The Michelson – Morley experiment provides evidence for the constancy of the speed of light – one of the postulates of the special theory of relativity. The original aim of the experiment though, since it was conducted before the special theory of relativity was published, was to find the speed of light through space. At the time of the experiment, many scientists thought space was filled with a ghostly substance called the ether.
The experiment involves a lightbeam from a coherent source split into tow parts made to travel at right angles as below.
The parts are brought together and made to interfere. Typically there will be a phase or path difference between the two paths and if one if the mirrors or the half – silvered mirror is allowed to move, the path difference will change, and constructive or destructive interference will occur successively.
The Galilean transformation, the speed of light through space is different in the two directions. If the earth is travelling to the right in the diagram above, the speed of the orange, horizontal beam, is
and
respectively, and the speed of the vertical beam is
Rotating the apparatus would change the interference pattern.
When the experiment was performed, no rotation of the apparatus would produce a change in the interference pattern, showing that the speed of light along both paths is the same, no matter how the apparatus is oriented.