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  • richardmitnick 9:49 am on September 11, 2014 Permalink | Reply
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    From Dr. Don Lincoln at FNAL: “Physics in a Nutshell Epic facepalm” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    FNAL Don Lincoln
    Don Lincoln

    face

    If you’re a science enthusiast, this week you have likely encountered outlandish headlines invoking Stephen Hawking, the Higgs boson and the end of the universe. I hope you had the presence of mind to react as the famous actor in the picture did. Let’s start with the answer first. The universe is safe and will be for a very long time — for trillions of years. This story as widely reported by the media is a jaw-dropping misrepresentation of science.

    To understand how abominably Hawking’s statement was twisted, first we need to understand the statement. To paraphrase just a little, Hawking said that in a world in which the Higgs boson and the top quark have the masses that scientists have measured, the universe is in a metastable state.

    So let’s take those pieces one at a time. What does “metastable” mean? Basically, metastable means “kind of stable.” So what does that mean? Let’s consider an example. Take a pool cue and lay it on the pool table. The cue is stable; it’s not going anywhere. Take the same cue and balance it on your finger. That’s unstable; under almost any circumstances, the cue will fall over. So the terms stable and unstable are easy and have familiar, real-world analogues. The analogy for a metastable object is a barstool. Under almost all circumstances, the stool will sit there for all eternity. However, if you bump the stool hard enough, it will fall over. When the stool has fallen over, it is now more stable than it was, just like the pool cue lying on the table.

    Now we need to bring in the universe and the laws that govern it. Here is an important guiding principle: The universe is lazy — a giant, cosmic couch potato. If at all possible, the universe will figure out a way to move to the lowest energy state it can. A simple analogy is a ball placed on the side of a mountain. It will roll down the mountainside and come to rest at the bottom of the valley. This ball would then be in a stable configuration. The universe is the same way. After the cosmos was created, the fields that make up the universe should arrange themselves into the lowest possible energy state.

    pool
    A stable thing is something that won’t change, like this pool cue on the table. An unstable thing is something that will quickly change, like this pool cue balanced on the man’s hand. A metastable thing will eventually change, but will not do so quickly or easily. An example is this stool, which is more stable when it is lying down, but it will stay upright for long periods of time.

    There is a proviso. Just as on a slope of a mountain, where there may be a little valley part way up the hill (above the real valley), it is possible that there could be little “valleys” in the energy slope. As the universe cooled, it could be that it might have been caught in one of those little valleys. Ideally, the universe would like to fall into the deeper valley below, but it could be trapped. This is an example of a metastable state. As long as the little valley is deep enough, it’s hard to get out of. Indeed, using classical physics, it is impossible to get out of it.

    However, we don’t live in a classical world. In our universe, we must take into account the nature of quantum mechanics. There are many ways to describe the quantum realm, but one of the properties most relevant here is “rare things happen.” In essence, if the universe was trapped in a little valley of metastability, it could eventually tunnel out of the valley and fall down into the deeper valley below.

    So what are the consequences of the universe slipping from one valley to another? Well, the rules of the universe are governed by the valley in which it finds itself. In the metastable valley that defines our familiar universe, we have the rules of physics and chemistry that allow matter to assemble into atoms and, eventually, us. If the universe slipped into a different valley, the rules that govern matter and energy would be different. This means, among other things, quarks and leptons might be impossible. The known forces might not apply. In short, there is no reason to think we’d exist at all.

    graph
    Whether our universe is in a stable configuration, an unstable configuration or a metastable one depends on the mass of the Higgs boson and the mass of the top quark. The dot shows tells us the value of those parameters in our universe. We see that it appears that the universe appears to be metastable but, as noted in the text, there is clearly a lot still to be understood before we can be sure.

    This leads us to ask how the transition would occur. Would we have any warning? Actually, we’d have no warning at all. If, somewhere in the cosmos, the universe made a transition from a metastable valley to a deeper one, the laws of physics would change and sweep away at the speed of light. As the shockwave passed over the solar system, we’d simply disappear as the laws that govern the matter that makes us up would just cease to apply. One second we’d be here; the next we’d be gone.

