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Difference between a red/blue shifted object and one in rest.

  1. Aug 6, 2009 #1

    I'm pretty much a pop science guy so I can't say I know much about the underlying math or physics, but after listening to these lectures about pretty much everything over and over again, there is one thing I just can't seem to find the answer to.

    The explanation of red/blue shift is pretty clear to me, however, how do we know the difference between the red shift from an object moving away from us and an object in rest emitting that same wavelength to begin with?

    If our only source of information from distant stars is light in an expanding universe, how do we know what they would look like at rest to begin with?

    Sorry if this is a dumb question, but I can't seem to find a good explanation.

    Thanks in advance!
  2. jcsd
  3. Aug 6, 2009 #2


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    This is actually a very good question with a simple answer: we don't. A redshifted photon looks exactly the same as one which is emitted in the lab under at the same energy.

    However, this doesn't mean we can't acquire information about distant stars. For example, we know that hydrogen emits a photon with wavelength 656.3 nm during the transition from n=3 to n=2. This is obviously independent of where the photon is being emitted! So, if we see a distant star which has the same line except at a wavelength of, say, 670nm, we can conclude something about the speed the star must be moving away from us. This is only one line, but in practice it happens with an entire array of spectrum lines, all of which are well known from earth laboratories, and all of which are shifted by the same amount.

    This is the general procedure for analyzing spectra from distant stars to obtain velocity measurements (and distance, for objects that hubble's law applies to).
  4. Aug 6, 2009 #3


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    Welcome to PF!

    Hi Cipz! Welcome to PF! :smile:
    We recognise the patterns …

    it's like a barcode, or tree-rings …

    different molecules produce lines at particular wavelengths, always the same wavelengths, and we can recognise those lines, by the way they're spaced. :wink:
  5. Aug 7, 2009 #4
    Thanks tiny-tim and thanks both of you for your answers, now I know what to look for when reading more about this :)
  6. Aug 7, 2009 #5
    Yeah this is a question i have been having for sometime, i found an explanation which i kind of liked

    You will need to take the spectra of a star/object and find different wavelengths corresponding to the different frequencies of the light emitted

    Now when this is seen, we will know that a particular wavelength, say red, would be either less or more; now the question is how are we so sure whether this is a red giant or is the star moving away from us.

    Here we get into a little bit of guesswork, now let us just say that the visual diameter of the star measured does not measure to be like a giant at all we can safely say its moving away.

    Also we will know that a particular wavelenght cannot be in excess by emission alone, what we would need to reason would be whether it is theoritically possible to have the concentrations to emit so much of light.

    So its a kind of intelligent guessing, but yes i dont think its possible to say for certain of course this applies only to confusion for redshifts, for intance when we see a blue star we know for sure its blue cos something cant be rocketing into earth :rofl:
  7. Aug 7, 2009 #6
    If the object was moving, the frequency/wavelength of light would be shifting (either towards more blue or more red depending on which direction it was moving). If the object is static and blaring a steady red light at us, the f/wave-l wouldn't change.

    Simple :)
  8. Aug 7, 2009 #7


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    That explanation isn't right; measurement of Doppler shifts isn't guesswork, but rather a very precise science. As Nabeshin said, scientists use spectral lines to calculate redshift/blueshift, and spectral line wavelengths are usually published with 8 or so digits. This works both for stars and for distant galaxies, except in the most distant, faint galaxies in which spectral lines can't be resolved. In such cases, telescopes take photos in different wavelengths, and the differences in brightness are compared to the differences expected given current models of galaxy evolution. Redshift is then inaccurately estimated.
  9. Aug 10, 2009 #8
    I think you got me wrong there, i said the interpretation is a little bit of guesswork, because we cannot really parametrically say that this is caused because of the redshift alone.

    Calculation of the doppler is certainly very precise, we however are not sure whats causing the doppler
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