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“But this result [that light would travel faster towards a moving observer] comes into conflict with the principle of relativity [the laws of physics are the same for all observers]", Einstein added. "For, like every other general law of nature, the law of the transmission of light must, according to the principle of relativity, be the same when the railway carriage is the reference body as it is when the enbamkment is the refernece body". [...] There should be no experiment you can do, including measuring the speed of light, to distinguish which inertial frame of refence is "at rest" and which is moving at a constant velocity.” — Walter Isaacson
But this result [that light would travel faster towards a moving observer] comes into conflict with the principle of relativity [the laws of physics are the same for all observers]", Einstein added. "For, like every other general law of nature, the law of the transmission of light must, according to the principle of relativity, be the same when the railway carriage is the reference body as it is when the enbamkment is the refernece body". [...]
There should be no experiment you can do, including measuring the speed of light, to distinguish which inertial frame of refence is "at rest" and which is moving at a constant velocity.