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  • richardmitnick 2:37 pm on September 15, 2018 Permalink | Reply
    Tags: , , Quanta Magazine, The End of Theoretical Physics As We Know It   

    From Quanta Magazine: “The End of Theoretical Physics As We Know It” 

    Quanta Magazine
    From Quanta Magazine

    August 27, 2018
    Sabine Hossenfelder

    1
    James O’Brien for Quanta Magazine

    Computer simulations and custom-built quantum analogues are changing what it means to search for the laws of nature.

    Theoretical physics has a reputation for being complicated. I beg to differ. That we are able to write down natural laws in mathematical form at all means that the laws we deal with are simple — much simpler than those of other scientific disciplines.

    Unfortunately, actually solving those equations is often not so simple. For example, we have a perfectly fine theory that describes the elementary particles called quarks and gluons, but no one can calculate how they come together to make a proton. The equations just can’t be solved by any known methods. Similarly, a merger of black holes or even the flow of a mountain stream can be described in deceptively simple terms, but it’s hideously difficult to say what’s going to happen in any particular case.

    Of course, we are relentlessly pushing the limits, searching for new mathematical strategies. But in recent years much of the pushing has come not from more sophisticated math but from more computing power.

    When the first math software became available in the 1980s, it didn’t do much more than save someone a search through enormous printed lists of solved integrals. But once physicists had computers at their fingertips, they realized they no longer had to solve the integrals in the first place, they could just plot the solution.

    In the 1990s, many physicists opposed this “just plot it” approach. Many were not trained in computer analysis, and sometimes they couldn’t tell physical effects from coding artifacts. Maybe this is why I recall many seminars in which a result was degraded as “merely numerical.” But over the past two decades, this attitude has markedly shifted, not least thanks to a new generation of physicists for whom coding is a natural extension of their mathematical skill.

    Accordingly, theoretical physics now has many subdisciplines dedicated to computer simulations of real-world systems, studies that would just not be possible any other way. Computer simulations are what we now use to study the formation of galaxies and supergalactic structures, to calculate the masses of particles that are composed of several quarks, to find out what goes on in the collision of large atomic nuclei, and to understand solar cycles, to name but a few areas of research that are mainly computer based.

    The next step of this shift away from purely mathematical modeling is already on the way: Physicists now custom design laboratory systems that stand in for other systems which they want to better understand. They observe the simulated system in the lab to draw conclusions about, and make predictions for, the system it represents.

    The best example may be the research area that goes by the name “quantum simulations.” These are systems composed of interacting, composite objects, like clouds of atoms. Physicists manipulate the interactions among these objects so the system resembles an interaction among more fundamental particles. For example, in circuit quantum electrodynamics, researchers use tiny superconducting circuits to simulate atoms, and then study how these artificial atoms interact with photons. Or in a lab in Munich, physicists use a superfluid of ultra-cold atoms to settle the debate over whether Higgs-like particles can exist in two dimensions of space (the answer is yes [Nature]).

    These simulations are not only useful to overcome mathematical hurdles in theories we already know. We can also use them to explore consequences of new theories that haven’t been studied before and whose relevance we don’t yet know.

    This is particularly interesting when it comes to the quantum behavior of space and time itself — an area where we still don’t have a good theory. In a recent experiment, for example, Raymond Laflamme, a physicist at the Institute for Quantum Computing at the University of Waterloo in Ontario, Canada, and his group used a quantum simulation to study so-called spin networks, structures that, in some theories, constitute the fundamental fabric of space-time. And Gia Dvali, a physicist at the University of Munich, has proposed a way to simulate the information processing of black holes with ultracold atom gases.

    A similar idea is being pursued in the field of analogue gravity, where physicists use fluids to mimic the behavior of particles in gravitational fields. Black hole space-times have attracted the bulk of attention, as with Jeff Steinhauer’s (still somewhat controversial) claim of having measured Hawking radiation in a black-hole analogue. But researchers have also studied the rapid expansion of the early universe, called “inflation,” with fluid analogues for gravity.

    In addition, physicists have studied hypothetical fundamental particles by observing stand-ins called quasiparticles. These quasiparticles behave like fundamental particles, but they emerge from the collective movement of many other particles. Understanding their properties allows us to learn more about their behavior, and thereby might also to help us find ways of observing the real thing.

    This line of research raises some big questions. First of all, if we can simulate what we now believe to be fundamental by using composite quasiparticles, then maybe what we currently think of as fundamental — space and time and the 25 particles that make up the Standard Model of particle physics — is made up of an underlying structure, too. Quantum simulations also make us wonder what it means to explain the behavior of a system to begin with. Does observing, measuring, and making a prediction by use of a simplified version of a system amount to an explanation?

    But for me, the most interesting aspect of this development is that it ultimately changes how we do physics. With quantum simulations, the mathematical model is of secondary relevance. We currently use the math to identify a suitable system because the math tells us what properties we should look for. But that’s not, strictly speaking, necessary. Maybe, over the course of time, experimentalists will just learn which system maps to which other system, as they have learned which system maps to which math. Perhaps one day, rather than doing calculations, we will just use observations of simplified systems to make predictions.

    At present, I am sure, most of my colleagues would be appalled by this future vision. But in my mind, building a simplified model of a system in the laboratory is conceptually not so different from what physicists have been doing for centuries: writing down simplified models of physical systems in the language of mathematics.

    See the full article here .

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    Please help promote STEM in your local schools.

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

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  • richardmitnick 3:39 am on August 15, 2018 Permalink | Reply
    Tags: , , , , Dark Energy May Be Incompatible With String Theory, , , Quanta Magazine,   

    From Quanta Magazine: “Dark Energy May Be Incompatible With String Theory” 

    Quanta Magazine
    From Quanta Magazine

    August 9, 2018
    Natalie Wolchover

    1
    String theory permits a “landscape” of possible universes, surrounded by a “swampland” of logically inconsistent universes. In all of the simple, viable stringy universes physicists have studied, the density of dark energy is either diminishing or has a stable negative value, unlike our universe, which appears to have a stable positive value. Maciej Rebisz for Quanta Magazine

    On June 25, Timm Wrase awoke in Vienna and groggily scrolled through an online repository of newly posted physics papers. One title startled him into full consciousness.

    The paper, by the prominent string theorist Cumrun Vafa of Harvard University and collaborators, conjectured a simple formula dictating which kinds of universes are allowed to exist and which are forbidden, according to string theory. The leading candidate for a “theory of everything” weaving the force of gravity together with quantum physics, string theory defines all matter and forces as vibrations of tiny strands of energy. The theory permits some 10500 different solutions: a vast, varied “landscape” of possible universes. String theorists like Wrase and Vafa have strived for years to place our particular universe somewhere in this landscape of possibilities.

    But now, Vafa and his colleagues were conjecturing that in the string landscape, universes like ours — or what ours is thought to be like — don’t exist. If the conjecture is correct, Wrase and other string theorists immediately realized, the cosmos must either be profoundly different than previously supposed or string theory must be wrong.

    After dropping his kindergartner off that morning, Wrase went to work at the Vienna University of Technology, where his colleagues were also buzzing about the paper. That same day, in Okinawa, Japan, Vafa presented the conjecture at the Strings 2018 conference, which was streamed by physicists worldwide. Debate broke out on- and off-site. “There were people who immediately said, ‘This has to be wrong,’ other people who said, ‘Oh, I’ve been saying this for years,’ and everything in the middle,” Wrase said. There was confusion, he added, but “also, of course, huge excitement. Because if this conjecture was right, then it has a lot of tremendous implications for cosmology.”

    Researchers have set to work trying to test the conjecture and explore its implications. Wrase has already written two papers, including one that may lead to a refinement of the conjecture, and both mostly while on vacation with his family. He recalled thinking, “This is so exciting. I have to work and study that further.”

    The conjectured formula — posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri and Lev Spodyneiko and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal and Paul Steinhardt — says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.

    Dark Energy in Question

    Since 1998, telescope observations have indicated that the cosmos is expanding ever-so-slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”

    In addition, it looks like the amount of dark energy infused in empty space stays constant over time (as best anyone can tell).

    But the new conjecture asserts that the vacuum energy of the universe must be decreasing.

    Vafa and colleagues contend that universes with stable, constant, positive amounts of vacuum energy, known as “de Sitter universes,” aren’t possible. String theorists have struggled mightily since dark energy’s 1998 discovery to construct convincing stringy models of stable de Sitter universes. But if Vafa is right, such efforts are bound to sink in logical inconsistency; de Sitter universes lie not in the landscape, but in the “swampland.” “The things that look consistent but ultimately are not consistent, I call them swampland,” he explained recently. “They almost look like landscape; you can be fooled by them. You think you should be able to construct them, but you cannot.”

    According to this “de Sitter swampland conjecture,” in all possible, logical universes, the vacuum energy must either be dropping, its value like a ball rolling down a hill, or it must have obtained a stable negative value. (So-called “anti-de Sitter” universes, with stable, negative doses of vacuum energy, are easily constructed in string theory.)

    The conjecture, if true, would mean the density of dark energy in our universe cannot be constant, but must instead take a form called “quintessence” — an energy source that will gradually diminish over tens of billions of years. Several telescope experiments are underway now to more precisely probe whether the universe is expanding with a constant rate of acceleration, which would mean that as new space is created, a proportionate amount of new dark energy arises with it, or whether the cosmic acceleration is gradually changing, as in quintessence models. A discovery of quintessence would revolutionize fundamental physics and cosmology, including rewriting the cosmos’s history and future. Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models, would eventually stop expanding and contract in either a Big Crunch or Big Bounce.

    Paul Steinhardt, a cosmologist at Princeton University and one of Vafa’s co-authors, said that over the next few years, “all eyes should be on” measurements by the Dark Energy Survey, WFIRST and Euclid telescopes of whether the density of dark energy is changing.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    NASA/WFIRST

    ESA/Euclid spacecraft

    “If you find it’s not consistent with quintessence,” Steinhardt said, “it means either the swampland idea is wrong, or string theory is wrong, or both are wrong or — something’s wrong.”

    Inflation Under Siege

    No less dramatically, the new swampland conjecture also casts doubt on the widely believed story of the universe’s birth: the Big Bang theory known as cosmic inflation.

    Inflation

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

    HPHS Owls

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

    Alan Guth’s notes:
    5

    According to this theory, a minuscule, energy-infused speck of space-time rapidly inflated to form the macroscopic universe we inhabit. The theory was devised to explain, in part, how the universe got so huge, smooth and flat.

    But the hypothetical “inflaton field” of energy that supposedly drove cosmic inflation doesn’t sit well with Vafa’s formula. To abide by the formula, the inflaton field’s energy would probably have needed to diminish too quickly to form a smooth- and flat-enough universe, he and other researchers explained. Thus, the conjecture disfavors many popular models of cosmic inflation. In the coming years, telescopes such as the Simons Observatory will look for definitive signatures of cosmic inflation, testing it against rival ideas.

    In the meantime, string theorists, who normally form a united front, will disagree about the conjecture. Eva Silverstein, a physics professor at Stanford University and a leader in the effort to construct string-theoretic models of inflation, thinks it is very likely to be false. So does her husband, the Stanford professor Shamit Kachru; he is the first “K” in KKLT, a famous 2003 paper (known by its authors’ initials) that suggested a set of stringy ingredients that might be used to construct de Sitter universes. Vafa’s formula says both Silverstein’s and Kachru’s constructions won’t work. “We’re besieged by these conjectures in our family,” Silverstein joked. But in her view, accelerating-expansion models are no more disfavored now, in light of the new papers, than before. “They essentially just speculate that those things don’t exist, citing very limited and in some cases highly dubious analyses,” she said.

    Matthew Kleban, a string theorist and cosmologist at New York University, also works on stringy models of inflation. He stresses that the new swampland conjecture is highly speculative and an example of “lamppost reasoning,” since much of the string landscape has yet to be explored. And yet he acknowledges that, based on existing evidence, the conjecture could well be true. “It could be true about string theory, and then maybe string theory doesn’t describe the world,” Kleban said. “[Maybe] dark energy has falsified it. That obviously would be very interesting.”

    Mapping the Swampland

    Whether the de Sitter swampland conjecture and future experiments really have the power to falsify string theory remains to be seen. The discovery in the early 2000s that string theory has something like 10^500 solutions killed the dream that it might uniquely and inevitably predict the properties of our one universe. The theory seemed like it could support almost any observations and became very difficult to experimentally test or disprove.

    In 2005, Vafa and a network of collaborators began to think about how to pare the possibilities down by mapping out fundamental features of nature that absolutely have to be true. For example, their “weak gravity conjecture” asserts that gravity must always be the weakest force in any logical universe. Imagined universes that don’t satisfy such requirements get tossed from the landscape into the swampland. Many of these swampland conjectures have held up famously against attack, and some are now “on a very solid theoretical footing,” said Hirosi Ooguri, a theoretical physicist at the California Institute of Technology and one of Vafa’s first swampland collaborators. The weak gravity conjecture, for instance, has accumulated so much evidence that it’s now suspected to hold generally, independent of whether string theory is the correct theory of quantum gravity.

    The intuition about where landscape ends and swampland begins derives from decades of effort to construct stringy models of universes. The chief challenge of that project has been that string theory predicts the existence of 10 space-time dimensions — far more than are apparent in our 4-D universe. String theorists posit that the six extra spatial dimensions must be small — curled up tightly at every point. The landscape springs from all the different ways of configuring these extra dimensions. But although the possibilities are enormous, researchers like Vafa have found that general principles emerge. For instance, the curled-up dimensions typically want to gravitationally contract inward, whereas fields like electromagnetic fields tend to push everything apart. And in simple, stable configurations, these effects balance out by having negative vacuum energy, producing anti-de Sitter universes. Turning the vacuum energy positive is hard. “Usually in physics, we have simple examples of general phenomena,” Vafa said. “De Sitter is not such a thing.”

    The KKLT paper, by Kachru, Renata Kallosh, Andrei Linde and Sandip Trivedi, suggested stringy trappings like “fluxes,” “instantons” and “anti-D-branes” that could potentially serve as tools for configuring a positive, constant vacuum energy. However, these constructions are complicated, and over the years possible instabilities have been identified. Though Kachru said he does not have “any serious doubts,” many researchers have come to suspect the KKLT scenario does not produce stable de Sitter universes after all.

    Vafa thinks a concerted search for definitely stable de Sitter universe models is long overdue. His conjecture is, above all, intended to press the issue. In his view, string theorists have not felt sufficiently motivated to figure out whether string theory really is capable of describing our world, instead taking the attitude that because the string landscape is huge, there must be a place in it for us, even if no one knows where. “The bulk of the community in string theory still sides on the side of de Sitter constructions [existing],” he said, “because the belief is, ‘Look, we live in a de Sitter universe with positive energy; therefore we better have examples of that type.’”

    His conjecture has roused the community to action, with researchers like Wrase looking for stable de Sitter counterexamples, while others toy with little-explored stringy models of quintessent universes. “I would be equally interested to know if the conjecture is true or false,” Vafa said. “Raising the question is what we should be doing. And finding evidence for or against it — that’s how we make progress.”

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 11:47 am on August 2, 2018 Permalink | Reply
    Tags: , , , , Quanta Magazine   

    From Quanta Magazine via Nautilus: “How Artificial Intelligence Can Supercharge the Search for New Particles” 

    Nautilus

    Nautilus

    Quanta Magazine
    From Quanta Magazine

    Jul 25, 2018
    Charlie Wood

    1
    In the hunt for new fundamental particles, physicists have always had to make assumptions about how the particles will behave. New machine learning algorithms don’t.
    Image by ATLAS Experiment © 2018 CERN

    The Large Hadron Collider (LHC) smashes a billion pairs of protons together each second.

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    Occasionally the machine may rattle reality enough to have a few of those collisions generate something that’s never been seen before. But because these events are by their nature a surprise, physicists don’t know exactly what to look for. They worry that in the process of winnowing their data from those billions of collisions to a more manageable number, they may be inadvertently deleting evidence for new physics. “We’re always afraid we’re throwing the baby away with the bathwater,” said Kyle Cranmer, a particle physicist at New York University who works with the ATLAS experiment at CERN.

    CERN ATLAS

    Faced with the challenge of intelligent data reduction, some physicists are trying to use a machine learning technique called a “deep neural network” to dredge the sea of familiar events for new physics phenomena.

    In the prototypical use case, a deep neural network learns to tell cats from dogs by studying a stack of photos labeled “cat” and a stack labeled “dog.” But that approach won’t work when hunting for new particles, since physicists can’t feed the machine pictures of something they’ve never seen. So they turn to “weakly supervised learning,” where machines start with known particles and then look for rare events using less granular information, such as how often they might take place overall.

    In a paper posted on the scientific preprint site arxiv.org in May, three researchers proposed applying a related strategy to extend “bump hunting,” the classic particle-hunting technique that found the Higgs boson. The general idea, according to one of the authors, Ben Nachman, a researcher at the Lawrence Berkeley National Laboratory, is to train the machine to seek out rare variations in a data set.

    Consider, as a toy example in the spirit of cats and dogs, a problem of trying to discover a new species of animal in a data set filled with observations of forests across North America. Assuming that any new animals might tend to cluster in certain geographical areas (a notion that corresponds with a new particle that clusters around a certain mass), the algorithm should be able to pick them out by systematically comparing neighboring regions. If British Columbia happens to contain 113 caribous to Washington state’s 19 (even against a background of millions of squirrels), the program will learn to sort caribous from squirrels, all without ever studying caribous directly. “It’s not magic but it feels like magic,” said Tim Cohen, a theoretical particle physicist at the University of Oregon who also studies weak supervision.

    By contrast, traditional searches in particle physics usually require researchers to make an assumption about what the new phenomena will look like. They create a model of how the new particles will behave—for example, a new particle might tend to decay into particular constellations of known particles. Only after they define what they’re looking for can they engineer a custom search strategy. It’s a task that generally takes a Ph.D. student at least a year, and one that Nachman thinks could be done much faster, and more thoroughly.

    The proposed CWoLa algorithm, which stands for Classification Without Labels, can search existing data for any unknown particle that decays into either two lighter unknown particles of the same type, or two known particles of the same or different type. Using ordinary search methods, it would take the LHC collaborations at least 20 years to scour the possibilities for the latter, and no searches currently exist for the former. Nachman, who works on the ATLAS project, says CWoLa could do them all in one go.

    Other experimental particle physicists agree it could be a worthwhile project. “We’ve looked in a lot of the predictable pockets, so starting to fill in the corners we haven’t looked in is an important direction for us to go in next,” said Kate Pachal, a physicist who searches for new particle bumps with the ATLAS project. She batted around the idea of trying to design flexible software that could deal with a range of particle masses last year with some colleagues, but no one knew enough about machine learning. “Now I think it might be the time to try this,” she said.

    The hope is that neural networks could pick up on subtle correlations in the data that resist current modeling efforts. Other machine learning techniques have successfully boosted the efficiency of certain tasks at the LHC, such as identifying “jets” made by bottom-quark particles. The work has left no doubt that some signals are escaping physicists’ notice. “They’re leaving information on the table, and when you spend $10 billion on a machine, you don’t want to leave information on the table,” said Daniel Whiteson, a particle physicist at the University of California, Irvine.

    Yet machine learning is rife with cautionary tales of programs that confused arms with dumbbells (or worse). At the LHC, some worry that the shortcuts will end up reflecting gremlins in the machine itself, which experimental physicists take great pains to intentionally overlook. “Once you find an anomaly, is it new physics or is it something funny that went on with the detector?” asked Till Eifert, a physicist on ATLAS.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 2:41 pm on July 22, 2018 Permalink | Reply
    Tags: , , , , , , , Quanta Magazine, Sau Lan Wu, ,   

    From LHC at CERN and University of Wisconsin Madison via WIRED and Quanta: Women in STEM “Meet the Woman Who Rocked Particle Physics—Three Times” Sau Lan Wu 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    U Wisconsin

    via

    Wired logo

    WIRED

    originated at

    Quanta Magazine
    Quanta Magazine

    7.22.18
    Joshua Roebke

    1
    Sau Lan Wu at CERN, the laboratory near Geneva that houses the Large Hadron Collider. The mural depicts the detector she and her collaborators used to discover the Higgs boson. Thi My Lien Nguyen/Quanta Magazine

    In 1963, Maria Goeppert Mayer won the Nobel Prize in physics for describing the layered, shell-like structures of atomic nuclei. No woman has won since.

    One of the many women who, in a different world, might have won the physics prize in the intervening 55 years is Sau Lan Wu. Wu is the Enrico Fermi Distinguished Professor of Physics at the University of Wisconsin, Madison, and an experimentalist at CERN, the laboratory near Geneva that houses the Large Hadron Collider.

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    Wu’s name appears on more than 1,000 papers in high-energy physics, and she has contributed to a half-dozen of the most important experiments in her field over the past 50 years. She has even realized the improbable goal she set for herself as a young researcher: to make at least three major discoveries.

    Wu was an integral member of one of the two groups that observed the J/psi particle, which heralded the existence of a fourth kind of quark, now called the charm. The discovery, in 1974, was known as the November Revolution, a coup that led to the establishment of the Standard Model of particle physics.

    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.


    Standard Model of Particle Physics from Symmetry Magazine

    Later in the 1970s, Wu did much of the math and analysis to discern the three “jets” of energy flying away from particle collisions that signaled the existence of gluons—particles that mediate the strong force holding protons and neutrons together. This was the first observation of particles that communicate a force since scientists recognized photons of light as the carriers of electromagnetism. Wu later became one of the group leaders for the ATLAS experiment, one of the two collaborations at the Large Hadron Collider that discovered the Higgs boson in 2012, filling in the final piece of the Standard Model.

    CERN ATLAS Higgs Event


    CERN/ATLAS detector

    She continues to search for new particles that would transcend the Standard Model and push physics forward.

    Sau Lan Wu was born in occupied Hong Kong during World War II. Her mother was the sixth concubine to a wealthy businessman who abandoned them and her younger brother when Wu was a child. She grew up in abject poverty, sleeping alone in a space behind a rice shop. Her mother was illiterate, but she urged her daughter to pursue an education and become independent of volatile men.

    Wu graduated from a government school in Hong Kong and applied to 50 universities in the United States. She received a scholarship to attend Vassar College and arrived with $40 to her name.

    Although she originally intended to become an artist, she was inspired to study physics after reading a biography of Marie Curie. She worked on experiments during consecutive summers at Brookhaven National Laboratory on Long Island, and she attended graduate school at Harvard University. She was the only woman in her cohort and was barred from entering the male dormitories to join the study groups that met there. She has labored since then to make a space for everyone in physics, mentoring more than 60 men and women through their doctorates.

    Quanta Magazine joined Sau Lan Wu on a gray couch in sunny Cleveland in early June. She had just delivered an invited lecture about the discovery of gluons at a symposium to honor the 50th birthday of the Standard Model. The interview has been condensed and edited for clarity.

    2
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    Wu’s office at CERN is decorated with mementos and photos, including one of her and her husband, Tai Tsun Wu, a professor of theoretical physics at Harvard.
    Thi My Lien Nguyen/Quanta Magazine

    You work on the largest experiments in the world, mentor dozens of students, and travel back and forth between Madison and Geneva. What is a normal day like for you?

