When the speed of light depends on its direction:
Paper
here.
Quote:
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Light does not travel at the same speed in all directions under the effect of an electromagnetic field. Although predicted by theory, this counter-intuitive effect has for the first time been demonstrated experimentally in a gas by a French team from the Laboratoire 'Collisions Agregats Reactivite' at CNRS. The researchers measured with extreme precision, of around one billionth m/s, the difference between the light propagation speeds in one direction and in the opposite direction.
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.. depends on the environment through which the light travels .. and 'space' is filled with EMFs .. the conditions/assumptions under which the question is framed, make all the difference.
Cheers