved.

    “In the morning we take the cage elevator half a mile down to the mine,” she says. Additionally, she and the other workers at WIPP have to take a 40-hour safety training to ensure their wellbeing, and wear protective gear in addition to normal lab clothes.

    “As part of the effort to minimize salt dust particles in our cleanroom, EXO-200 scientists also cover our hair and wear coveralls,” Lin adds.

    The sheer amount of earth over the detector shields it from electrons and other charged particles from space, which would make it too hard to spot the signal from double beta decay. WIPP is carved out of a sodium chloride deposit—the same stuff as table salt—that has very little uranium or the other radioactive minerals you find in solid rock caverns. But it has its drawbacks, too.

    “Salt is very dynamic: It moves at the level of centimeters a year, so you can’t build a nice concrete structure,” says Breidenbach. To compensate, the EXO-200 team has opted for a more modular design.

    The inadvertent shutdown provided extra challenges. EXO-200, like most experiments, isn’t well suited for being neglected for more than a few days at a time. However, Lin and other researchers worked hard to get the equipment running for new data this year, and the downtime also allowed researchers to install some upgraded equipment.

    The next phase of the experiment, nEXO, is at a conceptual stage based on what has been learned from EXO200. Experimenters are considering the benefits of moving the project deeper underground, perhaps at a facility like the Sudbury Neutrino Observatory (SNOlab) in Canada.
    SNOLAB, Sudbury, Ontario, Canada.
    SNOLAB
    SNOLAB, Sudbury, Ontario, Canada

    Dolinski is optimistic that if there are any neutrinoless double beta decays to see, nEXO or similar experiments should see them in the next 15 years or so.

    Then, maybe we’ll know if neutrinos and antineutrinos are the same and find out more about these weird low-mass particles.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 7:45 pm on April 28, 2016 Permalink | Reply
    Tags: , , Symmetry Magazine   

    From Symmetry: “A GUT feeling about physics” 

    Symmetry Mag

    Symmetry

    04/28/16
    Matthew R. Francis

    Scientists want to connect the fundamental forces of nature in one Grand Unified Theory.

    1
    Artwork by Sandbox Studio, Chicago

    The 1970s were a heady time in particle physics. New accelerators in the United States and Europe turned up unexpected particles that theorists tried to explain, and theorists in turn predicted new particles for experiments to hunt. The result was the Standard Model of particles and interactions, a theory that is essentially a catalog of the fundamental bits of matter and the forces governing them.

    While that Standard Model is a very good description of the subatomic world, some important aspects—such as particle masses—come out of experiments rather than theory.

    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    “If you write down the Standard Model, quite frankly it’s a mess,” says John Ellis, a particle physicist at King’s College London. “You’ve got a whole bunch of parameters, and they all look arbitrary. You can’t convince me that’s the final theory!”

    The hunt was on to create a grand unified theory, or GUT, that would elegantly explain how the universe works by linking three of the four known forces together. Physicists first linked the electromagnetic force, which dictates the structure of atoms and the behavior of light, and the weak nuclear force, which underlies how particles decay.

    But they didn’t want to stop there. Scientists began working to link this electroweak theory with the strong force, which binds quarks together into things like the protons and neutrons in our atoms. (The fourth force that we know, gravity, doesn’t have a complete working quantum theory, so it’s relegated to the realm of Theories of Everything, or ToEs.)

    Linking the different forces into a single theory isn’t easy, since each behaves a different way. Electromagnetism is long-ranged, the weak force is short-ranged, and the strong force is weak in high-energy environments such as the early universe and strong where energy is low. To unify these three forces, scientists have to explain how they can be aspects of a single thing and yet manifest in radically different ways in the real world.

    The electroweak theory unified the electromagnetic and weak forces by proposing they were aspects of a single interaction that is present only at very high energies, as in a particle accelerator or the very early universe. Above a certain threshold known as the electroweak scale, there is no difference between the two forces, but that unity is broken when the energy drops below a certain point.

    The GUTs developed in the mid-1970s to incorporate the strong force predicted new particles, just as the electroweak theory had before. In fact, the very first GUT showed a relationship between particle masses that allowed physicists to make predictions about the second-heaviest particle before it was detected experimentally.

    “We calculated the mass of the bottom quark before it was discovered,” says Mary Gaillard, a particle physicist at University of California, Berkeley. Scientists at Fermilab would go on to find the particle in 1977.

    GUTs also predicted that protons should decay into lighter particles. There was just one problem: Experiments didn’t see that decay.

