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  • richardmitnick 6:57 am on May 16, 2017 Permalink | Reply
    Tags: , , EPR paradox, , Gravity, , , Spooky action at a distance   

    From COSMOS: “Using Einstein’s ‘spooky action at a distance’ to hear ripples in spacetime” 

    Cosmos Magazine bloc


    16 May 2017
    Cathal O’Connell

    The new technique will aid in the detection of gravitational waves caused by colliding black holes. Henze / NASA

    In new work that connects two of Albert Einstein’s ideas in a way he could scarcely have imagined, physicists have proposed a way to improve gravitational wave detectors, using the weirdness of quantum physics.

    The new proposal, published in Nature Physics, could double the sensitivity of future detectors listening out for ripples in spacetime caused by catastrophic collisions across the universe.

    When the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves in late 2015 it was the first direct evidence of the gravitational waves Einstein had predicted a century before.

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

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

    Now it another of Einstein’s predictions – one he regarded as a failure – could potentially double the sensitivity of LIGOs successors.

    The story starts with his distaste for quantum theory – or at least for the fundamental fuzziness of all things it seemed to demand.

    Einstein thought the universe would ultimately prove predictable and exact, a clockwork universe rather than one where God “plays dice”. In 1935 he teamed up with Boris Podolsky and Nathan Rosen to publish a paper they thought would be a sort of reductio ad absurdum. They hoped to disprove quantum mechanics by following it to its logical, ridiculous conclusion. Their ‘EPR paradox’ (named for their initials) described the instantaneous influence of one particle on another, what Einstein called “spooky action at a distance” because it seemed at first to be impossible.

    Yet this sally on the root of quantum physics failed, as the EPR effect turned out not to be a paradox after all. Quantum entanglement, as it’s now known, has been repeatedly proven to exist, and features in several proposed quantum technologies, including quantum computation and quantum cryptography.

    Artistic rendering of the generation of an entangled pair of photons by spontaneous parametric down-conversion as a laser beam passes through a nonlinear crystal. Inspired by an image in Dance of the Photons by Anton Zeilinger. However, this depiction is from a different angle, to better show the “figure 8” pattern typical of this process, clearly shows that the pump beam continues across the entire image, and better represents that the photons are entangled.
    Date 31 March 2011
    Source Entirely self-generated using computer graphics applications.
    Author J-Wiki at English Wikipedia

    Now we can add gravity wave detection to the list.

    LIGO works by measuring the minute wobbling of mirrors as a gravitational wave stretches and squashes spacetime around them. It is insanely sensitive – able to detect wobbling down to 10,000th the width of a single proton.

    At this level of sensitivity the quantum nature of light becomes a problem. This means the instrument is limited by the inherent fuzziness of the photons bouncing between its mirrors — this quantum noise washes out weak signals.

    To get around this, physicists plan to use so-called squeezed light to dial down the level of quantum noise near the detector (while increasing it elsewhere).

    The new scheme aids this by adding two new, entangled laser beams to the mix. Because of the ‘spooky’ connection between the two entangled beams, their quantum noise is correlated – detecting one allows the prediction of the other.

    This way, the two beams can be used to probe the main LIGO beam, helping nudge it into a squeezed light state. This reduces the noise to a level that standard quantum theory would deem impossible.

    The authors of the new proposal write that it is “appropriate for all future gravitational-wave detectors for achieving sensitivities beyond the standard quantum limit”.

    Indeed, the proposal could as much as double the sensitivity of future detectors.

    Over the next 30 years, astronomers aim to improve the sensitivity of the detectors, like LIGO, by 30-fold. At that level, we’d be able to hear all black hole mergers in the observable universe.

    ESA/eLISA, the future of gravitational wave research

    However, along with improved sensitivity, the proposed system would also increase the number of photons lost in the detector. Raffaele Flaminio, a physicist at the National Astronomical Observatory of Japan, points out in a perspective piece for Nature Physics [no link], Flaminio that the team need to do more work to understand how this will affect ultimate performance.

    “But the idea of using Einstein’s most famous (mistaken) paradox to improve the sensitivity of gravitational-wave detectors, enabling new tests of his general theory of relativity, is certainly intriguing,” Flaminio writes. “Einstein’s ideas – whether wrong or right – continue to have a strong influence on physics and astronomy.”

    See the full article here .

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  • richardmitnick 9:54 am on April 23, 2017 Permalink | Reply
    Tags: , , , Computer modelling, , , , Gravity, Modified Newtonian Dynamics, or MOND, , Simulating galaxies,   

    From Durham: “Simulated galaxies provide fresh evidence of dark matter” 

    Durham U bloc

    Durham University

    21 April 2017
    No writer credit

    A simulated galaxy is pictured, showing the main ingredients that make up a galaxy: the stars (blue), the gas from which the stars are born (red), and the dark matter halo that surrounds the galaxy (light grey). No image credit.

    Further evidence of the existence of dark matter – the mysterious substance that is believed to hold the Universe together – has been produced by Cosmologists at Durham University.

    Using sophisticated computer modelling techniques, the research team simulated the formation of galaxies in the presence of dark matter and were able to demonstrate that their size and rotation speed were linked to their brightness in a similar way to observations made by astronomers.

    One of the simulations is pictured, showing the main ingredients that make up a galaxy: the stars (blue), the gas from which the stars are born (red), and the dark matter halo that surrounds the galaxy (light grey).

