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  • richardmitnick 12:33 pm on December 11, 2016 Permalink | Reply
    Tags: Alfred Einstein, Ernst Mach, General Relativity, Michele Besso, ,   

    From Nautilus: “When Einstein Tilted at Windmills” 

    Nautilus

    Nautilus

    December 1, 2016
    By Amanda Gefter

    1
    Illustrations by Jasu Hu

    The young physicist’s quest to prove the theories of Ernst Mach.

    When they met, Einstein wasn’t Einstein yet. He was just Albert Einstein, a kid, about 17, with a dark cloud of teenage angst and a violin. Michele Besso was older, 23, but a kindred spirit. Growing up in Trieste, Italy he had shown an impressive knack for mathematics, but he was kicked out of high school for insubordination and had to go live with his uncle in Rome.

    2

    Einstein could relate. At the Swiss Polytechnic, where he was now a student, his professors resented his intellectual arrogance, and had begun locking him out of the library out of spite.

    Their first encounter was on a Saturday night in Zurich, 1896. They were at Selina Caprotti’s house by the lake for one of her music parties. Einstein was handsome—dark hair, moustache, soulful brown eyes. Besso was short with narrow, pointed features and a thick pile of coarse black hair on his head and chin. Einstein had a look of cool detachment. Besso had the look of a nervous mystic. As they chatted, Einstein learned that Besso worked at an electrical machinery factory; Besso learned that Einstein was studying physics. Perhaps they recognized something in each other then: They both wanted to get to the truth of things.

    Besso would go on to become a sidekick, of sorts, to Einstein—a sounding board, as Einstein put it, “the best in Europe,” asking the right questions that would inspire Einstein to find the right answers. At times, though, he would seem to be something more—a collaborator, perhaps, making suggestions, working through calculations.

    At other times he’d be the perfect fool—a schlemiel, Einstein called him. Like the time Besso was sent on a job to inspect some newly installed power lines on the outskirts of Milan but missed his train and then forgot to go the following day. On the third day he finally made it to his destination, but by that time he’d completely forgotten what he was supposed to be doing there in the first place. He sent a postcard to his boss: “Instructions should be wired.”

    If Besso never seemed to know quite what he was doing, it wasn’t for a lack of smarts. “The great strength of Besso resides in his intelligence,” Einstein would write, “which is out of the ordinary, and in his endless devotion to both his moral and professional obligations; his weakness is his truly insufficient spirit of decision. This explains why his successes in life do not match up with his brilliant aptitudes and with his extraordinary scientific and technical knowledge.”

    Still other times, Besso would play the role of Einstein’s conscience—urging him to work things out with his future wife, Mileva, or to be a better father to his sons. Besso took care of those sons on Einstein’s behalf when Mileva was sick. “Nobody else is so close to me, nobody knows me so well,” Einstein would write in 1918.

    But there was something uncanny about Besso. Over the coming years, he would always show up at exactly the right moment, the perfect deus ex machina, handing Einstein books, innocently offering suggestions, prodding him, goading him, nudging him onto the right path, as if he had a plan. “I … watch my friend Einstein struggle with the great Unknown,” he would write, “the work and torment of a giant, of which I am the witness—a pygmy witness—but a pygmy witness endowed with clairvoyance.”

    That Saturday night, though, all of that lay in the future. For now, they became fast friends—best friends, really. They talked for hours on end. For his first act of camaraderie, Besso handed Einstein two books, insisting that he read them. They were the works of Ernst Mach, the final actor in this three-man play.

    Perhaps you’ve heard of Ernst Mach. Mach 1, Mach 2, Mach 3, that Mach.

    3
    Ernst Mach

    His name is a unit of speed, and—despite his beard—a brand of razors. He was a physicist, a physiologist, a philosopher. A little bit of everything, really. You could find the young Mach in the Austrian countryside carefully observing nature—staring at a leaf or a shadow or a cloud with the utmost concentration and scrutiny, then scrutinizing his scrutinizing, noting his every sensory glitch and glimmer, building a taxonomy of tricks that our eyes can play. He collected bugs and butterflies. He tested the reactions of various materials—in trying to see whether camphor would ignite, he burned off his eyelashes and eyebrows. But it was when he was 15 years old that a single moment changed everything.

    “On a bright summer day in the open air, the world with my ego suddenly appeared to me as one coherent mass of sensations,” he later wrote. He felt, in that moment, there was no reality sitting “out there,” independent of his sensations, and likewise that there was no self sitting “in here,” independent of its sensations. He grew certain that there could be no real difference between mind and matter, between perceiving subject and perceived object. “This moment was decisive for my whole view,” he wrote.

    From that day forward, he vehemently rejected any form of dualism: the idea that the external world was made up of substantial material objects—things—while the mind was made of something else, so that the world we experience in consciousness is a mere copy of an actual world that lies forever hidden from us. Instead he grew convinced that mind and matter were made of the same basic ingredient. It couldn’t be a physical ingredient, he argued, because how would bare matter ever give rise to subjective experience? But it couldn’t be a mental ingredient either, he said, because he was certain that the self was equally an illusion. The only way to unite mind and matter, he decided, was to presume that they were made not of objective atoms, and not of subjective qualia, but of some neutral thing, an “element,” he called it, which in one configuration would behave as material substance and in another as immaterial mentation, though in itself it would be neither and nothing.

    “There is no rift between the psychical and the physical, no inside and outside, no “sensation” to which an external “thing,” different from sensation, corresponds,” he wrote. “There is but one kind of elements, out of which this supposed inside and outside are formed—elements which are themselves inside or outside, according to the aspect in which, for the time being, they are viewed.” These elements “form the real, immediate, and ultimate foundation.”

    Mach’s view—neutral monism, it would later be called—required that every single aspect of reality, from physical objects to subjective sensations, be purely relational, so that whether something was “mind” or “matter” was determined solely by its relations with other elements and not by anything inherent to itself. It was a radical idea, but it seemed plausible. After all, Mach said, science is based on measurement, but “the concept of measurement is a concept of relation.” What we call length or weight, for instance, is really the relation between an object and a ruler, or an object and a scale.

    It dawned on Mach, then, that if we could rewrite science solely in terms of what can measured, then the world could be rendered entirely relational—entirely relative—and the mind and universe could be unified at last. But that was going to require a new kind of physics.

    By 1904, Don Quixote had become one of Einstein’s favorite books.

    Two years earlier, an unemployed Einstein had put an ad in the newspaper offering physics tutoring for three francs an hour, and a philosophy student named Maurice Solovine had shown up at his door. They started talking about physics and philosophy and didn’t stop; the whole tutoring thing never even came up. Soon Conrad Habicht, a mathematics student, joined the conversation, and the three young bohemians formed something of a book club for highbrowed degenerates. They read works of philosophy and literature and discussed them, sometimes until one in the morning, smoking, eating cheap food, getting rowdy and waking the neighbors. They met several nights a week. In mockery of stuffy academia, they dubbed themselves the Olympia Academy.