    Coming back to the original question, what does the Higgs boson tell us about this? It turns out that we can use the Standard Model to tell us whether we are in a stable, unstable or metastable universe. We know we don’t live in an unstable one, because we’re here, but the other two options are open. So, what is the answer? It depends on two parameters: the mass of the top quark and the mass of the Higgs boson. As we see in the figure [below], our universe appears to be in a metastable state, although it is quite close to the stable region. The size of the box reflects our uncertainty in our measurements.

    sm
    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    measure
    In the context of the cosmos, the universe prefers to be in the lowest energy state. However it is possible that our familiar cosmos is in a little valley higher up the slope. In this little valley, the rules of matter with which we are familiar reign supreme. However, if the universe ever transitions to the lower valley, the rules of physics might change entirely. Those new rules could be anything, including ones in which matter doesn’t exist. It probably doesn’t need saying, but for my Chicago readers, I should caution them that a universe in which the Cubs win the World Series is still exceedingly unlikely.

    So if we follow our understanding of the Standard Model, combined with our best measurements, it appears that we live in a metastable universe that could one day disappear without warning. You can be forgiven if you take that pronouncement as a reason to indulge in some sort of rare treat tonight. But before you splurge too much, take heed of a few words of caution. Using the same Standard Model we used to figure out whether the cosmos is metastable, we can predict how long it is likely to take for quantum mechanics to let the universe slip from the metastable valley to the stable one, and it will take trillions of years. Mankind has only existed for about 100,000 years, and the sun will grow to a red giant and incinerate the Earth in about five billion years. Since we’re talking about the universe existing as a metastable state for trillions of years, maybe overindulging tonight might be a bad idea.

    It is important to note that finding the Higgs boson has no effect on whether the universe is in a metastable state. If we live in a metastable cosmos, it has been that way since the universe was created. It’s like living in a century-old house that was built with a ticking time bomb hidden in its walls. Finding the Higgs boson is like hearing the ticking of the bomb that was always there. I must repeat: The discovery of the Higgs boson has no effect at all on whether the universe is in a metastable state.

    Returning to the original, overly hyped media stories, you can see that there was a kernel of truth and a barrel full of hysteria. There is no danger, and it’s completely OK to resume watching with great interest the news reports of the discovery and careful measurement of the Higgs boson. And, yes, you have to go to work tomorrow.

    See the full article here.

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  • richardmitnick 4:04 pm on September 7, 2014 Permalink | Reply
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    From Don Lincoln for JHU Press: “Damage to the Large Hadron Collider” 

    jhu

    FNAL Don Lincoln
    FNAL’s Dr. Don Lincoln

    September 5, 2014

    A spark. That’s all it was . . . just a little spark . . . in a vacuum, no less. It sounds so harmless. What could it hurt? Let’s see how the story unfolds.

    Well, time, which is measured in microseconds at this point, moved on. The spark jumped from copper conductor to copper conductor, causing copper atoms to be knocked off into the vacuum. As the amount of copper vapor grew, the vacuum became less of an insulator and more conductive, letting more electricity flow. That’s when things began to get interesting. Like opening a faucet completely, the trickle of the initial spark grew until it became a torrent of electricity: ten thousand amperes, enough to simultaneously start thirty or so cars in the dead of winter. The onslaught of electricity was enough to melt a chunk of copper the size an adult fist. This would be bad, but, if you will excuse the pun, things were just beginning to heat up.

    The tipping point from annoying incident to serious disaster occurred when the heat from the electrical arc punctured the volume filled with the liquid helium used to cool the Large Hadron Collider magnets to more than 450° Farenheit below zero. Luckily, helium is an inert gas, so an explosion in the usual sense of the word was impossible. However, the helium was in liquid form, and when it encountered more ordinary temperatures, it boiled and turned into gas. When any liquid turns to gas at atmospheric temperature, it expands in volume to 700 times its ordinary size. And the LHC magnets contain an awful lot of helium . . . as in 96 tons of helium. (Although, in the end, only six tons were released.)

