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  • richardmitnick 12:10 pm on April 23, 2019 Permalink | Reply
    Tags: "Falsifiability and physics", , , , , , Einstein’s theory of General Relativity, , Karl Popper (1902-1994) "The Logic of Scientific Discovery", , ,   

    From Symmetry: “Falsifiability and physics” 

    Symmetry Mag
    From Symmetry

    04/23/19
    Matthew R. Francis

    1
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Can a theory that isn’t completely testable still be useful to physics?

    What determines if an idea is legitimately scientific or not? This question has been debated by philosophers and historians of science, working scientists, and lawyers in courts of law. That’s because it’s not merely an abstract notion: What makes something scientific or not determines if it should be taught in classrooms or supported by government grant money.

    The answer is relatively straightforward in many cases: Despite conspiracy theories to the contrary, the Earth is not flat. Literally all evidence is in favor of a round and rotating Earth, so statements based on a flat-Earth hypothesis are not scientific.

    In other cases, though, people actively debate where and how the demarcation line should be drawn. One such criterion was proposed by philosopher of science Karl Popper (1902-1994), who argued that scientific ideas must be subject to “falsification.”

    Popper wrote in his classic book The Logic of Scientific Discovery that a theory that cannot be proven false—that is, a theory flexible enough to encompass every possible experimental outcome—is scientifically useless. He wrote that a scientific idea must contain the key to its own downfall: It must make predictions that can be tested and, if those predictions are proven false, the theory must be jettisoned.

    When writing this, Popper was less concerned with physics than he was with theories like Freudian psychology and Stalinist history. These, he argued, were not falsifiable because they were vague or flexible enough to incorporate all the available evidence and therefore immune to testing.

    But where does this falsifiability requirement leave certain areas of theoretical physics? String theory, for example, involves physics on extremely small length scales unreachable by any foreseeable experiment.

    String Theory depiction. Cross section of the quintic Calabi–Yau manifold Calabi yau.jpg. Jbourjai (using Mathematica output)

    Cosmic inflation, a theory that explains much about the properties of the observable universe, may itself be untestable through direct observations.

    Some critics believe these theories are unfalsifiable and, for that reason, are of dubious scientific value.

    At the same time, many physicists align with philosophers of science who identified flaws in Popper’s model, saying falsification is most useful in identifying blatant pseudoscience (the flat-Earth hypothesis, again) but relatively unimportant for judging theories growing out of established paradigms in science.

    “I think we should be worried about being arrogant,” says Chanda Prescod-Weinstein of the University of New Hampshire. “Falsifiability is important, but so is remembering that nature does what it wants.”

    Prescod-Weinstein is both a particle cosmologist and researcher in science, technology, and society studies, interested in analyzing the priorities scientists have as a group. “Any particular generation deciding that they’ve worked out all that can be worked out seems like the height of arrogance to me,” she says.

    Tracy Slatyer of MIT agrees, and argues that stringently worrying about falsification can prevent new ideas from germinating, stifling creativity. “In theoretical physics, the vast majority of all the ideas you ever work on are going to be wrong,” she says. “They may be interesting ideas, they may be beautiful ideas, they may be gorgeous structures that are simply not realized in our universe.”

    Particles and practical philosophy

    Take, for example, supersymmetry. SUSY is an extension of the Standard Model in which each known particle is paired with a supersymmetric partner.

    Standard Model of Supersymmetry via DESY

    The theory is a natural outgrowth of a mathematical symmetry of spacetime, in ways similar to the Standard Model itself. It’s well established within particle physics, even though supersymmetric particles, if they exist, may be out of scientists’ experimental reach.

    SUSY could potentially resolve some major mysteries in modern physics. For one, all of those supersymmetric particles could be the reason the mass of the Higgs boson is smaller than quantum mechanics says it should be.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    “Quantum mechanics says that [the Higgs boson] mass should blow up to the largest mass scale possible,” says Howard Baer of the University of Oklahoma. That’s because masses in quantum theory are the result of contributions from many different particles involved in interactions—and the Higgs field, which gives other particles mass, racks up a lot of these interactions. But the Higgs mass isn’t huge, which requires an explanation.

    “Something else would have to be tuned to a huge negative [value] in order to cancel [the huge positive value of those interactions] and give you the observed value,” Baer says. That level of coincidence, known as a “fine-tuning problem,” makes physicists itchy. “It’s like trying to play the lottery. It’s possible you might win, but really you’re almost certain to lose.”