    Besso was in Trieste working as an engineering consultant, but he came when he could, and as Einstein’s closest friend, he was made an honorary member of the Academy. Under Besso’s influence, the Olympians read and discussed Mach. Eventually Einstein landed a job at the Patent Office in Bern, and in 1904 he got Besso a job in the same office, so they could work side by side. In the evenings, the Academy read Don Quixote. It struck a chord with Einstein—later, when his sister Maja lay dying, he would read it to her. As for the Olympia boys, who can say whether they noticed it then: how Besso had become the Sancho Panza to Einstein’s Quixote. When Solovine and Habicht left, it was just Einstein and Besso, walking home together from the patent office, discussing the nature of space and time and, as always, Mach.

    Mach’s plan to unite matter and mind required that every last bit of world be rendered relative, with nothing left over. But there was one stubborn obstacle standing in the way: According to physics, all motion was defined relative to absolute space, but absolute space wasn’t defined relative to anything. It just existed, self-defined, like the basement level of reality—it wouldn’t budge. Mach knew of this obstacle, and it rankled. He criticized Newton’s “conceptual monstrosity of absolute space”—the idea of space as a thing unto itself. But how to get around it?

    For years it had been bugging Einstein that all attempts on an observer’s part to determine whether or not he was at rest relative to absolute space were doomed to fail. For every experiment he could think of, nature seemed to have a clever trick up its sleeve to hide any evidence of absolute motion. It was so downright conspiratorial that one might suspect, as Einstein did, that absolute space simply didn’t exist.

    Following Mach’s lead, Einstein wanted to assert that motion was not defined by reference to absolute space, but only relative to other motion. Unfortunately, the laws of physics seemed to suggest otherwise. The laws of electromagnetism, in particular, insisted that light had to travel at 186,000 miles per second regardless of the observer’s frame of reference. But if all motion was relative, the light’s motion would have to be relative too—traveling 186,000 miles per second in one reference frame and some other speed in another, in blatant violation of electromagnetic law.

    So Einstein went to see Besso. “Today I come here to battle against that problem with you,” he announced when he arrived.

    They discussed the situation from every angle. Einstein was ready to give up, but they hammered away.

    The next day, Einstein returned. “Thank you,” he said. “I’ve completely solved the problem.” Within five weeks, his Theory of Special Relativity was complete.

    What magic words had Besso uttered in that fateful conversation? It seems he reminded Einstein of Mach’s central idea: a measurement is always a relation.

    Einstein and Besso discussed this—what two quantities we compare in order to measure time. “All our judgments in which time plays a part are always judgments of simultaneous events,” Einstein realized. “If, for instance, I say, ‘That train arrives here at 7 o’clock,’ I mean something like this: ‘The pointing of the small hand of my watch to 7 and the arrival of the train are simultaneous events.”

    But how does one know that two events are simultaneous? Perhaps you’re standing still and you see two distant lights flash at precisely the same moment. They’re simultaneous. But what if you had been moving? If you happened to be moving in the direction of flash A and away from flash B, you’d see A happen first, because B’s light would take ever so slightly longer to reach you.

    Simultaneity is not absolute. There’s no single “now” in which all observers live. Time is relative. Space, too.

    It all dawned on Einstein then: It was possible for all observers to see light moving at exactly 186,000 miles per second regardless of their own state of motion. The light’s speed is a measure of how much distance it covers in a given amount of time. But time changes depending on your state of motion. So even if you’re moving relative to the light, time itself will slow down precisely long enough for you to measure light’s speed at the very one required by Maxwell’s equations.

    Einstein’s 1905 paper On the Electrodynamics of Moving Bodies introduced the world to the theory of relativity, in which time and space can slow and stretch to account for an observer’s relative motions. It included no references whatsoever, but it ended with this final paragraph: “In conclusion I wish to say that in working at the problem here dealt with I have had the loyal assistance of my friend and colleague M. Besso, and that I am indebted to him for several valuable suggestions.”

    Einstein proudly sent his work to Mach, and seemed almost giddy when Mach responded with his approval. “Your friendly letter gave me enormous pleasure,” Einstein replied. “I am very glad that you are pleased with the relativity theory … Thanking you again for your friendly letter, I remain, your student, A. Einstein.”

    Einstein had a long way to go, however, to see Mach’s vision through. The problem was that special relativity only relativized motion for observers moving at a constant speed. The question of accelerated observers—those who were changing speed or rotating—was far trickier. Within special relativity, there was no way to blame the force that comes with acceleration on relative motion. Absolute space lingered.

    In 1907, Einstein made a breakthrough. It was the happiest thought of his life, he would later say: In small regions of space, an observer would be unable to tell whether he was accelerating or at rest in a gravitational field. This suggested that it might be possible to do away with the absolute nature of acceleration—and with it absolute space—once and for all. Gravity, it seemed, was the secret ingredient that made all motion relative, just as Mach had wanted. And that gave a whole new meaning to the very nature of gravity: The path of an accelerated observer through spacetime traces a curve, so if acceleration was equivalent to gravity, then gravity was the curvature of spacetime. It would be some time before Einstein brought his General Theory of Relativity to fruition, but for now, he knew he was on the right track.

    Excited, Einstein wrote a letter to Mach informing him of his progress and the publication of his newest paper. A new theory of gravity was underway, he said, and as soon as he could prove it correct, “your inspired investigations into the foundations of mechanics … will receive a splendid confirmation.” In other words: I’ve done what you wanted. He published his theory of general relativity in 1915; the next year, Mach died.

    Einstein wrote a long and moving obituary, glowing with praise for Mach’s scientific vision, with its central point, as Einstein wrote, that “physics and psychology are to be distinguished from each other not by the objects they study but only by the manner of ordering and relating them.” He argued that Mach himself was close to coming up with the theory of relativity, and wrote, with palpable admiration and innocence, that Mach “helped me a lot, both directly and indirectly.”

    That, however, was the apogee of the kinship between Einstein and Mach’s philosophy. Einstein would eventually disavow the pure relativism of his mentor, and even to split from his Sancho. The rift begins with a most unlikely event: words from beyond the grave.

    In 1921, Mach’s book The Principles of Physical Optics was published posthumously, and contained a preface written by the author around 1913, shortly after Einstein had sent him the early paper on general relativity.

    “I am compelled in what may be my last opportunity, to cancel my views of the relativity theory,” Mach wrote. “I gather from the publications which have reached me, and especially from my correspondence, that I am gradually becoming regarded as the forerunner of relativity … I must as assuredly disclaim to be a forerunner of the relativists …”

    Mach had likely seen what Einstein would only later come to terms with—that the so-called general theory of relativity did not live up to its name. General relativity was an unprecedented intellectual feat—but it didn’t make everything relative, as Mach had dreamed. In the final version of the theory, the equivalence between acceleration and gravitation, which had seemed to make all motion relative, turned out to hold only for infinitesimally small regions of space. Patching together local regions into one big universe produced misalignments at their edges, like flat tiles on a round globe. The misalignments revealed the curvature of spacetime—a global geometry that couldn’t be transformed away by a mere change in perspective. Each local region—a self-consistent, relative world—turned out to be the tiny tip of an enormous, four-dimensional iceberg, forever hidden from sight and decidedly not relative.

    It must have been an unsettling feeling for Einstein—watching his theory gather steam and speed away from him, proving the very thing he had set out to disprove. The problem was that, according to the theory, spacetime geometry was not fully determined by the distribution of matter in the universe, so that even if you removed everything observable, some extra ingredient still remained—spacetime itself, dynamic yet absolute. It created an unbridgeable divide between the physical world and the mind, inviting, in its realist stance, a whiff of pure belief, even mysticism—the belief in a four-dimensional substratum, the paper on which reality is drawn, though the paper itself is invisible.