    As the helium vented from the storage volume, it jetted out with tremendous force. And by “tremendous force,” I mean enough force to move a 50-foot-long magnet weighing 35 tons and anchored to the concrete floor about two feet. As the helium gas expanded in the LHC tunnel, it pushed air out of the way. The boundary between an environment containing ordinary air and one containing only helium moved up the tunnel at incredible speed. It was possible for a human to outrun the helium monster, but only if the person could run a four-minute mile. Run any slower, and you would be overtaken by helium. Soon, you would fall down and die, suffocated by lack of oxygen.

    Luckily, there was nobody near the punctured helium volume to be in danger. Actually, luck had nothing to do with it. The CERN (European Organization for Nuclear Research) safety professionals were aware of the danger of a catastrophic failure. Although such an incident was extremely unlikely, people are allowed in the Large Hadron Collider tunnel only rarely. If they are allowed inside, they must have special training and carry oxygen tanks and protective clothing. In this case, however, the nearest CERN personnel were miles away from the incident, and even the civilians who lived above the LHC were separated by at least 300 feet of solid rock. No people were ever in danger.

    I was in the United States on the day in September 2008 when the LHC broke. My colleagues and I were getting reports second-hand, and I remember well the group sitting around a table, looking shell-shocked, and asking each other, “How bad can it be?”

    So now, in the fullness of time, we can answer that question. How bad was it? Pretty bad. Repairing the LHC cost tens of millions of dollars and took about a year. In the end, fifty-three magnets, each fifty feet long and weighing thirty-five tons, needed to be removed, repaired, cleaned, and replaced. While the true damage was relatively localized, among the collateral damage was a breeching of the LHC’s beam pipe, into which soot and debris spread for a mile or so. The technicians were busy.

    It is now six years later, and perhaps it is time for a broader viewpoint. Yes, the damage was grave, and yes, it took a year to repair. However, the repair costs were about two percent the cost of the entire LHC, and the delay was only about five percent of the schedule. Granted, if you were a graduate student who was hoping to graduate on the first year’s data, the incident was an awful delay. However, now, in 2014, what was the real consequence? Well, we now have an accelerator that is better instrumented against similar incidents. The damage of 2008 won’t occur again. We have studied billions of particle collisions and begun to explore the behavior of matter under conditions never before possible. We have discovered the Higgs boson and facilitated a Nobel Prize in physics. There have been some considerable successes, and the debacle of 2008 is now fading into distant memory.

    It’s all a matter of perspective. And, let us not forget, the data of 2015 beckons alluringly. Soon the universe will give up some more of her mysteries and scientists will do what they have for millennia: they will take up their pens and begin writing a new page in the book of knowledge, a book whose first pages were penned over two thousand years ago.

    Perspective.

    Don Lincoln is a senior scientist at Fermi National Accelerator Laboratory and an adjunct professor of physics at the University of Notre Dame. He is the author of

    The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind
    book

    , Alien Universe: Extraterrestrials in Our Minds and in the Cosmos
    book3

    and The Quantum Frontier: The Large Hadron Collider,
    book2

    all published by Johns Hopkins.

    See the full article here.

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  • richardmitnick 12:18 pm on August 30, 2014 Permalink | Reply
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    From Don Lincoln at Fermilab: “Particle Detectors Subatomic Bomb Squad ” a Great Video 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    The manner in which particle physicists investigate collisions in particle accelerators is a puzzling process. Using vaguely-defined “detectors,” scientists are able to somehow reconstruct the collisions and convert that information into physics measurements. In this video, Fermilab’s Dr. Don Lincoln sheds light on this mysterious technique. In a surprising analogy, he draws a parallel between experimental particle physics and bomb squad investigators and uses an explosive example to illustrate his points. Be sure to watch this video… it’s totally the bomb.

