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

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

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    RT

    23 Jun, 2014
    No Writer Credit

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

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

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

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

    Natural laws of space and time simply break down there.

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

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

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

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

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

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

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

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

    See the full article here.

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  • richardmitnick 10:31 am on July 18, 2015 Permalink | Reply
    Tags: , , , , , Quantum Mechanics   

    From NOVA: “How Time Got Its Arrow” 

    PBS NOVA

    NOVA

    15 Jul 2015

    1
    Lee Smolin, Perimeter Institute for Theoretical Physics

    I believe in time.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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

     
  • richardmitnick 3:32 pm on May 29, 2015 Permalink | Reply
    Tags: , , , Quantum Mechanics   

    From ANU: “Physicists solve quantum tunneling mystery” 

    ANU Australian National University Bloc

    Australian National University

    28 May 2015
    No Writer Credit

    1
    Professor Anatoli Kheifets’ theory tackles ultrafast physics. Composite image Stuart Hay, ANU

    An international team of scientists studying ultrafast physics have solved a mystery of quantum mechanics, and found that quantum tunneling is an instantaneous process.

    The new theory could lead to faster and smaller electronic components, for which quantum tunneling is a significant factor. It will also lead to a better understanding of diverse areas such as electron microscopy, nuclear fusion and DNA mutations.

    “Timescales this short have never been explored before. It’s an entirely new world,” said one of the international team, Professor Anatoli Kheifets, from The Australian National University (ANU).

    “We have modelled the most delicate processes of nature very accurately.”

    At very small scales quantum physics shows that particles such as electrons have wave-like properties – their exact position is not well defined. This means they can occasionally sneak through apparently impenetrable barriers, a phenomenon called quantum tunneling.

    Quantum tunneling plays a role in a number of phenomena, such as nuclear fusion in the sun, scanning tunneling microscopy, and flash memory for computers. However, the leakage of particles also limits the miniaturisation of electronic components.

    Professor Kheifets and Dr. Igor Ivanov, from the ANU Research School of Physics and Engineering, are members of a team which studied ultrafast experiments at the attosecond scale (10-18 seconds), a field that has developed in the last 15 years.

    Until their work, a number of attosecond phenomena could not be adequately explained, such as the time delay when a photon ionised an atom.

    “At that timescale the time an electron takes to quantum tunnel out of an atom was thought to be significant. But the mathematics says the time during tunneling is imaginary – a complex number – which we realised meant it must be an instantaneous process,” said Professor Kheifets.

    “A very interesting paradox arises, because electron velocity during tunneling may become greater than the speed of light. However, this does not contradict the special theory of relativity, as the tunneling velocity is also imaginary” said Dr Ivanov, who recently took up a position at the Center for Relativistic Laser Science in Korea.

    The team’s calculations, which were made using the Raijin supercomputer, revealed that the delay in photoionisation originates not from quantum tunneling but from the electric field of the nucleus attracting the escaping electron.

    The results give an accurate calibration for future attosecond-scale research, said Professor Kheifets.

    “It’s a good reference point for future experiments, such as studying proteins unfolding, or speeding up electrons in microchips,” he said.

    The research is published in Nature Physics.

    See the full article here.

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    ANU is a world-leading university in Australia’s capital city, Canberra. Our location points to our unique history, ties to the Australian Government and special standing as a resource for the Australian people.

    Our focus on research as an asset, and an approach to education, ensures our graduates are in demand the world-over for their abilities to understand, and apply vision and creativity to addressing complex contemporary challenges.

     
  • richardmitnick 4:29 pm on May 28, 2015 Permalink | Reply
    Tags: , Classical Mechanics, , , Quantum Mechanics   

    From NOVA: “Ultracold Experiment Could Solve One of Physics’s Biggest Contradictions” 

    PBS NOVA

    NOVA

    28 May 2015
    Allison Eck

    1
    A vortex structure emerges within a rotating Bose-Einstein condensate.

    There’s a mysterious threshold that’s predicted to exist beyond the limits of what we can see. It’s called the quantum-classical transition.

    If scientists were to find it, they’d be able to solve one of the most baffling questions in physics: why is it that a soccer ball or a ballet dancer both obey the Newtonian laws while the subatomic particles they’re made of behave according to quantum rules? Finding the bridge between the two could usher in a new era in physics.

    We don’t yet know how the transition from the quantum world to the classical one occurs, but a new experiment, detailed in Physical Review Letters, might give us the opportunity to learn more.

    The experiment involves cooling a cloud of rubidium atoms to the point that they become virtually motionless. Theoretically, if a cloud of atoms becomes cold enough, the wave-like (quantum) nature of the individual atoms will start to expand and overlap with one another. It’s sort of like circular ripples in a pond that, as they get bigger, merge to form one large ring. This phenomenon is more commonly known as a Bose-Einstein condensate, a state of matter in which subatomic particles are chilled to near absolute zero (0 Kelvin or −273.15° C) and coalesce into a single quantum object. That quantum object is so big (compared to the individual atoms) that it’s almost macroscopic—in other words, it’s encroaching on the classical world.

    The team of physicists cooled their cloud of atoms down to the nano-Kelvin range by trapping them in a magnetic “bowl.” To attempt further cooling, they then shot the cloud of atoms upward in a 10-meter-long pipe and let them free-fall from there, during which time the atom cloud expanded thermally. Then the scientists contained that expansion by sending another laser down onto the atoms, creating an electromagnetic field that kept the cloud from expanding further as it dropped. It created a kind of “cooling” effect, but not in the traditional way you might think—rather, the atoms have a lowered “effective temperature,” which is a measure of how quickly the atom cloud is spreading outward. At this point, then, the atom cloud can be described in terms of two separate temperatures: one in the direction of downward travel, and another in the transverse direction (perpendicular to the direction of travel).

    Here’s Chris Lee, writing for ArsTechnica:

    “This is only the start though. Like all lenses, a magnetic lens has an intrinsic limit to how well it can focus (or, in this case, collimate) the atoms. Ultimately, this limitation is given by the quantum uncertainty in the atom’s momentum and position. If the lensing technique performed at these physical limits, then the cloud’s transverse temperature would end up at a few femtoKelvin (10-15). That would be absolutely incredible.

    A really nice side effect is that combinations of lenses can be used like telescopes to compress or expand the cloud while leaving the transverse temperature very cold. It may then be possible to tune how strongly the atoms’ waves overlap and control the speed at which the transition from quantum to classical occurs. This would allow the researchers to explore the transition over a large range of conditions and make their findings more general.”

    Jason Hogan, assistant professor of physics at Stanford University and one of the study’s authors, told NOVA Next that you can understand this last part by using the Heisenberg Uncertainty Principle. As a quantum object’s uncertainty in momentum goes down, its uncertainty in position goes up. Hogan and his colleagues are essentially fine-tuning these parameters along two dimensions. If they can find a minimum uncertainty in the momentum (by cooling the particles as much as they can), then they could find the point at which the quantum-to-classical transition occurs. And that would be a spectacular discovery for the field of particle physics.

    See the full article here.

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

     
  • richardmitnick 8:18 am on April 8, 2015 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From NBC: “Great Scott! Reverse-Causality Research Ends in a Quantum Muddle” 

    NBC News

    NBC News

    April 6th 2015
    Alan Boyle

    One of the longest-running and weirdest examples of a crowdfunded scientific experiment is finally reaching the end of the road, and the results will come as a disappointment to anyone who wishes the “Back to the Future” movies could really happen: Quantum interference foils what once looked like a plausible strategy for influencing events in the past.

    “The trick doesn’t work in its present form,” John Cramer, a physics professor emeritus at the University of Washington in Seattle, told NBC News.

    Cramer suspected that would be the case, but back in 2006, he was interested in figuring out exactly why it wouldn’t work. The trick involved trying to flip a switch that would have an effect not only on photons going through a complicated set-up of lasers and mirrors, but also on entangled photons that had gone through the set-up about 50 microseconds earlier.

    “We were looking at whether there might be a loophole that would allow you to do this,” Cramer said.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    World’s Most Complex Machine Gets Ready for More Atom-Smashing

    He would describe the experiment as a study of quantum nonlocal communication. But conceptually, the effect would be a little like sending Marty McFly back in time to make sure his mom married his dad in “Back to the Future.”

    In 2007, two years before Kickstarter was founded, Cramer put out an appeal for private contributions that would help him buy the required equipment. The appeal raised $40,000, and the retrocausality experiment went forward. Over the years that followed, Cramer repeatedly tweaked the apparatus to get around roadblocks posed by quantum mechanics.

    Signals vs. anti-signals

    Past experiments showed that there was a complementary relationship between two characteristics of quantum systems: entanglement and coherence. When photons are entangled, “there’s a certain amount of noise that’s generated at the same time,” Cramer said. That makes reading the signal amid the noise more difficult. But anything you do to make the signal more coherent decreases the level of entanglement.

    Cramer thought he could use a wedge-shaped mirror to split a photon beam into two signals that were partly entangled and partly coherent. Theoretically, he should have been able to analyze the interference patterns to see how fiddling with one signal affected the other one microseconds earlier.

    It turned out not to be that easy.

    “We analyzed it up, down and sideways, and concluded that what happens is, yes, you have a switchable interference pattern,” Cramer said. “But because you have no coincidence measurement, you can’t look at just one interference pattern. You have to add up two patterns. And they always add up to no signal.”

    Translation: When you analyze the quantum signal from earlier in time, you have to include an “anti-signal” in your calculations. Thus, the future leaves no fingerprints on the past. “Nature appears to be well-protected from the possibility of nonlocal signaling,” according to Cramer and his co-author, Nick Herbert.

    Fact vs. fiction

    Cramer is a novelist as well as a physicist, and he plans to work the concept of backward causality into his fiction even if it’s not observable in reality. “I have the outline of a novel that I was thinking about writing along these lines,” he said.

    The plot calls for a pair of researchers to rig up Puget Sound with a huge fiber-optic network, in order to prove it’s possible to communicate backwards in time. The experiment makes a splash, but not the kind that the researchers intended.

    “The boat that contains the fiber-optics equipment gets destroyed,” Cramer said.

    Cramer and Herbert are the authors of An Inquiry Into the Possibility of Nonlocal Quantum Communication, which has been submitted to Foundations of Physics for publication. The research was supported in part by the U. S. Department of Energy Office of Scientific Research.

    See the full article here.

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  • richardmitnick 10:56 am on March 24, 2015 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From PI: “Quantum Cause and Effect” 

    Perimeter Institute
    Perimeter Institute

    March 23, 2015
    Erin Bow

    Correlation does not imply causation – unless it’s quantum. That’s the message of surprising new work from Perimeter Institute and the Institute for Quantum Computing.

    Does taking a drug and then getting better mean that the drug made you better? Did that tax cut really stimulate the economy or did it recover on its own? The problem of answering such questions – of inferring causal relationships from correlations – reaches across the sciences, and beyond.

    Normally, correlation by itself does not imply causation. But new research from Perimeter Institute and the Institute for Quantum Computing (IQC) has found that in the case of quantum variables, it sometimes can.

    The new work, just published in Nature Physics, is the result of a collaboration between Perimeter Faculty member Robert Spekkens, IQC Faculty member Kevin Resch, PhD student Katja Ried, MSc students Megan Agnew and Lydia Vermeyden, and Max Planck Institute senior research scientist Dominik Janzing.

    As a practical illustration of the difference between correlation and causation, consider a drug trial: some people take a drug, and some of them get better. Even more promising, the doctors find that among people who took the drug, 60 percent recover; among people who didn’t take the drug, only 40 percent recover. What conclusions can the doctors draw?

    At first blush, it may look as if the drug caused the recovery, but the doctors would need more information before drawing that conclusion. It might be that more men than women chose to take the drug, and more men than women tended to spontaneously recover. In that case, a common cause, gender, could potentially explain the correlation.

    This imaginary drug trial shows how tricky it can be to distinguish cause-effect correlations from correlations springing from common causes. That’s why the caution “correlation does not imply causation” is drilled into the heads of every researcher for whom statistics is of even passing importance.

    Over the last century, scientists, mathematicians, and philosophers have developed a powerful toolkit for untangling webs of cause, effect, and correlation in even the most complex evolving system. The case of systems with only two variables – like the drug trial above – turns out to be the hardest one. If you want to avoid introducing assumptions about what’s happening, you need to intervene on variable A – in this case, taking the drug. That’s why a real drug trial would be carefully randomized, assigning some people to take the drug and others to take a placebo. Only active intervention on variable A can establish its causal relationship with variable B.

    But what of quantum variables? This new research shows that certain kinds of quantum correlations do imply causation – even without the kind of active intervention that classical variables require.