    Einstein continued to push Mach’s view for several years after publishing general relativity in 1915, living in total denial of the fact that his own theory went against it. He tried everything under the sun to mold his theory into the shape of Mach’s philosophy—making the universe finite but unbounded, adding a cosmological constant—but it just wouldn’t fit. “The necessity to uphold [Mach’s principle] is by no means shared by all colleagues,” he said, “but I myself feel it is absolutely necessary to satisfy it.”

    So when Einstein first read Mach’s preface, it must have stung. We can hear his hurt in a comment he made at a lecture in Paris in 1922, shortly after Mach’s preface was published. Mach was un bon mecanicien, Einstein said bitterly, but a “deplorable philosophe.” He would no longer claim that his theory was one of Machian relativism, and by 1931 he would abandon Mach’s views completely. “The belief in an external world independent of the perceiving subject is the basis of all natural science,” he wrote. When asked how he could believe in anything beyond our sensory experience, he replied: “I cannot prove my conception is right, but that is my religion.” And in 1954, a year before his death: “We ought not to speak about the Machian Principle anymore.”

    What Mach had never known—couldn’t have known—was that his true devotee had never been Einstein. It was Besso.

    Besso, that pygmy witness endowed with clairvoyance, saw exactly where Einstein’s departure from Mach would soon lead him astray: in the realm of quantum mechanics.

    As Einstein came to grips with Mach’s rejection of relativity, the world of physics was rocked by quantum theory, a revolution Einstein had helped to spark but now refused to join. While he was making peace with an absolute spacetime—an absolute reality—quantum mechanics was rendering the world even more relative. The theory suggested that the outcomes of measurements could be defined only in relation to a given experiment: An electron might be a wave relative to one measuring apparatus and a particle relative to another, though in itself it was neither and nothing. In the words of Niels Bohr, the purpose of the theory was “to track down, so far as it is possible, relations between the manifold aspects of our experience”—relations and nothing more. In other words, quantum theory picked up Mach’s program right where Einstein left off, a point that both Bohr and Besso were quick to emphasize.

    When Einstein, complaining about a colleague’s work, joked to Besso that, “He rides Mach’s poor horse to exhaustion,” Besso replied, “As to Mach’s little horse, we should not insult it; did it not make possible the infernal journey through the relativities? And who knows—in the case of the nasty quanta, it may also carry Don Quixote de la Einsta through it all!”

    “I do not inveigh against Mach’s little horse,” Einstein responded, “but you know what I think about it. It cannot give birth to anything living.”

    The truth was, Einstein’s belief in a hidden reality had lain dormant for years, ever since he was a little boy—4, maybe 5—and his father had come to his bedside and handed him a compass. Einstein had held it in his hand, and found himself trembling in awe. The way the needle quivered, tugged northward by some invisible force, overwhelmed him with the feeling that “something deeply hidden had to be behind things.” Now he glimpsed it again in the mathematics of general relativity. With Mach’s approval moot, the awe he’d felt as a boy returned to him. When Besso tried to steer him away—toward Mach, toward the quantum— Einstein reproached his faithful squire: “It appears that you do not take the four-dimensionality of reality seriously.”

    The reinvention of Einstein as a young iconoclast who embraced Mach’s view and ran with it, determined to create a theory of pure relativity despite his natural realist leanings—was it actually Besso’s doing? Had the squire steered his master? In the short story “The Truth About Sancho Panza,” Franz Kafka suggests that this reversal is, in fact, the key to Cervantes’ tale. Don Quixote, he wrote, was Sancho Panza’s own creation, an alter ego invented to carry out some inner vision Panza himself was ill equipped to face. “I owe to you the scientific synthesis that without such a friendship one would never have acquired—at least, not without expending all one’s personal forces,” Besso wrote to Einstein—as if to say, thanks for working out that theory for me. But the synthesis was incomplete. Having guided Einstein to water, Besso appears to have failed to make him drink.

    Besso never gave up on luring Einstein back to Machian relativity. But Don Quixote had abandoned the knighthood for good, leaving Sancho to fend off the windmills for himself. In Princeton, New Jersey, his hair now white and wild, Einstein sat at a cluttered desk and struggled with reality while physics marched on without him. In Geneva, Switzerland, in the University mathematics library, his wiry beard now blanched with time, Besso sat hunched over his own pile of books, and worked—quietly, mysteriously—alone.

    See the full article here .

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  • richardmitnick 12:10 pm on April 26, 2016 Permalink | Reply
    Tags: , , CNES Microscope satellite, General Relativity,   

    From phys.org: “Einstein’s theory of relativity faces satellite test” 

    physdotorg
    phys.org

    April 26, 2016

    CNES Microscope satellite
    CNES Microscope satellite, which scientists hope will help find a gap in the general relativity theory developed by Albert Einstein

    Einstein’s theory of general relativity is to be put to the test by a newly launched satellite in an experiment that could upend our understanding of physics.

    The French “Microscope” orbiter will try to poke a hole in one of Einstein’s most famous theories, which provides the basis for our modern understanding of gravity.

    Scientists will use the kit to measure how two different pieces of metal—one titanium and the other a platinum-rhodium alloy—behave in orbit.

    “In space, it is possible to study the relative motion of two bodies in almost perfect and permanent free fall aboard an orbiting satellite, shielded from perturbations encountered on Earth,” said Arianespace, which put the satellite into orbit on Monday.

    Einstein’s theory suggests that in perfect free-fall, the two objects should move in exactly the same way. But if they are shown to behave differently “the principle will be violated: an event that would shake the foundations of physics”, Arianespace added.

    Also aboard the Russian Soyuz rocket launched from French Guiana was an Earth-observation satellite equipped with radar to monitor the planet’s surface to track climate and environmental change and help in disaster relief operations.

    That satellite, along with another launched two years ago, is part of the 3.8-billion-euro ($4.3-billion) Copernicus project, which will ultimately boast six orbiters in all.

    Three previous launches from Arianespace’s Spaceport in French Guiana, an overseas territory that borders Brazil, were delayed by poor weather and technical issues.

    A countdown on Sunday was halted after scientists observed an “anomaly”, the agency said in an earlier statement, while adverse weather conditions had thwarted other attempts.

    See the full article here .

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

    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*.

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

    NASA Chandra Telescope
    NASA/Chandra

    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.

    3
    Perimeter postdoctoral researcher Tim Johannsen.

    4
    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.

    6
    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
    NASA LISA
    LISA

    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 4:19 pm on January 11, 2016 Permalink | Reply
    Tags: , , General Relativity, ,   

    From Ethan Siegel: “A distant galaxy cluster and the power of Einstein’s gravity” 

    Starts with a bang
    Starts with a Bang

    1.11.16
    Ethan Siegel

    Temp 1
    Image credit: NASA, ESA, and G. Tremblay (European Southern Observatory).

    The ability for mass to bend and magnify background light is a unique feature of General Relativity. But it can fool us, too.

    “Gravitational and electromagnetic interactions are long-range interactions, meaning they act on objects no matter how far they are separated from each other.” -Francois Englert

    A century ago, [Albert] Einstein put forth a new theory of gravity: General Relativity. The solar eclipse of 1919 finally confirmed that mass gravitationally bent light around it.