    Very tiring! In principle, I am full-time at CERN, but I do go to Madison fairly often. So I do travel a lot.

    How do you manage it all?

    Well, I think the key is that I am totally devoted. My husband, Tai Tsun Wu, is also a professor, in theoretical physics at Harvard. Right now, he’s working even harder than me, which is hard to imagine. He’s doing a calculation about the Higgs boson decay that is very difficult. But I encourage him to work hard, because it’s good for your mental state when you are older. That’s why I work so hard, too.

    Of all the discoveries you were involved in, do you have a favorite?

    Discovering the gluon was a fantastic time. I was just a second- or third-year assistant professor. And I was so happy. That’s because I was the baby, the youngest of all the key members of the collaboration.

    The gluon was the first force-carrying particle discovered since the photon. The W and Z bosons, which carry the weak force, were discovered a few years later, and the researchers who found them won a Nobel Prize. Why was no prize awarded for the discovery of the gluon?

    Well, you are going to have to ask the Nobel committee that. [Laughs.] I can tell you what I think, though. Only three people can win a Nobel Prize. And there were three other physicists on the experiment with me who were more senior than I was. They treated me very well. But I pushed the idea of searching for the gluon right away, and I did the calculations. I didn’t even talk to theorists. Although I married a theorist, I never really paid attention to what the theorists told me to do.

    How did you wind up being the one to do those calculations?

    If you want to be successful, you have to be fast. But you also have to be first. So I did the calculations to make sure that as soon as a new collider at at DESY [the German Electron Synchrotron] turned on in Hamburg we could see the gluon and recognize its signal of three jets of particles.

    DESY Helmholtz Centres & Networks: DESY’s synchrotron radiation source: the PETRA III storage ring (in orange) with the three experimental halls (in blue) in 2015.

    We were not so sure in those days that the signal for the gluon would be clear-cut, because the concept of jets had only been introduced a couple of years earlier, but this seemed to be the only way to discover gluons.

    You were also involved in discovering the Higgs boson, the particle in the Standard Model that gives many other particles their masses. How was that experiment different from the others that you were part of?

    I worked a lot more and a lot longer to discover the Higgs than I have on anything else. I worked for over 30 years, doing one experiment after another. I think I contributed a lot to that discovery. But the ATLAS collaboration at CERN is so large that you can’t even talk about your individual contribution. There are 3,000 people who built and worked on our experiment [including 600 scientists at Brookhaven National Lab, NY, USA]. How can anyone claim anything? In the old days, life was easier.

    Has it gotten any easier to be a woman in physics than when you started?

    Not for me. But for younger women, yes. There is a trend among funding agencies and institutions to encourage younger women, which I think is great. But for someone like me it is harder. I went through a very difficult time. And now that I am established others say: Why should we treat you any differently?

    Who were some of your mentors when you were a young researcher?

    Bjørn Wiik really helped me when I was looking for the gluon at DESY.

    How so?

    Well, when I started at the University of Wisconsin, I was looking for a new project. I was interested in doing electron-positron collisions, which could give the clearest indication of a gluon. So I went to talk to another professor at Wisconsin who did these kinds of experiments at SLAC, the lab at Stanford. But he was not interested in working with me.

    So I tried to join a project at the new electron-positron collider at DESY. I wanted to join the JADE experiment [abbreviated from the nations that developed the detector: Japan, Germany (Deutschland) and England]. I had some friends working there, so I went to Germany and I was all set to join them. But then I heard that no one had told a big professor in the group about me, so I called him up. He said, “I am not sure if I can take you, and I am going on vacation for a month. I’ll phone you when I get back.” I was really sad because I was already in Germany at DESY.

    But then I ran into Bjørn Wiik, who led a different experiment called TASSO, and he said, “What are you doing here?” I said, “I tried to join JADE, but they turned me down.” He said, “Come and talk to me.” He accepted me the very next day.

    4
    TASSO detector at PETRA at DESY

    And the thing is, JADE later broke their chamber, and they could not have observed the three-jet signal for gluons when we observed it first at TASSO. So I have learned that if something does not work out for you in life, something else will.

    5
    Wu and Bjørn Wiik in 1978, in the electronic control room of the TASSO experiment at the German Electron Synchrotron in Hamburg, Germany. Dr. Ulrich Kötz

    You certainly turned that negative into a positive.

    Yes. The same thing happened when I left Hong Kong to attend college in the US. I applied to 50 universities after I went through a catalog at the American consulate. I wrote in every application, “I need a full scholarship and room and board,” because I had no money. Four universities replied. Three of them turned me down. Vassar was the only American college that accepted me. And it turns out, it was the best college of all the ones I applied to.

    If you persist, something good is bound to happen. My philosophy is that you have to work hard and have good judgment. But you also have to have luck.

    I know this is an unfair question, because no one ever asks men, even though we should, but how can society inspire more women to study physics or consider it as a career?

    Well, I can only say something about my field, experimental high-energy physics. I think my field is very hard for women. I think partially it’s the problem of family.

    My husband and I did not live together for 10 years, except during the summers. And I gave up having children. When I was considering having children, it was around the time when I was up for tenure and a grant. I feared I would lose both if I got pregnant. I was less worried about actually having children than I was about walking into my department or a meeting while pregnant. So it’s very, very hard for families.

    I think it still can be.

    Yeah, but for the younger generation it’s different. Nowadays, a department looks good if it supports women. I don’t mean that departments are deliberately doing that only to look better, but they no longer actively fight against women. It’s still hard, though. Especially in experimental high-energy physics. I think there is so much traveling that it makes having a family or a life difficult. Theory is much easier.

    You have done so much to help establish the Standard Model of particle physics. What do you like about it? What do you not like?

    It’s just amazing that the Standard Model works as well as it does. I like that every time we try to search for something that is not accounted for in the Standard Model, we do not find it, because the Standard Model says we shouldn’t.

    But back in my day, there was so much that we had yet to discover and establish. The problem now is that everything fits together so beautifully and the Model is so well confirmed. That’s why I miss the time of the J/psi discovery. Nobody expected that, and nobody really had a clue what it was.

    But maybe those days of surprise aren’t over.

    We know that the Standard Model is an incomplete description of nature. It doesn’t account for gravity, the masses of neutrinos, or dark matter—the invisible substance that seems to make up six-sevenths of the universe’s mass. Do you have a favorite idea for what lies beyond the Standard Model?

    Well, right now I am searching for the particles that make up dark matter. The only thing is, I am committed to working at the Large Hadron Collider at CERN. But a collider may or may not be the best place to look for dark matter. It’s out there in the galaxies, but we don’t see it here on Earth.

    Still, I am going to try. If dark matter has any interactions with the known particles, it can be produced via collisions at the LHC. But weakly interacting dark matter would not leave a visible signature in our detector at ATLAS, so we have to intuit its existence from what we actually see. Right now, I am concentrating on finding hints of dark matter in the form of missing energy and momentum in a collision that produces a single Higgs boson.

    What else have you been working on?What else have you been working on?

    Our most important task is to understand the properties of the Higgs boson, which is a completely new kind of particle. The Higgs is more symmetric than any other particle we know about; it’s the first particle that we have discovered without any spin. My group and I were major contributors to the very recent measurement of Higgs bosons interacting with top quarks. That observation was extremely challenging. We examined five years of collision data, and my team worked intensively on advanced machine-learning techniques and statistics.

    In addition to studying the Higgs and searching for dark matter, my group and I also contributed to the silicon pixel detector, to the trigger system [that identifies potentially interesting collisions], and to the computing system in the ATLAS detector. We are now improving these during the shutdown and upgrade of the LHC. We are also very excited about the near future, because we plan to start using quantum computing to do our data analysis.

    6
    Wu at CERN. Thi My Lien Nguyen/Quanta Magazine

    Do you have any advice for young physicists just starting their careers?

    Some of the young experimentalists today are a bit too conservative. In other words, they are afraid to do something that is not in the mainstream. They fear doing something risky and not getting a result. I don’t blame them. It’s the way the culture is. My advice to them is to figure out what the most important experiments are and then be persistent. Good experiments always take time.

    But not everyone gets to take that time.

    Right. Young students don’t always have the freedom to be very innovative, unless they can do it in a very short amount of time and be successful. They don’t always get to be patient and just explore. They need to be recognized by their collaborators. They need people to write them letters of recommendation.

    The only thing that you can do is work hard. But I also tell my students, “Communicate. Don’t close yourselves off. Try to come up with good ideas on your own but also in groups. Try to innovate. Nothing will be easy. But it is all worth it to discover something new.”

    See the full article here .

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    Please help promote STEM in your local schools.

    Stem Education Coalition

    In achievement and prestige, the University of Wisconsin–Madison has long been recognized as one of America’s great universities. A public, land-grant institution, UW–Madison offers a complete spectrum of liberal arts studies, professional programs and student activities. Spanning 936 acres along the southern shore of Lake Mendota, the campus is located in the city of Madison.

     
  • richardmitnick 8:19 am on July 9, 2018 Permalink | Reply
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    From Quanta Magazine: “Physicists Find a Way to See the ‘Grin’ of Quantum Gravity” 

    Quanta Magazine
    From Quanta Magazine

    March 6, 2018
    Natalie Wolchover

    Re-released 7.8.18

    A recently proposed experiment would confirm that gravity is a quantum force.

    1
    Two microdiamonds would be used to test the quantum nature of gravity. Olena Shmahalo/Quanta Magazine

    In 1935, when both quantum mechanics and Albert Einstein’s general theory of relativity were young, a little-known Soviet physicist named Matvei Bronstein, just 28 himself, made the first detailed study of the problem of reconciling the two in a quantum theory of gravity. This “possible theory of the world as a whole,” as Bronstein called it, would supplant Einstein’s classical description of gravity, which casts it as curves in the space-time continuum, and rewrite it in the same quantum language as the rest of physics.

    Bronstein figured out how to describe gravity in terms of quantized particles, now called gravitons, but only when the force of gravity is weak — that is (in general relativity), when the space-time fabric is so weakly curved that it can be approximated as flat. When gravity is strong, “the situation is quite different,” he wrote. “Without a deep revision of classical notions, it seems hardly possible to extend the quantum theory of gravity also to this domain.”

    His words were prophetic. Eighty-three years later, physicists are still trying to understand how space-time curvature emerges on macroscopic scales from a more fundamental, presumably quantum picture of gravity; it’s arguably the deepest question in physics.

    2
    To Solve the Biggest Mystery in Physics, Join Two Kinds of Law. Robbert Dijkgraaf . James O’Brien for Quanta Magazine.Reductionism breaks the world into elementary building blocks. Emergence finds the simple laws that arise out of complexity. These two complementary ways of viewing the universe come together in modern theories of quantum gravity. September 7, 2017

    Perhaps, given the chance, the whip-smart Bronstein might have helped to speed things along. Aside from quantum gravity, he contributed to astrophysics and cosmology, semiconductor theory, and quantum electrodynamics, and he also wrote several science books for children, before being caught up in Stalin’s Great Purge and executed in 1938, at the age of 31.

    The search for the full theory of quantum gravity has been stymied by the fact that gravity’s quantum properties never seem to manifest in actual experience. Physicists never get to see how Einstein’s description of the smooth space-time continuum, or Bronstein’s quantum approximation of it when it’s weakly curved, goes wrong.

    The problem is gravity’s extreme weakness. Whereas the quantized particles that convey the strong, weak and electromagnetic forces are so powerful that they tightly bind matter into atoms, and can be studied in tabletop experiments, gravitons are individually so weak that laboratories have no hope of detecting them. To detect a graviton with high probability, a particle detector would have to be so huge and massive that it would collapse into a black hole. This weakness is why it takes an astronomical accumulation of mass to gravitationally influence other massive bodies, and why we only see gravity writ large.

    Not only that, but the universe appears to be governed by a kind of cosmic censorship: Regions of extreme gravity — where space-time curves so sharply that Einstein’s equations malfunction and the true, quantum nature of gravity and space-time must be revealed — always hide behind the horizons of black holes.

    3
    Mike Zeng for Quanta Magazine. Where Gravity Is Weak and Naked Singularities Are Verboten. Natalie Wolchover Recent calculations tie together two conjectures about gravity, potentially revealing new truths about its elusive quantum nature.

    “Even a few years ago it was a generic consensus that, most likely, it’s not even conceivably possible to measure quantization of the gravitational field in any way,” said Igor Pikovski, a theoretical physicist at Harvard University.

    Now, a pair of papers recently published in Physical Review Letters has changed the calculus.

    Spin Entanglement Witness for Quantum Gravity https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240401
    Gravitationally Induced Entanglement between Two Massive Particles is Sufficient Evidence of Quantum Effects in Gravity https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240402

    The papers contend that it’s possible to access quantum gravity after all — while learning nothing about it. The papers, written by Sougato Bose at University College London and nine collaborators and by Chiara Marletto and Vlatko Vedral at the University of Oxford, propose a technically challenging, but feasible, tabletop experiment that could confirm that gravity is a quantum force like all the rest, without ever detecting a graviton. Miles Blencowe, a quantum physicist at Dartmouth College who was not involved in the work, said the experiment would detect a sure sign of otherwise invisible quantum gravity — the “grin of the Cheshire cat.”

    2
    A levitating microdiamond (green dot) in Gavin Morley’s lab at the University of Warwick, in front of the lens used to trap the diamond with light. Gavin W Morley

    The proposed experiment will determine whether two objects — Bose’s group plans to use a pair of microdiamonds — can become quantum-mechanically entangled with each other through their mutual gravitational attraction. Entanglement is a quantum phenomenon in which particles become inseparably entwined, sharing a single physical description that specifies their possible combined states. (The coexistence of different possible states, called a “superposition,” is the hallmark of quantum systems.) For example, an entangled pair of particles might exist in a superposition in which there’s a 50 percent chance that the “spin” of particle A points upward and B’s points downward, and a 50 percent chance of the reverse. There’s no telling in advance which outcome you’ll get when you measure the particles’ spin directions, but you can be sure they’ll point opposite ways.

    The authors argue that the two objects in their proposed experiment can become entangled with each other in this way only if the force that acts between them — in this case, gravity — is a quantum interaction, mediated by gravitons that can maintain quantum superpositions. “If you can do the experiment and you get entanglement, then according to those papers, you have to conclude that gravity is quantized,” Blencowe explained.

    To Entangle a Diamond

    Quantum gravity is so imperceptible that some researchers have questioned whether it even exists. The venerable mathematical physicist Freeman Dyson, 94, has argued since 2001 that the universe might sustain a kind of “dualistic” description, where “the gravitational field described by Einstein’s theory of general relativity is a purely classical field without any quantum behavior,” as he wrote that year in The New York Review of Books, even though all the matter within this smooth space-time continuum is quantized into particles that obey probabilistic rules.

    Dyson, who helped develop quantum electrodynamics (the theory of interactions beween matter and light) and is professor emeritus at the Institute for Advanced Study in Princeton, New Jersey, where he overlapped with Einstein, disagrees with the argument that quantum gravity is needed to describe the unreachable interiors of black holes. And he wonders whether detecting the hypothetical graviton might be impossible, even in principle. In that case, he argues, quantum gravity is metaphysical, rather than physics.

    He is not the only skeptic. The renowned British physicist Sir Roger Penrose and, independently, the Hungarian researcher Lajos Diósi have hypothesized that space-time cannot maintain superpositions. They argue that its smooth, solid, fundamentally classical nature prevents it from curving in two different possible ways at once — and that its rigidity is exactly what causes superpositions of quantum systems like electrons and photons to collapse. This “gravitational decoherence,” in their view, gives rise to the single, rock-solid, classical reality experienced at macroscopic scales.

    The ability to detect the “grin” of quantum gravity would seem to refute Dyson’s argument. It would also kill the gravitational decoherence theory, by showing that gravity and space-time do maintain quantum superpositions.

    Bose’s and Marletto’s proposals appeared simultaneously mostly by chance, though experts said they reflect the zeitgeist. Experimental quantum physics labs around the world are putting ever-larger microscopic objects into quantum superpositions and streamlining protocols for testing whether two quantum systems are entangled. The proposed experiment will have to combine these procedures while requiring further improvements in scale and sensitivity; it could take a decade or more to pull it off. “But there are no physical roadblocks,” said Pikovski, who also studies how laboratory experiments might probe gravitational phenomena. “I think it’s challenging, but I don’t think it’s impossible.”

    The plan is laid out in greater detail in the paper by Bose and co-authors — an Ocean’s Eleven cast of experts for different steps of the proposal. In his lab at the University of Warwick, for instance, co-author Gavin Morley is working on step one, attempting to put a microdiamond in a quantum superposition of two locations. To do this, he’ll embed a nitrogen atom in the microdiamond, next to a vacancy in the diamond’s structure, and zap it with a microwave pulse. An electron orbiting the nitrogen-vacancy system both absorbs the light and doesn’t, and the system enters a quantum superposition of two spin directions — up and down — like a spinning top that has some probability of spinning clockwise and some chance of spinning counterclockwise. The microdiamond, laden with this superposed spin, is subjected to a magnetic field, which makes up-spins move left while down-spins go right. The diamond itself therefore splits into a superposition of two trajectories.

    In the full experiment, the researchers must do all this to two diamonds — a blue one and a red one, say — suspended next to each other inside an ultracold vacuum. When the trap holding them is switched off, the two microdiamonds, each in a superposition of two locations, fall vertically through the vacuum. As they fall, the diamonds feel each other’s gravity. But how strong is their gravitational attraction?

    If gravity is a quantum interaction, then the answer is: It depends. Each component of the blue diamond’s superposition will experience a stronger or weaker gravitational attraction to the red diamond, depending on whether the latter is in the branch of its superposition that’s closer or farther away. And the gravity felt by each component of the red diamond’s superposition similarly depends on where the blue diamond is.

    In each case, the different degrees of gravitational attraction affect the evolving components of the diamonds’ superpositions. The two diamonds become interdependent, meaning that their states can only be specified in combination — if this, then that — so that, in the end, the spin directions of their two nitrogen-vacancy systems will be correlated.

    3
    Lucy Reading-Ikkanda/Quanta Magazine

    After the microdiamonds have fallen side by side for about three seconds — enough time to become entangled by each other’s gravity — they then pass through another magnetic field that brings the branches of each superposition back together. The last step of the experiment is an “entanglement witness” protocol developed by the Dutch physicist Barbara Terhal and others: The blue and red diamonds enter separate devices that measure the spin directions of their nitrogen-vacancy systems. (Measurement causes superpositions to collapse into definite states.) The two outcomes are then compared. By running the whole experiment over and over and comparing many pairs of spin measurements, the researchers can determine whether the spins of the two quantum systems are correlated with each other more often than a known upper bound for objects that aren’t quantum-mechanically entangled. In that case, it would follow that gravity does entangle the diamonds and can sustain superpositions.

    “What’s beautiful about the arguments is that you don’t really need to know what the quantum theory is, specifically,” Blencowe said. “All you have to say is there has to be some quantum aspect to this field that mediates the force between the two particles.”

    Technical challenges abound. The largest object that’s been put in a superposition of two locations before is an 800-atom molecule. Each microdiamond contains more than 100 billion carbon atoms — enough to muster a sufficient gravitational force. Unearthing its quantum-mechanical character will require colder temperatures, a higher vacuum and finer control. “So much of the work is getting this initial superposition up and running,” said Peter Barker, a member of the experimental team based at UCL who is improving methods for laser-cooling and trapping the microdiamonds. If it can be done with one diamond, Bose added, “then two doesn’t make much of a difference.”

    Why Gravity Is Unique

    Quantum gravity researchers do not doubt that gravity is a quantum interaction, capable of inducing entanglement. Certainly, gravity is special in some ways, and there’s much to figure out about the origin of space and time, but quantum mechanics must be involved, they say. “It doesn’t really make much sense to try to have a theory in which the rest of physics is quantum and gravity is classical,” said Daniel Harlow, a quantum gravity researcher at the Massachusetts Institute of Technology. The theoretical arguments against mixed quantum-classical models are strong (though not conclusive).

    On the other hand, theorists have been wrong before, Harlow noted: “So if you can check, why not? If that will shut up these people” — meaning people who question gravity’s quantumness — “that’s great.”

    Dyson wrote in an email, after reading the PRL papers, “The proposed experiment is certainly of great interest and worth performing with real quantum systems.” However, he said the authors’ way of thinking about quantum fields differs from his. “It is not clear to me whether [the experiment] would settle the question whether quantum gravity exists,” he wrote. “The question that I have been asking, whether a single graviton is observable, is a different question and may turn out to have a different answer.”

    In fact, the way Bose, Marletto and their co-authors think about quantized gravity derives from how Bronstein first conceived of it in 1935. (Dyson called Bronstein’s paper “a beautiful piece of work” that he had not seen before.) In particular, Bronstein showed that the weak gravity produced by a small mass can be approximated by Newton’s law of gravity. (This is the force that acts between the microdiamond superpositions.) According to Blencowe, weak quantized-gravity calculations haven’t been developed much, despite being arguably more physically relevant than the physics of black holes or the Big Bang. He hopes the new experimental proposal will spur theorists to find out whether there are any subtle corrections to the Newtonian approximation that future tabletop experiments might be able to probe.

    Leonard Susskind, a prominent quantum gravity and string theorist at Stanford University, saw value in carrying out the proposed experiment because “it provides an observation of gravity in a new range of masses and distances.” But he and other researchers emphasized that microdiamonds cannot reveal anything about the full theory of quantum gravity or space-time. He and his colleagues want to understand what happens at the center of a black hole, and at the moment of the Big Bang.

    Perhaps one clue as to why it is so much harder to quantize gravity than everything else is that other force fields in nature exhibit a feature called “locality”: The quantum particles in one region of the field (photons in the electromagnetic field, for instance) are “independent of the physical entities in some other region of space,” said Mark Van Raamsdonk, a quantum gravity theorist at the University of British Columbia. But “there’s at least a bunch of theoretical evidence that that’s not how gravity works.”

    In the best toy models of quantum gravity (which have space-time geometries that are simpler than those of the real universe), it isn’t possible to assume that the bendy space-time fabric subdivides into independent 3-D pieces, Van Raamsdonk said. Instead, modern theory suggests that the underlying, fundamental constituents of space “are organized more in a 2-D way.” The space-time fabric might be like a hologram, or a video game: “Even though the picture is three-dimensional, the information is stored in some two-dimensional computer chip,” he said. In that case, the 3-D world is illusory in the sense that different parts of it aren’t all that independent. In the video-game analogy, a handful of bits stored in the 2-D chip might encode global features of the game’s universe.

    The distinction matters when you try to construct a quantum theory of gravity. The usual approach to quantizing something is to identify its independent parts — particles, say — and then apply quantum mechanics to them. But if you don’t identify the correct constituents, you get the wrong equations. Directly quantizing 3-D space, as Bronstein did, works to some extent for weak gravity, but the method fails when space-time is highly curved.

    Witnessing the “grin” of quantum gravity would help motivate these abstract lines of reasoning, some experts said. After all, even the most sensible theoretical arguments for the existence of quantum gravity lack the gravitas of experimental facts. When Van Raamsdonk explains his research in a colloquium or conversation, he said, he usually has to start by saying that gravity needs to be reconciled with quantum mechanics because the classical space-time description fails for black holes and the Big Bang, and in thought experiments about particles colliding at unreachably high energies. “But if you could just do this simple experiment and get the result that shows you that the gravitational field was actually in a superposition,” he said, then the reason the classical description falls short would be self-evident: “Because there’s this experiment that suggests gravity is quantum.”