    The new research is both theoretical and experimental. Ried, Spekkens, and Janzing worked from the theoretical end. They considered the situation of an observer who has probed two quantum variables – say, the polarization properties of two photons – and found that they are correlated. The measurement is carried out at two points in time, but the observer doesn’t know if she’s looking at the same photon twice (that is, probing a cause-effect relationship) or looking at a pair of photons in an entangled state (that is, probing a common cause relationship).

    The theorists’ crucial insight was that the correlations measured between a photon at one time and the same photon at another time had a different pattern than the correlations measured between two entangled photons. In other words, they discovered that under the right circumstances, they could tell cause-effect from common cause.

    Meanwhile, at the Institute for Quantum Computing, Agnew, Vermeyden, and Resch had the tools to put this remarkable idea to the test. They built an apparatus that could generate two entangled photons, A and B. They measured A, and then sent the pair through a gate that either transmitted photon A, or switched photon A and photon B and transmitted B.

    Crucially, this gate could swap between the two scenarios, choosing one or the other based on the output of a random number generator. On the other side of this gate, the researchers conducted another measurement while blind to which photon they measured. Just as the theorists predicted, they saw two distinct patterns of correlation emerge.

    This means that researchers measuring quantum variables can do something researchers measuring classical variables cannot: tell the difference between cause-effect and common cause in a system with only two variables, without making an active intervention on the first variable.

    This discovery has significance for both quantum information and quantum foundations.

    The work establishes a new class of things that quantum systems can do which classical systems cannot. It’s too early to say how that may play out, but such quantum advantages underpin the promise of quantum technologies: quantum entanglement, for instance, underlies quantum cryptography, and quantum superposition underlies quantum computation.

    The discovery of new quantum advantages has historically led to interesting places, and the researchers are hopeful that this new quantum advantage will follow suit.

    For those interested in quantum foundations, this work provides a new framework to ask basic questions about quantum mechanics. There is a lively and long-standing debate in the field concerning which quantum concepts are about reality, and which are about our knowledge of reality – for instance, whether the quantum uncertainty about (say) the polarization of a photon means that the photon itself has no defined polarization, or if it means that the observer of such a photon has limited knowledge.

    Because correlations are about what observers can infer, while causal relations are about the physical relations among systems, this research opens a new window on such questions.

    The team describes the work as opening the door to many more lines of inquiry, such as: How can these techniques be generalized to scenarios involving more than two systems? Is the menu of possible causal relations between quantum systems larger than between classical systems? And most broadly and excitingly: How should we understand causality in a quantum world?

    See the full article here.

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    About Perimeter

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

     
  • richardmitnick 10:55 am on March 18, 2015 Permalink | Reply
    Tags: , Quantum Mechanics, ,   

    From ars technica: “Shining an X-Ray torch on quantum gravity” 

    Ars Technica
    ars technica

    Mar 17, 2015
    Chris Lee

    1
    This free electron laser could eventually provide a test of quantum gravity. BNL

    Quantum mechanics has been successful beyond the wildest dreams of its founders. The lives and times of atoms, governed by quantum mechanics, play out before us on the grand stage of space and time. And the stage is an integral part of the show, bending and warping around the actors according to the rules of general relativity. The actors—atoms and molecules—respond to this shifting stage, but they have no influence on how it warps and flows around them.

    This is puzzling to us. Why is it such a one directional thing: general relativity influences quantum mechanics, but quantum mechanics has no influence on general relativity? It’s a puzzle that is born of human expectation rather than evidence. We expect that, since quantum mechanics is punctuated by sharp jumps, somehow space and time should do the same.

    There’s also the expectation that, if space and time acted a bit more quantum-ish, then the equations of general relativity would be better behaved. In general relativity, it is possible to bend space and time infinitely sharply. This is something we simply cannot understand: what would infinitely bent space look like? To most physicists, it looks like something that cannot actually be real, indicating a problem with the theory. Might this be where the actors influence the stage?

    Quantum mechanics and relativity on the clock

    To try and catch the actors modifying the stage requires the most precise experiments ever devised. Nothing we have so far will get us close, so a new idea from a pair of German physicists is very welcome. They focus on what’s perhaps the most promising avenue for detecting quantum influences on space-time: time-dilation experiments. Modern clocks rely on the quantum nature of atoms to measure time. And the flow of time depends on relative speed and gravitational acceleration. Hence, we can test general relativity, special relativity, and quantum mechanics all in the same experiment.

