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Redshift effect And photon energy loss

  1. Oct 14, 2015 #1
    I've been searching around the web to figure out why photons shift towards the longer wavelengths as they travel from stars and other light sources but I haven't figured out why they loose energy as they travel ( and after reading some web pages I was told that they don't even loose the energy as they travel) so essentially I'm all confused on this topic. Here are my questions. How do photons loose the energy in the red shift effect? Where does that energy lost go?
  2. jcsd
  3. Oct 14, 2015 #2


    Staff: Mentor

  4. Oct 15, 2015 #3
    It's not clear if you talk about redshift due to speed or due to gravitation; you already got an answer about the effect of speed, and the discussion in the link is a good primer for the effect of gravitation.

    Concerning gravitation there is some disagreement, in part perhaps due to Einstein who in 1911 suggested that radiation can loose energy in transfer, but in that same paper clarified that radiation cannot change frequency on its travel through vacuum - and without a change in frequency there is also no "lost energy". That second consideration fits neatly with the reference about speed that Bill gave and leads to consistent descriptions.
    For example Einstein predicted the gravitational redshift effect of light from stars because it is emitted at lower frequencies:

    "Thus the clock goes more slowly if set up in the neighbourhood of ponderable masses. From this it follows that the spectral lines of light reaching us from the surface of large stars must appear displaced towards the red end of the spectrum."
    - p.198, Foundation of General Relativity, 1916. http://web.archive.org/web/20060829045130/http://www.alberteinstein.info/gallery/gtext3.html
    Last edited: Oct 15, 2015
  5. Oct 15, 2015 #4
    Alright thanks guys. You guys have given me enough info , thanks.
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