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Information for the uninformed

  1. Feb 26, 2008 #1
    I am in college and have no formal training in physics, but i really enjoy the field. I was reading about redshift and am currently stumped in regards to its relation to light. I have read that light is constant, but in redshift the wavelengths of the light lengthen. I was wondering if light is constant shouldn't the wavelengths remain the same length when traveling through space? Or do the different wavelengths of light still travel at the same speed? Again i am a layman so please help to shed some light on this topic. i would greatly appreciate it.
     
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  3. Feb 26, 2008 #2

    cristo

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    I'll try and have a go at answering your question, but feel free to jump in if I've not answered the correct question! Perhaps you recall from some introductory physics course (if you've taken one) that the wavelength, velocity and frequency of light are related by c=fl, where f is the frequency, l is the wavelength and c is the speed of light. Thus, when we talk about light being red-shifted, its wavelength increases, but its frequency decreases, thus leaving c constant.
     
  4. Feb 26, 2008 #3

    tiny-tim

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    Only the speed of light is constant.

    But its energy can change.

    And energy is dictated by wavelength (=1/frequency).

    So different photons of light can have different wavelengths, but they do all travel at the same speed! :smile:
     
  5. Feb 26, 2008 #4
    If l/c =T, in what frame does "T" apply, and what does it apply to?

    Regards,

    Bill
     
  6. Feb 26, 2008 #5
    ??? I thought wavelength = c/f.

    Regards,

    Bill
     
  7. Feb 26, 2008 #6
    I think that the redshift of distant galaxies is caused primarily by cosmological expansion. That might be part of the confusion. Essentially it's something like the lightwave has gotten physically stretched over astronomical lengths of time as it traveled (because the spatial dimensions of the universe itself have gotten stretched since the time the light wave began traveling.)
     
  8. Feb 26, 2008 #7
    I don't think so.

    I think the redshift is more commonly attributed to the recession velocity of the source(s) - which, consequently, is the basis for the "cosmological expansion".

    Regards,

    Bill
     
  9. Feb 26, 2008 #8
    The reason why distant galaxies are receding isn't because they're moving, like they were flung away from our galaxy by the Big Bang or something; it's because space is stretching. See this Misconceptions About the Big Bang article that marcus frequently posts:

    Sorry, I should have quoted that in the first place.
     
  10. Feb 26, 2008 #9
    That would imply that absorbtion bands shift as a function of time!

    Regard,

    Bill
     
  11. Feb 27, 2008 #10

    tiny-tim

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    Sorry! :smile:

    Some books measure time and space in units in which c = 1 - and some don't - and I'm used to the ones that do!

    Yes, red-shift (and blue-shift) are non-relativistic effects that Newton knew all about, and depends only on the recession velocity of the source.
     
  12. Feb 27, 2008 #11
    Absorption bands? Do you mean frequency and wavelength shift as a function of time? I think that would be a yes, unless the author of that Scientific American article was wrong.

    Cosmological expansion isn't a relativistic effect. Are you saying that the Doppler Effect is the only thing that can cause red-shift, even on a cosmological scale? I must respectfully point out that the above article I linked to seems to disagree with you.
     
  13. Feb 27, 2008 #12

    tiny-tim

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    How not to explain pool!

    (Scientific American articles are far too long.)

    I'm saying that the original post in this thread was asking about the relationship between wavelength and velocity.

    That relationship is redshift.

    It's a straightforward effect, familiar to us even before relativity.

    Introducing cosmological expansion to explain redshift is like introducing nuclear physics to explain pool! :smile:
     
  14. Feb 27, 2008 #13
    Yes…

    Redshift isn't a relationship, it's a phenomenon. The relationship between wavelength and velocity is v=λf or some other formulation of that like Bill mentioned.

    I don't understand why you keep mentioning relativity. No one else in this thread has mentioned relativity.

    Unless James23 is reading about astronomy and that's why he's asking about light “traveling through space.” In which case the important redshift, the one that lets us gauge how old the universe is and how distant astronomical objects are from us, is the one caused by cosmological expansion.

    In that case it's particularly unhelpful for you to make statements like saying that red-shift “depends only on the recession velocity of the source.” He's trying to understand how red-shift relates to the speed of light, so avoiding a frequently-mentioned cause of red-shift that involves no change in the speed of the wave seems more confusing than anything to me.

    If there's anything he doesn't understand he can just ask questions.
     
    Last edited: Feb 27, 2008
  15. Feb 27, 2008 #14

    tiny-tim

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    You introduced "cosmological expansion" - that is relativity! :smile:

    Are you saying that cosmological expansion changes the speed of light? :confused:
     
  16. Feb 27, 2008 #15
    I don't think it is, I thought it was cosmology. Do you think it's part of Special Relativity or General relativity?

    No, it changes the wavelength and frequency at the same time without changing the velocity, that's exactly why it's relevant to the question.
     
  17. Feb 27, 2008 #16
    He asked,

    Cosmological expansion is the only way I know of that the wavelength might change while the light is traveling through space. The wavelength of light red-shifted due only to Doppler effects would remain constant.
     
  18. Feb 27, 2008 #17

    tiny-tim

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    Cosmology is definitely part of General relativity! :smile:

    wavelength = c/frequency anyway, and you agree c is constant, so it's the same effect, but a far more obscure example.
     
  19. Feb 27, 2008 #18
    Well technically everything physical is affected by GR, including the wave mechanics of light we're discussing. But you don't need to understand relativity to understand cosmological expansion or the relationship between wave velocity and wavelength and frequency.

    As a non-physicist that is not my experience. I hear about astronomical applications far more frequently than I ever hear about redshift in a moving terrestrial source of light. And the use of redshift to measure the age of the universe and the distance of galaxies and quasars - the redshift caused by cosmological expansion - is by far the most frequent mention of redshift within astronomy that I hear of.

    And as I pointed out, the Doppler effect does not change the wavelength of light as it's moving through space, which is what he asked about.
     
    Last edited: Feb 27, 2008
  20. Feb 27, 2008 #19

    cristo

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    Everyone seems to be taking this discussion a little too far, if you ask me. I saw a question that asked about the redshift of light, not about cosmological expansion, or relativity. Perhaps we should wait and see what James thinks before guessing which question he want answering.

    This isn't correct: the doppler shift is defined as a change of wavelength and frequency of the wave (see here).

    I would say that, today, general relativity is a part of cosmology, in that cosmologists use GR since it is our current theory of gravity, but cosmology is not a subset of GR.
     
  21. Feb 27, 2008 #20
    Good point, I agree.

    I just think it's important to mention various possible sources of red-shift, particularly since, as I said as a non-physicist (and non-astronomer), it appears to me that one is much more likely to encounter mention of red-shift as a consequence of cosmological expansion. I think it was for something like ten years that I had the faulty understanding that the primary source of astronomical redshift was due to velocity.

    Oh, I didn't realize that - so even red-shift due to cosmological expansion rather than the velocity of either of the sources is considered doppler shift? If so I apologize.
     
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