Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Lost energy of red shifted photons

  1. Apr 10, 2010 #1
    Photons which have experienced a change in frequency (red shift) due to gravity(or other red shifting affects), have necessarily lost energy. But total energy is conserved. Can someone please explain in what form is the energy lost?

    Also related to the red shifted light subject:
    Electrons emit and absorb energy in quantum values. However, after an adequate amount of red shifting, photons of a given emission will lose the ability to excite the same (equivalent) electron to that higher energy state. What is the tolerance? For example say an electron in the outer shell of some atom can be excited to higher state by photon of 5500 angstroms. What would happen when the electron encounters a red shifted photon of 5505 angstroms. Will the electron be excited to a certain extent, but be unstable?
  2. jcsd
  3. Apr 10, 2010 #2


    User Avatar

    Staff: Mentor

    Imagine you're in a car accident: you get rear ended by a 1000 kg car going 10 m/s while you're stationary (in a really big truck). The energy of the collision is 50,000J.

    Now imagine you're in a similar collision, but this time you are moving at 5 m/s. What's the energy of the collision now? Did the universe lose energy somewhere?
  4. Apr 10, 2010 #3

    Jonathan Scott

    User Avatar
    Gold Member

    For gravity, red shift or blue shift is NOT due to any change in frequency of the photons, but is rather due to the time rates being different for observers in different potentials. No energy change occurs. If a photon of a given frequency is created by some atomic transition in the vicinity of a star, then it will appear red-shifted compared with the energy of the same transition at some distance from the star, but it hasn't changed frequency.

    Similar considerations apply for red or blue shift due to relative motion; the energy is unchanged relative to the location at which the photon was emitted, but appears different from a moving frame.

    The situation isn't quite so clear-cut about red shift due to cosmological expansion, because it depends on how you describe the expansion, but even in that case the frequency of the photon is unchanged relative to its original rest frame.
  5. Apr 11, 2010 #4


    User Avatar
    Science Advisor
    Gold Member

    Redshifted photons are time dilated, so no net energy loss - er, what jonathan said.
  6. Apr 11, 2010 #5
    Ahh, interesting. That makes the study of time dilation my next subject of interest.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook