cross-posted from: https://lemmy.ml/post/19504984
It’s all relative
OK I need someone to explain this to me cause AFAIK light speed is constant no matter how fast moving the source is
Light speed is constant but the apparent frequency and wavelength (which roughly corresponds to what colour we see) change due to the Doppler effect
So light is a wave. Shorter wavelengths are bluer longer wavelengths are redder. When you walk towards a wave you hit the peaks and troughs faster than had you been still. They come slower if you’re walking away from it. The Doppler effect is that but with light waves and velocities that make it relevant
Which is achieved by spacetime dilation which can in turn stretch or compress the light waves.
While it is true that space time dilation can cause red/blueshift, that is a distinct from the doppler effect which is the primary effect here.
(dilation plays only a small role: without time dilation our answer going from 700nm to 350nm would be 0.5c instead of the 0.6c calculated below)
Adding to the other comments; I once saw a interesting video of a visual demonstration of that effect and other weird things that happen close to the speed of light. It was this one if i remember correctly: https://www.youtube.com/watch?v=ge_j31Yx_yk