    If SUSY particles turn up in a certain mass range, their contributions to the Higgs mass “naturally” solve this problem, which has been an argument in favor of the theory of supersymmetry. So far, the Large Hadron Collider has not turned up any SUSY particles in the range of “naturalness.”

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    However, the broad framework of supersymmetry can accommodate even more massive SUSY particles, which may or may not be detectable using the LHC. In fact, if naturalness is abandoned, SUSY doesn’t provide an obvious mass scale at all, meaning SUSY particles might be out of range for discovery with any earthly particle collider. That point has made some critics queasy: If there’s no obvious mass scale at which colliders can hunt for SUSY, is the theory falsifiable?

    A related problem confronts dark matter researchers: Despite strong indirect evidence for a large amount of mass invisible to all forms of light, particle experiments have yet to find any dark matter particles. It could be that dark matter particles are just impossible to directly detect. A small but vocal group of researchers has argued that we need to consider alternative theories of gravity instead.

    Fritz Zwicky, the Father of Dark Matter research.No image credit after long search

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    U Washington ADMX Axion Dark Matter Experiment

    DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

    Dark Side-50 Dark Matter Experiment at Gran Sasso

    Slatyer, whose research involves looking for dark matter, considers the criticism partly as a problem of language. “When you say ‘dark matter,’ [you need] to distinguish dark matter from specific scenarios for what dark matter could be,” she says. “The community has not always done that well.”

    In other words, specific models for dark matter can stand or fall, but the dark matter paradigm as a whole has withstood all tests so far. But as Slatyer points out, no alternative theory of gravity can explain all the phenomena that a simple dark matter model can, from the behavior of galaxies to the structure of the cosmic microwave background.

    Prescod-Weinstein argues that we’re a long way from ruling out all dark matter possibilities. “How will we prove that the dark matter, if it exists, definitively doesn’t interact with the Standard Model?” she says. “Astrophysics is always a bit of a detective game. Without laboratory [detection of] dark matter, it’s hard to make definitive statements about its properties. But we can construct likely narratives based on what we know about its behavior.”

    Similarly, Baer thinks that we haven’t exhausted all the SUSY possibilities yet. “People say, ‘you’ve been promising supersymmetry for 20 or 30 years,’ but it was based on overly optimistic naturalness calculations,” he says. “I think if one evaluates the naturalness properly, then you find that supersymmetry is still even now very natural. But you’re going to need either an energy upgrade of LHC or an ILC [International Linear Collider] in order to discover it.”

    ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan

    Beyond falsifiability of dark matter or SUSY, physicists are motivated by more mundane concerns. “Even if these individual scenarios are in principle falsifiable, how much money would [it] take and how much time would it take?” Slatyer says. In other words, rather than try to demonstrate or rule out SUSY as a whole, physicists focus on particle experiments that can be performed within a certain number of budgetary cycles. It’s not romantic, but it’s true nevertheless.

    2
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Is it science? Who decides?

    Historically, sometimes theories that seem untestable turn out to just need more time. For example, 19th century physicist Ludwig Boltzmann and colleagues showed they could explain many results in thermal physics and chemistry if everything were made up of “atoms”—what we call particles, atoms, and molecules today—governed by Newtonian physics.

    Since atoms were out of reach of experiments of the day, prominent philosophers of science argued that the atomic hypothesis was untestable in principle, and therefore unscientific.

    However, the atomists eventually won the day: J. J. Thompson demonstrated the existence of electrons, while Albert Einstein showed that water molecules could make grains of pollen dance on a pond’s surface.

    Atoms provide a case study for how falsifiability proved to be the wrong criterion. Many other cases are trickier.

    For instance, Einstein’s theory of general relativity is one of the best-tested theories in all of science. At the same time, it allows for physically unrealistic “universes,” such as a “rotating” cosmos where movement back and forth in time is possible, which are contradicted by all observations of the reality we inhabit.

    General relativity also makes predictions about things that are untestable by definition, like how particles move inside the event horizon of a black hole: No information about these trajectories can be determined by experiment.

    The first image of a black hole, Messier 87 Credit Event Horizon Telescope Collaboration, via NSF 4.10.19

    Yet no knowledgeable physicist or philosopher of science would argue that general relativity is unscientific. The success of the theory is due to enough of its predictions being testable.

    Eddington/Einstein exibition of gravitational lensing solar eclipse of 29 May 1919

    Another type of theory may be mostly untestable, but have important consequences. One such theory is cosmic inflation, which (among other things) explains why we don’t see isolated magnetic monopoles and why the universe is a nearly uniform temperature everywhere we look.

    The key property of inflation—the extremely rapid expansion of spacetime during a tiny split second after the Big Bang—cannot be tested directly. Cosmologists look for indirect evidence for inflation, but in the end it may be difficult or impossible to distinguish between different inflationary models, simply because scientists can’t get the data. Does that mean it isn’t scientific?

    Inflation

    4
    Alan Guth, from Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    Alan Guth’s notes:
    5

    “A lot of people have personal feelings about inflation and the aesthetics of physical theories,” Prescod-Weinstein says. She’s willing to entertain alternative ideas which have testable consequences, but inflation works well enough for now to keep it around. “It’s also the case that the majority of the cosmology community continues to take inflation seriously as a model, so I have to shrug a little when someone says it’s not science.”

    On that note, Caltech cosmologist Sean M. Carroll argues that many very useful theories have both falsifiable and unfalsifiable predictions. Some aspects may be testable in principle, but not by any experiment or observation we can perform with existing technology. Many particle physics models fall into that category, but that doesn’t stop physicists from finding them useful. SUSY as a concept may not be falsifiable, but many specific models within the broad framework certainly are. All the evidence we have for the existence of dark matter is indirect, which won’t go away even if laboratory experiments never find dark matter particles. Physicists accept the concept of dark matter because it works.

    Slatyer is a practical dark matter hunter. “The questions I’m most interested asking are not even just questions that are in principle falsifiable, but questions that in principle can be tested by data on the timescale of less than my lifetime,” she says. “But it’s not only problems that can be tested by data on a timescale of ‘less than Tracy’s lifetime’ are good scientific questions!”

    Prescod-Weinstein agrees, and argues for keeping an open mind. “There’s a lot we don’t know about the universe, including what’s knowable about it. We are a curious species, and I think we should remain curious.”

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 9:23 pm on March 19, 2018 Permalink | Reply
    Tags: , , , , , Einstein’s theory of General Relativity, Gravastar model, ,   

    From Sky & Telescope: “Physicist Proposes Alternative to Black Holes” 

    SKY&Telescope bloc

    Sky & Telescope

    March 19, 2018
    Ben Skuse

    A physicist has incorporated a quantum mechanical idea with general relativity to arrive at a new alternative to black hole singularities.

    1
    An artist’s rendering of Cygnus X-1, an X-ray-emitting black hole that formed when a large star caved in. (We see its X-rays now as it feeds from its stellar companion.) But are black holes the inevitable next step after neutron stars? NASA / CXC / M.Weiss.

    What do you get when you cross two hypothetical alternatives to black holes? A self-consistent semiclassical relativistic star, according to Raúl Carballo-Rubio (International School for Advanced Studies, Trieste, Italy) whose recently published results in the February 6th Physical Review Letters describe a new mathematical model for the fate of massive stars.

    When a massive star comes to the end of its life, it goes supernova, leaving behind a dense core that — according to conventional thought — continues to collapse to form either a neutron star or black hole. To which fate a particular star is destined comes down to its mass. Neutron stars find a balance between the repulsive force of quantum mechanical degeneracy pressure and the attractive force of gravity, while more massive cores collapse into black holes, unable to fight the overwhelming pull of their own gravity.

    Repulsive Gravity

    Now, Carballo-Rubio adds an extra force into the mix: quantum fluctuations. Quantum mechanics has shown that virtual particles spontaneously pop into and out of existence — the effects can be measured best in a vacuum, but these fluctuations can happen anywhere in spacetime. These particles can be thought of as fluctuations of positive and negative energy that under normal conditions would cancel out. But the extreme gravity of compact objects breaks this balance, effectively generating negative energy. This negative energy creates a repulsive gravitational force.

    “The existence of quantum [fluctuations] due to gravitational fields has been known since the late 1970s,” explains Carballo-Rubio. But physicists didn’t know how to take this effect into account in collapsing stars.

    Carballo-Rubio derived equations that combine general relativity and quantum mechanics in a way that accounts for quantum fluctuations. Moreover, he found solutions that balance attractive and negative gravity for stellar masses that would otherwise have produced black holes. Dubbing them “semiclassical relativistic stars,” these compact objects do not fully collapse under their own weight to form an event horizon, and are therefore not black holes.

    Hybrid Star

    Interestingly, Carballo-Rubio’s semiclassical relativistic stars bear hallmarks of previously proposed black hole alternatives: gravastars and black stars.

    Gravastars and black stars also consist of ordinary matter and quantum fluctuations. But when these ideas were first conceived, equations incorporating quantum flluctuations were not yet known, so theorists Carballo-Rubio’s stars, on the other hand, emerge naturally from a consistent set of equations based on known physics.