    To get an idea of how this works, let’s take a look at the traditional atomic clock. In an atomic clock, we carefully prepare some atoms in a predefined superposition state: that is the atom is prepared such that it has a fifty percent chance of being in state A, and a fifty percent chance of being in state B. As time passes, the environment around the atom forces the superposition state to change. At some later point, it will have a seventy five percent chance of being in state A; even later, it will certainly be in state A. Keep on going, however, and the chance of being in state A starts to shrink, and it continues to do so until the atom is certainly in state B. Provided that the atom is undisturbed, these oscillations will continue.

    These periodic oscillations provide the perfect ticking clock. We simply define the period of an oscillation to be our base unit of time. To couple this to general relativity measurements is, in principle, rather simple. Build two clocks and place them beside each other. At a certain moment, we start counting ticks from both clocks. When one clock reaches a thousand (for instance), we compare the number of ticks from the two clocks. If we have done our job right, both clocks should have reached a thousand ticks.

    If we shoot one into space, however, and perform the same experiment, and relativity demands that the clock in orbit record more ticks than the clock on Earth. The way we record the passing of time is by a phenomena that is purely quantum in nature, while the passing of time is modified by gravity. These experiments work really well. But at present, they are not sensitive enough to detect any deviation from either quantum mechanics or general relativity.

    Going nuclear

    That’s where the new ideas come in. The researchers propose, essentially, to create something similar to an atomic clock, but instead of tracking the oscillation atomic states, they want to track nuclear states. Usually, when I discuss atoms, I ignore the nucleus entirely. Yes, it is there, but I only really care about the influence the nucleus has on the energetic states of the electrons that surround it. However, in one key way the nucleus is just like the electron cloud that surrounds it: it has its own set of energetic states. It is possible to excite nuclear states (using X-Ray radiation) and, afterwards, they will return the ground state by emitting an X-Ray.

    So let’s imagine that we have a crystal of silver sitting on the surface of the Earth. The silver atoms all experience a slightly different flow of time because the atoms at the top of the crystal are further away from the center of the Earth compared to the atoms at the bottom of the crystal.

    To kick things off, we send in a single X-Ray photon, which is absorbed by the crystal. This is where the awesomeness of quantum mechanics puts on sunglasses and starts dancing. We don’t know which silver atom absorbed the photon, so we have to consider that all of them absorbed a tiny fraction of the photon. This shared absorption now means that all of the silver atoms enter a superposition state of having absorbed and not absorbed a photon. This superposition state changes with time, just like in an atomic clock.

    In the absence of an outside environment, all the silver atoms will change in lockstep. And when the photon is re-emitted from the crystal, all the atoms will contribute to that emission. So each atom behaves as if it is emitting a partial photon. These photons add together, and a single photon flies off in the same direction as the absorbed photon had been traveling. Essentially because all the atoms are in lockstep, the charge oscillations that emit the photon add up in phase only in the direction that the absorbed photon was flying.

    Gravity, though, causes the atoms to fall out of lockstep. So when the time comes to emit, the charge oscillations are all slightly out of phase with each other. But they are not random: those at the top of the crystal are just slightly ahead of those at the bottom of the crystal. As a result, the direction for which the individual contributions add up in phase is not in the same direction as the flight path of the absorbed photon, but at a very slight angle.

    How big is this angle? That depends on the size of the crystal and how long it takes the environment to randomize the emission process. For a crystal of silver atoms that is less than 1mm thick, the angle could be as large as 100 micro-degrees, which is small but probably measurable.
    Spinning crystals

    That, however, is only the beginning of a seam of clever. If the crystal is placed on the outside of a cylinder and rotated during the experiment, then the top atoms of the crystal are moving faster than the bottom, meaning that the time-dilation experienced at the top of the crystal is greater than that at the bottom. This has exactly the same effect as placing the crystal in a gravitational field, but now the strength of that field is governed by the rate of rotation.

    In any case, by spinning a 10mm diameter cylinder very fast (70,000 revolutions per second), the angular deflection is vastly increased. For silver, for instance, it reaches 90 degrees. With such a large signal, even smaller deviations from the predictions of general relativity should be detectable in the lab. Importantly, these deviations happen on very small length scales, where we would normally start thinking about quantum effects in matter. Experiments like these may even be sensitive enough to see the influence of quantum mechanics on space and time.

