== Historical Background ==
The experiment was performed in the days when technology had just become sufficiently advanced to accurately measure the speed of light. At that time it was known that light behaved like a wave. Scientists believed that light waves (consisting of photons) traveled through a medium referred to as the [[aether (science)|aether]] (sometimes spelled ether). The aether was believed to be fixed in space and so was regarded as an absolute frame of reference, relative to which the velocity of any object could (in theory), be measured. The concept of the eather was in some ways similar to the modern concept of "space-time" though some differences exist. Thus the idea was that absolute movement existed.
Scientists at the time thought that if they could accurately measure the relative velocity between the earth and light waves, they might get different results for the speed of the light waves moving in different directions. They believed this would happen because the earth was constantly moving, and the aether was fixed. By examining the results they could measure the relative velocity between the earth and the aether, and hence determine the absolute velocity of the earth through space. The Michelson-Morley experiment was an attempt to do that.