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Light and gravity

  1. Apr 10, 2012 #1
    Hi All
    I understand that in GR big masses like sun just change the geometry of their neighbourhood, which means that some lines will be transformed to elliptic forms so the earth will use the new line and then will be forever turning around the sun.
    Is this true?
    Does this mean that even a photon will be stuck in that curve forever ?
  2. jcsd
  3. Apr 10, 2012 #2


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  4. Apr 10, 2012 #3
    Hi A.T.
    That is not my question.
    What I understand is that gravity changes the geometry so the planets will use the new straight lines which are so curved to become closed.
    no if a photon uses the same trajectory as the planet will it be turning forever around sun?
  5. Apr 10, 2012 #4


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    If the mass is great enough, yes. Such a situtation is a "black hole".

    If you are talking about "orbits" of planets, the space-time geometry is determined by both masses (more correctly "energies"). The joint energies of the sun and moon are sufficient to form a closed geodesic which the earth follows around the sun. The energy of the sun is not sufficient, alone, to form a closed geodesic.
  6. Apr 10, 2012 #5
    Thank you very much I think I understand.
    it depends on both masses not only the sun one.
    so there is not a fixed geometry with fixed geodesics?
    I thought that the sun for our example will create new geodesics that every other corps will use.
    Do you mean for the moon the line becomes an elliptic form but for the light it will be a slightly curved line but not closed?
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