Ether

During this class, we have looked at the propagation of waves through several different media, waves on a string or sound waves for example. Most importantly, we looked at electromagnetic waves. We said then that these waves propagate through the vacuum and do not need a medium.

This last fact was not always so clear. In fact, until the the end of the 19th century everybody believed that all waves need a physical medium to propagate in, that waves could not simply travel through a vacuum. Since one knows, of course, that sunlight moves through space for the 150 Million km long trip from the Sun to Earth, this required that all of the space between Earth and Sun was filled with some kind of medium. This medium that was proposed to exist was called "ether".

How can you detect the presence of the ether when you do not see it? Well, let's remind ourselves of the problem of a swimmer crossing a stream (the top graphic on the right was shown there and is reproduced to refresh your memory): as the swimmer moves through the river, he drifts downstream due to the motion of the medium he is moving in, in this case the river (see the animation). So if the Earth is moving through the ether, then one should be able to detect the ether from the change in the velocity of light moving in different directions, one perpendicular to the motion of the ether and one parallel. In particular, since the velocity vector of the Earth relative to the ether changes all the time, because the Earth is on orbit around the Sun (shown on the left at the present time and then 6 month later), that motion should leave a detectable effect if one performs that same experiment twice, six months apart.

This is exactly what Albert Michelson and Edward Morley at the Case Institute in Cleveland, Ohio, did in 1887. They used an extremely cleverly designed precision instrument, which is now called the Michelson-Morley interferometer, to measure the motion of Earth through the ether. (We have already discussed the workings of interferometers, and the picture on the left is shown again here to trigger your memory.)

The result of this experiment was an incredible surprise at the time: The speed of light is exactly the same in all directions, and no motion relative to the ether can be detected.

People struggled with an explanation for this astounding result, and Lorentz and Fitzgerald came up with the idea that objects moving through the ether become length-contracted when they move through the ether. It took the genius of Albert Einstein, however, to make the conceptional leap required for the new insight and it's astounding consequences: ether does not exist. Thinking through the fact that the speed of light is constant for all observers independent of the observer's motion led Einstein much further to the formulation of the theory of relativity, the topic of this chapter.