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This question already has an answer here:

https://www.youtube.com/watch?v=CYv5GsXEf1o&t=4m54s

The speed of light is constant as long as you are measuring it from a reference frame moving at a constant velocity, such as earth

What do we have to measure it from a reference frame moving at a constant velocity ? Isn't the speed of light always constant?

I don't really understand..Please explain.

Also sorry if this question seems dumb, im a 10th grader..

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marked as duplicate by lemon, ACuriousMind, David Z Sep 17 '16 at 15:31

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicate: Does the speed of light vary in noninertial frames? $\endgroup$ – lemon Sep 17 '16 at 8:58
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    $\begingroup$ This question is not dumb at all! (Unlike the answer to this question, the one with a bazillion downvotes). It all depends on how to define velocity in accelerated frames. +1 for the duplicate. $\endgroup$ – Prof. Legolasov Sep 17 '16 at 15:05
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Two points:

  1. The speed of light is not constant for inertial reference frames - it is easy to show that the speed of light relative to the observer varies with the speed of the observer. But since this is not an answer to your question, I am not going to elaborate.

  2. According to general relativity, the speed of light is variable for non-inertial systems. For instance, the speed of photons falling towards the source of gravity DECREASES - their acceleration is NEGATIVE (in the gravitational field of the Earth the acceleration is -2g). This is a fudge factor which is absurd but indispensable - without it the gravitational time dilation fabricated by Einstein in 1911 would be incompatible with the gravitational redshift. See more discussion here:

http://www.speed-light.info/speed_of_light_variable.htm "Einstein wrote this paper in 1911 in German. [...] ...you will find in section 3 of that paper Einstein's derivation of the variable speed of light in a gravitational potential, eqn (3). The result is: c'=c0(1+φ/c^2) where φ is the gravitational potential relative to the point where the speed of light c0 is measured. Simply put: Light appears to travel slower in stronger gravitational fields (near bigger mass). [...] You can find a more sophisticated derivation later by Einstein (1955) from the full theory of general relativity in the weak field approximation. [...] Namely the 1955 approximation shows a variation in km/sec twice as much as first predicted in 1911."

Speed of light in a gravitational field? (John Rennie's answer)

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    $\begingroup$ What you say in point 1. is simply wrong. The fact that the speed of light is constant in all inertial reference frames is one of the cornerstones of the special theory of relativity. $\endgroup$ – DelCrosB Sep 17 '16 at 11:38

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