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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: , , EHT, 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).

  • richardmitnick 10:04 am on November 7, 2014 Permalink | Reply
    Tags: , , , , Gravity,   

    From Ethan Siegel: ” How far does gravity reach?” 

    Starts with a bang
    Starts with a Bang

    Nov 7, 2014
    Ethan Siegel

    “In my dreams and visions, I seemed to see a line, and on the other side of that line were green fields, and lovely flowers, and beautiful white ladies, who stretched out their arms to me over the line, but I couldn’t reach them no-how. I always fell before I got to the line.” –Harriet Tubman

    It’s the end of the week once again, which means we get to take a look at your reader submissions for Ask Ethan, and choose my favorite to share with you. This week’s entry comes from one of my readers who’s been following Starts With A Bang the longest (since my first few months of blogging way back in 2008): Frank Burdge. He asks the following head-scratcher:

    If the Universe is not infinite, then what can be said about the range of the gravitational and electromagnetic forces?

    This is actually an incredibly profound question, when you think about it. Consider what we know about the gravitational and electromagnetic forces, first off.

    Image credit: Regents Physics, via http://aplusphysics.com/wordpress/regents/tag/inverse-square-law/.

    It isn’t the magnitude of the forces, or the nature of attraction/repulsion that I want you to consider, but rather the fact that — at long ranges — these are inverse-square forces, meaning that as you double the distance between two such objects, the force between them drops to one-fourth what it was at the previous distance. This is a very special relationship, and not only do gravitation and electromagnetism, so do some other important physical properties, such as the effects of light, sound, and radiation.

    Why is this very special? Consider how any of these things — light from a star, for example — spread out as you move farther away from the source.

    Image credit: Wikimedia Commons user Borb.

    They move radially outward in a sphere, so that a pulse — of light, again, for example — would spread out in an ever expanding spherical shell. Now, why is this special? Because the surface area of a sphere has a very specific formula: A = 4πr^2, where the “r^2” is the important part.

    Why? Because if the area expands according to r^2, but the force drops according to an inverse square law (i.e., r^-2), then we can say that phenomenon has an infinite range. This means you can draw a sphere of any size centered around an object — a gravitational source (a mass), an electric source (a charge), a light source (a star), etc. — and the total amount of stuff acting on that sphere, whether it be force or flux, will be the same when you add up all the different parts of the sphere. This is true not only for the whole sphere, but for any given amount of solid angle on the sphere!

    Image credit: R Nave of Hyperphysics, via http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html.

    So that’s what we mean by gravitation being an “infinite range” force.

    But the Universe — at the very least, the part of it observable and accessible to us — is not infinite at all! Quite to the contrary, despite the fact that we can see the starlight coming from hundreds of billions of galaxies, that’s not even such a big number. It’s a beautiful sight, and it boggles the mind to look at. (Seriously, take some time and look at it!) But in no way — despite its grandeur — should you conflate its vastness with infinity.

    Image credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski (Arizona State University, Tempe), R. O’Connell (University of Virginia), P. McCarthy (Carnegie Observatories), N. Hathi (University of California, Riverside), R. Ryan (University of California, Davis), H. Yan (Ohio State University), and A. Koekemoer (Space Telescope Science Institute).

    Sure, it seems pretty big, but when you consider that there are billions of times that many grains of sand on Earth’s beaches as there are galaxies in the observable Universe, it doesn’t seem so big. The reason we can’t access more has to do with the fact that the Universe hasn’t been around forever, but rather has only had about 13.8 billion years since the Big Bang for things like light (and gravitation) to reach out towards us.

    Image credit: James Schombert of University of Oregon, via http://abyss.uoregon.edu/~js/ast121/lectures/lec06.html.

    Or more accurately, to reach spherically outward away from their sources. We do the same thing: the gravitation from every particle in our bodies, on our planet and in our galaxy has been reaching out, “grabbing” at the speed of gravity (which is the speed of light) with a force falling off in magnitude as ~1 / r^2 (but reaching out with an increasing area according to ~r^2) since what we conceive as “our Universe” began.

    Image credit: WiseGEEK, via http://www.wisegeek.org/what-happened-after-the-big-bang.htm#.

    Now, this — the observable Universe, the part reachable by light, gravity, and other finite-speed phenomenon — is not to be confused with however big (or possibly infinite) the entire Universe, including the unobservable part, actually is.

    Images credit: Rob Knop, via http://www.galacticinteractions.org/?p=1623, of the observable Universe at the moment the Universe could first be described by the Big Bang (L), and of the unobservable Universe as well at the same moment (R).

    We have good reasons to believe that the entire Universe is far bigger than the observable parts of it are, but our signals are unable to reach it, due to the fact that there’s only been a finite amount of time that we’ve been emitting signals and that the speed at which these signals propagate is also finite.

    But this is not a bug with our Universe: it’s a feature!

    Image credit: European Gravitational Observatory, Lionel BRET/EUROLIOS.

