Paul Ehrenfest suggested the following thought experiment to illustrate the dramatically different observations that might be expected, dependent on whether light moved relative to a stationary ether or according to Einstein’s second postulate:
Suppose that you are seated at the center of a huge dark sphere with a radius of 3 x10⁸ m and with its inner surface highly reflective. A source at the center emits a very brief flash of light which moves outward through the darkness with uniform intensity as an expanding spherical wave.
What would you see during the first 3 seconds after the emission of the flash if 

(a) The sphere moved through the ether at a constant 30 km/s 

(b) If Einstein’s second postulate is correct?