    Gravastars and black stars are structured differently: In gravastar cores, large quantum fluctuations push ordinary matter outward to form an ultra-thin shell at the surface. Black stars, on the other hand, balance matter and the quantum fluctuations throughout their structure.

    Carballo-Rubio’s stars are like a hybrid of the two previous ideas. “On the one hand, both matter and the quantum [fluctuations] are present throughout the structure, as in the black star model,” he says. “On the other, the star displays two distinct elements, namely a core and an ultra-thin shell, as in the gravastar model.”

    2
    Artist’s drawing of a neutron star. Casey Reed / Penn State University.

    The Question of Stability

    Whether these hybrid stars exist in the real world is a matter of debate. Carballo-Rubio’s solutions do not incorporate time, so it isn’t clear if a such a star would remain stable or rapidly morph into something else . . . like a black hole.

    “Equilibrium solutions can be found for a pen standing on its tip,” remarks relativistic astrophysicist Luciano Rezzolla (Institute of Theoretical Physics, Germany). “Such a solution is obviously unstable to small perturbations.”

    However, if Carballo-Rubio can show that his semiclassical relativistic stars are indeed dynamically stable — which he will start work on next — the next generation of gravitational wave observatories should offer the level of precision necessary in the coming decades to distinguish unconventional compact bodies from black holes, potentially providing evidence to support the existence of this new type of star.

    See the full article here .

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    Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

    Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

    Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

    “Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”

     
  • richardmitnick 2:39 pm on December 7, 2017 Permalink | Reply
    Tags: A space mission to test how objects fall in a vacuum has released its first results providing an improved foundation for Einstein's famous theory, , , , , , Einstein’s theory of General Relativity, , , , The theory is fundamentally incompatible with another well-tested theory: quantum mechanics which describes the physics of the extremely small, The theory of general relativity and the conclusions it draws about gravity have been shown to be true wherever tested, This first result is going to shake the world of physics and will certainly lead to a revision of alternative theories to general relativity   

    From ICL: “European satellite confirms general relativity with unprecedented precision” 

    Imperial College London
    Imperial College London

    07 December 2017
    Hayley Dunning

    A space mission to test how objects fall in a vacuum has released its first results, providing an improved foundation for Einstein’s famous theory.

    The first results of the ‘Microscope’ satellite mission were announced this week by a group of researchers led by the French space agency CNES and including Imperial scientists. The findings are published in the journal Physical Review Letters.

    CNES Microscope satellite

    Launched in April 2016, the mission set out to test the ‘equivalence principle’, the founding assumption of Einstein’s theory of general relativity. The theory poses that gravity is not a ‘pulling’ force, but is the result of large bodies, like the Earth, bending spacetime.

    As a result, when two objects are dropped in a vacuum under the same force of gravity, they fall at the same rate, no matter what their difference in weight or composition. This principle was demonstrated by Apollo 15 astronaut David Scott, who dropped a hammer and a feather on the Moon and showed them both reaching the ground at the same time.

    However, dropping household objects on the lunar surface does not allow very precise measurements – it could be that they reach the ground fractions of a second apart. This is important for scientists to know, because if the equivalence principle does not hold absolutely, then it could provide clues to a unifying theory of physics.

    Finding a single theory

    The theory of general relativity, and the conclusions it draws about gravity, have been shown to be true wherever tested. However, the theory is fundamentally incompatible with another well-tested theory: quantum mechanics, which describes the physics of the extremely small.

    The major goal for 21st Century physics is a single theory that ties them all together neatly. Certain candidate theories predict that the equivalence principle may be violated at very weak levels.

    The new results have measured the equivalence principle with ten times the precision of any previous experiment, and show that objects in a vacuum fall with the same acceleration.

    Professor Timothy Sumner, from the Department of Physics at Imperial was involved in the early discussions for the project thirty years ago, which led to the current mission. He more recently joined the Science Working Team. Commenting on the latest results, he said: “The equivalence principle has proven unshakeable yet again.

    “This result is the first new measurement for several years and demonstrates the possibility of taking such difficult ‘laboratory’ experiments into the quiet and interference-free space environment. There will more results from this impressive experiment.”

    1,900 orbits of the Earth

    To test the principle, the Microscope satellite contains a series of ‘test masses’: blocks of metals of different weights with very precisely measured properties. These masses are isolated from any other influence and are monitored as they freefall in space while orbiting the Earth.

    This means their acceleration due to the freefall can be measured and compared to test the equivalence principle. If two test masses of equal size but different composition are accelerated differently during the freefall, then the equivalence principle is violated.

