redshift - relativity?

Question

If the speed of light is the same if we move or not, according to Einstein, then why do we see a redshift if the light source is moving away at great speed? Or am i confusing speed with wavelength now?

I understand the doppler effect for normal speeds, but claim that:

"redshift contradicts the relativity concept. "

Consider what it would mean if the source was moving away at speed of light?

I would expect no shift of colour of light if the source was moving or not.
this contrary to sound/ moving sources.

Answer 1

Yes you are confusing speed with wavelength. Careful also about equating with sound, which travels through and is controlled by the medium of the air. Light is different because it always reaches the observer at a constant speed (in a vacuum), regardless of the speed of the source or the observer. It does not need a medium - it works just fine in a vacuum. This does not affect the change in wavelength due to the Doppler effect, which is a separate effect. If the source is moving away at near c and is emitting pulses at one per second, then those pulses will reach us at intervals of greater than 1 second because of the greater distance traversed by each pulse, even though they still pass by at c. The wave peaks can be thought of as pulses.

ADDED: If the source was moving away at the speed of light, then your premise violates the laws of relativity to begin with, since that speed cannot be attained by anything that is not light.

If the source is traveling away from you at 0.9c, then the speed of the light from that source will still pass by you at exactly c. The same is true if the source is approaching you at 0.9c. This is true as far as we can tell based on observations made since 1900 or so, and it's why relativity works.

However, consider the light from a source that is receding at 0.9c. It emits a yellow light at a fixed frequency (in his frame of reference). This means that every second, he puts out - say 1000 light waves. But since he is traveling away from you, each of these waves leaves from a location in space that is further and further distant from you. Therefore, each wave has to travel a longer and longer distance to reach your eyeball. Since all these waves are traveling at the same speed, they will therefore arrive at a later and later time. This will result in a frequency that is measured by you to be - not 1000 waves per second - but something less - like 500 waves per second - because each wave took longer to cross it's longer distance. So, at 500 waves per second, you will not see the light as yellow, but as red, because it has shifted in the spectrum.

Please understand that the frequencies I used here were not real but just picked as an example. I would recommend "Modern Physics" by Paul Tipler - a great text on the effects of relativity etc. Hope this helps a little.

Answer 2

the speed of light is constant yes.
Consider the Doppler effect when considering sound.
An ambulance drives past you and away, the sound of the ambulances' siren decreases in pitch, because it is moving away, and its sound waves are reaching you less often than they would if you were next to it.
In otherwords, its frequency is decreasing.
Consider light and a light source moving away from us,
the waves/particles of the light that it emits, is reaching us less often because of its motion away. Hence the frequency is 'shifted' down, or into the red part of spectrum. (red has a longer wavelength and lower frequency than light in the middle of the electromagnetic spectrum. You could search this on wiki, or in a textbook, and also look up blueshifts while youre at it.