    Alternative theories

    Until now, theories of dark matter have predicted a much more complex relationship between the size, mass and brightness (or luminosity) of galaxies than is actually observed, which has led to dark matter sceptics proposing alternative theories that are seemingly a better fit with what we see.

    The research led by Dr Aaron Ludlow of the Institute for Computational Cosmology, is published in the academic journal, Physical Review Letters.

    Most cosmologists believe that more than 80 per cent of the total mass of the Universe is made up of dark matter – a mysterious particle that has so far not been detected but explains many of the properties of the Universe such as the microwave background measured by the Planck satellite.

    CMB per ESA/Planck


    Convincing explanations

    Alternative theories include Modified Newtonian Dynamics, or MOND. While this does not explain some observations of the Universe as convincingly as dark matter theory it has, until now, provided a simpler description of the coupling of the brightness and rotation velocity, observed in galaxies of all shapes and sizes.

    The Durham team used powerful supercomputers to model the formation of galaxies of various sizes, compressing billions of years of evolution into a few weeks, in order to demonstrate that the existence of dark matter is consistent with the observed relationship between mass, size and luminosity of galaxies.

    Long-standing problem resolved

    Dr Ludlow said: “This solves a long-standing problem that has troubled the dark matter model for over a decade. The dark matter hypothesis remains the main explanation for the source of the gravity that binds galaxies. Although the particles are difficult to detect, physicists must persevere.”

    Durham University collaborated on the project with Leiden University, Netherlands; Liverpool John Moores University, England and the University of Victoria, Canada. The research was funded by the European Research Council, the Science and Technology Facilities Council, Netherlands Organisation for Scientific Research, COFUND and The Royal Society.

    See the full article here .

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    Durham U campus

    Durham University is distinctive – a residential collegiate university with long traditions and modern values. We seek the highest distinction in research and scholarship and are committed to excellence in all aspects of education and transmission of knowledge. Our research and scholarship affect every continent. We are proud to be an international scholarly community which reflects the ambitions of cultures from around the world. We promote individual participation, providing a rounded education in which students, staff and alumni gain both the academic and the personal skills required to flourish.

  • richardmitnick 8:38 am on December 29, 2016 Permalink | Reply
    Tags: , , , , DDM hypothesis, , , Gravity, Institute for Nuclear Research in Moscow, , The Universe is losing dark matter and researchers have finally measured how much   

    From Science Alert: “The Universe is losing dark matter, and researchers have finally measured how much” 


    Science Alert

    28 DEC 2016


    Researchers from Russia have, for the first time, been able to measure the amount of dark matter the Universe has lost since the Big Bang some 13.7 billion years ago, and calculate that as much as 5 percent of dark matter could have deteriorated.

    The finding could explain one of the biggest mysteries in physics – why our Universe appears to function in a slightly different way than it did in the years just after the Big Bang, and it could also shed insight into how it might continue to evolve in future.

    “The discrepancy between the cosmological parameters in the modern Universe and the Universe shortly after the Big Bang can be explained by the fact that the proportion of dark matter has decreased,” said co-author Igor Tkachev, from the Institute for Nuclear Research in Moscow.

    “We have now, for the first time, been able to calculate how much dark matter could have been lost, and what the corresponding size of the unstable component would be.”

    The mystery surrounding dark matter was first brought up way back in the 1930s, when astrophysicists and astronomers observed that galaxies moved in weird ways, appearing to be under the effect of way more gravity than could be explained by the visible matter and energy in the Universe.

    This gravitational pull has to come from somewhere. So, researchers came up with a new type of ‘dark matter’ to describe the invisible mass responsible for the things they were witnessing.

    As of right now, the current hypothesis states that the Universe is made up of 4.9 percent normal matter – the stuff we can see, such as galaxies and stars – 26.8 percent dark matter, and 68.3 percent dark energy, a hypothetical type of energy that’s spread throughout the Universe, and which might be responsible for the Universe’s expansion.

    But even though the majority of matter predicted to be in the Universe is actually dark, very little is known about dark matter – in fact, scientists still haven’t been able to prove that it actually exists.

    One of the ways scientists study dark matter is by examining the cosmic microwave background (CMB), which some call the ‘echo of the Big Bang’.

    CMB per ESA/Planck
    CMB per ESA/Planck

    The CMB is the thermal radiation left over from the Big Bang, making it somewhat of an astronomical time capsule that researchers can use to understand the early, newly born Universe.

    The problem is that the cosmological parameters that govern how our Universe works – such as the speed of light and the way gravity works – appear to differ ever so slightly in the CMB compared to the parameters we know to exist in the modern Universe.

    “This variance was significantly more than margins of error and systematic errors known to us,” Tkachev explains. “Therefore, we are either dealing with some kind of unknown error, or the composition of the ancient universe is considerably different to the modern Universe.”

    One of the hypotheses that might explain why the early Universe was so different is the ‘decaying dark matter‘ [Nature] (DDM) hypothesis – the idea that dark matter has slowly been disappearing from the Universe.

    And that’s exactly what Tkachev and his colleagues set out to analyse on a mathematical level, looking for just how much dark matter might have decayed since the creation of the Universe.

    The study’s lead author, Dmitry Gorbunov, also from the Institute for Nuclear Research, explains:

    “Let us imagine that dark matter consists of several components, as in ordinary matter (protons, electrons, neutrons, neutrinos, photons). And one component consists of unstable particles with a rather long lifespan.

