# Speed of light and red shift

## Main Question or Discussion Point

My basic understanding is that no matter how you observe light, it always a constant, it always travels at the speed of light.

So even travelling at near the speed of light you will always measure light travelling at the speed of light. If this is the case? Why do we observe red shift?

## Answers and Replies

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Matterwave
Science Advisor
Gold Member
A redshift is a change in the frequency of the light, not the speed of light. I'm not sure what your concern is?

We observe redshift, because it takes more time for a long wave pulse to pass us, if we are fleeing the pulse.

If we want to know the speed of that wave pulse, we must measure how long it takes for one point on that wave pulse to pass us.

speed = length of observer / time it takes for the middle point of the wave pulse to pass the observer

Last edited:
ghwellsjr
Science Advisor
Gold Member
My basic understanding is that no matter how you observe light, it always a constant, it always travels at the speed of light.
I'm genuinely curious: what is your understanding of how you observe light?

So even travelling at near the speed of light you will always measure light travelling at the speed of light.
I'm curious about this too: how do you measure how fast light travels?

If this is the case?
It depends on your answers to my previous two questions.

Why do we observe red shift?
Because the object that was emitting the light was moving away from us when the light started traveling towards us and/or because we are moving away from it when the light reached us. Do you think this question is related to your previous comments?