    2
    Images credit: New York Times, 10 November 1919 (L); Illustrated London News, 22 November 1919 (R).

    But only much later was the phenomenon of gravitational lensing confirmed: where a distant galaxy cluster acted as a lens, magnifying and distorting the background galaxies behind it.


    view the mp4 video here.

    In 2014, the Hubble Space Telescope imaged an ultra-massive galaxy cluster found by the Sloan Digital Sky Survey3 [SDSS], and unveiled what appeared to be a spectacular, multiply-imaged distortion of blue, star-forming background galaxies.

    NASA Hubble Telescope
    NASA/ESA Hubble

    SDSS Telescope
    SDSS telescope at Apache Point, NM, USA

    3
    Image credit: NASA, ESA, and G. Tremblay (European Southern Observatory).

    The multiple images of similar structures, the distortions and the similar colorations all pointed to gravitational lensing.

    Temp 4
    Image credit: NASA, ESA, and G. Tremblay (European Southern Observatory).

    But a careful analysis of the data showed that while the outer arcs are indeed lensed background galaxies…

    5
    Image credit: K. Sharon et al., 2014, via http://arxiv.org/abs/1407.2266.

    the brightest blue lights, interconnecting the two giant ellipticals at the cluster’s center, come from the merger of the galaxies and the surrounding gas themselves.

    6
    Image credit: NASA, ESA, and G. Tremblay (European Southern Observatory).

    What we’re looking at is a combination of the stars and galaxies of the foregrounds cluster, some 4,000 times as massive as the Milky Way, a transient burst of star formation, and only a few background objects.


    view mp4 video here.

    Despite our excellent intuition, there’s no substitute for good data.

    7
    Image credit: NASA, ESA, and G. Tremblay (European Southern Observatory).

    See the full article here .

    Please help promote STEM in your local schools.

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    “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,” says Ethan

     
  • richardmitnick 4:24 pm on December 24, 2015 Permalink | Reply
    Tags: , General Relativity, , , ,   

    From Ethan Siegel: “What Are Quantum Gravity’s Alternatives To String Theory?” 

    Starts with a bang
    Starts with a Bang

    12.24.15
    Ethan Siegel

    1
    Image credit: CPEP (Contemporary Physics Education Project), NSF/DOE/LBNL.

    If there is a quantum theory of gravity, is String Theory the only game in town?

    “I just think too many nice things have happened in string theory for it to be all wrong. Humans do not understand it very well, but I just don’t believe there is a big cosmic conspiracy that created this incredible thing that has nothing to do with the real world.” –Edward Witten

    The Universe we know and love — with [Albert] Einstein’s General Relativity as our theory of gravity and quantum field theories of the other three forces — has a problem that we don’t often talk about: it’s incomplete, and we know it. Einstein’s theory on its own is just fine, describing how matter-and-energy relate to the curvature of space-and-time. Quantum field theories on their own are fine as well, describing how particles interact and experience forces. Normally, the quantum field theory calculations are done in flat space, where spacetime isn’t curved. We can do them in the curved space described by Einstein’s theory of gravity as well (although they’re harder — but not impossible — to do), which is known as semi-classical gravity. This is how we calculate things like Hawking radiation and black hole decay.

    2
    Image credit: NASA, via http://www.nasa.gov/topics/universe/features/smallest_blackhole.html

    But even that semi-classical treatment is only valid near and outside the black hole’s event horizon, not at the location where gravity is truly at its strongest: at the singularities (or the mathematically nonsensical predictions) theorized to be at the center. There are multiple physical instances where we need a quantum theory of gravity, all having to do with strong gravitational physics on the smallest of scales: at tiny, quantum distances. Important questions, such as:

    What happens to the gravitational field of an electron when it passes through a double slit?
    What happens to the information of the particles that form a black hole, if the black hole’s eventual state is thermal radiation?
    And what is the behavior of a gravitational field/force at and around a singularity?

    3
    Image credit: Nature 496, 20–23 (04 April 2013) doi:10.1038/496020a, via http://www.nature.com/news/astrophysics-fire-in-the-hole-1.12726.

    In order to explain what happens at short distances in the presence of gravitational sources — or masses — we need a quantum, discrete, and hence particle-based theory of gravity. The known quantum forces are mediated by particles known as bosons, or particles with integer spin. The photon mediates the electromagnetic force, the W-and-Z bosons mediate the weak force, while the gluons mediate the strong force. All these types of particles have a spin of 1, which for massive (W-and-Z) particles mean they can take on spin values of -1, 0, or +1, while for massless ones (like gluons and photons), they can take on values of -1 or +1 only.

    The Higgs boson is also a boson, although it doesn’t mediate any forces, and has a spin of 0. Because of what we know about gravitation — General Relativity is a tensor theory of gravity — it must be mediated by a massless particle with a spin of 2, meaning it can take on a spin value of -2 or +2 only.

    This is fantastic! It means that we already know a few things about a quantum theory of gravity before we even try to formulate one! We know this because whatever the true quantum theory of gravity turns out to be, it must be consistent with General Relativity when we’re not at very small distances from a massive particle or object, just as — 100 years ago — we knew that General Relativity needed to reduce to Newtonian gravity in the weak-field regime.

    4
    Image credit: NASA, of an artist’s concept of Gravity Probe B orbiting the Earth to measure space-time curvature.

    NASA Gravity Probe B
    Gravity Probe B

    The big question, of course is how? How do you quantize gravity in a way that’s correct (at describing reality), consistent (with both GR and QFT), and hopefully leads to calculable predictions for new phenomena that might be observed, measured or somehow tested. The leading contender, of course, is something you’ve long heard of: String Theory.

    String Theory is an interesting framework — it can include all of the standard model fields and particles, both the fermions and the bosons.

    0
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    It includes also a 10-dimensional Tensor-Scalar theory of gravity: with 9 space and 1 time dimensions, and a scalar field parameter. If we erase six of those spatial dimensions (through an incompletely defined process that people just call compactification) and let the parameter (ω) that defines the scalar interaction go to infinity, we can recover General Relativity.

    5
    Image credit: NASA/Goddard/Wade Sisler, of Brian Greene presenting on String Theory.

    But there are a whole host of phenomenological problems with String Theory. One is that it predicts a large number of new particles, including all the supersymmetric ones, none of which have been found.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    It claims to not need to need “free parameters” like the standard model has (for the masses of the particles), but it replaces that problem with an even worse one. String theory refers to “10⁵⁰⁰ possible solutions,” where these solutions refer to the vacuum expectation values of the string fields, and there’s no mechanism to recover them; if you want String Theory to work, you need to give up on dynamics, and simply say, “well, it must’ve been anthropically selected.” There are frustrations, drawbacks, and problems with the very idea of String Theory. But the biggest problem with it may not be these mathematical ones. Instead, it may be that there are four other alternatives that may lead us to quantum gravity instead; approaches that are completely independent of String Theory.

    6
    Image credit: Wikimedia Commons user Linfoxman, of an illustration of a quantized “fabric of space.”

    1.) Loop Quantum Gravity [reader, please take the time to visit this link and read the article]. LQG is an interesting take on the problem: rather than trying to quantize particles, LQG has as one of its central features that space itself is discrete. Imagine a common analogy for gravity: a bedsheet pulled taut, with a bowling ball in the center. Rather than a continuous fabric, though, we know that the bedsheet itself is really quantized, in that it’s made up of molecules, which in turn are made of atoms, which in turn are made of nuclei (quarks and gluons) and electrons.

