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### Is speed of gravitational waves constant? [duplicate]

Is the speed of gravitational waves constant in all inertial frames? If so, why isn't this included as one of the postulates of special relativity?
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### Speed of light in a perfect vacuum

Lets' say that something is in a universe with a perfect vacuum, where nothing exists but that object. This universe has an infinite amount of space and time. Now that object emits a single photon. ...
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### Does the speed of light change? [duplicate]

I know that there is a similar questions, but I think mine is a bit different. I wonder if with the expansion of the universe the speed of light changes. It seems that the speed of light is very ...
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### Does the equivalence principle imply that light must move slower when moving away from a massive object?

Thought experiment: Elevator going up at an extreme acceleration, pulse of light bouncing up, and down between mirrors on the floor, and the ceiling. Won't it take light longer to travel from the ...
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### Speed of light in gravitational field [duplicate]

Why has the speed of light in a gravitational field always the same value $c$ when measured locally? More precise: When you are far away from any masses, that means when the spacetime is flat, and ...
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### Is there variation in the speed of light? [duplicate]

I read recently that there may be (very slight) variations in the speed of light due to the nature of spacetime itself, I had the idea that this may be due to the photon having mass, but that's ...
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### Is there a difference between what Schwarzschild vs Gullstrand-Painleve predict for c?

When we set the interval(ds) for a given coordinate(i.e. Schwarzschild coordinates) to 0 to calculate for the velocity of light in a gravitational field, we can arrive at an equation such as the one ...