Please can someone help me as I really don't understand the Michelson-Morley experiment - there seem to be (from what I have read) two different versions on it. Both versions assume the same apparatus where two light beams are sent out perpendicular to one another. One version says that if a light beam is sent in the same direction as the earth is travelling then it will have a speed c-v (v being the velocity of the earth) and then after it reflects from the mirror its return journey will be c+v. The other beam is sent perpendicular to this. It puts emphasis on the fact that everything is stationary with respect to the so called "ether". In the second version, it describes the motion of the earth through the ether as creating an "ether wind" which is OPPOSITE to the direction of the earth moving through it (like a car creating a backward wind despite the air being still outside). This version says that if you send out a beam of light in the same direction as the wind (and therefore opposite direction to the earth) you will have a speed c+v and in the opposite direction a speed c-v. These versions seem completely different to me! The first one makes no mention at all about an ether wind. Both these versions come from reliable books; first version - Great Scientific Experiments (Rom Harre), second version - Why does E=mc2 (Brian Cox). Please clarify.