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  • richardmitnick 1:57 pm on August 15, 2014 Permalink | Reply
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    From Fermilab: “Physics in a Nutshell What is a WIMP?” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Friday, Aug. 15, 2014
    Fermilab Don Lincoln
    Don Lincoln

    If you want to understand dark matter, you need to understand terms such as MACHO and WIMP. It’s enough to recall one of those 1970s comic book advertisements for Charles Atlas’ body building program (well, for those of us of a certain age anyway).

    To understand the term WIMP, we need to go back to the idea of dark matter and why we think it exists. The easiest-to-understand evidence for the existence of dark matter involves spinning galaxies. As early as the 1930s, scientists combined measurements of the rotational speed of galaxies with [Isaac] Newton’s theory of gravity and determined that something was awry. The galaxies were spinning so fast that they could not be held together by the gravitational force of the observed matter and should have torn themselves apart. After decades of studies, scientists have determined that the most probable explanation is that there exists another form of matter that we now call dark matter. It is generally imagined that dark matter is essentially a diffuse gas of massive subatomic particles.

    wimp
    Possibility

    Astronomical evidence has allowed us to determine a fairly specific list of properties for dark matter, if it exists. Because this matter neither emits nor absorbs light, it neither is charged nor contains charge within it. This is why we call it dark. It is also stable. We know this because galaxies persist for billions of years. It does not interact via the strong force, as we see no evidence of cosmic rays (made of protons) interacting with it. And because this matter causes galaxies to rotate quickly, we know it both contains mass and participates in the gravitational force.

    That last point is crucial. There are four known forces: the strong and weak nuclear forces, electromagnetism and gravity. We know that dark matter does not experience the strong or electromagnetic forces. We know it does experience gravity. We don’t know about the weak force.

    So let’s think about that for a bit. While the weak force is … well, weak … gravity is incredibly weak, about a trillion trillion trillion times weaker than the weak force. We have never measured the force due to gravity between two subatomic particles (and we probably never will). So if gravity is the only force that dark matter feels, we will likely never detect it, nor will we ever make it any conceivable particle accelerator.

    So how is it that Fermilab (and others) have a vibrant research program looking for dark matter? Is it all wishful thinking?

    The answer is, “Of course not.” However, it does bring forward an assumption buried inside most dark matter searches. This assumption is that dark matter also experiences the weak nuclear force. Like weakly interacting neutrinos, maybe dark matter will occasionally experience an interaction with ordinary matter and be detected.

    So why would scientists postulate that dark matter experiences the weak force? One answer is that if it doesn’t, we’ll never detect it. But that’s not a very good answer. A better answer involves the Higgs boson. Because the Higgs field gives mass to ordinary matter, maybe it also gives mass to dark matter. Further, since the Higgs field was invented to solve a problem with early attempts to unify the weak and electromagnetic forces, maybe the interaction of the Higgs boson with dark matter also ties dark matter to the weak force. And this would be great, as we know from experience that we can detect weak force interactions.

    So this leads us to the meaning of the term “WIMP.” It is a weakly interacting massive particle — the name is quite literal. It is not necessary that dark matter interact via the weak force, and dark matter may not be a WIMP. If dark matter does not interact via the weak force, we’ll probably never detect it directly. In short, the success of all direct dark matter searches depends crucially on dark matter being WIMP-y.

    —Don Lincoln

    See the full article here.

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  • richardmitnick 2:53 pm on July 31, 2014 Permalink | Reply
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    From Don Lincoln of Fermilab: The Origin of Mass, a Really Cool Video 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Don Lincoln at his best.

    Learn and enjoy

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  • richardmitnick 11:29 am on June 6, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: CMS Connecting the dots” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Friday, June 6, 2014

    Fermilab Don Lincoln
    Dr. Don Lincoln wrote this article

    When two protons collide in the center of the CMS detector, the collision energy can create hundreds of electrically charged particles. These particles roar through the apparatus, crossing individual detector elements. Each particle marks the location of its passage, leaving a string of dots that can be seen on a computer screen.

    CERN CMS New
    CMS at CERN

    One of the trickiest jobs in particle physics is to teach a computer how to connect the dots and reconstruct the tracks of all of the particles that exited the collision. That’s correct: The child’s simple pastime of connect-the-dots can consume the efforts of many of the finest minds in an experiment like CMS. The difficulty stems from the fact that there are hundreds of tracks and that, in a bit of an inconvenient oversight, nobody bothered to put numbers beside the dots to tell the computer which to connect.

    Reconstructing tracks is one of the first tasks that an experiment must accomplish in order to begin to analyze the data. Before the tracks are identified, the data is a mess of little dots. Once the tracks are determined, scientists can begin to sort out the physical process that occurred by figuring out that this particle went this way while another particle went that.

    In addition to reconstructing the tracks of particles, scientists also reconstruct the origin of the particles. This is the location at which the collision between two protons occurred. Until you know the origin and trajectory of the particles, you can’t even begin to understand what sort of collision was recorded.

    CMS scientists have worked long and hard to develop the algorithms to accomplish these challenging tasks. In a recent paper, they described the result of their efforts. Particles leaving the collision at angles near 90 degrees measured from the beam can be reconstructed about 94 percent of the time. For the special case of isolated muons, the reconstruction probability rises to 100 percent. The location of the origin of the collision can be localized with a precision about 0.01 millimeters, or about half the size of the finest human hair. These algorithms are fast and flexible, and scientists continue to improve on them in anticipation of the resumption of operations in early 2015.

    See the full article here.

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  • richardmitnick 7:21 pm on May 14, 2014 Permalink | Reply
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    From Don Lincoln via PBS NOVA 

    Fermilab Don Lincoln
    Don Lincoln of Fermilab

    Wed, 14 May 2014
    In a contest for the least contentious statement a person can make, “What goes up must come down” is surely a strong contender. Of the four known fundamental forces—gravity, the electromagnetic force, and the strong and weak nuclear forces—we have the most intuitive understanding of gravity. From our first experiments dropping Cheerios from our high chair, we spend our lives coming to grips with the limitations that gravity imposes on us.

    grav
    Credit: Christoph Zurbuchen/Flickr, adapted under a Creative Commons license.

    In the late 1600s, Isaac Newton devised the first serious theory of gravity. He described gravity as a field that could reach out across great distances and dictate the path of massive objects like the Earth. Newton’s theory was stunningly effective, yet the nature of the gravitational field remained a mystery. In 1915, Albert Einstein’s theory of general relativity gave theorists their first look “under the hood” of gravity. What we call gravity, Einstein argued, is actually the distortion of space and time. The Earth looks like it’s rounding the Sun in an ellipse, but it’s actually following a straight line through warped spacetime.

    Einstein’s theory of gravity is very good at explaining the behavior of large objects. But just a few years later, physicists opened up the world of the ultra-small, revealing that the other fundamental forces are due to the exchange of specialized force-carrying particles: photons convey electromagnetism, the strong nuclear force is transmitted by gluons and the weak nuclear force is imparted by the movement of the W and Z bosons. Is gravity due to the same kind of particle exchange?

    We actually don’t know the answer to that question, but we have a name for that hypothetical particle if it does exist: It is called the graviton. And even though we have never observed a graviton, we know a great deal about them, if they are real. First, since the range of the force due to gravity is infinite and the force due to gravity weakens as one over the square of the distance between two objects (i.e. 1/r2), the graviton must have zero mass. We know this because if the photon had mass, it would change the “2” in the exponent and that “2” has been established with incredible precision. Like massless photons, gravitons should travel at the speed of light.

    General relativity also gives us some insight into the nature of gravitons. In general relativity, the distribution of mass and energy in the universe is described by a four-by-four matrix that mathematicians call a tensor of rank two. This is important because if the tensor is the source of gravitation, you can show that the graviton must be a particle with a quantum mechanical spin of two. Another nice fallout of this correspondence is that the graviton is the only possible massless, spin two particle. If you observe a massless, spin two particle, you have found the graviton.

