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

Understanding Redshift

  1. Sep 20, 2014 #1
    Hello guys!

    I know what redshift is and all concerning it, so i have a good understanding of it. Though there is something that my 15 year old little head cant really seem to understand... When a star emits its photon, it travels to us and due to the accelerating distance between it and us, the light is redshifted because it has to strech out its wave and therefore the wavelengt increases. To me, for this to make sence, the photon would have to have a set amount of frequences associated with it in the moment it gets emitted, and therefore the wavelenght has to get streched out.

    For example the first light year this light beam travels, it has a frequency and wavelenght. When this photon has travelled the distance that was between the star and us, the moment it was emitted, it havent reached us due to expansion. But at this point it still has the same frequency and wavelength. But when it then travels further and reaches us, it is redshifted or streched out sort of. What makes this happen!? Why doesnt it just continue with its frequency and wavelength.

    I know im gonna get some responses saying that i should try not to think of it as either a photon or a wave, and i have no matter what i imagine i dont see why this has to happen.

    Thanks in advance, Mathias
  2. jcsd
  3. Sep 20, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    This isn't what happens.

    The wavelength gets stretched continually as the light travels - by 1/144th of a percent per million light-years, iirc. Every bit of distance it covers adds a bit to its wavelength(therefore reducing frequency). It's as if the space was being stretched underneath, similar to a guitar string getting longer as you move your fingers up the instrument's neck - the standing wave in the string gets longer with it, producing lower frequency sounds.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook