Why the ether steps in

Maxwell’s theory predicted that radio or light waves should travel at a certain fixed speed. But Newton’s theory had got rid of the idea of absolute rest, so if light was supposed to travel at a fixed speed, one would have to say what that fixed speed was to be measured relative to. It was therefore suggested that there was a substance called the “ether” that was present everywhere, even in “empty” space. Light waves should travel through the ether as sound waves travel through air, and their speed should therefore be relative to the ether. Different observers, moving relative to the ether, would see light coming toward them at different speeds, but light’s speed relative to the ether would remain fixed.