    So why hasn’t anyone found a graviton yet? The problem with searching for gravitons is that gravity is incredibly weak. For instance, the electromagnetic force between an electron and a proton in a hydrogen atom is 1039 times larger than the gravitational force between the same two particles. Perhaps a more intuitive example is the behavior of a magnet and a paperclip. A magnet will hold a paperclip against the Earth’s gravity. Think about what that means. A little magnet, like the one that held your art to your parent’s refrigerator when you were a kid, pulls the paperclip upwards, while the gravity of an entire planet pulls downward, and the magnet wins.

    Individual gravitons interact very feebly, and we are only held to the planet because the Earth emits so many of them. Because a single graviton is so weak, it is impossible for us to directly detect individual classical gravitons.

    However, there are new and innovative ideas about gravity in which other forms of gravitons might exist. Some of these exotic gravitons might be detectable, but they require significant modifications to our understanding of our universe. This is where things get a bit mind-bending.

    If “what goes up, must come down” might be a catch phrase for Captain Obvious, “we live in three dimensions” could be the rallying cry of his sidekick, Lieutenant Duh. However, some scientists have proposed the idea that gravity might have access to more than three dimensions. In that case, gravity might not actually be as weak as we think it is. It only appears weak because, unlike the other fundamental forces, it has extra dimensions into which it can “spread out.”

    On the face of it, this seems silly. The 1/r2 nature of gravity is an incontrovertible sign that gravity operates in three dimensions, and this behavior has been directly verified down to distances smaller than a millimeter. But this leaves open the possibility of extra dimensions smaller than 150 micrometers or so. One can imagine these small dimensions by thinking of a tightrope. To a tightrope walker, who can only walk forward and backward on the rope, the rope is one-dimensional. But to an ant, which can also crawl around the rope’s circumference, the rope seems to be two-dimensional. What appears to be one-dimensional to a large being is two-dimensional to a smaller one. These smaller dimensions are cyclical in that if you travel around the outside of one, you will end up back in the same place.

    Quantum mechanics tells us that every particle is also a vibrating wave, and it has been proposed that gravitons could vibrate in these extra dimensions, wrapping around the small dimension like bracelets encircling a slender wrist. However, the cyclical nature of the extra dimension imposes limits on how a graviton can vibrate. Only an integer number of wavelengths can fit evenly in the extra dimension. And this brings us to a couple of interesting consequences. In theories with extra dimensions, more than one type of graviton can exist. One way to see that is to imagine taking a sine wave and wrapping it around a cylinder. In order for it to fit perfectly, you must use one wavelength or two or three or any integer number of wavelengths. Each of these instances is a distinct graviton; the ones with more vibrations can actually have mass. Particles of this kind are called Kaluza-Klein gravitons after physicists Theodor Kaluza and Oskar Klein, who first proposed the idea of additional small spatial dimensions. On tiny scales, Kaluza-Klein gravitons can have mass, but on larger scales, they reduce to the familiar massless gravitons of classical theory.

    Using particle accelerators like the Large Hadron Collider, physicists are already searching for these small extra dimensions, in part by looking for the expected decay products of massive gravitons. They haven’t found anything yet, which means that if extra dimensions exist, they must be a thousand times smaller than a proton, although there are many caveats to how one interprets the data.

    Gravity is the one known fundamental force that has resisted study in the quantum realm and finding gravitons of any kind would be a huge step forward in our understanding of the phenomenon. Devising a successful theory of quantum gravity is one of the hottest goals of modern physics and ongoing experimental searches for gravitons will play a central role.


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  • richardmitnick 9:17 am on May 2, 2014 Permalink | Reply
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    From Fermilab: “Physics in a Nutshell – The twin paradox” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.
    Friday, May 2, 2014

    Fermilab Don Lincoln
    This column was written by Dr. Don Lincoln

    In my last column, I discussed the fact that time passes slower for clocks that are moving at high velocity, and I showed that the Fermilab MINOS beamline proves that the predictions of relativity are right.