    The problem with protons

    GUTs predicted that all quarks could potentially change into lighter particles, including the quarks making up protons. In fact, GUTs said that protons would be unstable over a period much longer than the lifetime of the universe. To maximize the chances of seeing that rare proton decay, physicists needed to build detectors with a lot of atoms.

    However, the first Kamiokande experiment in Japan didn’t detect any proton decays, which meant a proton lifetime longer than that predicted by the simplest GUT theory. More complicated GUTs emerged with longer predicted proton lifetimes – and more complicated interactions and additional particles.

    Super-Kamiokande Detector
    Super-Kamiokande Detector

    More complicated GUTs emerged with longer predicted proton lifetimes – and more complicated interactions and additional particles.

    Most modern GUTs mix in supersymmetry (SUSY), a way of thinking about the structure of space-time that has profound implications for particle physics. SUSY uses extra interactions to adjust the strength of the three forces in the Standard Model so that they meet at a very high energy known as the GUT scale.

    Standard model of Supersymmetry Illustration: CERN & IES de SAR
    Standard model of Supersymmetry Illustration: CERN & IES de SAR

    “Supersymmetry gives more particles that are involved via virtual quantum effects in the decay of the proton,” says JoAnne Hewett, a physicist at the Department of Energy’s SLAC National Accelerator Laboratory. That extends the predicted lifetime of the proton beyond what previous experiments were able to test. Yet SUSY-based GUTs also have some problems.

    “They’re kinda messy,” Gaillard says. Particularly, these theories predict more Higgs-like particles and different ways the Higgs boson from the Standard Model should behave. For that reason, Gaillard and other physicists are less enamored of GUTs than they were in the 1970s and ’80s. To make matters worse, no supersymmetric particles have been found yet. But the hunt is still on.

    “The basic philosophical impulse for grand unification is still there, just as important as ever,” Ellis says. “I still love SUSY, and I also am enamored of GUTs.”

    Hewett agrees that GUTs aren’t dead yet.

    “I firmly believe that an observation of proton decay would affect how every person would think about the world,” she says. “Everybody can understand that we’re made out of protons and ‘Oh wow! They decay.’”

    Upcoming experiments like the proposed Hyper-K in Japan and the Deep Underground Neutrino Experiment in the United States will probe proton decay to greater precision than ever.

    Hyper-Kamiokande
    Hyper-Kamiokande

    FNAL LBNF/DUNE
    FNAL LBNF/DUNE

    Seeing a proton decay will tell us something about the unification of the forces of nature and whether we ultimately can trust our GUTs.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 3:03 pm on April 19, 2016 Permalink | Reply
    Tags: , , Symmetry Magazine, The wonders of photons of light   

    From Symmetry: “Eight things you might not know about light” 

    Symmetry Mag

    Symmetry

    04/19/16
    Matthew R. Francis

    Light is all around us, but how much do you really know about the photons speeding past you?

    1
    Illustration by Sandbox Studio, Chicago with Kimberly Boustead

    There’s more to light than meets the eye. Here are eight enlightening facts about photons:

    1. Photons can produce shock waves in water or air, similar to sonic booms.

    Nothing can travel faster than the speed of light in a vacuum. However, light slows down in air, water, glass and other materials as photons interact with atoms, which has some interesting consequences.

    The highest-energy gamma rays from space hit Earth’s atmosphere moving faster than the speed of light in air.

    Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions
    Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions.

    NASA/Fermi Telescope
    NASA/Fermi Telescope

    These photons produce shock waves in the air, much like a sonic boom, but the effect is to make more photons instead of sound. Observatories like VERITAS in Arizona look for those secondary photons, which are known as Cherenkov radiation. Nuclear reactors also exhibit Cherenkov light in the water surrounding the nuclear fuel.

    CfA/VERITAS
    CfA/VERITAS Cherenkov telescope installation

    Nuclear reactors also exhibit Cherenkov light in the water surrounding the nuclear fuel.

    2. Most types of light are invisible to our eyes.

    Colors are our brains’ way of interpreting the wavelength of light: how far the light travels before the wave pattern repeats itself. But the colors we see—called “visible” or “optical” light—are only a small sample of the total electromagnetic spectrum.

    Red is the longest wavelength light we see, but stretch the waves more and you get infrared, microwaves (including the stuff you cook with) and radio waves. Wavelengths shorter than violet span ultraviolet, X-rays and gamma rays. Wavelength is also a stand-in for energy: The long wavelengths of radio light have low energy, and the short-wavelength gamma rays have the highest energy, a major reason they’re so dangerous to living tissue.

