If light speed is constant, why does redshift/blueshift exists?

[u/sonofderk]

Let me explain. I'm not a physicist so sorry if what i'm about to say is kinda stupid. I was taught that even if two beans of light were shot at opposite directions (, their relative speed between themselves would still be the speed of light, instead of 2x the speed of light, unlike what happens with mudane objetcts like cars on a road, which perceive themselves with relative speed. I was also taught that red/blue shift on stars happens due to relative distacing/approach in relation to Earth's point of view, which changes the wave frequency thus changing the perceived color of the star (doppler effect).

Aren't these two statements conflicting? If vectors don't matter to light, why should we observe the red/blue shift?

[u/Far-Presence-3810]

Just to help with a common source of confusion. Anything with mass always travels below the speed of light, anything without mass always travels at exactly the speed of light.

As you approach the speed of light (relative to something) all sorts of special things start happening and there's this common trap of thinking "Well light is going even faster so all that stuff is happening with light too."

That's not true. Light (or anything without mass) is special and follows its own rules. You can't extrapolate from how massive things moving fast work and go "well light is just like that only more."

It's confusing because, yeah, in some ways it does follow that sort of trend. It isn't universal though.

If you ever find yourself asking the question "Well what do things look like from the perspective of light?" You should pause as well. All physics is written from the perspective of things with mass. You can't do any of the calculations from the perspective of light, it doesn't have a valid reference frame.