    Fermilab NuMI Tunnel
    Fermilab NuMI Tunnel

    ae
    The twin paradox is a classic seeming conundrum of Einstein’s theory of special relativity. Today’s column explains why it is important that the word “seeming” is added. In reality, there is no paradox.

    However, one must be very careful. The “relativity” in the theory’s name comes from the absolute core premise of [Albert] Einstein’s idea, which is that nothing is absolute. If you are standing on a train platform and a train whizzes by, you would say that a person on the train is moving. On the other hand, a person sitting on the train would say that he is stationary and that you are moving. Relativity says that both of you are right. Who is moving and who is stationary is just a matter of perspective, and the laws of physics must work equally well for both people.

    But this raises a conundrum when applied to the question of moving clocks. How can moving clocks tick more slowly than stationary ones if the question of who is moving is a matter of opinion? If I can say you are moving and your clock is slow, and if you can say I am moving and my clock is slow, something is inconsistent.

    This longstanding question about special relativity is called the twin paradox. Suppose one in a set of twins sets off in a spaceship, travels to a distant star and then returns. On both legs of the trip, he accelerates to high velocity and then coasts for most of the journey. According to the “moving clocks tick slower” premise, the twin who stays on Earth will have experienced one duration, while the traveling twin will have experienced another, slower duration. The spacefaring twin will return to Earth younger than his homebody brother.

    “But wait,” says the traveling twin, “according to my definition, I was just sitting there on my stationary spaceship while the Earth zoomed away from me and then zoomed back. By all rights, Earth twin should be younger!”

    The solution to this seeming paradox has to do with the idea of a reference frame, which is central to special relativity. “Reference frame” is just a fancy term that means “the world according to me,” putting each person at the center of his or her universe. All “inertial” observers — those who don’t experience any acceleration — will agree that the homebody never changed his reference frame. He just sat there. Similarly, all observers will agree that the traveler lived in two reference frames, one moving away from Earth and one returning. Any third observer coasting through space will see that the homebody’s velocity doesn’t change while the traveler’s velocity does.

    The law of relativity takes the traveler’s two reference frames into account. Thus the so-called paradox isn’t really a paradox. While the question of who is moving is a matter of opinion, the question of who has experienced two reference frames is not.

    Some readers, probably including some of my doctoral-holding colleagues at Fermilab, will claim that the difference between the two twins is that one of the two has experienced an acceleration. (After all, that’s how he slowed down and reversed direction.) However, the relativistic equations don’t include that acceleration phase; they include just the coasting time at high velocity. For the professional (or the brave), I work out the predictions of relativity. That one twin inhabits two frames is the only thing that matters.

    The twin paradox is one of those mind-bending questions you encounter as you first begin to explore the predictions of relativity, but don’t be fooled: It really isn’t a paradox at all. Keep that in mind as you explore other explanations that may resonate with you — the well-known ones posted by physicist John Baez, a video by Neil deGrasse Tyson, the idea of acceleration, or my own description here of one twin being in one frame.

    Embracing the twin paradox is an important first step as you dip your toe into the nonintuitive world of special relativity. If you dig a little deeper into the links given here (and show a little determination), hopefully you’ll begin to be more comfortable that Einstein really was right.

    See the full article here.

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.


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    • Lubos Motl 9:37 am on May 2, 2014 Permalink | Reply

      ““Reference frame” is just a fancy term that means “the world according to me,” putting each person at the center of his or her universe.

      We’re lucky that my blog is called “The Reference Frame” including the article, “The”, which means that mine is the only true and objectively correct center of *the* Universe. ;-)

      Like

    • Richard Mitnick 11:20 am on May 2, 2014 Permalink | Reply

      Thanks for reading.

      Like

  • richardmitnick 7:31 pm on April 28, 2014 Permalink | Reply
    Tags: , , Don Lincoln, , , ,   

    From Don Lincoln at Fermilab: “Big Mysteries: The Higgs Mass” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Don Lincoln brings us another interesting video about the Higgs boson.
    Learn and enjoy.