    In the era of the formation of hydrogen, hundreds of thousands of years after the Big Bang, they are still in the Universe, but by now (billions of years later), they have disappeared, having decayed into neutrinos or hypothetical relativistic particles. In that case, the amount of dark matter in the era of hydrogen formation and today will be different.”

    To come up with a figure, the team analysed data taken from the Planck Telescope observations on the CMB, and compared it to different dark matter models like DDM.


    They found that the DDM model accurately depicts the observational data found in the modern Universe over other possible explanations for why our Universe looks so different today compared to straight after the Big Bang.

    The team was able to take the study a step further by comparing the CMB data to the modern observational studies of the Universe and error-correcting for various cosmological effects – such as gravitational lensing, which can amplify regions of space thanks to the way gravity can bend light.

    In the end, they suggest that the Universe has lost somewhere between 2 and 5 percent of its dark matter since the Big Bang, as a result of these hypothetical dark matter particles decaying over time.

    “This means that in today’s Universe, there is 5 percent less dark matter than in the recombination era,” Tkachev concludes.

    “We are not currently able to say how quickly this unstable part decayed; dark matter may still be disintegrating even now, although that would be a different and considerably more complex model.”

    These findings suggest that dark matter decays over time, making the Universe move in different ways than it had in the past, though the findings call for more outside research before anything is said for certain.

    Even so, this research is another step closer to potentially understanding the nature of dark matter, and solving one of science’s greatest mysteries – why the Universe looks the way it does, and how it will evolve in the future.

    The team’s work was published in Physical Review D.

    See the full article here .

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  • richardmitnick 10:43 am on November 12, 2016 Permalink | Reply
    Tags: , , , Gravity,   

    From EarthSky: “No need for dark matter?” 



    November 10, 2016
    Deborah Byrd

    Erik Verlinde just released the latest installment of his new theory of gravity. He now says he doesn’t need dark matter to explain the motions of stars in galaxies.

    Theoretical physicist Erik Verlinde has a new theory of gravity, which describes gravity not a force but as an illusion. The theory says gravity is an emergent phenomenon, possible to be derived from the microscopic building blocks that make up our universe’s entire existence. This week, he published the latest installment of his theory showing that – if he’s correct – there’s no need for dark matter to describe the motions of stars in galaxies.

    Verlinde, who is at the University of Amsterdam, first released his new theory in 2010. According to a statement released this week (November 8, 2016):

    … gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.

    Dark matter – the invisible “something” that most modern physicists believe makes up a substantial fraction of our universe – came to be necessary when astronomers found in the mid-20th century they couldn’t explain why stars in galaxies moved as they did. The outer parts of galaxies, including our own Milky Way, rotate much faster around their centers than they should, according to the theories of gravity as explained by Isaac Newton and Albert Einstein. According to these very accepted theories, there must be more mass in galaxies than that we can see, and thus scientists began speaking of invisible matter, which they called dark matter.

    They’ve been speaking of it, and trying to understand it, ever since.

    Verlinde is now saying we don’t need dark matter to explain what’s happening in galaxies. He says his new theory of gravity accurately predicts star velocities in the Milky Way and other galaxies. In his statement, he said:

    “We have evidence that this new view of gravity actually agrees with the observations. At large scales, it seems, gravity just doesn’t behave the way Einstein’s theory predicts.

    If true, it’s a revolution in science, since essentially all of modern cosmology – including the Big Bang theory that describes how our universe began – is based on Einstein’s theory of gravity. In recent decades, dark matter and its cousin dark energy have been bugaboos to the accepted theories; despite searches, for example, no one has ever actually observed dark matter.

    If Verlinde’s theory of gravity is true, it doesn’t mean Einstein’s theory is wrong, just as Einstein’s description of gravity didn’t exactly nullify Isaac Newton’s theory of gravity from two centuries before. Newton’s theory is still taught in physics classes, but Einstein’s theory was a refinement – a major one – in our way of thinking about gravity. Likewise, Verlinde’s theory, if correct, would be a refinement of Einstein’s ideas and a chance to have a deeper understanding of the way our universe works. Verlinde commented in his statement:

    “Many theoretical physicists like me are working on a revision of the [accepted modern theories of gravity], and some major advancements have been made. We might be standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity.”

    See the full article here .

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  • richardmitnick 10:13 am on July 22, 2016 Permalink | Reply
    Tags: , , Gravity   

    from ars technica: “Gravity doesn’t care about quantum spin” 

    Ars Technica
    ars technica

    Chris Lee

    An atomic clock based on a fountain of atoms. NSF

    Physics, as you may have read before, is based around two wildly successful theories. On the grand scale, galaxies, planets, and all the other big stuff dance to the tune of gravity. But, like your teenage daughter, all the little stuff stares in bewildered embarrassment at gravity’s dancing. Quantum mechanics is the only beat the little stuff is willing get down to. Unlike teenage rebellion, though, no one claims to understand what keeps relativity and quantum mechanics from getting along.

    Because we refuse to believe that these two theories are separate, physicists are constantly trying to find a way to fit them together. Part and parcel with creating a unifying model is finding evidence of a connection between the gravity and quantum mechanics. For example, showing that the gravitational force experienced by a particle depended on the particle’s internal quantum state would be a great sign of a deeper connection between the two theories. The latest attempt to show this uses a new way to look for coupling between gravity and the quantum property called spin.

    I’m free, free fallin’

    One of the cornerstones of general relativity is that objects move in straight lines through a curved spacetime. So, if two objects have identical masses and are in free fall, they should follow identical trajectories. And this is what we have observed since the time of Galileo (although I seem to recall that Galileo’s public experiment came to an embarrassing end due to differences in air resistance).

    The quantum state of an object doesn’t seem to make a difference. However, if there is some common theory that underlies general relativity and quantum mechanics, at some level, gravity probably has to act differently on different quantum states.

    To see this effect means measuring very tiny differences in free fall trajectories. Until recently, that was close to impossible. But it may be possible now thanks to the realization of Bose-Einstein condensates. The condensates themselves don’t necessarily provide the tools we need, but the equipment used to create a condensate allows us to manipulate clouds of atoms with exquisite precision. This precision is the basis of a new free fall test from researchers in China.

    Surge like a fountain, like tide

    The basic principle behind the new work is simple. If you want to measure acceleration due to gravity, you create a fountain of atoms and measure how long it takes for an atom to travel from the bottom of the fountain to the top and back again. As long as you know the starting velocity of the atoms and measure the time accurately, then you can calculate the force due to gravity. To do that, you need to impart a well-defined momentum to the cloud at a specific time.

    Quantum superposition

    Superposition is nothing more than addition for waves. Let’s say we have two sets of waves that overlap in space and time. At any given point, a trough may line up with a peak, their peaks may line up, or anything in between. Superposition tells us how to add up these waves so that the result reconstructs the patterns that we observe in nature.

    Then you need to measure the transit time. This is done using the way quantum states evolve in time, which also means you need to prepare the cloud of atoms in a precisely defined quantum state.

    If I put the cloud into a superposition of two states, then that superposition will evolve in time. What do I mean by that? Let’s say that I set up a superposition between states A and B. Now, when I take a measurement, I won’t get a mixture of A and B; I only ever get A or B. But the probability of obtaining A (or B) oscillates in time. So at one moment, the probability might be 50 percent, a short time later it is 75 percent, then a little while later it is 100 percent. Then it starts to fall until it reaches zero and then it starts to increase again.

    This oscillation has a regular period that is defined by the environment. So, under controlled circumstances, I set the superposition state as the atomic cloud drifts out the top of the fountain, and at a certain time later, I make a measurement. Each atom reports either state A or state B. The ratio of the amount of A and B tells me how much time has passed for the atoms, and, therefore, what the force of gravity was during their time in the fountain.

    Once you have that working, the experiment is dead simple (he says in the tone of someone who is confident he will never have to actually build the apparatus or perform the experiment). Essentially, you take your atomic cloud and choose a couple of different atomic states. Place the atoms in one of those states and measure the free fall time. Then repeat the experiment for the second state. Any difference, in this ideal case, is due to gravity acting differently on the two quantum states. Simple, right?

    Practically speaking, this is kind-a-sorta really, really difficult.

    I feel like I’m spinnin’

    Obviously, you have to choose a pair of quantum states to compare. In the case of our Chinese researchers, they chose to test for coupling between gravity and a particle’s intrinsic angular momentum, called spin. This choice makes sense because we know that in macroscopic bodies, the rotation of a body (in other words, its angular momentum) modifies the local gravitational field. So, depending on the direction and magnitude of the angular momentum, the local gravitational field will be different. Maybe we can see this classical effect in quantum states, too?

    However, quantum spin is, confusingly, not related to the rotation of a body. Indeed, if you calculate how fast an electron needs to rotate in order to generate its spin angular momentum, you’ll come up with a ridiculous number (especially if you take the idea of the electron being a point particle seriously). Nevertheless, particles like electrons and protons, as well as composite particles like atoms, have intrinsic spin angular momentum. So, an experiment comparing the free fall of particles with the same spin, but oriented in different directions, makes perfect sense.

    Except for one thing: magnetic fields. The spin of a particle is also coupled to its magnetic moment. That means that if there are any changes in the magnetic field around the atom fountain, the atomic cloud will experience a force due to these variations. Since the researchers want to measure a difference between two spin states that have opposite orientations, this is bad. They will always find that the two spin populations have different fountain trajectories, but the difference will largely be due to variations in the magnetic field, rather than to differences in gravitational forces.

    So the story of this research is eliminating stray magnetic fields. Indeed, the researchers spend most of their paper describing how they test for magnetic fields before using additional electromagnets to cancel out stray fields. They even invented a new measurement technique that partially compensates for any remaining variations in the magnetic fields. To a large extent, the researchers were successful.

    So, does gravity care about your spin?

    Short answer: no. The researchers obtained a null result, meaning that, to within the precision of their measurements, there was no detectable difference in atomic free falls when atoms were in different spin states.

    But this is really just the beginning of the experiment. We can expect even more sensitive measurements from the same researchers within the next few years. And the strategies that they used to increase accuracy can be transferred to other high-precision measurements.

    Physical Review Letters, 2016, DOI: 10.1103/PhysRevLett.117.023001

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    Ars Technica was founded in 1998 when Founder & Editor-in-Chief Ken Fisher announced his plans for starting a publication devoted to technology that would cater to what he called “alpha geeks”: technologists and IT professionals. Ken’s vision was to build a publication with a simple editorial mission: be “technically savvy, up-to-date, and more fun” than what was currently popular in the space. In the ensuing years, with formidable contributions by a unique editorial staff, Ars Technica became a trusted source for technology news, tech policy analysis, breakdowns of the latest scientific advancements, gadget reviews, software, hardware, and nearly everything else found in between layers of silicon.