    The science phase of the mission began in December 2016 and has already collected data from 1,900 orbits of the Earth. This means that altogether the objects have been freefalling in space for the equivalent of 85 million kilometres, or half the Earth-Sun distance.

    The mission’s principle investigator, Pierre Touboul from France’s national aerospace research centre, ONERA, said: “The satellite’s performance is far exceeding expectations. Data from more than 1,900 additional orbits are already available and more are to come.

    “This first result is going to shake the world of physics and will certainly lead to a revision of alternative theories to general relativity.”

    The Microscope experiment is continuing to collect data, and the team hope that the final analysis will have a precision within a tenth of a trillionth of a percent.

    See the full article here .

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    Imperial College London

    Imperial College London is a science-based university with an international reputation for excellence in teaching and research. Consistently rated amongst the world’s best universities, Imperial is committed to developing the next generation of researchers, scientists and academics through collaboration across disciplines. Located in the heart of London, Imperial is a multidisciplinary space for education, research, translation and commercialisation, harnessing science and innovation to tackle global challenges.

     
  • richardmitnick 9:28 pm on November 24, 2017 Permalink | Reply
    Tags: , Einstein’s theory of General Relativity, Free-fall experiment could test if gravity is a quantum force, , , , ,   

    From New Scientist: “Free-fall experiment could test if gravity is a quantum force” 

    NewScientist

    New Scientist

    22 November 2017
    Anil Ananthaswamy

    1
    Free-falling. Manuela Schewe-Behnisch/EyeEm/Getty

    Despite decades of effort, a theory of quantum gravity is still out of grasp. Now a group of physicists have proposed an experimental test of whether gravity is quantum or not, to settle questions about the force’s true nature.

    The search for quantum gravity is an effort to reconcile Einstein’s general relativity with quantum mechanics, which is a theory of all the fundamental particles and the forces that act on them – except gravity. Both are needed to explain what happens inside black holes and what happened at the big bang. But the two theories are incompatible, leading to apparent paradoxes and things like singularities, where the theories break down.

    If gravity is a quantum mechanical force, adjacent free-falling masses, each of which is in a superposition of being in two places at once, could get entangled by gravity such that measuring the properties of one mass could instantly influence the other. To test this, Sougato Bose of University College London and his colleagues have proposed an experiment.

    Branching paths

    It starts with a neutrally charged mass weighing about 10^-14 kilograms. Embedded within the mass is some material with a property called spin, which can be up or down. This mass falls through a continuously varying magnetic field, which changes the path of the mass depending on its spin. It is like the mass encounters a fork in the road and takes one path if its spin is up, and another if its spin is down.

    As it falls, the mass is in a superposition of being on both paths. Next, a series of microwave pulses manipulate the spin at various stages of descent and thus the paths the mass takes. At the bottom, the paths then come together again and the mass is brought to its original state.

    To use this set-up to test the quantum nature of gravity, two such masses would be dropped through the magnetic field. Each mass has two possible paths. This results in four possible states for the two masses combined. One of these states represents paths in which the masses come closest together.

    This distance should be no less than 200 micrometres to avoid other interactions that can dominate gravity. Once the masses are back to their original state, a test to see if their spin components are entangled should tell us if gravity is indeed a quantum force. The assumption, of course, is that the experiment ensures there are no other ways in which the masses can get entangled – such as via electromagnetic interactions or the Casimir force.

    Bose points out, however, that a null result – in which no entanglement is observed – wouldn’t constitute proof that gravity is classical, unless the experiment can definitively rule out all other interactions with the environment that can destroy entanglement, such as collisions with stray photons or molecules.

    Quantum roots?

    Antoine Tilloy at the Max Planck Institute of Quantum Optics in Germany is impressed. But he points out that a positive result will falsify only some classes of theories of classical gravity. “That said, the class is sufficiently large that I think the result would still be amazing,” he says.

    Even a verifiable null result would be exciting because it would mean gravity doesn’t have quantum roots, says Maaneli Derakhshani of Utrecht University in the Netherlands. “This would then raise tough but interesting questions about how and when exactly gravity ‘turns on’ in the quantum-classical transition for ordinary matter,” says Derakhshani. “A null result would be the most surprising and interesting outcome.”

    The biggest hurdle to carrying out the experiment for real would be putting such relatively large masses in a superposition. The most massive objects that have been observed to be in two places at once are still orders of magnitude smaller than what is required here. But efforts to go higher are ongoing.

    This work is soon to be published in Physical Review Letters.

    Reference: arxiv.org/abs/1707.06050

    See the full article here .

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