    3. Scientists can perform measurements on single photons.

    Light is made of particles called photons, bundles of the electromagnetic field that carry a specific amount of energy. With sufficiently sensitive experiments, you can count photons or even perform measurements on a single one. Researchers have even frozen light temporarily.

    But don’t think of photons like they are pool balls. They’re also wave-like: they can interfere with each other to produce patterns of light and darkness. The photon model was one of the first triumphs of quantum physics; later work showed that electrons and other particles of matter also have wave-like properties.

    4. Photons from particle accelerators are used in chemistry and biology.

    Visible light’s wavelengths are larger than atoms and molecules, so we literally can’t see the components of matter. However, the short wavelengths of X-rays and ultraviolet light are suited to showing such small structure. With methods to see these high-energy types of light, scientists get a glimpse of the atomic world.

    Particle accelerators can make photons of specific wavelengths by accelerating electrons using magnetic fields; this is called “synchrotron radiation.” Researchers use particle accelerators to make X-rays and ultraviolet light to study the structure of molecules and viruses and even make movies of chemical reactions.

    CERN Proton Synchrotron
    CERN Proton Synchrotron

    5. Light is the manifestation of one of the four fundamental forces of nature.

    Photons carry the electromagnetic force, one of the four fundamental forces (along with the weak force, the strong force, and gravity). As an electron moves through space, other charged particles feel it thanks to electrical attraction or repulsion. Because the effect is limited by the speed of light, other particles actually react to where the electron was rather than where it actually is. Quantum physics explains this by describing empty space as a seething soup of virtual particles. Electrons kick up virtual photons, which travel at the speed of light and hit other particles, exchanging energy and momentum.

    6. Photons are easily created and destroyed.

    Unlike matter, all sorts of things can make or destroy photons. If you’re reading this on a computer screen, the backlight is making photons that travel to your eye, where they are absorbed—and destroyed.

    The movement of electrons is responsible for both the creation and destruction of the photons, and that’s the case for a lot of light production and absorption. An electron moving in a strong magnetic field will generate photons just from its acceleration.

    Similarly, when a photon of the right wavelength strikes an atom, it disappears and imparts all its energy to kicking the electron into a new energy level. A new photon is created and emitted when the electron falls back into its original position. The absorption and emission are responsible for the unique spectrum of light each type of atom or molecule has, which is a major way chemists, physicists, and astronomers identify chemical substances.

    7. When matter and antimatter annihilate, light is a byproduct.

    An electron and a positron have the same mass, but opposite quantum properties such as electric charge. When they meet, those opposites cancel each other, converting the masses of the particles into energy in the form of a pair of gamma ray photons.

    8. You can collide photons to make particles.

    Photons are their own antiparticles. But here’s the fun bit: the laws of physics governing photons are symmetric in time. That means if we can collide an electron and a positron to get two gamma ray photons, we should be able to collide two photons of the right energy and get an electron-positron pair.

    In practice that’s hard to do: successful experiments generally involve other particles than just light. However, inside the LHC, the sheer number of photons produced during collisions of protons means that some of them occasionally hit each other.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Some physicists are thinking about building a photon-photon collider, which would fire beams of photons into a cavity full of other photons to study the particles that come out of collisions.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 3:23 pm on April 7, 2016 Permalink | Reply
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    From Symmetry: “Physicists build ultra-powerful accelerator magnet” 

    Symmetry Mag

    Symmetry

    04/07/16
    Sarah Charley

    Magnet built for LHC

    The next generation of cutting-edge accelerator magnets is no longer just an idea. Recent tests revealed that the United States and CERN have successfully co-created a prototype superconducting accelerator magnet that is much more powerful than those currently inside the Large Hadron Collider.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    CERN/LHC

    Engineers will incorporate more than 20 magnets similar to this model into the next iteration of the LHC, which will take the stage in 2026 and increase the LHC’s luminosity by a factor of ten. That translates into a ten-fold increase in the data rate.

    “Building this magnet prototype was truly an international effort,” says Lucio Rossi, the head of the High-Luminosity (HighLumi) LHC project at CERN. “Half the magnetic coils inside the prototype were produced at CERN, and half at laboratories in the United States.”

    During the original construction of the Large Hadron Collider, US Department of Energy national laboratories foresaw the future need for stronger LHC magnets and created the LHC Accelerator Research Program (LARP): an R&D program committed to developing new accelerator technology for future LHC upgrades.

