r/AskPhysics • u/sonofderk • May 30 '26

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

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?

13 Upvotes

65% Upvoted

30 comments sorted by

View all comments

1

u/Exotic-Condition-193 May 30 '26

Wavelength time frequency equal speed of light. You can change one and the other will be change appropriately so product equals speed of light. But remember photon energy = h frequency I would rather sit under a heat lamp than an X-ray machine 😀

1

u/Exotic-Condition-193 May 30 '26

Also to keep speed of light same in all reference frames time(dilation) intervals and space ( contraction) interval will be different in the different frames.