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Basically you are saying that the light must be moving slower towards you in
order to get a red shift. This isn't the case. Red light and blue light
both move forward at the same speed, they just have different frequencies of
wave fronts. So if the light is emitted at a different frequency, or if the
wave fronts hit you (with each wave front going at the same speed c) with
more space in between them, then you have a redder (lower frequency) light.
If it helps you, imagine light as being a series of fronts. Each front is
emitted at the speed of C, but if the fronts are emitted further away from
you, the light fronts have longer to travel, meaning that there is more
distance and time between them arriving to you. This lowers the frequency
of the light we perceive - but each wave front was still moving at the speed
of c.
If you want to talk more about this off-line then drop me a note. I
pondered the same question prior to seriously studying relativity. Because
although Doppler effects happen without time dilation, I don't think we
could have argued for time dilation without a Doppler effect.
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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
ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related
institutions.
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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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