    Space might be the same way! Perhaps it acts like a fabric, but perhaps it’s made up of finite, quantized entities. And perhaps it’s woven out of “loops,” which is where the theory gets it name from. Weave these loops together and you get a spin network, which represents a quantum state of the gravitational field. In this picture, not just the matter itself but space itself is quantized. The way to go from this idea of a spin network to a perhaps realistic way of doing gravitational computations is an active area of research, one that saw a tremendous leap forward made in just 2007/8, so this is still actively advancing.

    7
    Image credit: Wikimedia Commons user & reasNink, generated with Wolfram Mathematica 8.0.

    2.) Asymptotically Safe Gravity. This is my personal favorite of the attempts at a quantum theory of gravity. Asymptotic freedom was developed in the 1970s to explain the unusual nature of the strong interaction: it was a very weak force at extremely short distances, then got stronger as (color) charged particles got farther and farther apart. Unlike electromagnetism, which had a very small coupling constant, the strong force has a large one. Due to some interesting properties of QCD, if you wound up with a (color) neutral system, the strength of the interaction fell off rapidly. This was able to account for properties like the physical sizes of baryons (protons and neutrons, for example) and mesons (pions, for example).

    Asymptotic safety, on the other hand, looks to solve a fundamental problem that’s related to this: you don’t need small couplings (or couplings that tend to zero), but rather for the couplings to simply be finite in the high-energy limit. All coupling constants change with energy, so what asymptotic safety does is pick a high-energy fixed point for the constant (technically, for the renormalization group, from which the coupling constant is derived), and then everything else can be calculated at lower energies.

    At least, that’s the idea! We’ve figured out how to do this in 1+1 dimensions (one space and one time), but not yet in 3+1 dimensions. Still, progress has been made, most notably by Christof Wetterich, who had two ground breaking papers in the 1990s. More recently, Wetterich used asymptotic safety — just six years ago — to calculate a prediction for the mass of the Higgs boson before the LHC found it. The result?

    9
    Image credit: Mikhail Shaposhnikov & Christof Wetterich.

    Amazingly, what it indicated was perfectly in line with what the LHC wound up finding.

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

    It’s such an amazing prediction that if asymptotic safety is correct, and — when the error bars are beaten down further — the masses of the top quark, the W-boson and the Higgs boson are finalized, there may not even be a need for any other fundamental particles (like SUSY particles) for physics to be stable all the way up to the Planck scale. It’s not only very promising, it has many of the same appealing properties of string theory: quantizes gravity successfully, reduces to GR in the low energy limit, and is UV-finite. In addition, it beats string theory on at least one account: it doesn’t need the addition of new particles or parameters that we have no evidence for! Of all the string theory alternatives, this one is my favorite.

    3.) Causal Dynamical Triangulations. This idea, CDT, is one of the new kids in town, first developed only in 2000 by Renate Loll and expanded on by others since. It’s similar to LQG in that space itself is discrete, but is primarily concerned with how that space itself evolves. One interesting property of this idea is that time must be discrete as well! As an interesting feature, it gives us a 4-dimensional spacetime (not even something put in a priori, but something that the theory gives us) at the present time, but at very, very high energies and small distances (like the Planck scale), it displays a 2-dimensional structure. It’s based on a mathematical structure called a simplex, which is a multi-dimensional analogue of a triangle.

    10
    Image credit: screenshot from the Wikipedia page for Simplex, via https://en.wikipedia.org/wiki/Simplex.

    A 2-simplex is a triangle, a 3-simplex is a tetrahedron, and so on. One of the “nice” features of this option is that causality — a notion held sacred by most human beings — is explicitly preserved in CDT. (Sabine has some words on CDT here, and its possible relation to asymptotically safe gravity.) It might be able to explain gravity, but it isn’t 100% certain that the standard model of elementary particles can fit suitably into this framework. It’s only major advances in computation that have enabled this to become a fairly well-studied alternative of late, and so work in this is both ongoing and relatively young.

    4.) Emergent gravity. And finally, we come to what’s probably the most speculative, recent of the quantum gravity possibilities. Emergent gravity only gained prominence in 2009, when Erik Verlinde proposed entropic gravity, a model where gravity was not a fundamental force, but rather emerged as a phenomenon linked to entropy. In fact, the seeds of emergent gravity go back to the discoverer of the conditions for generating a matter-antimatter asymmetry, Andrei Sakharov, who proposed the concept back in 1967. This research is still in its infancy, but as far as developments in the last 5–10 years go, it’s hard to ask for more than this.

    11
    Image credit: flickr gallery of J. Gabas Esteban.

    We’re sure we need a quantum theory of gravity to make the Universe work at a fundamental level, but we’re not sure what that theory looks like or whether any of these five avenues (string theory included) are going to prove fruitful or not. String Theory is the best studied of all the options, but Loop Quantum Gravity is a rising second, with the others being given serious consideration at long last. They say the answer’s always in the last place you look, and perhaps that’s motivation enough to start looking, seriously, in newer places.

    See the full article here .

    Please help promote STEM in your local schools.

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    “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,” says Ethan

     
  • richardmitnick 8:20 am on December 12, 2015 Permalink | Reply
    Tags: , , General Relativity,   

    From Daily Galaxy: “”Gravity Alters the Quantum Nature of Particles on Earth” –What Does It Imply at Cosmological Scales?” 

    Daily Galaxy
    The Daily Galaxy

    December 11, 2015
    University of Vienna

    1

    “It is quite surprising that gravity can play any role in quantum mechanics“, says Igor Pikovski, a theoretical physicist working at the Harvard-Smithsonian Center for Astrophysics:”Gravity is usually studied on astronomical scales, but it seems that it also alters the quantum nature of the smallest particles on Earth”. “It remains to be seen what the results imply on cosmological scales, where gravity can be much stronger”, adds Caslav Brukner University Professor at the University of Vienna and Director of the Institute for Quantum Optics and Quantum Information.

    In 1915 Albert Einstein formulated the theory of general relativity which fundamentally changed our understanding of gravity. He explained gravity as the manifestation of the curvature of space and time. Einstein’s theory predicts that the flow of time is altered by mass. This effect, known as “gravitational time dilation“, causes time to be slowed down near a massive object. It affects everything and everybody; in fact, people working on the ground floor will age slower than their colleagues a floor above, by about 10 nanoseconds in one year. This tiny effect has actually been confirmed in many experiments with very precise clocks.

    This past June, a team of researchers from the University of Vienna, Harvard University and the University of Queensland discovered that the slowing down of time can explain another perplexing phenomenon: the transition from quantum behavior to our classical, everyday world.

    The image below is an illustration of a molecule in the presence of gravitational time dilation. The molecule is in a quantum superposition of being several places at the same time.

    2

    Quantum theory, the other major discovery in physics in the early 20th century, predicts that the fundamental building blocks of nature show fascinating and mind-boggling behavior. Extrapolated to the scales of our everyday life quantum theory leads to situations such as the famous example of Schroedinger’s cat: the cat is neither dead nor alive, but in a so-called quantum superposition of both.

    4

    Yet such a behavior has only been confirmed experimentally with small particles and has never been observed with real-world cats. Therefore, scientists conclude that something must cause the suppression of quantum phenomena on larger, everyday scales. Typically this happens because of interaction with other surrounding particles.

    The research team, headed by ?aslav Brukner from the University of Vienna and the Institute of Quantum Optics and Quantum Information, found that time dilation also plays a major role in the demise of quantum effects. They calculated that once the small building blocks form larger, composite objects – such as molecules and eventually larger structures like microbes or dust particles -, the time dilation on Earth can cause a suppression of their quantum behavior.

    The tiny building blocks jitter ever so slightly, even as they form larger objects. And this jitter is affected by time dilation: it is slowed down on the ground and speeds up at higher altitudes. The researchers have shown that this effect destroys the quantum superposition and, thus, forces larger objects to behave as we expect in everyday life.

    The results of Pikovski and his co-workers reveal how larger particles lose their quantum behavior due to their own composition, if one takes time dilation into account. This prediction should be observable in experiments in the near future, which could shed some light on the fascinating interplay between the two great theories of the 20th century, quantum theory and general relativity.

    Publication in Nature Physics: “Universal decoherence due to gravitational time dilation”. I. Pikovski, M. Zych, F. Costa, C. Brukner. Nature Physics (2015) doi:10.1038/nphys3366

    See the full article here .

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  • richardmitnick 11:05 pm on November 24, 2015 Permalink | Reply
    Tags: , General Relativity, Leonard Susskind, ,   

    From Nature: “Theoretical physics: Complexity on the horizon” 2014 

    Nature Mag
    Nature

    28 May 2014
    Amanda Gefter

    Temp 1

    When physicist Leonard Susskind gives talks these days, he often wears a black T-shirt proclaiming “I ♥ Complexity”. In place of the heart is a Mandelbrot set, a fractal pattern widely recognized as a symbol for complexity at its most beautiful.

    1
    Initial image of a Mandelbrot set zoom sequence with a continuously colored environment

    That pretty much sums up his message. The 74-year-old Susskind, a theorist at Stanford University in California, has long been a leader in efforts to unify quantum mechanics with the general theory of relativityAlbert Einstein’s framework for gravity. The quest for the elusive unified theory has led him to advocate counter-intuitive ideas, such as superstring theory or the concept that our three-dimensional Universe is actually a two-dimensional hologram. But now he is part of a small group of researchers arguing for a new and equally odd idea: that the key to this mysterious theory of everything is to be found in the branch of computer science known as computational complexity.

    This is not a subfield to which physicists have tended to look for fundamental insight. Computational complexity is grounded in practical matters, such as how many logical steps are required to execute an algorithm. But if the approach works, says Susskind, it could resolve one of the most baffling theoretical conundrums to hit his field in recent years: the black-hole firewall paradox, which seems to imply that either quantum mechanics or general relativity must be wrong. And more than that, he says, computational complexity could give theorists a whole new way to unify the two branches of their science — using ideas based fundamentally on information.

    Behind a firewall

    It all began 40 years ago, when physicist Stephen Hawking at the University of Cambridge, UK, realized that quantum effects would cause a black hole to radiate photons and other particles until it completely evaporates away.

    As other researchers were quick to point out, this revelation brings a troubling contradiction. According to the rules of quantum mechanics, the outgoing stream of radiation has to retain information about everything that ever fell into the black hole, even as the matter falling in carries exactly the same information through the black hole’s event horizon, the boundary inside which the black hole’s gravity gets so strong that not even light can escape. Yet this two-way flow could violate a key law of quantum mechanics known as the no-cloning theorem, which dictates that making a perfect copy of quantum information is impossible.

    Happily, as Susskind and his colleagues observed (1) in 1995, nature seemed to sidestep any such violation by making it impossible to see both copies at once: an observer who remains outside the horizon cannot communicate with one who has fallen in. But in 2012, four physicists at the University of California, Santa Barbara — Ahmed Almheiri, Donald Marolf, Joseph Polchinski and James Sully, known collectively as AMPS — spotted a dangerous exception to this rule (2). They found a scenario in which an observer could decode the information in the radiation, jump into the black hole and then compare that information with its forbidden duplicate on the way down.

    AMPS concluded that nature prevents this abomination by creating a blazing firewall just inside the horizon that will incinerate any observer — or indeed, any particle — trying to pass through. In effect, space would abruptly end at the horizon, even though Einstein’s gravitational theory says that space must be perfectly continuous there. If AMPS’s theory is true, says Raphael Bousso, a theoretical physicist at the University of California, Berkeley, “this is a terrible blow to general relativity”.

    Does not compute

    Fundamental physics has been in an uproar ever since, as practitioners have struggled to find a resolution to this paradox. The first people to bring computational complexity into the debate were Stanford’s Patrick Hayden, a physicist who also happens to be a computer scientist, and Daniel Harlow, a physicist at Princeton University in New Jersey. If the firewall argument hinges on an observer’s ability to decode the outgoing radiation, they wondered, just how hard is that to do?

    Impossibly hard, they discovered. A computational-complexity analysis showed that the number of steps required to decode the outgoing information would rise exponentially with the number of radiation particles that carry it. No conceivable computer could finish the calculations until long after the black hole had radiated all of its energy and vanished, along with the forbidden information clones. So the firewall has no reason to exist: the decoding scenario that demands it cannot happen, and the paradox disappears.

    “The black hole’s interior is protected by an armour of computational complexity.”

    Hayden was sceptical of the result at first. But then he and Harlow found much the same answer for many types of black hole (3). “It did seem to be a robust principle,” says Hayden: “a conspiracy of nature preventing you from performing this decoding before the black hole had disappeared on you.”

    The Harlow–Hayden argument made a big impression on Scott Aaronson, who works on computational complexity and the limits of quantum computation at the Massachusetts Institute of Technology in Cambridge. “I regard what they did as one of the more remarkable syntheses of physics and computer science that I’ve seen in my career,” he says.

    It also resonated strongly among theoretical physicists. But not everyone is convinced. Even if the calculation is correct, says Polchinski, “it is hard to see how one would build a fundamental theory on this framework”. Nevertheless, some physicists are trying to do just that. There is a widespread belief in the field that the laws of nature must somehow be based on information. And the idea that the laws might actually be upheld by computational complexity — which is defined entirely in terms of information — offers a fresh perspective.

    It certainly inspired Susskind to dig deeper into the role of complexity. For mathematical clarity, he chose to make his calculations in a theoretical realm known as anti-de Sitter space (AdS). This describes a cosmos that is like our own Universe in the sense that everything in it, including black holes, is governed by gravity. Unlike our Universe, however, it has a boundary — a domain where there is no gravity, just elementary particles and fields governed by quantum physics. Despite this difference, studying physics in AdS has led to many insights, because every object and physical process inside the space can be mathematically mapped to an equivalent object or process on its boundary. A black hole in AdS, for example, is equivalent to a hot gas of ordinary quantum particles on the boundary. Better still, calculations that are complicated in one domain often turn out to be simple in the other. And after the calculations are complete, the insights gained in AdS can generally be translated back into our own Universe.