    A physical implementation of this experiment will be challenging but not impossible. The biggest issue is probably the X-Ray source and doing single photon experiments in the X-Ray regime. Following that, the crystals need to be extremely pure, and something called a coherent state needs to be created within them. This is certainly not trivial. Given that it took atomic physicists a long time to achieve this for electronic transitions, I think it will take a lot more work to make it happen at X-Ray frequencies.

    On the upside free electron lasers have come a very long way, and they have much better control over beam intensities and stability. This is, hopefully, the sort of challenge that beam-line scientists live for.

    See the full article here.

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

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

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

     
  • richardmitnick 10:05 am on March 17, 2015 Permalink | Reply
    Tags: , , , Quantum Mechanics   

    From phys.org: “Confirming Einstein, scientists find ‘spacetime foam’ not slowing down photons from faraway gamma-ray burst (Update)” 

    physdotorg
    phys.org

    Mar 16, 2015
    No Writer Credit

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    This is the “South Pillar” region of the star-forming region called the Carina Nebula. Like cracking open a watermelon and finding its seeds, the infrared telescope “busted open” this murky cloud to reveal star embryos tucked inside finger-like pillars of thick dust. Credit: NASA

    One hundred years after Albert Einstein formulated the General Theory of Relativity, an international team has proposed another experimental proof. In a paper published today in Nature Physics, researchers from the Hebrew University of Jerusalem, the Open University of Israel, Sapienza University of Rome, and University of Montpellier in France, describe a proof for one of the theory’s basic assumptions: the idea that all light particles, or photons, propagate at exactly the same speed.

    The researchers analyzed data, obtained by NASA’s Fermi Gamma-ray Space Telescope, of the arrival times of photons from a distant gamma-ray burst [GRB]. The data showed that photons traveling for billions of years from the distant burst toward Earth all arrived within a fraction of a second of each other.

    NASA Fermi Telescope
    Fermi

    This finding indicates that the photons all moved at the same speed, even though different photons had different energies. This is one of the best measurements ever of the independence of the speed of light from the energy of the light particles.

    Beyond confirming the general theory of relativity, the observation rules out one of the interesting ideas concerning the unification of general relativity and quantum theory. While these two theories are the pillars of physics today, they are still inconsistent, and there is an intrinsic contradiction between the two that is partially based on Heisenberg’s uncertainty principle that is at the heart of quantum theory.

    One of the attempts to reconcile the two theories is the idea of “space-time foam.” According to this concept, on a microscopic scale space is not continuous, and instead it has a foam-like structure. The size of these foam elements is so tiny that it is difficult to imagine and is at present impossible to measure directly. However light particles that are traveling within this foam will be affected by the foamy structure, and this will cause them to propagate at slightly different speeds depending on their energy.

    Yet this experiment shows otherwise. The fact that all the photons with different energies arrived with no time delay relative to each other indicates that such a foamy structure, if it exists at all, has a much smaller size than previously expected.

    “When we began our analysis, we didn’t expect to obtain such a precise measurement,” said Prof. Tsvi Piran, the Schwartzmann University Chair at the Hebrew University’s Racah Institute of Physics and a leader of the research. “This new limit is at the level expected from quantum gravity theories and can direct us how to combine quantum theory and relativity.”

    See the full article here.

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
  • richardmitnick 7:54 pm on March 2, 2015 Permalink | Reply
    Tags: , , , , Quantum Mechanics   

    From EPFL: “The first ever photograph of light as both a particle and wave” 

    EPFL bloc

    Ecole Polytechnique Federale Lausanne

    1

    March 2, 2015

    Light behaves both as a particle and as a wave. Since the days of [Albert] Einstein, scientists have been trying to directly observe both of these aspects of light at the same time. Now, scientists at EPFL have succeeded in capturing the first-ever snapshot of this dual behavior.

    Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, EPFL scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle. The breakthrough work is published in Nature Communications.

    When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this “photoelectric” effect by proposing that light – thought to only be a wave – is also a stream of particles. Even though a variety of experiments have successfully observed both the particle- and wave-like behaviors of light, they have never been able to observe both at the same time.

    A research team led by Fabrizio Carbone at EPFL has now carried out an experiment with a clever twist: using electrons to image light. The researchers have captured, for the first time ever, a single snapshot of light behaving simultaneously as both a wave and a stream of particles particle.

