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How is the speed of light constant in all directions for all observers?

Is the speed of light constant regardless of frame of reference? If not, why is it still measured as such, and if it is, why isn't it measured as being different in different directions?

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marked as duplicate by dmckee Jan 24 '13 at 1:10

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Possible duplicate/related? physics.stackexchange.com/q/49862/11053 –  Kyle Jan 23 '13 at 19:33
    
@Kyle No, because I'm asking just about the measured speed and its relation to relativity, while that question is dealing with the pulses used to measure that speed. –  Garan Jan 23 '13 at 19:57
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The speed of light is indeed constant regardless of the frame of reference of the observer. If you're on a train travelling 1000 m/s and I'm on the earth and we both observe the same light wave, we will both measure it's speed to be the same (3e8 m/s). This is from Einstein's relativity. It also means that you and I view time differently.

Is this what you were asking?

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If I understand what you are saying correctly: If our reference frame is moving, then since time is measured differently in the direction we are moving, the speed of light will still be measured as 3e8 m/s in that direction? –  Garan Jan 23 '13 at 19:50
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Exactly. I wouldn't use the word direction since it has to do with your reference frames velocity. Time is measured differently between your reference frame and the reference from of someone moving much slower than you, but that is the principle. And for this to be true, light is the constant of the universe, not time. –  Matthew Jan 23 '13 at 20:02
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