In a book, it's as follows. Direction of propagation of electromagnetic wave and flow of energy is the same. Energy is a vector quantity. If so, when the prism (in the outer space) is pointed at the direction from where plane waves of a star light are coming, value of energy that enter both faces of the prism will be different generally. And when the prism is turned on the direction of light path, it will be renewal of the M-M experiment.