    MQXF1 quadrupole 1.5-meter prototype magnet sits at Fermilab before testing.
    MQXF1 quadrupole 1.5-meter prototype magnet sits at Fermilab before testing. G. Ambrosio (US-LARP and Fermilab), P. Ferracin and E. Todesco (CERN TE-MSC)

    This 1.5-meter-long model, which is a fully functioning accelerator magnet, was developed by scientists and engineers at Fermilab [FNAL], Brookhaven National Laboratory [BNL], Lawrence Berkeley National Laboratory [LBL], and CERN.

    FNAL II photo
    FNAL

    BNL Logo (2)
    BNL

    LBL Big
    LBL

    CERN
    CERN

    The magnet recently underwent an intense testing program at Fermilab, which it passed in March with flying colors. It will now undergo a rigorous series of endurance and stress tests to simulate the arduous conditions inside a particle accelerator.

    This new type of magnet will replace about 5 percent of the LHC’s focusing and steering magnets when the accelerator is converted into the High-Luminosity LHC, a planned upgrade which will increase the number and density of protons packed inside the accelerator. The HL-LHC upgrade will enable scientists to collect data at a much faster rate.

    The LHC’s magnets are made by repeatedly winding a superconducting cable into long coils. These coils are then installed on all sides of the beam pipe and encased inside a superfluid helium cryogenic system. When cooled to 1.9 Kelvin, the coils can carry a huge amount of electrical current with zero electrical resistance. By modulating the amount of current running through the coils, engineers can manipulate the strength and quality of the resulting magnetic field and control the particles inside the accelerator.

    The magnets currently inside the LHC are made from niobium titanium, a superconductor that can operate inside a magnetic field of up to 10 teslas before losing its superconducting properties. This new magnet is made from niobium-three tin (Nb3Sn), a superconductor capable of carrying current through a magnetic field of up to 20 teslas.

    “We’re dealing with a new technology that can achieve far beyond what was possible when the LHC was first constructed,” says Giorgio Apollinari, Fermilab scientist and Director of US LARP. “This new magnet technology will make the HL-LHC project possible and empower physicists to think about future applications of this technology in the field of accelerators.”

    High-Luminosity LHC coil
    High-Luminosity LHC coil similar to those incorporated into the successful magnet prototype shows the collaboration between CERN and the LHC Accelerator Research Program, LARP.
    Photo by Reidar Hahn, Fermilab

    This technology is powerful and versatile—like upgrading from a moped to a motorcycle. But this new super material doesn’t come without its drawbacks.

    “Niobium-three tin is much more complicated to work with than niobium titanium,” says Peter Wanderer, head of the Superconducting Magnet Division at Brookhaven National Lab. “It doesn’t become a superconductor until it is baked at 650 degrees Celsius. This heat-treatment changes the material’s atomic structure and it becomes almost as brittle as ceramic.”

    Building a moose-sized magnet from a material more fragile than a teacup is not an easy endeavor. Scientists and engineers at the US national laboratories spent 10 years designing and perfecting a new and internationally reproducible process to wind, form, bake and stabilize the coils.

    “The LARP-CERN collaboration works closely on all aspects of the design, fabrication and testing of the magnets,” says Soren Prestemon of the Berkeley Center for Magnet Technology at Berkeley Lab. “The success is a testament to the seamless nature of the collaboration, the level of expertise of the teams involved, and the ownership shown by the participating laboratories.”

    This model is a huge success for the engineers and scientists involved. But it is only the first step toward building the next big supercollider.

    “This test showed that it is possible,” Apollinari says. “The next step is it to apply everything we’ve learned moving from this prototype into bigger and bigger magnets.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 9:36 am on April 5, 2016 Permalink | Reply
    Tags: , , , Symmetry Magazine   

    From Symmetry: “Six weighty facts about gravity” 

    Symmetry Mag

    Symmetry

    1
    Illustration by Sandbox Studio, Chicago with Ana Kova

    04/05/16
    Matthew R. Francis

    Perplexed by gravity? Don’t let it get you down.

    Gravity: we barely ever think about it, at least until we slip on ice or stumble on the stairs. To many ancient thinkers, gravity wasn’t even a force—it was just the natural tendency of objects to sink toward the center of Earth, while planets were subject to other, unrelated laws.

    Of course, we now know that gravity does far more than make things fall down. It governs the motion of planets around the Sun, holds galaxies together and determines the structure of the universe itself. We also recognize that gravity is one of the four fundamental forces of nature, along with electromagnetism, the weak force and the strong force.

