# Speed of light in non inertial frames

## Main Question or Discussion Point

sorry to bring this up again, i have just started special relativity and the constancy of c is a bit confusing, i get how c is the same in all inertial frames but what about accelerating frames? do observers in acceleration still see c as a constant? i have seen many forum posts and they always have two people saying the opposite things, can somebody set me straight on this?

also, if the observer is moving at 0.99c and he will see c still being c, but what about a stationary observer looking at both? will he see both of them travelling almost at the same speed?

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pervect
Staff Emeritus
It really depends on which clocks and rulers you use. If you have an accelerating frame, by the equivalence principle you'll have something that looks like a local gravitational field. This means that clocks that are "up" higher in this field will run faster than clocks that are "lower" - they won't all run at the same rate.

If you use local clocks and local rulers, things are simple - the speed of light is always 'c', it doesn't matter whether you accelerate or not.

However, you will not find that the path that light travels is given by the equation x=ct in an accelerated frame - the coordinate expression for light's path is more complex than that.

Light always travels at C in a vacuum. The postulate of SR states the invariance of the speed of light. This invariance is without caveats barring the medium travelled through.

Doc Al
Mentor
sorry to bring this up again, i have just started special relativity and the constancy of c is a bit confusing, i get how c is the same in all inertial frames but what about accelerating frames? do observers in acceleration still see c as a constant? i have seen many forum posts and they always have two people saying the opposite things, can somebody set me straight on this?