    Imagine the Universe if signals propagated infinitely fast, or if we could feel the gravitation, see the light, or otherwise experience the effects of things from far beyond what ought to be accessible to us.

    We’d find ourselves — and likely many other parts of our Universe — catastrophically pulled in a random direction, seemingly without cause. We’d have the entire night sky lit up by sources that ought never to have reached our eyes.

    Image credit: James Schombert of University of Oregon, via http://abyss.uoregon.edu/~js/ast123/lectures/lec15.html.

    And we, ourselves, would be exerting far more force that we’ve had any physical right to do. In short, the laws of physics would break down, as General Relativity could no longer describe where we live, there would be a catastrophe of too much energy density in our Universe, and — as a consequence of that gravitational binding energy — the Universe itself would inevitably recollapse in short order.

    Image credit: Shashi M. Kanbur of SUNY Oswego, via http://oswego.edu/~kanbur/.

    To quote Egon from Ghostbusters, “It would be bad.”

    So what we have, instead, is gravitation, electromagnetism, and all the other “infinite range” forces reaching the distances they could have reached after traveling at the speed of light in a 13.8 billion year old Universe that’s expanded according to our own unique history, or about 46 billion light years. This is — since the speed of gravity and the speed of light are identical — also equal to the size of our observable Universe in all directions!

    Image credit: Wikimedia Commons user Azcolvin429.

    And that’s how far the range of the gravitational and electromagnetic forces extend! Thanks for a tough but informative question, Frank, and if you want to see your question featured on Ask Ethan, send in your questions and suggestions here. The next column could be all about whatever you choose!

    See the full article here.

    Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible.

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  • richardmitnick 9:39 pm on March 30, 2014 Permalink | Reply
    Tags: , , , , Gravity,   

    From Science@NASA: “NASA Announces Results of Epic Space-Time Experiment” 2011 

    NASA Science Science News

    [This is from 2011, but it just hit NASA’s Twitter feed]

    May 4, 2011
    Dr. Tony Phillips

    [Albert Einstein] was right again. There is a space-time vortex around Earth, and its shape precisely matches the predictions of Einstein’s theory of gravity.

    Dr. Albert Einstein, Nobel Laureate

    Researchers confirmed these points at a press conference today at NASA headquarters where they announced the long-awaited results of Gravity Probe B (GP-B).

    NASA Gravity Probe B
    Gravity Probe B (GP-B)

    “The space-time around Earth appears to be distorted just as general relativity predicts,” says Stanford University physicist Francis Everitt, principal investigator of the Gravity Probe B mission.

    An artist’s concept of GP-B measuring the curved spacetime around Earth.

    “This is an epic result,” adds Clifford Will of Washington University in St. Louis. An expert in Einstein’s theories, Will chairs an independent panel of the National Research Council set up by NASA in 1998 to monitor and review the results of Gravity Probe B. “One day,” he predicts, “this will be written up in textbooks as one of the classic experiments in the history of physics.”

    Time and space, according to Einstein’s theories of relativity, are woven together, forming a four-dimensional fabric called “space-time.” The mass of Earth dimples this fabric, much like a heavy person sitting in the middle of a trampoline. Gravity, says Einstein, is simply the motion of objects following the curvaceous lines of the dimple.

    If Earth were stationary, that would be the end of the story. But Earth is not stationary. Our planet spins, and the spin should twist the dimple, slightly, pulling it around into a 4-dimensional swirl. This is what GP-B went to space in 2004 to check.

    The idea behind the experiment is simple:

    Put a spinning gyroscope into orbit around the Earth, with the spin axis pointed toward some distant star as a fixed reference point. Free from external forces, the gyroscope’s axis should continue pointing at the star–forever. But if space is twisted, the direction of the gyroscope’s axis should drift over time. By noting this change in direction relative to the star, the twists of space-time could be measured.

    In practice, the experiment is tremendously difficult.

    One of the super-spherical gyroscopes of Gravity Probe B.

    The four gyroscopes in GP-B are the most perfect spheres ever made by humans. These ping pong-sized balls of fused quartz and silicon are 1.5 inches across and never vary from a perfect sphere by more than 40 atomic layers. If the gyroscopes weren’t so spherical, their spin axes would wobble even without the effects of relativity.

    According to calculations, the twisted space-time around Earth should cause the axes of the gyros to drift merely 0.041 arcseconds over a year. An arcsecond is 1/3600th of a degree. To measure this angle reasonably well, GP-B needed a fantastic precision of 0.0005 arcseconds. It’s like measuring the thickness of a sheet of paper held edge-on 100 miles away.

    “GP-B researchers had to invent whole new technologies to make this possible,” notes Will.

    They developed a “drag free” satellite that could brush against the outer layers of Earth’s atmosphere without disturbing the gyros. They figured out how to keep Earth’s magnetic field from penetrating the spacecraft. And they created a device to measure the spin of a gyro–without touching the gyro. More information about these technologies may be found in the Science@NASA story “A Pocket of Near-Perfection.”