    Increasing complexity

    Susskind decided to look at a black hole sitting at the centre of an AdS universe, and to use the boundary description to explore what happens inside a black hole’s event horizon. Others had attempted this and failed, and Susskind could see why after he viewed the problem through the lens of computational complexity. Translating from the boundary of the AdS universe to the interior of a black hole requires an enormous number of computational steps, and that number increases exponentially as one moves closer to the event horizon (4). As Aaronson puts it, “the black hole’s interior is protected by an armour of computational complexity”.

    Furthermore, Susskind noticed, the computational complexity tends to grow with time. This is not the increase of disorder, or entropy, that is familiar from everyday physics. Rather, it is a pure quantum effect arising from the way that interactions between the boundary particles cause an explosive growth in the complexity of their collective quantum state.

    If nothing else, Susskind argued, this growth means that complexity behaves much like a gravitational field. Imagine an object floating somewhere outside the black hole. Because this is AdS, he said, the object can be described by some configuration of particles and fields on the boundary. And because the complexity of that boundary description tends to increase over time, the effect is to make the object move towards regions of higher complexity in the interior of the space. But that, said Susskind, is just another way of saying that the object will be pulled down towards the black hole. He captured that idea in a slogan (4): “Things fall because there is a tendency toward complexity.”

    Another implication of increasing complexity turns out to be closely related to an argument (5) that Susskind made last year in collaboration with Juan Maldacena, a physicist at the Institute for Advanced Study in Princeton, New Jersey, and the first researcher to recognize the unique features of AdS. According to general relativity, Susskind and Maldacena noted, two black holes can be many light years apart yet still have their interiors connected by a space-time tunnel known as a wormhole. But according to quantum theory, these widely separated black holes can also be connected by having their states entangled, meaning that information about their quantum states is shared between them in a way that is independent of distance.

    After exploring the many similarities between these connections, Susskind and Maldacena concluded that they were two aspects of the same thing — that the black hole’s degree of entanglement, a purely quantum phenomenon, will determine the wormhole’s width, a matter of pure geometry.

    With his latest work, Susskind says, it turns out that the growth of complexity on the boundary of AdS shows up as an increase in the wormhole’s length. So putting it all together, it seems that entanglement is somehow related to space, and that computational complexity is somehow related to time.

    Susskind is the first to admit that such ideas by themselves are only provocative suggestions; they do not make up a fully fledged theory. But he and his allies are confident that the ideas transcend the firewall paradox.

    “I don’t know where all of this will lead,” says Susskind. “But I believe these complexity–geometry connections are the tip of an iceberg.”

    See the full article for References

    See the full article here .

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    Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

     
  • richardmitnick 11:22 am on July 26, 2015 Permalink | Reply
    Tags: , , General Relativity, , Time Travel   

    From RT: “Time-traveling photons connect general relativity to quantum mechanics” 

    RT Logo

    RT

    23 Jun, 2014
    No Writer Credit

    1
    Space-time structure exhibiting closed paths in space (horizontal) and time (vertical). A quantum particle travels through a wormhole back in time and returns to the same location in space and time. (Photo credit: Martin Ringbauer)

    Scientists have simulated time travel by using particles of light acting as quantum particles sent away and then brought back to their original space-time location. This is a huge step toward marrying two of the most irreconcilable theories in physics.

    Since traveling all the way to a black hole to see if an object you’re holding would bend, break or put itself back together in inexplicable ways is a bit of a trek, scientists have decided to find a point of convergence between general relativity and quantum mechanics in lab conditions, and they achieved success.

    Australian researchers from the UQ’s School of Mathematics and Physics wanted to plug the holes in the discrepancies that exist between two of our most commonly accepted physics theories, which is no easy task: on the one hand, you have Einstein’s theory of general relativity, which predicts the behavior of massive objects like planets and galaxies; but on the other, you have something whose laws completely clash with Einstein’s – and that is the theory of quantum mechanics, which describes our world at the molecular level. And this is where things get interesting: we still have no concrete idea of all the principles of movement and interaction that underpin this theory.

    Natural laws of space and time simply break down there.

    The light particles used in the study are known as photons, and in this University of Queensland study, they stood in for actual quantum particles for the purpose of finding out how they behaved while moving through space and time.

    The team simulated the behavior of a single photon that travels back in time through a wormhole and meets its older self – an identical photon. “We used single photons to do this but the time-travel was simulated by using a second photon to play the part of the past incarnation of the time traveling photon,” said UQ Physics Professor Tim Ralph asquotedby The Speaker.

    The findings were published in the journal Nature Communications and gained support from the country’s key institutions on quantum physics.

    Some of the biggest examples of why the two approaches can’t be reconciled concern the so-called space-time loop. Einstein suggested that you can travel back in time and return to the starting point in space and time. This presented a problem, known commonly as the ‘grandparents paradox,’ theorized by Kurt Godel in 1949: if you were to travel back in time and prevent your grandparents from meeting, and in so doing prevent your own birth, the classical laws of physics would prevent you from being born.

    But Tim Ralph has reminded that in 1991, such situations could be avoided by harnessing quantum mechanics’ flexible laws: “The properties of quantum particles are ‘fuzzy’ or uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations,” he said.

    There are still ways in which science hasn’t tested the meeting points between general relativity and quantum mechanics – such as when relativity is tested under extreme conditions, where its laws visibly seem to bend, just like near the event horizon of a black hole.

    But since it’s not really easy to approach one, the UQ scientists were content with testing out these points of convergence on photons.

    “Our study provides insights into where and how nature might behave differently from what our theories predict,” Professor Ralph said.

    See the full article here.

    Please help promote STEM in your local schools.

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  • richardmitnick 10:31 am on July 18, 2015 Permalink | Reply
    Tags: , , , General Relativity, ,   

    From NOVA: “How Time Got Its Arrow” 

    PBS NOVA

    NOVA

    15 Jul 2015

    1
    Lee Smolin, Perimeter Institute for Theoretical Physics

    I believe in time.

    I haven’t always believed in it. Like many physicists and philosophers, I had once concluded from general relativity and quantum gravity that time is not a fundamental aspect of nature, but instead emerges from another, deeper description. Then, starting in the 1990s and accelerated by an eight year collaboration with the Brazilian philosopher Roberto Mangabeira Unger, I came to believe instead that time is fundamental. (How I came to this is another story.) Now, I believe that by taking time to be fundamental, we might be able to understand how general relativity and the standard model emerge from a deeper theory, why time only goes one way, and how the universe was born.

    2
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    1
    Flickr user Robert Couse-Baker, adapted under a Creative Commons license.

    The story starts with change. Science, most broadly defined, is the systematic study of change. The world we observe and experience is constantly changing. And most of the changes we observe are irreversible. We are born, we grow, we age, we die, as do all living things. We remember the past and our actions influence the future. Spilled milk is hard to clean up; a cool drink or a hot bath tend towards room temperature. The whole world, living and non-living, is dominated by irreversible processes, as captured mathematically by the second law of thermodynamics, which holds that the entropy of a closed system usually increases and seldom decreases.

    It may come as a surprise, then, that physics regards this irreversibility as a cosmic accident. The laws of nature as we know them are all reversible when you change the direction of time. Film a process described by those laws, and then run the movie backwards: the rewound version is also allowed by the laws of physics. To be more precise, you may have to change left for right and particles for antiparticles, along with reversing the direction of time, but the standard model of particle physics predicts that the original process and its reverse are equally likely.

