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Thread: Dirac and Anderson

  1. #1 Dirac and Anderson 
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    Five years after Dirac proposed the concept of a positive electron, just such a particle was found by American physicist Carl Anderson (1905–1991). Anderson named the newly found particle a positron, for posi tive elec tron.

    Anderson's discovery raised an obvious question: If an antielectron exists, could there also be an antiproton, a proton with a negative charge? That question took much longer to answer than did Dirac's original problem. It was not until 1955 that Italian-American physicist Emilio Segrè (1905–1989) and American physicist Owen Chamberlain (1920– ) produced antiprotons by colliding normal protons with each other inside a powerful cyclotron (atom-smashing machine).

    If antielectrons and antiprotons exist, is it possible that antimatter also exists? Antimatter would consist of antiatoms made of antiprotons and antielectrons. The idea may seem bizarre because we have no experience with antimatter in our everyday lives. Scientists now believe that antimatter is common in the universe, but we don't have any direct contact with it.

    If antimatter does exist, locating it may be a problem. Scientists know that the collision of an antiparticle with its mirror image—an electron with a positron, for example—results in the annihilation of both, with the release of huge amounts of energy. Thus, any time matter comes into contact with antimatter, both are destroyed and converted into energy.

    If? IF it exists?! Oh come on!
    Last edited by SpeedFreek; 09-05-2014 at 09:47 PM. Reason: removed link
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    If? IF it exists?! Oh come on!
    This is a major open question in current physics. Specifically the big bang should have produced an equal amount of matter and anti-matter. What happened to the anti-matter? There is no solid explanation yet, only the knowledge that there is a slight difference in certain reactions.
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    *Every* elementary particle has an antiparticle, though some particles and their antiparticles are identical, like the photon.

    Various physicists have proposed a variety of "baryogenesis" mechanisms, ranging in energy scale from electroweak to GUT scales. However, the mechanisms satisfy some criteria proposed by Andrei Sakharov in 1967:
    1. C and CP violation.
    2. Baryon-number violation.
    3. Departure from thermal equilibrium
    The first one implies a time asymmetry, and it must combine with the second one to make baryon-number asymmetry being dependent on the time direction. The third one supplies the time direction.

    Sakharov imagined very massive particles or "maximons" being produced when the Universe was hot enough. As the Universe expanded, it became too cold to produce them, and the existing ones decayed. Their decays had baryon-number asymmetry, thus producing matter-antimatter asymmetry. Some more recent scenarios picture a false vacuum state instead of particles, but other than that, they work essentially the same.
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    How do we know that some of the galaxies are not made of antimatter? Sorry if this is a daft question......
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    Quote Originally Posted by Jilan View Post
    How do we know that some of the galaxies are not made of antimatter? Sorry if this is a daft question......
    It's not a daft question at all. The short answer is that we don't know if some antimatter galaxies exist. However, we can say that if there are indeed antimatter galaxies, they must be very far removed from ordinary matter galaxies, or we would see signature radiation from their interaction. The crude data we have so far is consistent with the "there are no antimatter galaxies" hypothesis, but does not rule out the existence of same.

    Your question is so important that a project is currently underway to look for those signatures. Nobelist Samuel C.C. Ting is leading the project, which placed a special instrument on the ISS. In a couple more years, we should have a far richer data set with which to constrain our theories more tightly.
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    Thanks tk421. How can we tell the difference?
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    Quote Originally Posted by Jilan View Post
    Thanks tk421. How can we tell the difference?
    The current methods are based on the detection of antiparticles close to us (e.g., by seeing which way they bend in a magnetic field, we can tell whether they're positively or negatively charged; the radius of curvature tells us the mass), as well as by characteristic radiations produced during mutual annihilation of matter and antimatter. By looking at the statistics of what we find, we can place bounds on the abundance of antimatter. Yes, the calculations rely on a chain of inferences about homogeneity, etc., but one must start somewhere.

    The first datasets were released about a year ago. My understanding is that another paper is forthcoming "real soon now". The instrument on the ISS has been, or shortly will be, upgraded to allow a much-extended observation lifetime, so we may have some higher-fidelity answers relatively soon.
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    Are you saying that particles make it here from other galaxies? That seems sort of unlikely.
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    Quote Originally Posted by Jilan View Post
    Are you saying that particles make it here from other galaxies? That seems sort of unlikely.
    It is indeed highly unlikely, so that's not what is assumed to be measured. As I said, the measurements are inputs to a chain of inferences. For example, if we assume homogeneity (and a few other things), measurements of, say, antihelium-to-helium ratios here tell us something about antimatter's distribution there (where "there" is way, way over there). That, in turn, allows us to rule out certain cosmological theories. In the absence of the ability to send instruments to the other side of the galaxy, that's about the best we can do.
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    Why do we assume homogeneity? Sorry I am new to all this. ....
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    Quote Originally Posted by Jilan View Post
    Why do we assume homogeneity? Sorry I am new to all this. ....

    Simple: What else would we assume?

    As I said, one must start somewhere. In the absence of evidence of inhomogeneity, homogeneity is the best operating assumption. If evidence accumulates that points to inhomogeneity, then we would modify how we proceed.

    It's no different from the many other assumptions we make. For example, we assume that all electrons are the same. But when was the last time you checked them all? To make progress, we have to start somewhere.
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    Sorry, I don't get that all. If you assume absolute homogeneity then there would be perfect annihilation of matter and anti-matter. Why would that be assumed at all? Would not small fluctuations be able to explain the universe we see today if the regions of matter and antimatter were separated ? I am betting that some of them other galaxies are made of antimatter. Nothing makes much sense otherwise
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    Quote Originally Posted by tk4210
    Simple: What else would we assume?
    Simple: inhomogeneity. Nature is rarely uniform. Look at the matter distribution in the universe on a galactic level. It's highly non-uniform. So in the absence of anything its best to assume inhomogeneity.
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    Quote Originally Posted by Physicist View Post
    Simple: inhomogeneity. Nature is rarely uniform. Look at the matter distribution in the universe on a galactic level. It's highly non-uniform. So in the absence of anything its best to assume inhomogeneity.
    No, sorry, that's simply wrong. On galactic scales [ETA: ~250Mly], the universe is actually highly uniform by almost any quantitative measure. That's reflected in, inter alia, the remarkably isotropic nature of the CMB.

    In physics, one always begins with the simplest cases and explores the consequences. Suppose we were to follow your and Jilan's reasonable-sounding prescription and assume inhomogeneity. Because there are infinitely many types of inhomogeneity, and we have no data about any putative inhomogeneity, you can't make any progress toward a theory.

    So we start with the best assumption we can make, and go from there. If we run into a buzz-saw, we pivot. But there's no data, no buzz-saw, so merely assuming inhomogeneity is an empty exercise.

    ETA: added quantitative info re: "galactic scales"
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    Quote Originally Posted by Jilan View Post
    Sorry, I don't get that all. If you assume absolute homogeneity then there would be perfect annihilation of matter and anti-matter. Why would that be assumed at all? Would not small fluctuations be able to explain the universe we see today if the regions of matter and antimatter were separated ? I am betting that some of them other galaxies are made of antimatter. Nothing makes much sense otherwise
    Be careful: Just because it doesn't make sense to you personally does not mean that it doesn't make sense. Don't assume that scientists are so stupid as to assume obviously foolish things. Any time you think you have identified such foolishness, the likelihood is that you haven't considered the story with sufficient care. Remember: Lots of really smart people -- all determined to prove that they're smarter than the others -- spend careers thinking of these things. Very, very few obviously stupid things slip through the sieve.

    First, no one is assuming "absolute" homogeneity. That would be silly -- we see that there is clumpiness to matter, obviously. There's a moon here, but not there. What is meant by homogeneity in the cosmological sense is that on large scales, one region of the universe looks pretty much like every other.

    Now, the question about how a homogeneous universe could lead to such an apparent deficit of antimatter is one that has grabbed the attention of many cosmologists. Sometimes it seems that there are more theories than there are theorists! That's why gathering data is so important -- without it, we would not be able to rule out many of these competing ideas.
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    If some galaxies are made of antimatter, then there would be separate ordinary-matter and antimatter regions of the Universe, and their boundaries would produce characteristic annihilation radiation, like the 511-keV two-photon positronium-annihilation line. We'd see only one of each of those pairs of photons, of course.
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    Quote Originally Posted by lpetrich View Post
    If some galaxies are made of antimatter, then there would be separate ordinary-matter and antimatter regions of the Universe, and their boundaries would produce characteristic annihilation radiation, like the 511-keV two-photon positronium-annihilation line. We'd see only one of each of those pairs of photons, of course.
    This is an interesting thread. What sort of matter would you expect to see at the boundaries? If anything was at the boundaries wouldn't it have been anhiliated long ago?
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    Quote Originally Posted by tk421 View Post
    Be careful: Just because it doesn't make sense to you personally does not mean that it doesn't make sense. Don't assume that scientists are so stupid as to assume obviously foolish things. Any time you think you have identified such foolishness, the likelihood is that you haven't considered the story with sufficient care. Remember: Lots of really smart people -- all determined to prove that they're smarter than the others -- spend careers thinking of these things. Very, very few obviously stupid things slip through the sieve.
    Be careful: I didn't call anything foolish. Is there a good explanation of why there is more matter than anti-matter? If so it will than all make sense.......
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  19. #19  
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    Quote Originally Posted by Jilan View Post
    If anything was at the boundaries wouldn't it have been anhiliated long ago?
    Don't forget that even if it was annihilated long ago, the photons produced might only be just reaching us now.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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    Quote Originally Posted by KJW View Post
    Don't forget that even if it was annihilated long ago, the photons produced might only be just reaching us now.
    Indeed. I wonder how red shifted they would be?
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  21. #21  
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    Quote Originally Posted by Jilan View Post
    I wonder how red shifted they would be?
    An important point. One can't simply be looking for 511 keV photons.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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    Quote Originally Posted by Jilan View Post
    Be careful: I didn't call anything foolish.
    True, but carefully note that I didn't say that you did. However, your dismissive attitude toward the quite-reasonable provisional assumption of homogeneity allowed one to draw an inference, just as you were able to draw a certain inference from my admonition.

    Is there a good explanation of why there is more matter than anti-matter? If so it will than all make sense.......
    No, there is no good, widely accepted explanation as of yet, as I mentioned earlier. There are a great many theories, but without experimental data, we aren't able to narrow the choices. A reasonable summary of the situation can be found here: Baryon asymmetry - Wikipedia, the free encyclopedia As you can see from the article, possible explanations span the gamut from CP and PT violations, to positing widely separated matter- and antimatter-dominated regions.

    And apropos of KJW's comment above, you may wish to read up on the capabilities of the next-gen AMS instrument being deployed to search for antimatter and refine bounds on where it could be hiding. See Alpha Magnetic Spectrometer - Wikipedia, the free encyclopedia (this is Samuel Ting's baby that I referred to earlier). In acknowledgment of KJW's observation, the AMS looks at other than 511keV gammas (although the latter are inputs to the overall equation).
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