A Quora question asks: **Can you deduce time is dilated based on blue shifted light? Or can light be blue shifted without the phenomenon of time dilation?**

Here’s my answer:

Yes, and yes.

Taking the questions in reverse order, the non-relativistic Doppler effect (ie neglecting time dilation) is a blue shift for an approaching source and a red shift for one that is receding; and the non-relativistic calculation is reasonably accurate for speeds that are not too great. So, yes, light would be blue shifted without the phenomenon of time dilation. (But this is just a qualified “yes” because in fact no relative motion can occur without the phenomenon of time dilation, so I had to say “would be” rather than “can be”.)

With regard to the first question, the relativistic correction (attributable to time dilation) is basically to make a small red shift adjustment to the non-relativistic calculation (ie to make red shifts slightly stronger and blue shifts slightly weaker). So, by noting the difference between an observed blue shift and what would be predicted classically from the observed rate of approach, we can see evidence of the time dilation effect.