    The experiment is set up like this: A pulse of laser light is fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

    This is where the experiment’s trick comes in: The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using the ultrafast microscope to image the position where this change in speed occurred, Carbone’s team could now visualize the standing wave, which acts as a fingerprint of the wave-nature of light.

    While this phenomenon shows the wave-like nature of light, it simultaneously demonstrated its particle aspect as well. As the electrons pass close to the standing wave of light, they “hit” the light’s particles, the photons. As mentioned above, this affects their speed, making them move faster or slower. This change in speed appears as an exchange of energy “packets” (quanta) between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

    “This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” says Fabrizio Carbone. In addition, the importance of this pioneering work can extend beyond fundamental science and to future technologies. As Carbone explains: “Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.”

    This work represents a collaboration between the Laboratory for Ultrafast Microscopy and Electron Scattering of EPFL, the Department of Physics of Trinity College (US) and the Physical and Life Sciences Directorate of the Lawrence Livermore National Laboratory. The imaging was carried out EPFL’s ultrafast energy-filtered transmission electron microscope – one of the two in the world.

    See the full article here.

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    EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

     
  • richardmitnick 9:54 am on December 18, 2014 Permalink | Reply
    Tags: , Quantum Mechanics, ,   

    From Ethan Siegel: “Quantum Immortality” 

    Starts with a bang
    Starts with a Bang

    This article was written by Paul Halpern, the author of Einstein’s Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics.

    Observers are the necessary, but unliked, bouncers in the elegant nightclub of quantum physics. While, no one is entirely comfortable with having doormen checking IDs, they persist; otherwise everyone and everything gets in, contrary to ordinary experience.

    2
    Image credit: AIP Emilio Segre Visual Archives, Physics Today Collection of [Paul]Dirac and [Werner] Heisenberg;

    v
    © Los Alamos National Laboratory of [John] von Neumann.

    In the late 1920s and early 1930s, Heisenberg, Dirac, and John von Neumann, codified the formalism of quantum mechanics as a two-step process. One part involves the continous evolution of states via the deterministic

    e
    Schrödinger equation.
    Image credit: Wikimedia Commons user YassineMrabet.

    Map out a system’s potential energy distribution — in the form of a well, for example — and the spectrum of possible quantum states is set. If the states are time-dependent, then they predictably transform. That could set out, for instance, a superposition of states that spreads out in position space over time, like an expanding puddle of water.

    Yet experiments show that if an apparatus is designed to measure a particular quantity, such as the position, momentum or spin-state of a particle, quantum measurements yield specific values of that respective physical parameter. Such specificity requires a second type of quantum operation that is instantaneous and discrete, rather than gradual and continuous: the process of collapse.

    c
    Image credit: A Friedman, via http://blogs.scientificamerican.com/the-curious-wavefunction/2014/01/15/what-scientific-idea-is-ready-for-retirement/.

    Collapse occurs when a measurement of a certain physical parameter — position, let’s say — precipitates a sudden transformation into one of the “eigenstates” (solution states) of the operator (mathematical function) corresponding to that parameter — the position operator, in that case.

    g
    Image credit: Nick Trefethen, via http://www.chebfun.org/examples/ode-eig/Eigenstates.html.

    Then the measured value of that quantity is the “eigenvalue” associated with that eigenstate — the specific position of the particle, for instance. Eigenstates represent the spectrum of possible states and eigenvalues the measurements associated with those states.

    We can imagine the situation of quantum collapse as being something like a slot machine with a mixture of dollar coins and quarters; some old enough to be valuable, others shining new.

    m
    Image credit: © 2014 Marco Jewelers, via http://marcojewelers.net/sell-buy-silver-gold-coins.

    Its front panel has two buttons: one red and the other blue. Press the red button and the coins instantly become sorted according to denomination. A number of dollar coins drop out (a mixture of old and new). Press the blue button and the sorting is instantly done by date. A bunch of old coins (of both denominations) are released. While someone seeking quick bucks might press red, a coin collector might push blue. The machine is set that you are not permitted to press both buttons. Similarly, in quantum physics, according to Heisenberg’s famous uncertainty principle certain quantities such as position and momentum are not measurable at once with any degree of precision.

    Over the years, a number of critics have attacked this interpretation.

    a
    Albert Einstein
    Image credit: Oren Jack Turner, Princeton, N.J., via Wikimedia Commons user Jaakobou.

