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Light bundles?

  1. Mar 17, 2003 #1
    i was wondering what happens where an infinite number of lightbeams meet in one point/knot in open space or anywhere..
    how come they do'nt mess up one another? is this on account of their being massless?
    ..anything we see, any lightbeam that reaches our eye is crossed infinite times by beams from any direction..

    i could'nt surf an answer for this..

    [added:] even though they're massless, there must be a reason why all the involved electromagnetic fields do'nt mess..
     
    Last edited by a moderator: Mar 17, 2003
  2. jcsd
  3. Mar 17, 2003 #2
    Well, they are massless, so they don't exactly "run into each other".
     
  4. Mar 17, 2003 #3
    The "Principle of Superposition."

    Lots of fields, EM being one of them, obey a principle where if you add up two valid solutions you always get a third valid one. In physical terms, this means light waves pass through each other without even 'seeing' each other.
     
  5. Mar 17, 2003 #4
    have you ever thought of the concept"anti light".anti light stays in one spot,has no mass in motion,but has rest mass.can't be seen unless your moving relavent to it.i mean as you move through them then they enter your eyes then you see things if you were in the dark and they where around you.but in order for you to see them they would have had to hit matter so the image is encoded on the anti light photon.and that only if you anti light interacts with you eyes like normal light,because its anti matter!

    chosenone,

    Did you read the post "read before posting"? Re; The part of not using someone else's question as an avenue for pushing non-conventional theories
     
    Last edited by a moderator: Mar 17, 2003
  6. Mar 17, 2003 #5
    Do you mean to refer to fermions?
     
  7. Mar 17, 2003 #6

    Tom Mattson

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    roeighty,

    There are 4 fundamental forces in nature, and these four are the only known means by which particles interact. Photons do not carry electric charge, color, or flavor, so they cannot interact via the EM, strong, or weak forces respectively.

    That leaves gravity.

    In principle, photons can interact gravitationally, because in GR, all matter and energy shows up in the energy-momentum tensor, which determines the curvature of spacetime. Thus, there should be a weak gravitational interaction between photons. However, this interaction is predicted to be very weak, which is why it is not noticeable.
     
  8. Mar 17, 2003 #7
    Holy cow!
     
  9. Mar 18, 2003 #8
    Tom: I thought photons did exhibit electromagnetic properties. Isn't it true that when a photon stops, before it's energy goes down to 0, there is an EM chain reaction? Or have I got it all confused with something else?
     
  10. Mar 18, 2003 #9

    Tom Mattson

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    Heh. OK, I saw the picture, and I don't get it.
     
  11. Mar 18, 2003 #10

    Tom Mattson

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    All's I'm saying is that they don't interact with each other via the EM interaction. That's what the original question was asking.
     
  12. Mar 18, 2003 #11
    Photons are the particles which "carry" EM interraction...
     
  13. Mar 18, 2003 #12

    Njorl

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    Doesn't it require two gamma rays to form an electron-positron pair (to conserve energy and momentum)? If so, wouldn't light be undergoing weak interactions?

    Njorl
     
  14. Mar 18, 2003 #13

    arivero

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    Well, there are some probability for photons to change, during a short time, into some pairs electron-positron, then interact, then forming back a bunch of photons. So, yes, at high energies one could consider photon-photon scattering. This effect should be noticeable higher than gravity scatter.

    On other hand, there is not hardball scattering due to impenetrability, because photons are bosons, thus a pair of bosons can lie in the same state. Roeighty argument was raised first time by atomist, who called those strange particles reaching our eyes eidolons.
     
  15. Mar 18, 2003 #14
    Yup... also happens with the photons splitting into hadrons, just the cross sections are tiny. There have been a lot of papers recently about the structure functions and cross-sections of these processes.... crazy stuff.

    cf http://xxx.lanl.gov/abs/hep-ph/0205301
    also Physics Reports 332:165-317
     
  16. Mar 19, 2003 #15

    Tom Mattson

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    Yes, now I remember a discussion of that in the beginning of Jackson's Classical Electrodynamics. I guess my answer was too simple, so I'll rephrase:

    Photons only couple to matter currents, not to other photons.

    Better?
     
  17. Mar 19, 2003 #16

    arivero

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    A lot better :)

    Outside of the ambit of electromagnetism one still raises a doubt, as the W+ and W- are charged bosons, a very surprising object... surely they will be able to emit photons too, but I have no checked. In any case, SU(2)xU(1) is probably out of the ambit of the original question.
     
  18. Mar 20, 2003 #17
    Perfect...well...maybe just gravitationaly...
     
  19. Mar 20, 2003 #18
    Feriendly tutorial

    Hi Njorl,
    FYI: It takes only one gamma ray of energy exceeding 1.02 MeV that is stopped cold by massive matter to threshold electron/positron pair production. Gamma energies less than that energy produce only photoelectrons. The electron produced disappears into a sea of electrons in the stopping material while each positron mates up in three distinct ways with an electron to annihilate with it, thus becomeing two or three actual singular photons of maximum individual energy of 0.511 MeV. It is noteworthy that pair-production is quantized in that a 5 Mev gamma, e.g. can peel off, one after another, in increments of energy, 1.02 MeV, up to 4 individual electron-positron pairs plus an iota of photoelectric electron creation. Cheers,

    "Logic is easy when done Nature's way." Jim
     
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