    The same is true of Einstein’s theory of general relativity, which describes gravity and cosmology. If the whole universe were observed to run backwards in time, so that it heated up while it collapsed, rather than cooled as it expanded, that would be equally consistent with these fundamental laws, as we currently understand them.

    This leads to a fundamental question: Why, if the laws are reversible, is the universe so dominated by irreversible processes? Why does the second law of thermodynamics hold so universally?

    Gravity is one part of the answer. The second law tells us that the entropy of a closed system, which is a measure of disorder or randomness in the motions of the atoms making up that system, will most likely increase until a state of maximum disorder is reached. This state is called equilibrium. Once it is reached, the system is as mixed as possible, so all parts have the same temperature and all the elements are equally distributed.

    But on large scales, the universe is far from equilibrium. Galaxies like ours are continually forming stars, turning nuclear potential energy into heat and light, as they drive the irreversible flows of energy and materials that characterize the galactic disks. On these large scales, gravity fights the decay to equilibrium by causing matter to clump,,creating subsystems like stars and planets. This is beautifully illustrated in some recent papers by Barbour, Koslowski and Mercati.

    But this is only part of the answer to why the universe is out of equilibrium. There remains the mystery of why the universe at the big bang was not created in equilibrium to start with, for the picture of the universe given us by observations requires that the universe be created in an extremely improbable state—very far from equilibrium. Why?

    So when we say that our universe started off in a state far from equilibrium, we are saying that it started off in a state that would be very improbable, were the initial state chosen randomly from the set of all possible states. Yet we must accept this vast improbability to explain the ubiquity of irreversible processes in our world in terms of the reversible laws we know.

    In particular, the conditions present in the early universe, being far from equilibrium, are highly irreversible. Run the early universe backwards to a big crunch and they look nothing like the late universe that might be in our future.

    In 1979 Roger Penrose proposed a radical answer to the mystery of irreversibility. His proposal concerned quantum gravity, the long-searched-for unification of all the known laws, which is believed to govern the processes that created the universe in the big bang—or transformed it from whatever state it was in before the big bang.

    Penrose hypothesized that quantum gravity, as the most fundamental law, will be unlike the laws we know in that it will be irreversible. The known laws, along with their time-reversibility, emerge as approximations to quantum gravity when the universe grows large and cool and dilute, Penrose argued. But those approximate laws will act within a universe whose early conditions were set up by the more fundamental, irreversible laws. In this way the improbability of the early conditions can be explained.

    In the intervening years our knowledge of the early universe has been dramatically improved by a host of cosmological observations, but these have only deepened the mysteries we have been discussing. So a few years ago, Marina Cortes, a cosmologist from the Institute for Astronomy in Edinburgh, and I decided to revive Penrose’s suggestion in the light of all the knowledge gained since, both observationally and theoretically.

    Dr. Cortes argued that time is not only fundamental but fundamentally irreversible. She proposed that the universe is made of processes that continuously generate new events from present events. Events happen, but cannot unhappen. The reversal of an event does not erase that event, Cortes says: It is a new event, which happens after it.

    In December of 2011, Dr. Cortes began a three-month visit to Perimeter Institute, where I work, and challenged me to collaborate with her on realizing these ideas. The first result was a model we developed of a universe created by events, which we called an energetic causal set model.

    This is a version of a kind of model called a causal set model, in which the history of the universe is considered to be a discrete set of events related only by cause-and-effect. Our model was different from earlier models, though. In it, events are created by a process which maximizes their uniqueness. More precisely, the process produces a universe created by events, each of which is different from all the others. Space is not fundamental, only the events and the causal process that creates them are fundamental. But if space is not fundamental, energy is. The events each have a quantity of energy, which they gain from their predecessors and pass on to their successors. Everything else in the world emerges from these events and the energy they convey.

    We studied the model universes created by these processes and found that they generally pass through two stages of evolution. In the first stage, they are dominated by the irreversible processes that create the events, each unique. The direction of time is clear. But this gives rise to a second stage in which trails of events appear to propagate, creating emergent notions of particles. Particles emerge only when the second, approximately reversible stage is reached. These emergent particles propagate and appear to interact through emergent laws which seem reversible. In fact, we found, there are many possible models in which particles and approximately reversible laws emerge after a time from a more fundamental irreversible, particle-free system.

    This might explain how general relativity and the standard model emerged from a more fundamental theory, as Penrose hypothesized. Could we, we wondered, start with general relativity and, staying within the language of that theory, modify it to describe an irreversible theory? This would give us a framework to bridge the transition between the early, irreversible stage and the later, reversible stage.

    In a recent paper, Marina Cortes, PI postdoc Henrique Gomes and I showed one way to modify general relativity in a way that introduces a preferred direction of time, and we explored the possible consequences for the cosmology of the early universe. In particular, we showed that there were analogies of dark matter and dark energy, but which introduce a preferred direction of time, so a contracting universe is no longer the time-reverse of an expanding universe.

    To do this we had to first modify general relativity to include a physically preferred notion of time. Without that there is no notion of reversing time. Fortunately, such a modification already existed. Called shape dynamics, it had been proposed in 2011 by three young people, including Gomes. Their work was inspired by Julian Barbour, who had proposed that general relativity could be reformulated so that a relativity of size substituted for a relativity of time.

    Using the language of shape dynamics, Cortes, Gomes and I found a way to gently modify general relativity so that little is changed on the scale of stars, galaxies and planets. Nor are the predictions of general relativity regarding gravitational waves affected. But on the scale of the whole universe, and for the early universe, there are deviations where one cannot escape the consequences of a fundamental direction of time.

    Very recently I found still another way to modify the laws of general relativity to make them irreversible. General relativity incorporates effects of two fixed constants of nature, Newton’s constant, which measures the strength of the gravitational force, and the cosmological constant [usually denoted by the Greek capital letter lambda: Λ], which measures the density of energy in empty space. Usually these both are fixed constants, but I found a way they could evolve in time without destroying the beautiful harmony and consistency of the Einstein equations of general relativity.

    These developments are very recent and are far from demonstrating that the irreversibility we see around us is a reflection of a fundamental arrow of time. But they open a way to an understanding of how time got its direction that does not rely on our universe being a consequence of a cosmic accident.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

     
  • richardmitnick 7:59 am on June 19, 2015 Permalink | Reply
    Tags: , General Relativity,   

    From NOVA: “Do We Need to Rewrite General Relativity?” 

    PBS NOVA

    NOVA

    18 Jun 2015
    Matthew Francis

    1
    A cosmological computer simulation shows dark matter density overlaid with a gas velocity field. Credit: Illustris Collaboration/Illustris Simulation

    General relativity, the theory of gravity Albert Einstein published 100 years ago, is one of the most successful theories we have. It has passed every experimental test; every observation from astronomy is consistent with its predictions. Physicists and astronomers have used the theory to understand the behavior of binary pulsars, predict the black holes we now know pepper every galaxy, and obtain deep insights into the structure of the entire universe.

    Yet most researchers think general relativity is wrong.

    To be more precise: most believe it is incomplete. After all, the other forces of nature are governed by quantum physics; gravity alone has stubbornly resisted a quantum description. Meanwhile, a small but vocal group of researchers thinks that phenomena such as dark matter are actually failures of general relativity, requiring us to look at alternative ideas.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

     
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