    Fermilab Don Lincoln
    Dr. Don Lincoln

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.


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  • richardmitnick 12:23 pm on April 18, 2014 Permalink | Reply
    Tags: , , Don Lincoln, , , , ,   

    From Fermilab: “Proving special relativity: episode 3″ 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Friday, April 18, 2014
    This column was written by Dr. Don Lincoln
    Fermilab Don Lincoln

    “Time waits for no man” goes the saying, and it appears to be true. Inexorably the moments of our lives tick away until we have none left and slip away into the darkness. However, as painful as that truth is, we have some comfort in the fact that time marches on equally for all of us — pauper and prince. Time plays no favorites.

    [Albert] Einstein turned this comforting truism on its head in 1905 when he published his theory of special relativity. In one of the most nonintuitive consequences of his theory, time does not march at the same pace for us all — it depends on a person’s velocity. Slow-moving objects age more quickly than their speedy brethren.

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    One of the most nonintuitive consequences of Einstein’s theory of special relativity is the idea that different people will experience time at different rates. This has no analog in classical theory, and yet it is easy to observe at laboratories such as Fermilab and CERN.

    That just didn’t seem even possible.

    Luckily, at particle accelerator laboratories, it is pretty easy to increase the velocity of subatomic particles and put Einstein’s idea to a strict test. Let me immediately get to the punch line: As bizarre as it seems, Einstein is right.

    There are a ton of examples I can give from every particle accelerator laboratory on the planet, and they all confirm the theory of special relativity beyond a shadow of a doubt. Let’s use one to illustrate the point: the Fermilab MINOS beamline, which shoots neutrinos in the direction of Minnesota.

    numi

    Fermilab makes neutrinos by slamming high-energy protons into a target, creating a spray of particles. The most common are pions, which then decay into muons and neutrinos. Since the pions come flying out of the collision, they move while they are decaying.

    To see the effect of relativity, we need to see just how long of a tunnel is needed to let them decay. To do that, we need to know the pions’ velocity and how long they live. In the same way that you can combine the speed of a car and the time it travels to determine the distance of its trip, you can figure out how far a pion will travel before it can decay.

    We know very well how long stationary pions live. Because pion decay is essentially a form of radioactive decay, individual pions don’t have a fixed lifetime any more than people do — some live longer and some shorter. But we can certainly say 95 percent of pions decay in 80 billionths of a second.

    Let’s say the pions have an energy of 14 GeV, traveling at 99.995 percent the speed of light (186,000 miles per second). Combining velocity and time, we would predict that the NuMI/MINOS decay tunnel would need to be about 76 feet long to contain all of the pion’s decay. Yet the actual tunnel is 2,320 feet long — almost half a mile. You know that Fermilab wouldn’t dig a much-longer-than-needed tunnel just for fun. There had to be a reason for its length, and that reason is Einstein’s theory of special relativity.

    One of the predictions of relativity is that moving clocks tick more slowly than stationary ones. There are many forms of clocks, from an old-style grandfather clock to the beat of a human heart. The steady decay of particles such as pions forms its own clock, and because of the effects of relativity, the moving-pion clock is slower than the stationary-pion clock, which means Fermilab scientists had to design the NuMI/MINOS tunnel to be long enough to accommodate the longer lifetime of the moving, decaying pion.

    Using the velocities and lifetimes described here, classical physics says that every pion would have decayed in the 2,320-foot-long tunnel — after all, it really only needed 76 feet anyway. Yet Fermilab physicists know that only about 40 percent of the pions will decay before they smash into the end of the tunnel about half a mile away. This is exactly what is predicted by relativity.

    While the fact that clocks tick more slowly if they are moving is not at all intuitive, every time we shoot a beam of neutrinos at Minnesota, we conclusively prove that the universe can be nonintuitive. Relativity works.

    See the full article here.

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.


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