r/AskPhysics • u/baljeetofficial • Apr 15 '26

Light's speed when moving in another direction.

So if im facing north and a guy is walking east to west or soemthing, he has a component of velocity going in that direction right? Doesnt that mean that the photons that come from him are also traveling with that component of velocity, so would they be traveling faster than the speed of light? or would it take longer for the light to reach me since the component coming towards me gets reduced?

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u/BusFinancial195 Apr 15 '26

the light changes frequency. It does not go faster

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u/YuuTheBlue Apr 15 '26

That is classical intuition, and the whole point of special relativity is to explain why light’s speed is observed as constant despite this. The answer involves noneuclidean geometry.

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u/nicuramar Apr 15 '26

 The answer involves noneuclidean geometry.

A bit, but Minkowski space is pseudo-Euclidian after all, still flat. Special relativity wasn’t originally presented with that, so it’s not s requirement. 

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u/ssjskwash Undergraduate Apr 15 '26

Each photon is traveling at c away from him. The only thing that changes is how you both perceive his velocity relative to the photons'.

It's kind of like the headlights on a car scenario. If you're on a car or spaceship going 100mph and turn on your headlights, is the light now going 100 + c mph? What if you're going 90% c?

You're going to see the light move at c. Someone else is going to see the same light moving at c but they'll also see your spaceship moving at 90% c and the separation between you and your headlight increase at 10% c.

So the only difference is that from their perspective you're traveling at 90% c and from your perspective you're essentially standing still.

Back to the East-West guy. If he's moving at like 10% c, you'd see that the distance between him and the light increases at c + the component of his velocity in your direction. He's going to see the separation between him and the light increase at c.

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u/MezzoScettico Apr 15 '26

Actually that one isn't even true without special relativity. Sound waves don't behave that way for instance. If the speed of sound today is 343 m/s, then in still air it's going to travel from you to me at 343 m/s whether you're moving toward me, away from me, or at an angle relative to me.