Michelson-Morley Experiment

I cannot see how the Michelson Morley result has anything to do with motivating SR's claim of constant light speed regardless of observer's frame. MM experiment happens entirely in one inertial frame. Where is the observer moving relative to the source in this experiment? And MM doesn't even measure the speed of light, just shows that two paths in the same inertial frame take the same time. Please help me understand what I am missing here. Thanks, Paul
- Paul Oeuvray
Vancouver Canada

The M-M experiment doesn't just happen in one inertial frame. Over the course of a day, the Earth's rotation changes the local velocity (relative to, say, the Sun) a bit. Over the course of a year the earth's orbit changes that velocity more. So the experiment ran long enough to check that there was no sign of absolute motion at any of those times.

You might wonder how they stabilized the apparatus long enough to check for little changes over long times. They didn't. They mounted the whole thing on a rigid table, floated it on mercury, and rotated it fast enough so that the orientation of the two arms with respect to any hypothetical absolute motion would change fairly quickly. There was no sign of any effects at any time of day or year.

You mention velocity with respect to the light source, as opposed to with respect to some medium. Other experiments checked for any such effect. I believe that a version of M-M has been run using the Sun as the source. At any rate, if velocity were fixed with respect to the source, Maxwell's equations would break down, an effect not observed in any experiment.

Mike W.

(published on 12/16/2013)