    Ars Technica innovates by listening to its core readership. Readers have come to demand devotedness to accuracy and integrity, flanked by a willingness to leave each day’s meaningless, click-bait fodder by the wayside. The result is something unique: the unparalleled marriage of breadth and depth in technology journalism. By 2001, Ars Technica was regularly producing news reports, op-eds, and the like, but the company stood out from the competition by regularly providing long thought-pieces and in-depth explainers.

    And thanks to its readership, Ars Technica also accomplished a number of industry leading moves. In 2001, Ars launched a digital subscription service when such things were non-existent for digital media. Ars was also the first IT publication to begin covering the resurgence of Apple, and the first to draw analytical and cultural ties between the world of high technology and gaming. Ars was also first to begin selling its long form content in digitally distributable forms, such as PDFs and eventually eBooks (again, starting in 2001).

  • richardmitnick 7:26 am on July 11, 2016 Permalink | Reply
    Tags: , , Gravity,   

    From COSMOS: “Gravity shrugs off differences of quantum spin” 

    Cosmos Magazine bloc


    11 July 2016
    Cathal O’Connell

    Italian astronomer and scientist Galileo Galilei performs his legendary experiment – dropping a cannonball and a wooden ball from the top of the Leaning Tower of Pisa, circa 1620.
    Hulton Archive/Getty Images

    A new experiment, based on measuring the free fall of rubidium atoms in a vacuum, confirms that atoms of different quantum spin experience identical acceleration due to gravity.

    The result is important in the quest to unify general relativity with quantum mechanics, and may already rule out some proposed theories of quantum gravity.

    Resolving the mismatch between the two great pillars of physics – the theory of gravity, and the theory of matter – is probably the grandest challenge in contemporary physics.

    Einstein’s theory of general relativity tells how gravity arises from mass bending space and time. The theory describes the universe on the largest of scales, from the orbits of the planets to the rotation of galaxies to the Big Bang itself.

    Quantum mechanics, on the other hand, describes the microscopic world of particles and how they join together to make the matter around us.

    The problem is these two ideas don’t seem to mesh. The very large and the very small seem to play by different rules.

    Now, a new breed of experiments is allowing physicists to measure the force of gravity at the scale of quantum objects and so test, for the first time, some of the theories proposing to bridge the chasm between gravity and quantum mechanics.

    In the new work, a team of Chinese scientists from Huazhong University of Science and Technology in Wuhan has compared the acceleration of rubidium atoms due to gravity and found it to be identical regardless of the orientation of the atom’s spin.

    This research is published in Physical Review Letters.

    At heart, this experiment is a test of the equivalence principle, which says the acceleration due to gravity is identical for any object.

    Tests of this principle have been performed in various guises over the centuries, from renaissance Europe to the surface of the moon.

    One of the most famous images in all of physics is that of the Italian scientist Galileo Galilei, atop the leaning Tower of Pisa, letting go of two metal balls of different masses to show they fell at the same rate. Although this account may be apocryphal, Galileo certainly did describe an experiment rolling balls down a slope, which showed the same thing.

    And in 1971, Commander David Scott famously tested the equivalence principle by dropping a hammer and feather at the same time, while standing on the moon.

    The hammer and the feather on the Moon
    Access mp4 video here .

    Although the equivalence principle is central to general relativity, many quantum theories of gravity, which attempt to describe gravity using quantum mechanics, predict that the equivalence principle could be violated.

    In particular, some quantum properties, such as the spin of an atom, might affect free fall the theories say.

    To test this, the Chinese team, led by physicist Zhong-Kun Hu, set up an intricate experiment, which measured the rate of free fall of atoms of rubidium.

    The experiment is based on atom interferometry, which exploits the wave nature of atoms to monitor their motion extremely precisely.

    First, the team isolated and cooled a collection of rubidium atoms to few millionths of a degree above absolute zero.

    The atoms started out at the bottom of a tube that had been emptied completely of air.

    The team then pointed a laser beam from below and using the light to give the cold atoms a kick, propelling them upwards in the tube. But what goes up, must come down. This set up a “fountain” of atoms, rising and falling.

    The scientists found that the free fall acceleration of the rubidium atoms with opposite spins agreed to within one part in 10 million.

    In the past decade, similar experiments have already verified universality of free fall for different atoms, and for different isotopes of the same element.

    But this is the first time gravity has been tested in terms of quantum spin. It means that several exotic theories which had predicted a significant interaction between quantum spin and gravity will have to be modified, or thrown out.

    Back to the drawing board.

    See the full article here .

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

    From Symmetry: “Six weighty facts about gravity” 

    Symmetry Mag


    Illustration by Sandbox Studio, Chicago with Ana Kova

    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.

    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.

    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.

    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 .

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 6:26 pm on January 19, 2016 Permalink | Reply
    Tags: , , , eLISA, , Gravity,   

    From PI: “Preparing for a cosmological challenge” 

    Perimeter Institute
    Perimeter Institute

    January 19, 2016
    Rose Simone

    Einstein’s theory of general relativity may soon be put to the ultimate test through measurements of a black hole’s shadow, say a pair of Perimeter researchers.
    Even though it is over 100 years old, Albert Einstein’s theory of general relativity is still a formidable prizefighter.