    Pulling off the experiment was an exceptional challenge. But after a year of data-taking and nearly five years of analysis, the GP-B scientists appear to have done it.

    “We measured a geodetic precession of 6.600 plus or minus 0.017 arcseconds and a frame dragging effect of 0.039 plus or minus 0.007 arcseconds,” says Everitt.

    For readers who are not experts in relativity: Geodetic precession is the amount of wobble caused by the static mass of the Earth (the dimple in spacetime) and the frame dragging effect is the amount of wobble caused by the spin of the Earth (the twist in spacetime). Both values are in precise accord with Einstein’s predictions.

    “In the opinion of the committee that I chair, this effort was truly heroic. We were just blown away,” says Will.

    An artist’s concept of twisted spacetime around a black hole. Credit: Joe Bergeron of Sky & Telescope magazine.

    The results of Gravity Probe B give physicists renewed confidence that the strange predictions of Einstein’s theory are indeed correct, and that these predictions may be applied elsewhere. The type of spacetime vortex that exists around Earth is duplicated and magnified elsewhere in the cosmos–around massive neutron stars, black holes, and active galactic nuclei.

    “If you tried to spin a gyroscope in the severely twisted space-time around a black hole,” says Will, “it wouldn’t just gently precess by a fraction of a degree. It would wobble crazily and possibly even flip over.”

    In binary black hole systems–that is, where one black hole orbits another black hole–the black holes themselves are spinning and thus behave like gyroscopes. Imagine a system of orbiting, spinning, wobbling, flipping black holes! That’s the sort of thing general relativity predicts and which GP-B tells us can really be true.

    The scientific legacy of GP-B isn’t limited to general relativity. The project also touched the lives of hundreds of young scientists:

    “Because it was based at a university many students were able to work on the project,” says Everitt. “More than 86 PhD theses at Stanford plus 14 more at other Universities were granted to students working on GP-B. Several hundred undergraduates and 55 high-school students also participated, including astronaut Sally Ride and eventual Nobel Laureate Eric Cornell.”

    NASA funding for Gravity Probe B began in the fall of 1963. That means Everitt and some colleagues have been planning, promoting, building, operating, and analyzing data from the experiment for more than 47 years—truly, an epic effort.

    What’s next?

    Everitt recalls some advice given to him by his thesis advisor and Nobel Laureate Patrick M.S. Blackett: “If you can’t think of what physics to do next, invent some new technology, and it will lead to new physics.”

    “Well,” says Everitt, “we invented 13 new technologies for Gravity Probe B. Who knows where they will take us?”

    This epic might just be getting started, after all….

    See the full article here.

    NASA leads the nation on a great journey of discovery, seeking new knowledge and understanding of our planet Earth, our Sun and solar system, and the universe out to its farthest reaches and back to its earliest moments of existence. NASA’s Science Mission Directorate (SMD) and the nation’s science community use space observatories to conduct scientific studies of the Earth from space to visit and return samples from other bodies in the solar system, and to peer out into our Galaxy and beyond. NASA’s science program seeks answers to profound questions that touch us all:

    This is NASA’s science vision: using the vantage point of space to achieve with the science community and our partners a deep scientific understanding of our planet, other planets and solar system bodies, the interplanetary environment, the Sun and its effects on the solar system, and the universe beyond. In so doing, we lay the intellectual foundation for the robotic and human expeditions of the future while meeting today’s needs for scientific information to address national concerns, such as climate change and space weather. At every step we share the journey of scientific exploration with the public and partner with others to substantially improve science, technology, engineering and mathematics (STEM) education nationwide.


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  • richardmitnick 4:57 pm on April 30, 2013 Permalink | Reply
    Tags: , , , , Gravity,   

    From Symmetry: “Matter, antimatter, we all fall down—right?” 

    April 30, 2013
    Ashley WennersHerron

    Scientists perform the first direct investigation into how antimatter interacts with gravity.

    What goes up must come down, the saying goes. But things might work a little differently with antimatter.
    A CERN-based experiment has taken the first step in investigating exactly how antimatter interacts with gravity.

    Photo: CERN

    Atimatter particles should mimic those of matter particles. If it turns out that there is a difference, it will be a sign of dramatically new physics.
    CERN ALPHA NewSo far, no one has been able to test directly how antimatter interacts with gravity—but the ALPHA experiment has begun to try.

    The ALPHA experiment’s main purpose is to trap and study antihydrogen atoms, the antimatter partners of hydrogen atoms. The antihydrogen atoms are held in place inside a tube by magnetic forces. Physicists on ALPHA have trapped more than 500 antiatoms since 2010. They keep them in their trap for up to about 16 minutes. When they turn off their magnets, the antiatoms fall out of the trap. A highly sensitive detector tracks the antiatoms and records where they first come in contact with matter and annihilate.”

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.

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