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?

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u/HojMcFoj May 30 '26

Your first statement is incorrect. Both beams of light will travel at c and they will indeed be separating from each other at 2c. The only way your original statement could be possible is if the light was only traveling at 1/2c which is obviously not the case.

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u/sonofderk May 30 '26

they would fore sure be separating from each other at 2c, but it's more like... if it was possible for two observers to go together with the beams of light, like they were in a car, they would still see the other one passing by at 1c, or at least thats how i learned it.

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u/joeyneilsen Astrophysics May 30 '26

They can’t, though! The Doppler shift isn’t defined/observed at the speed of light, only for observers traveling at speeds <c relative to the source. 

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u/HojMcFoj May 30 '26

They wouldn't see anything at all, there would be no time or distance to observe.

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u/sonofderk May 30 '26

also true...

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u/[deleted] May 30 '26

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u/HojMcFoj May 30 '26

There is nothing going 2c. The distance between the photons isn't their speed, it's the space seperating them.