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William and Wayne are right: the frequency changes, not the speed of light. The light from distant, redshifted galaxies has always been moving at the same speed ever since it started out on its trip to us.

But... calling it a "doppler shift" is actually not *quite* right, and it's one of those things that gets on the nerves of cosmologists when it's explained this way.

We do observe a doppler shift when we study the light from a rotating galaxy, or the radio signals from a speeding space probe, or detect a distant planet by how the planet's gravity makes its parent star swing around in a comparatively tiny circle. These are all proper examples of doppler effect. The source of a wave disturbance -- ripples in water, sound in air, light through space -- "chases" or "falls behind" it's own disturbance, so the waves are either crammed together or spread apart. Something moving toward you will seem to be emitting high-frequency waves, and something moving away from you will seem to be emitting lower-frequency waves.

But the reason distant galaxies look redshifted is because, in the time since the light was emitted, **the space between us and that galaxy has expanded.** the waves themselves have been stretched out by this expansion. It's as if you were happily playing with a giant bedsheet, sending ripples across its surface, and someone grabbed one end and stretched the sheet. The waves get stretched too, since they are part and parcel of the material. (Yes, light is a little weirder than that, but it's a working analogy, okay?) You'll hear people talk about galactic "recession velocities" but this is a convenient model, not the best explanation.

The afterglow from the earliest epochs of the universe, the first light that could travel through space at all, finally was free to travel after the Universe was 380,000 years old. This light was originally hellishly intense and high-frequency, but in the intervening 3.7 billion years, the expansion of space has stretched this light out so much that now it's microwave radiation. This cosmic background radiation corresponds to a brisk 3 degrees above absolute zero.

and by the way... if it's suggested that the Universe could be, say, only a few thousand years old because the speed of light could have changed, remember that the behavior of atoms and all matter is described by mathematics which crucially includes the speed of light. Change the speed of light and matter as we know it could not exist.

Marc Taylor
Coordinator, Andrus Planetarium
Hudson River Museum
511 Warburton Avenue
Yonkers, NY 10701
914 963 4550 x223
Fax 963 8558
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> -----Original Message-----
> From: Informal Science Education Network
> [mailto:[log in to unmask]]On Behalf Of Jack Cannon
> Sent: Friday, July 18, 2008 11:59 AM
> To: [log in to unmask]
> Subject: Doppler Shift
> 
> It seems to be very well settled that the speed of light (c) 
> is a fixed 
> constant that is not influenced by the relative speed of the 
> observer.  It 
> is about 186,282.397 miles per second in a vacuum.  c remains 
> a constant 
> irrespective of the relative speed of the observer with 
> respect to the light 
> source.  In other words, an observer moving towards the light source 
> measures the same c as well as that measured by an observer 
> moving away from 
> the light source.
> 
> It is also true that ancient galaxies exhibit a red shift in 
> the observed 
> light as viewed from earth.  This is due to the doppler shift which 
> indicates that the galaxy is moving away from us.
> 
> The question that I have is that a doppler shift (or red 
> shift) is not 
> possible unless the light being observed is moving slower 
> (causing a longer 
> wavelength) at the point of observation than the light that 
> is emitted from 
> the point source.  However it is not possible for the speed 
> of light to be 
> slower than c at the point of observation irrespective of the 
> relative 
> velocities between the source and observer.
> 
> Can anyone explain how a doppler shift can the observed from 
> an ancient 
> galaxy if the speed of light does not change?
> 
> Jack Cannon
> [log in to unmask]

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