    The modern theory of gravity—Einstein’s general theory of relativity—is one of the most successful theories we have. At the same time, we still don’t know everything about gravity, including the exact way it fits in with the other fundamental forces. But here are six weighty facts we do know about gravity.

    2
    Illustration by Sandbox Studio, Chicago with Ana Kova

    1. Gravity is by far the weakest force we know.

    Gravity only attracts—there’s no negative version of the force to push things apart. And while gravity is powerful enough to hold galaxies together, it is so weak that you overcome it every day. If you pick up a book, you’re counteracting the force of gravity from all of Earth.

    For comparison, the electric force between an electron and a proton inside an atom is roughly one quintillion (that’s a one with 30 zeroes after it) times stronger than the gravitational attraction between them. In fact, gravity is so weak, we don’t know exactly how weak it is.

    3
    Illustration by Sandbox Studio, Chicago with Ana Kova

    2. Gravity and weight are not the same thing.

    Astronauts on the space station float, and sometimes we lazily say they are in zero gravity. But that’s not true. The force of gravity on an astronaut is about 90 percent of the force they would experience on Earth. However, astronauts are weightless, since weight is the force the ground (or a chair or a bed or whatever) exerts back on them on Earth.

    Take a bathroom scale onto an elevator in a big fancy hotel and stand on it while riding up and down, ignoring any skeptical looks you might receive. Your weight fluctuates, and you feel the elevator accelerating and decelerating, yet the gravitational force is the same. In orbit, on the other hand, astronauts move along with the space station. There is nothing to push them against the side of the spaceship to make weight. Einstein turned this idea, along with his special theory of relativity, into general relativity.

    3. Gravity makes waves that move at light speed.

    General relativity predicts gravitational waves.

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    If you have two stars or white dwarfs or black holes locked in mutual orbit, they slowly get closer as gravitational waves carry energy away. In fact, Earth also emits gravitational waves as it orbits the sun, but the energy loss is too tiny to notice.

    We’ve had indirect evidence for gravitational waves for 40 years, but the Laser Interferometer Gravitational-wave Observatory (LIGO) only confirmed the phenomenon this year.

    MIT/Caltech Advanced aLIGO Hanford Washington USA installation
    MIT/Caltech Advanced aLIGO Hanford Washington USA installation

    The detectors picked up a burst of gravitational waves produced by the collision of two black holes more than a billion light-years away.

    Black holes merging Swinburne Astronomy Productions
    Black holes merging Swinburne Astronomy Productions

    One consequence of relativity is that nothing can travel faster than the speed of light in vacuum. That goes for gravity, too: If something drastic happened to the sun, the gravitational effect would reach us at the same time as the light from the event.

    4. Explaining the microscopic behavior of gravity has thrown researchers for a loop.

    The other three fundamental forces of nature are described by quantum theories at the smallest of scales— specifically, the Standard Model. However, we still don’t have a fully working quantum theory of gravity, though researchers are trying.

    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    One avenue of research is called loop quantum gravity, which uses techniques from quantum physics to describe the structure of space-time. It proposes that space-time is particle-like on the tiniest scales, the same way matter is made of particles. Matter would be restricted to hopping from one point to another on a flexible, mesh-like structure. This allows loop quantum gravity to describe the effect of gravity on a scale far smaller than the nucleus of an atom.

    A more famous approach is string theory, where particles—including gravitons—are considered to be vibrations of strings that are coiled up in dimensions too small for experiments to reach. Neither loop quantum gravity nor string theory, nor any other theory is currently able to provide testable details about the microscopic behavior of gravity.

    4
    Illustration by Sandbox Studio, Chicago with Ana Kova

    5. Gravity might be carried by massless particles called gravitons.

    In the Standard Model, particles interact with each other via other force-carrying particles. For example, the photon is the carrier of the electromagnetic force. The hypothetical particles for quantum gravity are gravitons, and we have some ideas of how they should work from general relativity. Like photons, gravitons are likely massless. If they had mass, experiments should have seen something—but it doesn’t rule out a ridiculously tiny mass.

    6. Quantum gravity appears at the smallest length anything can be.

    Gravity is very weak, but the closer together two objects are, the stronger it becomes. Ultimately, it reaches the strength of the other forces at a very tiny distance known as the Planck length, many times smaller than the nucleus of an atom.

    That’s where quantum gravity’s effects will be strong enough to measure, but it’s far too small for any experiment to probe. Some people have proposed theories that would let quantum gravity show up at close to the millimeter scale, but so far we haven’t seen those effects. Others have looked at creative ways to magnify quantum gravity effects, using vibrations in a large metal bar or collections of atoms kept at ultracold temperatures.

    It seems that, from the smallest scale to the largest, gravity keeps attracting scientists’ attention. Perhaps that’ll be some solace the next time you take a tumble, when gravity grabs your attention too.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:28 am on March 29, 2016 Permalink | Reply
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    From Symmetry: “The Milky Way’s hot spot” 

    Symmetry Mag

    Symmetry

    03/29/16
    Ali Sundermier

    Credits: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)
    Milky Way map. Credits: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

    When you look up at night, the Milky Way appears as a swarm of stars arranged in a misty white band across the sky.

    But from an outside perspective, our galaxy looks more like a disk, with spiral arms of stars reaching out into the universe. At the center of this disk is a small region around which the entire pinwheel of our galaxy rotates, a region packed with exotic astronomical phenomena ranging from dark matter and newborn stars to a supermassive black hole. Astronomers call this region of the Milky Way the galactic center.

    SGR A* NASA's Chandra X-Ray Observatory
    Milky Way’s supermassive black hole SGR A* NASA’s Chandra X-Ray Observatory

    It’s a strange neighborhood, and scientists have reason to believe it’s one of the best places to hunt for dark matter.

    1
    The Spitzer Space Telescope provides an infrared view of the galactic center region.
    Courtesy of: NASA/JPL-Caltech/ESA/CXC/STScI

    NASA/Spitzer Telescope
    NASA/Spitzer Telescope

    Phenomena in our galaxy’s heart

    In the ’70s, scientists hypothesized that a supermassive black hole might be lurking in the center of the Milky Way. Black holes are points of space-time where gravity is so strong that not even light can escape.

    After decades of trying to indirectly identify the mysterious object in the galactic center by tracing the orbits of stars and gas, astronomers were finally able to calculate its mass in 2008. It weighed more than 4 million times as much as the sun, making it a prime supermassive black hole candidate.

    About 10 percent of all new star formation in the galaxy occurs in the galactic center. This is strange because local conditions produce an extreme environment in which it should be difficult for stars to form.

    Scientists believe that at least some of the new stars being formed should explode and transform into pulsars, but they aren’t seeing any. Pulsars emit a regular pulsating signal, like a lighthouse. One early explanation for the apparent lack of pulsars in the galactic center was that the magnetic fields there could be bending their radio waves on their way to us, hiding their pulsating signals. But recently scientists measured the strength of the fields and realized the bending was much less than they had anticipated. The mystery of the missing pulsars remains unsolved.

    The galactic center also has a notably high concentration of cosmic rays, high-energy charged particles that hurtle through outer space. Scientists still don’t understand where these particles come from or how they reach such intense energies.

    2
    The Hubble Space Telescope, though better known for its visible light images, also captured an infrared light picture of the galactic center (the bright patch in the lower right).
    Courtesy of: NASA/JPL-Caltech/ESA/CXC/STScI

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    Hunting for dark matter

    We know that the Milky Way is rotating because when we look along it, we see some stars moving towards us and some stars moving away. But the speed at which our galaxy rotates is faster than it should be for the amount of matter we can see.

    This leads scientists to believe that there is matter located in the center of our galaxy that we cannot see. Despite all of the other stuff going on there, this makes the inner galaxy the perfect hunting ground for this “dark matter,” an invisible substance that makes up most of the matter in the universe.

    Scientists looking for dark matter take advantage of the fact that it likely interacts with itself. Researchers predict that when dark matter particles run into each other, they annihilate. They believe that this might produce a distinctive spectrum of gamma rays.

    Over the past few years, scientists have detected an excess of gamma rays from the Milky Way’s galactic center. Many scientists believe that this could be a very strong signal for dark matter. The events look the way they would expect dark matter to look, and the energy spectrum and the way the gamma rays are concentrated resemble what scientists would expect from dark matter.

    Other scientists believe that it is pulsars, not dark matter, that create this signal. Because the excess appears clumped, instead of smooth, scientists believe that it could be coming from compact sources like an ancient population of pulsars.

    To determine whether this excess is a dark matter signal, scientists are looking for similar signatures elsewhere in the universe, in places like dwarf galaxies. These small galaxies are cleaner places to look for dark matter with a lot less going on, but the trade-off is that they do not produce as much gamma radiation.

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
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