    Correction March 6, 2018: An earlier version of this article referred to Dartmouth University. Despite the fact that Dartmouth has multiple individual schools, including an undergraduate college as well as academic and professional graduate schools, the institution refers to itself as Dartmouth College for historical reasons.

    See the full article here .


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    Please help promote STEM in your local schools.

    Stem Education Coalition

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 2:27 pm on May 20, 2018 Permalink | Reply
    Tags: , , , , , Quanta Magazine   

    From Quanta Magazine: “A New World’s Extraordinary Orbit Points to Planet Nine” 

    Quanta Magazine
    From Quanta Magazine

    May 15, 2018
    Shannon Hall

    Astronomers argue that there’s an undiscovered giant planet far beyond the orbit of Neptune. A newly discovered rocky body has added evidence to the circumstantial case for it.

    1
    Olena Shmahalo/Quanta Magazine

    In early 2016, two planetary scientists declared that a ghost planet is hiding in the depths of the solar system, well beyond the orbit of Pluto. Their claim, which they made based on the curious orbits of distant icy worlds, quickly sparked a race to find this so-called Planet Nine — a planet that is estimated to be about 10 times the mass of Earth. “It has a real magnetism to it,” said Gregory Laughlin, an astronomer at Yale University. “I mean, finding a 10-Earth-mass planet in our own solar system would be a discovery of unrivaled scientific magnitude.”

    Now, astronomers are reporting [The Astronomical Journal] that they have spotted another distant world — perhaps as large as a dwarf planet — whose orbit is so odd that it is likely to have been shepherded by Planet Nine.

    The Extreme Trans-Neptunian object orbits
    2
    6 original and 8 new TNO object orbits with current positions near their perihelion in purple, with hypothetical Planet Nine orbit in green. https://en.wikipedia.org/wiki/Planet_Nine. No image credit found.

    The object confirms a specific prediction made by Konstantin Batygin and Michael Brown, the astronomers at the California Institute of Technology who first argued for Planet Nine’s existence. “It’s not proof that Planet Nine exists,” said David Gerdes, an astronomer at the University of Michigan and a co-author on the new paper. “But I would say the presence of an object like this in our solar system bolsters the case for Planet Nine.”

    3
    Lucy Reading-Ikkanda/Quanta Magazine

    Gerdes and his colleagues spotted the new object in data from the Dark Energy Survey, a project that probes the acceleration in the expansion of the universe by surveying a region well above the plane of the solar system.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam

    This makes it an unlikely tool for finding objects inside the solar system, since they mostly orbit within the plane. But that is exactly what makes the new object unique: Its orbit is tilted 54 degrees with respect to the plane of the solar system. It’s something Gerdes did not expect to see. Batygin and Brown, however, predicted it.

    Two years ago, Batygin and Brown made a case [The Astronomical Journal] for Planet Nine’s existence based on the peculiar orbits of a handful of distant worlds known as Kuiper belt objects.

    Kuiper Belt. Minor Planet Center

    That small population loops outward toward the same quadrant of the solar system, a phenomenon that would be extremely unlikely to happen by chance. Batygin and Brown argued that a ninth planet must be shepherding those worlds into their strange orbits.

    What’s more, Batygin and Brown also predicted that over time, Planet Nine’s gravity would push these Kuiper belt objects out of their current plane and into ever-higher orbital inclinations. Although astronomers have already spotted a bizarre population of worlds that orbit the sun perpendicularly to the plane of the solar system, they had never caught an object transitioning between the two populations. “There’s no real way to put something on an orbit like that — except that it’s exactly what we predicted from Planet Nine,” Brown said. Batygin notes that the new object fits so perfectly with their model that it almost looks like one of the data points in their simulations. “A good theory reproduces data — but a great theory predicts new data,” he said.

    The Dark Energy Survey first detected evidence for the new object in late 2014. Gerdes and his colleagues have spent the years since then tracking its orbit and trying to understand its origins. In the new paper, they describe how they ran many simulations of the object within the known solar system, letting the clock run forward and backward 4.5 billion years at a time. Nothing could explain how the object landed in such a tilted orbit. It wasn’t until they added in a ninth planet — a planet with characteristics that perfectly match Batygin and Brown’s predictions — that the wacky orbit finally made sense. “The second you put Planet Nine in the simulations, not only can you form objects like this object, but you absolutely do,” said Juliette Becker, a graduate student at Michigan and the lead author on the new paper. A strong and sustained interaction with Planet Nine appears to be the only way to pump up the object’s inclination, pushing it away from the plane of the solar system. “There is no other reasonable way to populate the Kuiper belt with such highly inclined bodies,” Batygin said. “I think the case for the existence of Planet Nine is now genuinely excellent.”

    Other astronomers aren’t so certain — in part because the early solar system remains a mystery. Scientists suspect that the sun was born within a cluster of stars, meaning that the early planets might have had many close encounters with other stars that sent them on paths that seem impossible today. And even once the stars dispersed, the early solar system likely contained tens of thousands of dwarf planets that could have provided the gravitational nudges needed to push 2015 BP519, as the new object is called, into such an odd orbit. “To me, Planet Nine is one of a number of ways that the solar system could have unfolded,” said Michele Bannister, an astronomer at Queen’s University Belfast who was not involved in the study. “It’s a potential idea.” But at the moment it is just that — an idea.

    Yet when astronomers examine the larger universe, the idea doesn’t seem all that surprising. Planets between two and 10 times the mass of Earth are incredibly common throughout the galaxy, which makes it odd that our solar system doesn’t harbor one. “If it wasn’t in our own solar system — if the stakes weren’t so high — I think that the hypothesis would almost certainly be correct,” Laughlin said. “It’s only the fact that it’s so amazing that tends to give me pause.” Finding a ninth planet within our solar system would be both transformative and extraordinarily inspiring, he said. “It would be this dramatic confirmation of the scientific method, which would be pretty refreshing in the current age where the truth is on trial.”

    See the full article here .

    Please help promote STEM in your local schools.

    stem

    Stem Education Coalition

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 4:30 pm on March 22, 2018 Permalink | Reply
    Tags: , , , , , , , Quanta Magazine, Squishy or Solid? A Neutron Star’s Insides Open to Debate   

    From Quanta Magazine: “Squishy or Solid? A Neutron Star’s Insides Open to Debate” 

    Quanta Magazine
    Quanta Magazine

    October 30, 2017 [Just now in social media]
    Joshua Sokol

    The core of a neutron star is such an extreme environment that physicists can’t agree on what happens inside. But a new space-based experiment — and a few more colliding neutron stars — should reveal whether neutrons themselves break down.

    1
    Maciej Rebisz for Quanta Magazine

    The alerts started in the early morning of Aug. 17. Gravitational waves produced by the wreck of two neutron stars — dense cores of dead stars — had washed over Earth. The thousand-plus physicists of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) rushed to decode the space-time vibrations that rolled across the detectors like a drawn-out peal of thunder. Thousands of astronomers scrambled to witness the afterglow. But officially, all this activity was kept secret. The data had to be collected and analyzed, the papers written. The outside world wouldn’t know for two more months.

    See https://sciencesprings.wordpress.com/2017/10/20/from-ucsc-neutron-stars-gravitational-waves-and-all-the-gold-in-the-universe/

    The strict ban put Jocelyn Read and Katerina Chatziioannou, two members of the LIGO collaboration, in a bit of an awkward situation. In the afternoon on the 17th, the two were scheduled to lead a panel at a conference dedicated to the question of what happens under the almost unfathomable conditions in a neutron star’s interior. Their panel’s topic? What a neutron-star merger would look like. “We sort of went off at the coffee break and sat around just staring at each other,” said Read, a professor at California State University, Fullerton. “OK, how are we going to do this?”

    Physicists have spent decades debating whether or not neutron stars contain new forms of matter, created when the stars break down the familiar world of protons and neutrons into new interactions between quarks or other exotic particles. Answering this question would also illuminate astronomical mysteries surrounding supernovas and the production of the universe’s heavy elements, such as gold.

    In addition to watching for collisions using LIGO, astrophysicists have been busy developing creative ways to probe neutron stars from the outside. The challenge is then to infer something about the hidden layers within. But this LIGO signal and those like it — emitted as two neutron stars pirouette around their center of mass, pull on each other like taffy, and finally smash together — offers a whole new handle on the problem.

    Strange Matter

    A neutron star is the compressed core of a massive star — the super dense cinders left over after a supernova. It has the mass of the sun, but squeezed into a space the width of a city. As such, neutron stars are the densest reservoirs of matter in the universe — the “last stuff on the line before a black hole,” said Mark Alford, a physicist at Washington University in St. Louis.

    To drill into one would bring us to the edge of modern physics. A centimeter or two of normal atoms — iron and silicon, mostly — encrusts the surface like the shiny red veneer on the universe’s densest Gobstopper. Then the atoms squeeze so close together that they lose their electrons, which fall into a shared sea. Deeper, the protons inside nuclei start turning into neutrons, which cluster so close together that they start to overlap.

    2
    Lucy Reading-Ikkanda/Quanta Magazine; Source: Feryal Özel

    But theorists argue about what happens farther in, when densities creep past two or three times higher than the density of a normal atomic nucleus. From the perspective of nuclear physics, neutron stars could just be protons and neutrons — collectively called nucleons — all the way in. “Everything can be explained by variations of nucleons,” said James Lattimer, an astrophysicist at Stony Brook University.

    Other astrophysicists suspect otherwise. Nucleons aren’t elementary particles. They’re made up of three quarks. Under immense pressure, these quarks might form a new state of quark matter. “Nucleons are not billiard balls,” said David Blaschke, a physicist at the University of Wroclaw in Poland. “They are like cherries. So you can compress them a little bit, but at some point you smash them.”

    But to some, the prospect of a quark jam like this is a relatively vanilla scenario. Theorists have long speculated that layers of other weird particles might arise inside a neutron star. As neutrons are jostled closer together, all that extra energy might go into creating heavier particles that contain not just the “up” and “down” quarks that exclusively make up protons and neutrons, but heavier and more exotic “strange” quarks.

    For example, neutrons might be replaced by hyperons, three-quark particles that include at least one strange quark. Laboratory experiments can make hyperons, but they vanish almost immediately. Deep inside neutron stars, they might be stable for millions of years.

    Alternatively, the hidden depths of neutron stars might be filled with kaons — also made with strange quarks — that collect into a single lump of matter sharing the same quantum state.

    For decades, though, the field has been stuck. Theorists invent ideas about what might be going on inside neutron stars, but that environment is so extreme and unfamiliar that experiments here on Earth can’t reach the right conditions. At Brookhaven National Laboratory and CERN, for example, physicists smash together heavy nuclei like those of gold and lead.

    That creates a soupy state of matter made up of released quarks, known as a quark-gluon plasma. But this stuff is rarefied, not dense, and at billions or trillions of degrees, it’s far hotter than the inside of neutron star, which sits in the comparatively chilly millions.

    Quark gluon plasma. Duke University

    Even the decades-old theory of quarks and nuclei — “quantum chromodynamics,” or QCD — can’t really provide answers. The computations needed to study QCD in relatively cold, dense environments are so devastatingly difficult that not even computers can calculate the results. Researchers are forced to resort to oversimplification and shortcuts.

    The only other option is for astronomers to study neutron stars themselves. Unfortunately, neutron stars are distant, thus dim, and difficult to measure for anything but the very basic bulk properties. Even worse, the truly interesting physics is happening under the surface. “It’s a bit like there’s this lab that’s doing amazing things,” Alford said, “but all you’re allowed to do is see the light coming out of the window.”

