## Trying to measure the Ether

Imagine yourself in a boat (blue rectangle below) traveling through the water.  If you drop a stone into the water near the middle of the boat, a wave will propagate through the water toward the front and back of the boat.  If the boat is moving toward the right, the wave will reach the back of the boat first.  This is because the back of the boat is moving toward the source of the wave, and the front is running away from the source.   If you look at the upper frame in the movie below (click the triangle in the lower-left corner to start the movie), you will see that the wave reaches the back (left) of the boat first.

The lower frame in the movie shows the exact same situation from the point of view of the boat (the upper frame is from the point of view of the water).  In this frame, the boat is not moving, and the water (medium) is moving from right to left.  We see that from the point of view of the boat, the wave moving to the left is faster than the one moving to the right.   This is because the speed of the water is added to the speed of the wave (relative to the water) for the left moving wave, and the speed of the water is subtracted from the speed of the wave (relative to the water) for the right moving wave.

At the end of the 19th century, it was thought that light traveled throught a medium, in an analogous way that water waves travel through water.  Michelson and Morley attempted to measure the  motion of the earth through the ether (which was what the medium for light waves was called) by measuring the difference in the speed of light in different directions, similar to the situation in the lower frame of the movie.  They found that the speed of light was always the same in both directions.    This result produced great confusion until Einstein postulated that there is no ether (or medium throught which light travels), and that all observers measure the same speed of light no matter what their state of motion.  This conclusion lead to the special theory of relativity with its strange consequences, such as the fact that the passage of time can be different for different observers, and two events that are simultaneous for one observer, are not necessarily simultaneous for another observer.