    Suggesting that quantum physics, though experimentally correct, must be incomplete, Einstein argued that random, instantaneous transitions had no place in a fundamental description of nature. Schrödinger cleverly developed his well-known feline thought experiment to demonstrate the absurdity of the observer’s role in quantum collapse. In his hypothetical scheme, he imagined a set-up in which a cat in a closed box, whose survival (or not) was tied to the random decay of a radioactive material, was in a mixed state of life and death until the box was opened and the system observed.


    Image credit: retrieved from Øystein Elgarøy at http://fritanke.no/index.php?page=vis_nyhet&NyhetID=8513.

    More recently, physicist Bryce DeWitt, who theorized how quantum mechanics might apply to gravity and the dynamics of the universe itself, argued that because there are presumably no observers outside the cosmos to view it (and trigger collapse into quantum gravity eigenstates), a complete accounting of quantum physics could not include observers.

    Instead, DeWitt, until his death in 2004, was an ardent advocate of an alternative to the Copenhagen (standard) interpretation of quantum mechanics that he dubbed the Many Worlds Interpretation (MWI).

    m
    Image credit: University of Texas of Bryce DeWitt;

    h
    Professor Jeffrey A. Barrett and UC Irvine, of Hugh Everett III.

    He based his views on the seminal work of Hugh Everett, who as a graduate student at Princeton, developed a way of avoiding the need in quantum mechanics for an observer. Instead, each time a quantum measurement is taken, the universe, including any observers, seamlessly and simultaneously splits into the spectrum of possible values for that measurement. For example, in the case of the measurement of the spin of an electron, in one branch it has spin up, and all observers see it that way; in the other it has spin down. Schrödinger’s cat would be happily alive in one reality, to the joy of its owner, while cruelly deceased in the other, much to the horror of the same owner (but in a different branch). Each observer in each branch would have no conscious awareness of his near-doppelgangers.

    As Everett wrote to DeWitt in explaining his theory:

    “The theory is in full accord with our experience (at least insofar as ordinary quantum mechanics is)… because it is possible to show that no observer would ever be aware of any ‘branching.’”

    If Schrödinger’s thought experiment were repeated each day, there would always be one branch of the universe in which the cat survives. Hypothetically, rather than the proverbial “nine lives,” the cat could have an indefinite number of “lives” or at least chances at life. There would always be one copy of the experimenter who is gratified, but perplexed, that his cat has beaten the odds and lived to see another day. The other copy, in mourning, would lament that the cat’s luck had finally run out.

    c
    Image credit: Ethan Zuckerman, from Garrett Lisi’s talk (2008), via http://www.ethanzuckerman.com/blog/2008/02/28/ted2008-garrett-lisi-looks-for-balance/.

    What about human survival? We are each a collection of particles, governed on the deepest level by quantum rules. If each time a quantum transition took place, our bodies and consciousness split, there would be copies that experienced each possible result, including those that might determine our life or death. Suppose in one case a particular set of quantum transitions resulted in faulty cell division and ultimately a fatal form of cancer. For each of the transitions, there would always be an alternative that did not lead to cancer. Therefore, there would always be branches with survivors. Add in the assumption that our conscious awareness would flow only to the living copies, and we could survive any number of potentially hazardous events related to quantum transitions.

    Everett reportedly believed in this kind of “quantum immortality.” Fourteen years after his death in 1982, his daughter Liz took her own life, explaining in her suicide note that in some branch of the universe, she hoped to reunite with her father.

    There are major issues with the prospects for quantum immortality however. For one thing the MWI is still a minority hypothesis. Even if it is true, how do we know that our stream of conscious thought would flow only to branches in which we survive? Are all possible modes of death escapable by an alternative array of quantum transitions? Remember that quantum events must obey conservation laws, so there could be situations in which there was no way out that follows natural rules. For example, if you fall out of a spaceship hatch into frigid space, there might be no permissible quantum events (according to energy conservation) that could lead you to stay warm enough to survive.

    Finally, suppose you do somehow manage to achieve quantum immortality — with your conscious existence following each auspicious branch. You would eventually outlive all your friends and family members — because in your web of branches you would eventually encounter copies of them that didn’t survive. Quantum immortality would be lonely indeed!

    See the full article here.

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

     
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