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Two-photon physics

  1. Aug 26, 2009 #1

    NJV

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    I read that when two photons interact with sufficient energy, matter is created. Are the photons absorbed in the process?
     
  2. jcsd
  3. Aug 26, 2009 #2
    Certainly two 0.511 MeV photons are emitted when a positron in the positronium singlet state annihilates with an electron. Time reversal invariance predicts that the same two photons (plus about 6.8 eV of positronium binding energy) could produce a free electron and positron. I do not know if any experiment has observed two photons creating matter.
     
  4. Aug 26, 2009 #3

    NJV

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    Elegantly explained! So in theory, (if the theory is correct) the photons would be lost in the process, much like the positron and electron in the reverse process. Thanks. :)

    So, the photons can produce almost any kind of matter? Most particles have an antiparticle, and combining the two will always yield photons.

    Speaking of time reversal transformations, though, where do the neutrinos come in? As far as I'm aware, combining matter with antimatter still yields 50% neutrinos, right?
     
    Last edited: Aug 26, 2009
  5. Aug 26, 2009 #4

    Vanadium 50

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    Many experiments have used two-photon processes to produce electron, muon or pion pairs.
     
  6. Aug 26, 2009 #5

    Nabeshin

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    No, combining antimatter with matter yields two photons. The example that has been raised of an electron and positron produces two .511MeV photons, no neutrinos. Neutrinos are produced in certain types of nuclear decays and reactions though. The first example that comes to mind is the fusion of two hydrogen atoms into a deuterium, neutrino, and positron, simply because I deal with this one a lot. There are tons of other interactions, not necessarily fusion, which produces neutrinos.
     
  7. Aug 27, 2009 #6

    NJV

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    Apparently, that would be the case only in the combination of nucleons and antinucleons:

    "Not all of that energy can be utilized by any realistic propulsion technology, because as much as 50% of energy produced in reactions between nucleons and antinucleons is carried away by neutrinos in these applications, so, for all intents and purposes, it can be considered lost.[15]"

    Wikipedia cited this as source: http://gltrs.grc.nasa.gov/reports/1996/TM-107030.pdf [Broken]

    Why is this?
     
    Last edited by a moderator: May 4, 2017
  8. Aug 28, 2009 #7
    Hello NJV-
    The initial reaction products do not include many neutrinos- most come from pion decay, which has a half life of 26 nsec. Have you thought seriously about how to store a fuel of antiprotons or antihydrogen in a space ship? Fermilab usually has roughly 1012 antiprotons stored in a ring 6,280 meters in circumference. How would you carry them? How much energy does 1012 antiprotons represent? Only about 2/3 of the initial (strong interaction) annihilation products are charged, and these are half positive and half negative. How do you direct all this energy to produce thrust?
    Bob S
     
    Last edited by a moderator: May 4, 2017
  9. Aug 28, 2009 #8

    NJV

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    So, this would have to do more with the current technology available to extract the energy from antimatter than with the fundamental nature of the antimatter reactions itself? Ideally, you could effectively get 100% energy conversion out of antimatter?
     
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