Purpose of MM experiment was to calculate the velocity of the aether (ether) relative to the earth. Well u know how they set up the apparatus rite? if u dont i'll quikly tell ya.they wanted to see the interferrence patterns wen light was travelling WITH and AGAINST the ether because at that time (pre-einstein) they thought light wuld travel faster wen with the ether wind. light was direcected into the experiment towards the half-silvered mirror which allows sum light to go thru and reflects sum light. these 2 light beams reflect of other mirrors and go to the observor who detects an interference pattern (fig A). The apparatus was then rotated about 90degrees. They expected a different interference pattern...instead they got a null result. MM tried this experiment at different times of year and altitudes all times they got null results. This was later explaind by Einstein and his special theory of relativity which states light travels at same speed irrelevant of the frame of reference of the observor. Hope this clears things up for ya?