    With a new generation of experiments coming online, though, theorists might soon get their best look yet.

    6
    The NICER instrument, shown here before it was launched to the International Space Station, monitors the X-ray emissions of neutron stars. NASA/Goddard/Keith Gendreau

    Squishy or Hard?

    Whatever might be inside the core of a neutron star — loose quarks, or kaon condensates, or hyperons, or just regular old nucleons — the material must be able to hold up to the crushing weight of more than a sun’s worth of gravity. Otherwise, the star would collapse into a black hole. But different materials will compress to different degrees when squeezed by gravity’s vise, determining how heavy the star can be at a given physical size.

    Stuck on the outside, astronomers work backwards to figure out what neutron stars are made of. For this purpose, it helps to know how squishy or stiff they are when squeezed. And for that, astronomers need to measure the masses and radii of various neutron stars.

    In terms of mass, the most easily weighed neutron stars are pulsars: neutron stars that rotate quickly, sweeping a radio beam across Earth with each spin. About 10 percent of the 2,500 known pulsars belong to binary systems. As these pulsars move with their partners, what should be a constant tick-tock of pulses hitting Earth will vary, betraying the pulsar’s motion and its location in its orbit. And from the orbit, astronomers can use Kepler’s laws and the additional rules imposed by Einstein’s general relativity to solve for the masses of the pair.

    So far, the biggest breakthrough has been the discovery of surprisingly hefty neutron stars. In 2010, a team led by Scott Ransom at the National Radio Astronomy Observatory in Virginia announced that they had measured a pulsar weighing about two solar masses — making it far bigger than any previously seen. Some people doubted whether such a neutron star could exist; that it does has had immense consequences for our understanding of how nuclei behave. “Now it’s like the most cited observational pulsar paper ever, because of the nuclear physicists,” Ransom said.

    According to some neutron-star models, which hold that gravity should strongly compress neutron stars, an object at that mass should collapse all the way into a black hole. That would be bad news for kaon condensates, which would be especially squishy, and it bodes poorly for some versions of quark matter and hyperons that would also compress too much. The measurement has been confirmed with the discovery of another neutron star of two solar masses in 2013.

    Radii are trickier. Astrophysicists like Feryal Özel at the University of Arizona have devised various tricks to calculate the physical size of neutron stars by observing the X-rays emitted at their surfaces. Here’s one way: You can look at the overall X-ray emission, use it to estimate the temperature of the surface, and then figure out how big the neutron star needs to be to emit the observed light (correcting for how the light bends through space-time warped by gravity). Or you can look for hot spots on the neutron star’s surface that spin in and out of view. The neutron star’s strong gravitational field will modify the pulses of light from these hot spots. And once you understand the star’s gravitational field, you can reconstruct its mass and radius.

    Taken at face value, these X-ray measurements suggest that even though neutron stars can be heavy, they are on the small end of predictions: only about 20 to 22 kilometers wide, according to Özel.

    Accepting that neutron stars are both small and massive “kind of locks you in, in a good way,” Özel said. Neutron stars stuffed with interacting quarks would look like this, she said, while neutron stars made up of only nucleons would have larger radii.

    But Lattimer, among other critics, has reservations about the assumptions that go into the X-ray measurements, which he calls flawed. He thinks they make the radii look smaller they really are.

    Both sides expect that a resolution to the dispute will soon arrive. This past June, SpaceX’s 11th resupply mission to the International Space Station brought with it a 372-kilogram box containing an X-ray telescope called the Neutron Star Interior Composition Explorer (NICER).

    7
    NICER before launch.

    Now taking data, NICER is designed to find the size of neutron stars by watching for hot spots on their surfaces. The experiment should produce better radii measurements of neutron stars, including pulsars that have already had their masses measured.

    “We look so much forward to it,” Blaschke said. A well-measured mass and radius for even a single neutron star would knock out many possible theories of their interior structure, keeping in play only the ones that could produce that particular combination of size and weight.

    And now, finally chiming in, there’s LIGO.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

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

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    As a first pass, the signal that Read huddled over coffee to discuss on Aug. 17 had been processed as if it were a merger of two black holes, not two neutron stars. This wasn’t unreasonable. LIGO’s previous signals had all come from black holes, which are more tractable beasts from a computational standpoint. But this signal involved lighter objects and went on for much longer than the black hole mergers. “It’s immediately obvious that this was not the same kind of system that we were practiced on,” Read said.

    When two black holes spiral together, they bleed orbital energy into space-time as gravitational waves. But in the final second or so of the new 90-second-long LIGO signal, each object did something black holes don’t do: It deformed. The pair started to stretch and squeeze each other’s matter, generating tides that stole energy from their orbits. This drove them to collide faster than they would have otherwise.

    After a frantic few months of running computer simulations, Read’s group inside LIGO has released their first measurement of the effect of those tides on the signal. So far, the team can set only an upper limit — meaning the tides have a weak or even unnoticeable effect. In turn, that means that neutron stars are physically small, with their matter held very tightly around their centers and thus more resistant to getting yanked by tides. “I think the first gravitational-wave measurement is in a sense really kind of confirming the kinds of things that X-ray observations have been saying,” Read said. But this isn’t the last word. She expects that more sophisticated modeling of the same signal will yield a more precise estimate.

    With NICER and LIGO both offering new ways to look at neutron-star stuff, many experts are optimistic that the next few years will provide unambiguous answers to the question of how the material stands up to gravity. But theorists like Alford caution that measuring neutron-star matter’s squishiness alone won’t fully reveal what it is.

    Perhaps other signatures can say more. Ongoing observations of the rate at which neutron stars cool, for example, should let astrophysicists speculate about the particles inside them and their ability to radiate away energy. Or observations of how their spins slow over time could help determine the viscosity of their insides.

    Ultimately, just knowing when dense matter changes phase and what it changes into is a worthy goal, Alford argues. “Mapping the properties of matter under different conditions,” he said, “kind of is physics”.

    See the full article here .

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 2:10 pm on March 21, 2018 Permalink | Reply
    Tags: , , , , , , Quanta Magazine   

    From Quanta Magazine: “Science’s Path From Myth to Multiverse” 

    Quanta Magazine
    Quanta Magazine

    In his latest book, the Nobel Prize winner Steven Weinberg explores how science made the modern world, and where it might take us from here.

    March 17, 2015 [Just found this in social media.]
    Dan Falk

    Steven Weinberg, U Texas


    Steven Weinberg

    Steven Weinberg, a physicist at the University of Texas, Austin, won a Nobel Prize in 1979 for work that became a cornerstone of particle physics.

    We can think of the history of physics as an attempt to unify the world around us: Gradually, over many centuries, we’ve come to see that seemingly unrelated phenomena are intimately connected. The physicist Steven Weinberg of the University of Texas, Austin, received his Nobel Prize in 1979 for a major breakthrough in that quest — showing how electromagnetism and the weak nuclear force are manifestations of the same underlying theory (he shared the prize with Abdus Salam and Sheldon Glashow). That work became a cornerstone of the Standard Model of particle physics, which describes how the fundamental building blocks of the universe come together to create the world we see.

    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.


    Standard Model of Particle Physics from Symmetry Magazine

    In his new book To Explain the World: The Discovery of Modern Science, Weinberg examines how modern science was born.

    2

    By tracing the development of what we now call the “scientific method” — an approach, developed over centuries, that emphasizes experiments and observations rather than reasoning from first principles — he makes the argument that science, unlike other ways of interpreting the world around us, can offer true progress. Through science, our understanding of the world improves over time, building on what has come before. Mistakes can happen, but are eventually corrected. Weinberg spoke with Quanta Magazine about the past and future of physics, the role of philosophy within science, and the startling possibility that the universe we see around us is a tiny sliver of a much larger multiverse. An edited and condensed version of the interview follows.

    QUANTA MAGAZINE: As a physicist, how is your perspective on the history of science different from that of a historian?

    STEVEN WEINBERG: One difference, of course, is that they know more than I do — at least, in their particular field of specialization. Real historians have a much better grasp of the original sources than I could possibly have. If they’re historians of the ancient world, they’ll be experts in Greek and Latin, which I’m not even remotely knowledgeable about.

    But there’s also a difference in attitude. Many historians are strongly opposed to the so-called “Whig interpretation” of history, in which you look at the past and try to pick out the threads that lead to the present. They feel it’s much more important to get into the frame of mind of the people who lived at the time you’re writing about. And they have a point. But I would argue that, when it comes to the history of science, a Whig interpretation is much more justifiable. The reason is that science, unlike, say, politics or religion, is a cumulative branch of knowledge. You can say, not merely as a matter of taste, but with sober judgment, that Newton knew more about the world than Aristotle did, and Einstein knew more than Newton did. There really has been progress. And to trace that progress, it makes sense to look at the science of the past and try to pick out modes of thought that either led to progress, or impeded progress.

    Why did you focus on the history of physics and astronomy?

    Well, that’s what I know about; that’s where I have some competence. But there’s another reason: It’s in physics and astronomy that science first became “modern.” Actually, it’s physics as applied to astronomy. Newton gave us the modern approach to physics in the late 17th century. Other branches of science became modern only more recently: chemistry in the early 19th century; biology in the mid-19th century, or perhaps the early 20th century. So if you want to understand the discovery of modern science — which is the subtitle of my book — that discovery was made in the context of physics, especially as applied to astronomy.

    Theoretical physics is often seen as a quest for unification — we think of Newton, unifying terrestrial and celestial physics, or James Clerk Maxwell, unifying electricity, magnetism, and light. And of course your own work. Where does this quest for unification stand today?

    It hasn’t advanced very much, except for the fact that the theories we speculated about in the 1960s have been confirmed by observation. In the theory I developed in 1967 — Abdus Salam developed essentially the same theory, independently, in 1968 — a symmetry-breaking field played a fundamental role, manifest in a particle called the Higgs boson, whose properties we predicted, except for its mass. Now, thanks to experiments performed at CERN, the Higgs has been verified.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    So we’re on much more solid ground. But we haven’t gone any further. There have been enormous efforts to take further steps, especially in the context of string theory. String theory would unify all of the forces — the strong and weak nuclear forces, and the electromagnetic force, together with gravity. String theory has provided some deep mathematical ideas about how that might work. But we’re far from being able to verify the theory — much further than we were from verifying the electroweak theory 40 years ago.

    The Large Hadron Collider (LHC) is scheduled to start up again this year [2015], with twice the power it had during its initial run. What do you hope it’ll find — I’m not sure if “hope” is the right word — when it’s turned on?

    “The Standard Model is so complex that it would be hard to put it on a T-shirt.”

    Hope is exactly the right word! It depends on what new particles might have masses in the range that the LHC can probe. There are certainly things to look for. The most obvious thing is the dark-matter particle. We know from astronomy that five-sixths of the matter in the universe is something that doesn’t fit in the Standard Model of particle physics. But we have no idea what its mass is. Astronomers can tell us the total mass of this dark matter, but not the mass carried by each particle. If it’s a conventional dark-matter particle, known as a WIMP — “weakly interacting massive particle” — then the LHC might find it. It depends on how heavy it is, and on how it decays, because you never see the particle itself, you only see the products of its decay.

    The LHC might also find signs of supersymmetry, a theory positing that known particles each have a partner particle — but again, we don’t know what the mass of those partner particles would be. And here, there’s an even deeper uncertainty: We don’t know if supersymmetry has anything to do with the real world. There could also be heavier quarks, perhaps even heavier versions of the Higgs particle.

    It’s sometimes said that supersymmetry would be a kind of thumbs-up for string theory, which has been impossible to test in any direct way. If the LHC finds no evidence for supersymmetry, what happens to string theory?

    Standard model of Supersymmetry DESY

    Damned if I know! Unfortunately, string theory doesn’t make very specific predictions about physics at the energies that are accessible to us. The kind of energies of the structures that string theory deals with are so high, we’ll probably never be able to reproduce them in the lab. But those energies were common in the very early universe. So by making cosmological observations, we may get a handle on the physics of those incredibly high energies. For example, if the matter-energy density at the time of inflation was of the order of magnitude that is characteristic of string theory, then a great deal of gravitational radiation would have been produced at that time, and it would have left an imprint on the cosmic microwave background. Last year, scientists working with the BICEP2 telescope announced that they had found these gravitational waves; now it seems they were actually measuring interstellar dust. Further observations with the Planck satellite may be able to settle this question. I think that’s one of the most exciting things going on in all of physical science right now.

    BICEP 2

    Gravitational Wave Background from BICEP 2 which ultimately failed to be correct. The Planck team determined that the culprit was cosmic dust.

    For theorists, is the ultimate goal a set of equations we could put on a T-shirt?

    That’s the aim. The Standard Model is so complex that it would be hard to put it on a T-shirt — though not impossible; you’d just have to write kind of small. Now, it wouldn’t take gravity into account, so it wouldn’t be a “theory of everything.” But it would be a theory of all the other things we study in our physics laboratories. The Standard Model is sufficiently complicated, and has so many arbitrary features, that we know it’s not the final answer. The goal would be to have a much simpler theory with fewer arbitrary features — maybe even none at all — that would fit on a T-shirt. We’re not there yet.

    Some physicists suggest that we may have to settle for an array of different theories, perhaps representing different solutions to string theory’s equations. Maybe each solution represents a different universe — part of some larger “multiverse.”

    I am not a proponent of the idea that our Big Bang universe is just part of a larger multiverse. It has to be taken seriously as a possibility, though. And it does lead to interesting consequences. For example, it would explain why some constants of nature, particularly the dark energy, have values that seem to be very favorable to the appearance of life.

    Dark energy depiction. Image: Volker Springle/Max Planck Institute for Astrophysics/SP)

    Suppose you have a multiverse in which constants like dark energy vary from one big bang to another. Then, if you ask why it takes the value it does in our Big Bang, you have to take into account that there’s a selection effect: It’s only in big bangs where the dark energy takes a value favorable to the appearance of life that there’s anybody around to ask the question.

    “You don’t have to verify every prediction to know that a theory is correct.”

    This is very closely analogous to a question that astronomers have discussed for thousands of years, concerning the Earth and the sun. Why is the sun the distance that it is from us? If it were closer, the Earth would be too hot to harbor life; if it were further away, the Earth would be too cold. Why is it at just the right distance? Most people, like Galen, the Roman physician, thought that it was due to the benevolence of the gods, that it was all arranged for our benefit. A much better answer — the answer we would give today — is that there are billions of planets in our galaxy, and billions of galaxies in the universe. And it’s not surprising that a few of them, out of all those billions, are positioned in a way that’s favorable for life.

    But at least we can see some of those other planets. That’s not the case with the universes that are said to make up the multiverse.

    It’s not part of the requirement of a successful physical theory that everything it describes be observable, or that all possible predictions of the theory be verifiable. For example, we have a very successful theory of the strong nuclear forces, called quantum chromodynamics [QCD], which is based on the idea that quarks are bound together by forces that increase with distance, so that we will never, even in principle, be able to observe a quark in isolation.
    All we can observe are other successful predictions of QCD. We can’t actually detect quarks, but it doesn’t matter; we know QCD is correct, because it makes predictions that we can verify.

    Similarly, string theory, which predicts a multiverse, can’t be verified by detecting the other parts of the multiverse. But it might make other predictions that can be verified. For example, it may say that in all of the big bangs within the multiverse, certain things will always be true, and those things may be verifiable. It may say that certain symmetries will always be observed, or that they’ll always be broken according to a certain pattern that we can observe. If it made enough predictions like that, then we would say that string theory is correct. And if the theory predicted a multiverse, then we’d say that that’s correct too. You don’t have to verify every prediction to know that a theory is correct.

    When we talk about the multiverse, it seems as though physics is brushing up against philosophy. A number of physicists, including Stephen Hawking and Lawrence Krauss, have angered philosophers by describing philosophy as useless. In your new book, it sounds as if you agree with them. Is that right?

    I think academic philosophy is helpful only in a negative sense — that is, sometimes physicists get impressed with philosophical ideas, so that it can be helpful to hear from experts that those ideas have been challenged within the philosophical community. One example is positivism, which decrees that you should only talk about things that are directly detectable or observable. I think philosophers themselves have challenged that, and it’s good to know that.

    On the other hand, a kind of philosophical discussion does go on among physicists themselves. For example, the discussion we were having earlier about the multiverse raised the issue of what we expect from a scientific theory — when do we reject it as being outside of science; when do we accept it as being confirmed. Those are meta-scientific questions; they’re philosophical questions. The scientists never seem to reach an agreement about those things — like in the case of the multiverse — but then, neither do the professional philosophers.

    And sometimes, as with the example of positivism, the work of professional philosophers actually stands in the way of progress. That’s also the case with the approach known as constructivism — the idea that every society’s scientific theories are a social construct, like its political institutions, and have to be understood as coming out of a particular cultural milieu. I don’t know whether you’d call it a philosophical theory or a historical theory, but at any rate, I think that view is wrong, and I also think it could impede the work of science, because it takes away one of science’s great motivations, which is to discover something that, in an absolute sense, divorced from any cultural milieu, is actually true.

    You’re 81. Many people would be thinking about retirement, but you’re very active. What are you working on now?

    There’s something I’ve been working on for more than a year — maybe it’s just an old man’s obsession, but I’m trying to find an approach to quantum mechanics that makes more sense than existing approaches. I’ve just finished editing the second edition of my book, Lectures on Quantum Mechanics, in which I think I strengthen the argument that none of the existing interpretations of quantum mechanics are entirely satisfactory.

    I don’t intend to retire, because I enjoy doing what I’m doing. I enjoy teaching; I enjoy following research; and I enjoy doing a little research on my own. The year before last, before I got onto this quantum mechanics kick, I was writing papers about down-to-earth problems in elementary particle theory; I was also working on cosmology. I hope I go back to that.

    See the full article here .

    Please help promote STEM in your local schools.

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 7:14 am on March 20, 2018 Permalink | Reply
    Tags: , , , , Cosmological-constant problem, , , In 1998 astronomers discovered that the expansion of the cosmos is in fact gradually accelerating, , Quanta Magazine, , , Saul Perlmutter UC Berkeley Nobel laureate, , Why Does the Universe Need to Be So Empty?, Zero-point energy of the field   

    From The Atlantic Magazine and Quanta: “Why Does the Universe Need to Be So Empty?” 

    Quanta Magazine
    Quanta Magazine

    Atlantic Magazine

    The Atlantic Magazine

    Mar 19, 2018
    Natalie Wolchover

    Physicists have long grappled with the perplexingly small weight of empty space.

    The controversial idea that our universe is just a random bubble in an endless, frothing multiverse arises logically from nature’s most innocuous-seeming feature: empty space. Specifically, the seed of the multiverse hypothesis is the inexplicably tiny amount of energy infused in empty space—energy known as the vacuum energy, dark energy, or the cosmological constant. Each cubic meter of empty space contains only enough of this energy to light a light bulb for 11 trillionths of a second. “The bone in our throat,” as the Nobel laureate Steven Weinberg once put it [http://hetdex.org/dark_energy.html
    ], is that the vacuum ought to be at least a trillion trillion trillion trillion trillion times more energetic, because of all the matter and force fields coursing through it.

    1

    Somehow the effects of all these fields on the vacuum almost equalize, producing placid stillness. Why is empty space so empty?

    While we don’t know the answer to this question—the infamous “cosmological-constant problem”—the extreme vacuity of our vacuum appears necessary for our existence. In a universe imbued with even slightly more of this gravitationally repulsive energy, space would expand too quickly for structures like galaxies, planets, or people to form. This fine-tuned situation suggests that there might be a huge number of universes, all with different doses of vacuum energy, and that we happen to inhabit an extraordinarily low-energy universe because we couldn’t possibly find ourselves anywhere else.

    Some scientists bristle at the tautology of “anthropic reasoning” and dislike the multiverse for being untestable. Even those open to the multiverse idea would love to have alternative solutions to the cosmological constant problem to explore. But so far it has proved nearly impossible to solve without a multiverse. “The problem of dark energy [is] so thorny, so difficult, that people have not got one or two solutions,” says Raman Sundrum, a theoretical physicist at the University of Maryland.

    To understand why, consider what the vacuum energy actually is. Albert Einstein’s general theory of relativity says that matter and energy tell space-time how to curve, and space-time curvature tells matter and energy how to move. An automatic feature of the equations is that space-time can possess its own energy—the constant amount that remains when nothing else is there, which Einstein dubbed the cosmological constant. For decades, cosmologists assumed its value was exactly zero, given the universe’s reasonably steady rate of expansion, and they wondered why. But then, in 1998, astronomers discovered that the expansion of the cosmos is in fact gradually accelerating, implying the presence of a repulsive energy permeating space. Dubbed dark energy by the astronomers, it’s almost certainly equivalent to Einstein’s cosmological constant. Its presence causes the cosmos to expand ever more quickly, since, as it expands, new space forms, and the total amount of repulsive energy in the cosmos increases.

    However, the inferred density of this vacuum energy contradicts what quantum-field theory, the language of particle physics, has to say about empty space. A quantum field is empty when there are no particle excitations rippling through it. But because of the uncertainty principle in quantum physics, the state of a quantum field is never certain, so its energy can never be exactly zero. Think of a quantum field as consisting of little springs at each point in space. The springs are always wiggling, because they’re only ever within some uncertain range of their most relaxed length. They’re always a bit too compressed or stretched, and therefore always in motion, possessing energy. This is called the zero-point energy of the field. Force fields have positive zero-point energies while matter fields have negative ones, and these energies add to and subtract from the total energy of the vacuum.

    The total vacuum energy should roughly equal the largest of these contributing factors. (Say you receive a gift of $10,000; even after spending $100, or finding $3 in the couch, you’ll still have about $10,000.) Yet the observed rate of cosmic expansion indicates that its value is between 60 and 120 orders of magnitude smaller than some of the zero-point energy contributions to it, as if all the different positive and negative terms have somehow canceled out. Coming up with a physical mechanism for this equalization is extremely difficult for two main reasons.

    First, the vacuum energy’s only effect is gravitational, and so dialing it down would seem to require a gravitational mechanism. But in the universe’s first few moments, when such a mechanism might have operated, the universe was so physically small that its total vacuum energy was negligible compared to the amount of matter and radiation. The gravitational effect of the vacuum energy would have been completely dwarfed by the gravity of everything else. “This is one of the greatest difficulties in solving the cosmological-constant problem,” the physicist Raphael Bousso wrote in 2007. A gravitational feedback mechanism precisely adjusting the vacuum energy amid the conditions of the early universe, he said, “can be roughly compared to an airplane following a prescribed flight path to atomic precision, in a storm.”

    Compounding the difficulty, quantum-field theory calculations indicate that the vacuum energy would have shifted in value in response to phase changes in the cooling universe shortly after the Big Bang. This raises the question of whether the hypothetical mechanism that equalized the vacuum energy kicked in before or after these shifts took place. And how could the mechanism know how big their effects would be, to compensate for them?

