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From: PD on 27 Feb 2010 11:47
On Feb 26, 10:37 am, Dougie Excel <DouglasWilliamSm...(a)Yahoo.Com>
wrote:
> I need an explanation:
>
> It's well known that the fact that most objects in the universe have a
> spectral signature that is shifted to the 'blue' end of the spectrum

Red.

> is interpreted as proof most objects are moving away from us.

In the leading model, yes.

> Usually
> this is analogized to the doppler effect with regard to sound waves,
> where the wavelengths of sound from a source moving towards you are
> shorter than when the source is moving away from you.

But the analogy is weak and should not be taken too far. Sound is
transmitted by a medium and this is important for explaining the shift
that occurs in that case. With light, there is no medium and the
explanation is far different. About the only thing that is common in
both is that objects receding are lowered in frequency. Beyond that,
the analogy breaks down.

>
> However, isn't the sound coming at you from a source that is moving
> towards you compressed into a shorter wavelength because it is
> actually moving towards you at a higher rate of speed (ie it's moving
> towards you at the speed of sound + the speed of the source)?

No. The speed of the traveling sound is still the same. But the
wavecrests are bunched together because the source follows the
preceding wave crest as it emits a new one, and so the distance
between the wavecrests is shorter. Our ears are actually tuned to this
wavelength, and so we hear the shorter wavelength as a higher pitch.

>
> If this is the case, I don't see how it could function with regard to
> light - the rare 'red shift' of a object moving towards the earth
> would then be explained by light being compressed into a shorter
> wavelength because it is moving at the speed of light + the speed of
> the source; however, due to relativity the light approaches us at the
> same speed no matter of the relative speed of the source, so wouldn't
> any red shift in fact reflect not the object's movement relative to us
> but rather than actual elemental make-up of the object?
>
>  = Doug

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