    The theory, which successfully describes gravity as a consequence of the curvature of spacetime itself, has withstood all the experimental tests that physicists have been able to throw at it over the decades.

    So now, to have any hope of challenging general relativity, they need to bring in a heavyweight. Enter the closest challenger: the smallish but still formidable 4.5-million- at the centre of our own Milky Way galaxy.

    The challenge will be assisted by the Event Horizon Telescope (EHT), a radio telescope array as large as the Earth, being configured to take precise images of the silhouette (or the shadow) of that black hole, known as Sagittarius A*.

    Sag A*. This image was taken with NASA’s Chandra X-Ray Observatory. Ellipses indicate light echoes.

    NASA Chandra Telescope

    Event Horizon Telescope map
    EHT map

    Meanwhile, Tim Johannsen, a postdoctoral fellow at Perimeter Institute and the University of Waterloo, who works with Avery Broderick, an Associate Faculty member at Perimeter Institute jointly appointed at Waterloo, has led a group of researchers in calculating the measurements that will be used to determine whether general relativity really does stand up in the strong gravity regime of that black hole.

    Perimeter postdoctoral researcher Tim Johannsen.

    Perimeter Associate Faculty member Avery Broderick.

    Their paper was recently published in Physical Review Letters, along with an accessible synopsis of the work.

    When the images from the black hole come in and the measurements outlined in the recent paper are actually taken, it will be the first truly broad test of general relativity in the strong gravity regime.

    “That is very exciting and we expect to be able to do that within the next few years,” Johannsen says.

    Black holes are regions of spacetime, where gravity is so strong that not even light can escape once it has passed the threshold of no return − the event horizon. So as the name implies, they are dark.

    But owing to its immense gravity, the black hole pulls in vast quantities of dust and gas from surrounding stars. These accrete into a hot swirling plasma disk that illuminates the silhouette of the black hole. The EHT will be able to capture this, in images that will be historic firsts.

    A lot of physics will be done with the data gleaned from those images, but putting general relativity to the test is perhaps the most exciting challenge.

    General relativity has been fantastically successful. In every experiment that has been done to test how the sun and stars in our cosmos affect spacetime and exert gravitational pull on other objects, its predictions have held up.

    But the question is whether the theory will continue to hold up in a strong gravity environment, such as the surroundings of a black hole.

    Black holes are so massive and compact that the spacetime-warping effects, predicted by general relativity, would be more evident than around the sun or other stars. They are “orders of magnitude” different as gravitational environments go, Broderick says.

    “That means that this is terra incognita and we don’t know what we are going to find,” Broderick says. The EHT provides “an opportunity to begin probing in a critical way the non-linear nature of general relativity in the strong gravity regime.”

    This is important to physicists because even though general relativity has been enormously successful in explaining the cosmos that we can see, there are a number of difficulties with it. “It is not clear, for example, exactly how it should be combined with the quantum theory that we have, and in fact, it is very difficult to reconcile the two in a grand unification scheme,” Johannsen says.

    Moreover, there is the problem of the mysterious “dark energy” driving the accelerated expansion of spacetime, as well as the conundrum about the nature of “dark matter,” unseen mass theorized as an explanation for observed galaxy rotation rates that prevent galaxy clusters from flying apart. Physicists are hoping for some insights about general relativity in the strong gravity regime to make sense of these mysteries.

    Johannsen’s team has developed a way of checking how much the gravitational environment of this black hole might deviate from the theory of general relativity and other gravity theories.

    The paper sets constraints on the parameters of the size of the shadow to fit with general relativity. Other gravity models also propose modifications to the theory of general relativity, such as the Modified Gravity Theory (MOG) and the Randall-Sundrum-type braneworld model (RS2). The paper sets the constraints for the black hole to fit with these gravity models as well.

    “We have made the first realistic estimate of the high precision with which the EHT can detect the size of the shadow,”Johannsen says. “We show that such a measurement can be a precise test of general relativity.”

    A nice bonus from this work is that researchers will also get much more precise measurements of the mass of the black hole and its distance. “Sharpening the precision is great because that will enable us to get even more precise constraints on deviations from general relativity,” Johannsen adds.

    There are already good measurements of how far away Sagittarius A* is and how massive it is, based on other experiments that have looked at the motion of stars as they orbit the black hole, as well as of masers throughout the Milky Way, Johannsen explains. “People have been doing this for about 20 years.”

    This can be used to figure out what it should look like. But once the images from the EHT are available, it will be possible to check: “Do we get what we expect? Or do we get something else?” Johannsen says.

    Getting the measurements is really a matter of drawing a series of lines from the centre of the black hole image to the edge of its shadow. On the image, it looks like a pie shape with slices. Measuring the lines of each slice and calculating an average “gives us the angular radius of the shadow and then we know how big it is,” Johannsen says.

    A reconstructed image of Sgr A* for an EHT observation at 230 GHz with a seven-station array.

    From the measurements of the size of the shadow, it is possible to see how closely the gravity in the black hole environment matches the predictions of general relativity and of other theories of gravity.

    “If general relativity is not correct, there can be significant change in the size. The shadow can also become asymmetric so that it is no longer circular, but egg-shaped, for example,” Johannsen says.

    Getting to the point of making these measurements will take a couple more years because at least seven or eight of the telescopes in the EHT array must be coordinated to get the data at the same time in a massive worldwide collaboration.

    The amount of raw data that has to be gathered to get the images is so enormous, it can’t even be transmitted over the internet.

    “These are humongous data sets. So they literally have to save all this data on hard drives and put them in a box and ship them,” Johannsen says.

    The hard drives get shipped to the MIT Haystack Observatory, which is the headquarters for the EHT. From there, the raw data is analyzed and the images are produced.

    After the images are produced, Johannsen gets to use his measurement technique to find out if general relativity is correct for the strong gravity environment around this black hole.

    This isn’t the only test of general relativity in the strong gravity regime in the works. There are other sophisticated experiments to detect, for example, the gravitational waves that are predicted by general relativity. But the prime experimental candidate to confirm the existence of gravitational waves would be the Evolved Laser Interferometer Space Antenna (eLISA), a space-based telescope with an estimated launch date of 2034.

    LISA graphic

    The EHT will produce images in the next few years.

    If it turns out that the measurements yield what was expected and general relativity holds up, that would be interesting, “because Einstein had this theory 100 years ago, and then we will know that it is true,” Johannsen says.

    But if the challenger should prevail, and strong gravity does strike a blow to the theory of general relativity, “that would be big,” he adds.

    See the full article here .

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    About Perimeter

    Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

  • richardmitnick 3:42 pm on August 6, 2015 Permalink | Reply
    Tags: , , Gravity,   

    From NRAO: “Gravitational Constant Appears Universally Constant, Pulsar Study Suggests” 

    NRAO Icon
    National Radio Astronomy Observatory

    NRAO Banner

    August 6, 2015
    Contact: Charles E. Blue
    (434) 296-0314; cblue@nrao.edu

    A 21-year study of a pair of ancient stars — one a pulsar and the other a white dwarf — helps astronomers understand how gravity works across the cosmos. The study was conducted with the NSF’s Green Bank Telescope and the Arecibo Observatory. Credit: B. Saxton (NRAO/AUI/NSF)

    Gravity, one of the four fundamental forces of nature, appears reassuringly constant across the Universe, according to a decades-long study of a distant pulsar. This research helps to answer a long-standing question in cosmology: Is the force of gravity the same everywhere and at all times? The answer, so far, appears to be yes.

    Astronomers using the National Science Foundation’s (NSF) Green Bank Telescope (GBT) in West Virginia and its Arecibo Observatory in Puerto Rico conducted a 21-year study to precisely measure the steady “tick-tick-tick” of a pulsar known as PSR J1713+0747.


    Arecibo Observatory

    This painstaking research produced the best constraint ever of the gravitational constant measured outside of our Solar System.

    Pulsars are the rapidly spinning, superdense remains of massive stars that detonated as supernovas. They are detected from Earth by the beams of radio waves that emanate from their magnetic poles and sweep across space as the pulsar rotates. Since they are phenomenally dense and massive, yet comparatively small – a mere 20–25 kilometers across – some pulsars are able to maintain their rate of spin with a consistency that rivals the best atomic clocks on Earth. This makes pulsars exceptional cosmic laboratories to study the fundamental nature of space, time, and gravity.

    This particular pulsar is approximately 3,750 light-years from Earth. It orbits a companion white dwarf star and is one of the brightest, most stable pulsars known. Previous studies show that it takes about 68 days for the pulsar to orbit its white dwarf companion, meaning they share an uncommonly wide orbit. This separation is essential for the study of gravity because the effect of gravitational radiation – the steady conversion of orbital velocity to gravitational waves as predicted by [Albert]Einstein – is incredibly small and would have negligible impact on the orbit of the pulsar. A more pronounced orbital change would confound the accuracy of the pulsar timing experiment.

    “The uncanny consistency of this stellar remnant offers intriguing evidence that the fundamental force of gravity – the big ‘G’ of physics – remains rock-solid throughout space,” said Weiwei Zhu, an astronomer formerly with the University of British Columbia in Canada and lead author on a study accepted for publication in the Astrophysical Journal. “This is an observation that has important implications in cosmology and some of the fundamental forces of physics.”

    “Gravity is the force that binds stars, planets, and galaxies together,” said Scott Ransom, a co-author and astronomer with the National Radio Astronomy Observatory in Charlottesville, Va. “Though it appears on Earth to be constant and universal, there are some theories in cosmology that suggest gravity may change over time or may be different in different corners of the Universe.”

    The data taken throughout this experiment are consistent with an unchanging gravitational constant in a distant star system. Earlier related research in our own Solar System, which was based on precise laser ranging studies of the Earth-Moon distance, found the same consistency over time.

    “These results – new and old – allow us to rule out with good confidence that there could be ‘special’ times or locations with different gravitational behavior,” added Ingrid Stairs, a co-author from the University of British Columbia in Canada. “Theories of gravity that are different from general relativity often make such predictions, and we have put new restrictions on the parameters that describe these theories.”

    Zhu concluded: “The gravitational constant is a fundamental constant of physics, so it is important to test this basic assumption using objects at different places, times, and gravitational conditions. The fact that we see gravity perform the same in our Solar System as it does in a distant star system helps to confirm that the gravitational constant truly is universal.”

    This work was part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a Physics Frontiers Center funded by the NSF.

