Michaelson-Morley Experiment

The period from 1860 to 1905 saw a number of attempts to detect one’s own or the earth’s motion through space by measuring the speed of pulses of light. Actually it was easier and far more accurate to compare the speeds of light traveling in different directions. If you were moving forward through space (like Bill in our thought experiment), you should see light coming from in front of you traveling faster than light from behind or even from the side. Michaelson and Morley used a device called a Michaelson interferometer which compared the speeds of pulses of light traveling at right angles to each other.
A detailed analysis of their device is not hard, just a bit lengthy. But the result was that the device should be able to detect small differences in speeds, small enough differences so that the motion of the earth through space should be observable -- even the motion caused by the earth orbiting the sun.
At this point we can summarize volumes of the history of science by pointing out that no experiment using the Michaelson interferometer, or any device based on measuring or comparing the speed of light pulses, ever succeeded in detecting the motion of the earth.​