    So far, these obstacles have thwarted attempts to explain the tiny weight of empty space without resorting to a multiverse lottery. But recently, some researchers have been exploring one possible avenue: If the universe did not bang into existence, but bounced instead, following an earlier contraction phase, then the contracting universe in the distant past would have been huge and dominated by vacuum energy. Perhaps some gravitational mechanism could have acted on the plentiful vacuum energy then, diluting it in a natural way over time. This idea motivated the physicists Peter Graham, David Kaplan, and Surjeet Rajendran to discover a new cosmic bounce model, though they’ve yet to show how the vacuum dilution in the contracting universe might have worked.

    In an email, Bousso called their approach “a very worthy attempt” and “an informed and honest struggle with a significant problem.” But he added that huge gaps in the model remain, and “the technical obstacles to filling in these gaps and making it work are significant. The construction is already a Rube Goldberg machine, and it will at best get even more convoluted by the time these gaps are filled.” He and other multiverse adherents see their answer as simpler by comparison.

    See the full article here .

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  • richardmitnick 8:22 pm on March 18, 2018 Permalink | Reply
    Tags: , , Mathematics vs Physics, , , Quanta Magazine, Shake a Black Hole, , The black hole stability conjecture   

    From Quanta: “To Test Einstein’s Equations, Poke a Black Hole” 

    Quanta Magazine
    Quanta Magazine

    mathematical physics
    https://sciencesprings.wordpress.com/2018/03/17/from-ethan-siegel-where-is-the-line-between-mathematics-and-physics/

    March 8, 2018
    Kevin Hartnett

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    Fantastic animation. Olena Shmahalo/Quanta Magazine

    In November 1915, in a lecture before the Prussian Academy of Sciences, Albert Einstein described an idea that upended humanity’s view of the universe. Rather than accepting the geometry of space and time as fixed, Einstein explained that we actually inhabit a four-dimensional reality called space-time whose form fluctuates in response to matter and energy.

    Einstein elaborated this dramatic insight in several equations, referred to as his “field equations,” that form the core of his theory of general relativity. That theory has been vindicated by every experimental test thrown at it in the century since.

    Yet even as Einstein’s theory seems to describe the world we observe, the mathematics underpinning it remain largely mysterious. Mathematicians have been able to prove very little about the equations themselves. We know they work, but we can’t say exactly why. Even Einstein had to fall back on approximations, rather than exact solutions, to see the universe through the lens he’d created.

    Over the last year, however, mathematicians have brought the mathematics of general relativity into sharper focus. Two groups have come up with proofs related to an important problem in general relativity called the black hole stability conjecture. Their work proves that Einstein’s equations match a physical intuition for how space-time should behave: If you jolt it, it shakes like Jell-O, then settles down into a stable form like the one it began with.

    “If these solutions were unstable, that would imply they’re not physical. They’d be a mathematical ghost that exists mathematically and has no significance from a physical point of view,” said Sergiu Klainerman, a mathematician at Princeton University and co-author, with Jérémie Szeftel, of one of the two new results [https://arxiv.org/abs/1711.07597].

    To complete the proofs, the mathematicians had to resolve a central difficulty with Einstein’s equations. To describe how the shape of space-time evolves, you need a coordinate system — like lines of latitude and longitude — that tells you which points are where. And in space-time, as on Earth, it’s hard to find a coordinate system that works everywhere.

    Shake a Black Hole

    General relativity famously describes space-time as something like a rubber sheet. Absent any matter, the sheet is flat. But start dropping balls onto it — stars and planets — and the sheet deforms. The balls roll toward one another. And as the objects move around, the shape of the rubber sheet changes in response.

    Einstein’s field equations describe the evolution of the shape of space-time. You give the equations information about curvature and energy at each point, and the equations tell you the shape of space-time in the future. In this way, Einstein’s equations are like equations that model any physical phenomenon: This is where the ball is at time zero, this is where it is five seconds later.

    “They’re a mathematically precise quantitative version of the statement that space-time curves in the presence of matter,” said Peter Hintz, a Clay research fellow at the University of California, Berkeley, and co-author, with András Vasy, of the other recent result [https://arxiv.org/abs/1606.04014].

    In 1916, almost immediately after Einstein released his theory of general relativity, the German physicist Karl Schwarzschild found an exact solution to the equations that describes what we now know as a black hole (the term wouldn’t be invented for another five decades). Later, physicists found exact solutions that describe a rotating black hole and one with an electrical charge.

    These remain the only exact solutions that describe a black hole. If you add even a second black hole, the interplay of forces becomes too complicated for present-day mathematical techniques to handle in all but the most special situations.

    Yet you can still ask important questions about this limited group of solutions. One such question developed out of work in 1952 by the French mathematician Yvonne Choquet-Bruhat. It asks, in effect: What happens when you shake a black hole?

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    Lucy Reading-Ikkanda/Quanta Magazine

    This problem is now known as the black hole stability conjecture. The conjecture predicts that solutions to Einstein’s equations will be “stable under perturbation.” Informally, this means that if you wiggle a black hole, space-time will shake at first, before eventually settling down into a form that looks a lot like the form you started with. “Roughly, stability means if I take special solutions and perturb them a little bit, change data a little bit, then the resulting dynamics will be very close to the original solution,” Klainerman said.

    So-called “stability” results are an important test of any physical theory. To understand why, it’s useful to consider an example that’s more familiar than a black hole.

    Imagine a pond. Now imagine that you perturb the pond by tossing in a stone. The pond will slosh around for a bit and then become still again. Mathematically, the solutions to whatever equations you use to describe the pond (in this case, the Navier-Stokes equations) should describe that basic physical picture. If the initial and long-term solutions don’t match, you might question the validity of your equations.

    “This equation might have whatever properties, it might be perfectly fine mathematically, but if it goes against what you expect physically, it can’t be the right equation,” Vasy said.

    For mathematicians working on Einstein’s equations, stability proofs have been even harder to find than solutions to the equations themselves. Consider the case of flat, empty Minkowski space — the simplest of all space-time configurations. This solution to Einstein’s equations was found in 1908 in the context of Einstein’s earlier theory of special relativity. Yet it wasn’t until 1993 that mathematicians managed to prove that if you wiggle flat, empty space-time, you eventually get back flat, empty space-time. That result, by Klainerman and Demetrios Christodoulou, is a celebrated work in the field.

    One of the main difficulties with stability proofs has to do with keeping track of what is going on in four-dimensional space-time as the solution evolves. You need a coordinate system that allows you to measure distances and identify points in space-time, just as lines of latitude and longitude allow us to define locations on Earth. But it’s not easy to find a coordinate system that works at every point in space-time and then continues to work as the shape of space-time evolves.

    “We don’t know of a one-size-fits-all way to do this,” Hintz wrote in an email. “After all, the universe does not hand you a preferred coordinate system.”

    The Measurement Problem

    The first thing to recognize about coordinate systems is that they’re a human invention. The second is that not every coordinate system works to identify every point in a space.

    Take lines of latitude and longitude: They’re arbitrary. Cartographers could have anointed any number of imaginary lines to be 0 degrees longitude.

    2

    And while latitude and longitude work to identify just about every location on Earth, they stop making sense at the North and South poles. If you knew nothing about Earth itself, and only had access to latitude and longitude readings, you might wrongly conclude there’s something topologically strange going on at those points.

    This possibility — of drawing wrong conclusions about the properties of physical space because the coordinate system used to describe it is inadequate — is at the heart of why it’s hard to prove the stability of space-time.

    “It could be the case that stability is true, but you’re using coordinates that are not stable and thus you miss the fact that stability is true,” said Mihalis Dafermos, a mathematician at the University of Cambridge and a leading figure in the study of Einstein’s equations.

    In the context of the black hole stability conjecture, whatever coordinate system you’re using has to evolve as the shape of space-time evolves — like a snugly fitting glove adjusting as the hand it encloses changes shape. The fit between the coordinate system and space-time has to be good at the start and remain good throughout. If it doesn’t, there are two things that can happen that would defeat efforts to prove stability.

    First, your coordinate system might change shape in a way that makes it break down at certain points, just as latitude and longitude fail at the poles. Such points are called “coordinate singularities” (to distinguish them from physical singularities, like an actual black hole). They are undefined points in your coordinate system that make it impossible to follow an evolving solution all the way through.

    Second, a poorly fitting coordinate system might disguise the underlying physical phenomena it’s meant to measure. To prove that solutions to Einstein’s equations settle down into a stable state after being perturbed, mathematicians must keep careful track of the ripples in space-time that are set in motion by the perturbation. To see why, it’s worth considering the pond again. A rock thrown into a pond generates waves. The long-term stability of the pond results from the fact that those waves decay over time — they grow smaller and smaller until there’s no sign they were ever there.

    The situation is similar for space-time. A perturbation will set off a cascade of gravitational waves, and proving stability requires proving that those gravitational waves decay. And proving decay requires a coordinate system — referred to as a “gauge” — that allows you to measure the size of the waves. The right gauge allows mathematicians to see the waves flatten and eventually disappear altogether.

    “The decay has to be measured relative to something, and it’s here where the gauge issue shows up,” Klainerman said. “If I’m not in the right gauge, even though in principle I have stability, I can’t prove it because the gauge will just not allow me to see that decay. If I don’t have decay rates of waves, I can’t prove stability.”

    The trouble is, while the coordinate system is crucial, it’s not obvious which one to choose. “You have a lot of freedom about what this gauge condition can be,” Hintz said. “Most of these choices are going to be bad.”

    Partway There

    A full proof of the black hole stability conjecture requires proving that all known black hole solutions to Einstein’s equations (with the spin of the black hole below a certain threshold) are stable after being perturbed. These known solutions include the Schwarzschild solution, which describes space-time with a nonrotating black hole, and the Kerr family of solutions, which describe configurations of space-time empty of everything save a single rotating black hole (where the properties of that rotating black hole — its mass and angular momentum — vary within the family of solutions).

    Both of the new results make partial progress toward a proof of the full conjecture.

    Hintz and Vasy, in a paper posted to the scientific preprint site arxiv.org in 2016 [see above 1606.04014], proved that slowly rotating black holes are stable. But their work did not cover black holes rotating above a certain threshold.

    Their proof also makes some assumptions about the nature of space-time. The original conjecture is in Minkowski space, which is not just flat and empty but also fixed in size. Hintz and Vasy’s proof takes place in what’s called de Sitter space, where space-time is accelerating outward, just like in the actual universe. This change of setting makes the problem simpler from a technical point of view, which is easy enough to appreciate at a conceptual level: If you drop a rock into an expanding pond, the expansion is going to stretch the waves and cause them to decay faster than they would have if the pond were not expanding.

    “You’re looking at a universe undergoing an accelerated expansion,” Hintz said. “This makes the problem a little easier as it appears to dilute the gravitational waves.”

    Klainerman and Szeftel’s work has a slightly different flavor. Their proof, the first part of which was posted online last November [see above 1711.07597], takes place in Schwarzschild space-time — closer to the original, more difficult setting for the problem. They prove the stability of a nonrotating black hole, but they do not address solutions in which the black hole is spinning. Moreover, they only prove the stability of black hole solutions for a narrow class of perturbations — where the gravitational waves generated by those perturbations are symmetric in a certain way.

    Both results involve new techniques for finding the right coordinate system for the problem. Hintz and Vasy start with an approximate solution to the equations, based on an approximate coordinate system, and gradually increase the precision of their answer until they arrive at exact solutions and well-behaved coordinates. Klainerman and Szeftel take a more geometric approach to the challenge.

    The two teams are now trying to build on their respective methods to find a proof of the full conjecture. Some expert observers think the day might not be far off.

    “I really think things are now at the stage that the remaining difficulties are just technical,” Dafermos said. “Somehow one doesn’t need new ideas to solve this problem.” He emphasized that a final proof could come from any one of the large number of mathematicians currently working on the problem.

    For 100 years Einstein’s equations have served as a reliable experimental guide to the universe. Now mathematicians may be getting closer to demonstrating exactly why they work so well.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
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