    The GBT is located in the National Radio Quiet Zone, which protects the incredibly sensitive telescope from unwanted radio interference, enabling it to study pulsars and other astronomical objects.

    The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

    See the full article here.

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    The NRAO operates a complementary, state-of-the-art suite of radio telescope facilities for use by the scientific community, regardless of institutional or national affiliation: the Very Large Array (VLA), the Robert C. Byrd Green Bank Telescope (GBT), and the Very Long Baseline Array (VLBA)*.

    ALMA Array




    The NRAO is building two new major research facilities in partnership with the international community that will soon open new scientific frontiers: the Atacama Large Millimeter/submillimeter Array (ALMA), and the Expanded Very Large Array (EVLA). Access to ALMA observing time by the North American astronomical community will be through the North American ALMA Science Center (NAASC).
    *The Very Long Baseline Array (VLBA) comprises ten radio telescopes spanning 5,351 miles. It’s the world’s largest, sharpest, dedicated telescope array. With an eye this sharp, you could be in Los Angeles and clearly read a street sign in New York City!

    Astronomers use the continent-sized VLBA to zoom in on objects that shine brightly in radio waves, long-wavelength light that’s well below infrared on the spectrum. They observe blazars, quasars, black holes, and stars in every stage of the stellar life cycle. They plot pulsars, exoplanets, and masers, and track asteroids and planets.

  • richardmitnick 3:21 pm on January 6, 2015 Permalink | Reply
    Tags: Gravity,   

    From Physics: “Focus: First Direct Measurement of Gravity’s Curvature” 

    Physics LogoAbout Physics

    Physics Logo 2


    January 5, 2015
    Philip Ball

    By measuring gravity with cold atoms at three different heights simultaneously, a team determined a new property of a gravitational field.

    Earth’s gravitational pull gradually decreases with increasing altitude, and researchers have detected the differences even over several vertical feet within a lab, using the extreme sensitivity of cold atoms. Now a team has taken the next step by measuring the change in this gravity gradient produced by a large mass, using measurements at three different heights. They say their technique could improve gravity-based mapping of variations in rock density in geology and prospecting, and it could also boost the precision of tests of general relativity and measurements of the gravitational constant.

    Physics guides oil exploration. Measuring the curvature of the gravitational field could be useful in the oil industry, where prospectors perform sensitive gravity measurements to search for underground deposits. (iStockphoto.com/SGV)

    The technique of atom interferometry enables distance measurements with extremely high precision, by exploiting the atoms’ quantum-mechanical wavelike nature. It has been used previously to measure the strength of gravitational fields and also the rate of change in those fields over some distance (the gradient). Together such measurements permit Newton’s gravitational constant G to be determined. It is currently known to within about 100 parts per million, a much lower precision than other fundamental constants. More accurate measurements would allow higher-precision tests of the theory of general relativity.

    Atom Interferometry

    Measuring gravity at two close locations gives the gradient as the difference between the two divided by their separation distance; measuring at three locations gives the rate of change of the gradient, which is also called the curvature of the field. This experiment was proposed in 2002, and now a team in Italy, led by Guglielmo Tino of the University of Florence and the National Institute of Nuclear Physics (INFN), has carried it out. Previously, Tino and his colleagues determined G by measuring gravity at two different heights with a similar experiment.

    To measure gravity at three locations simultaneously, the team launched three clouds of ultracold atoms to three different heights inside a meter-long vertical pipe. Surrounding the top half of the pipe was 516kg of tungsten alloy weights, to increase the variation in the gravitational field. Near the peaks of their trajectories, the atoms were irradiated with a rapid series of laser pulses from the top and bottom of the pipe.

    In the team’s technique, the first pulse separates each cloud into two populations—one that absorbs two photons, sending it into an excited state and also providing a momentum boost, and a second population that remains in the ground state. The extra momentum causes the first population to fall a different distance during a fixed time, which leads to a gravity-dependent difference in the number of quantum wave cycles that elapse, compared with the ground-state population. Two more wave pulses recombine the populations, allowing them to interfere. From the interference effects the researchers can calculate the difference in the lengths of the two populations’ trajectories, a difference that depends on the gravitational acceleration. The team measured variations in the gravitational acceleration of a few millionths of a percent and calculated the average curvature to be 1.4×10−5s−2m−1, which is virtually identical to the value they predicted.

    Measuring the curvature of a gravitational field could improve the measurement of G, says Tino. A common method involves measuring the field strength and gradient as a heavy mass is moved between one detector and another. But by making two separate measurements of the gradient at different positions simultaneously, the new technique could eliminate systematic sources of error without having to move the mass, which can introduce errors from shifts of the apparatus.

    The curvature could also be useful for mapping gravity changes in the earth, which are used to deduce buried geological structures and to find oil reservoirs. Even if the density changes are small, the curvature can alter dramatically if the density change is abrupt, like a step edge. So measuring gravity curvature could improve the spatial resolution of such density maps.

    “Measuring the gravitational force is sensitive to everything underground,” says Holger Müller of the University of California at Berkeley, who uses atom interferometry to make ultraprecise measurements for probing fundamental physics. “Measuring the gravity gradient enhances the sensitivity to nearby objects, and measuring the [curvature] does so even more.” A practical, curvature-measuring device would be “a great achievement,” Müller says.

    This research is published in Physical Review Letters.

    See the full article here.

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    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

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