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Why Does Redshift Occur?; Other Factors That Cause Elongation Of Wavelength?

  1. Jun 29, 2011 #1
    Me and my Dad were having a conversation about the elongation of EMR wavelengths over an extended period of time. He began to explain to me what Red Shift was but I couldn't understand what he was trying to get at (he's a poor explainer, or I'm bad listener). But his explanation did spark an idea I had that goes like this:

    Since light is affected by gravity and also must maintain its velocity constancy, to overcome gravitational effects energy must be given up. In doing so, it must also elongate its wavelength. Over time, these gravitational effects from surrounding bodies would lower the energy of the photon.

    I asked him if that was Red Shift and he said no. So what is Red Shift, and much of a role does it play compared to the phenomena explained above?
  2. jcsd
  3. Jun 29, 2011 #2


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    "Redshift" is simply a generic term for any effect that shifts an electromagnetic wave's frequency downward.

    In special relativity, you get redshifts because the source and receiver are in relative motion away from each other, blueshifts when approaching. For instance, we get this when we receive radio waves from a space probe that's moving away from us. Roughly speaking, the reasons for this type of shift are the same as the reasons for any other Doppler shift, http://en.wikipedia.org/wiki/Doppler_shift , but you also get a shift when the motion is transverse, which can be interpreted as coming from time dilation.

    In general relativity, in the context of a static gravitational field, you get a redshift whenever an electromagnetic wave moves from a lower gravitational potential to a higher one, a blueshift in the opposite case. An example would the 1962 Pound-Rebka experiment: http://en.wikipedia.org/wiki/Pound-Rebka_experiment. One way to understand this is in terms of the equivalence principle http://en.wikipedia.org/wiki/Equivalence_principle . Another way of looking at it is that there is gravitational time dilation. E.g., time flows at a different rate aboard a GPS satellite than it does on earth.

    Also in general relativity, you can get cosmological redshifts. These can be verbally described in more than one way. You can describe them as kinematic redshifts, or you can say that both the source's galaxy and the observer's galaxy are at rest, but the space between them is expanding, causing the light waves to stretch. These are just two different verbal descriptions of the same mathematical result in GR. There is no way to say which one is right, because GR doesn't have a well-defined way of talking about the relative velocities of cosmologically distant objects.
    Last edited by a moderator: Apr 26, 2017
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