THE LUMINIFEROUS ETHER

In the 1800s, physicists determined that light has wavelike properties and in some ways resembles sound. But light travels much faster than sound. However, light can travel through a vacuum, whereas sound cannot. Sound waves require a material medium such as air, water, or metal to propagate. Most scientists thought light also must require some sort of medium, but what? What could exist everywhere, even in a jar from which all the air was pumped out? This mysterious medium was called luminiferous ether, or simply ether. It turned out to be nothing but a figment of the imagination.
If the ether exists, some scientists wondered, how could it pass right through everything, even the entire Earth, and get inside an evacuated chamber? How could the ether be detected? One idea was to see if the ether “blows” against the Earth as our planet orbits around the Sun, and as the Solar System orbits around the center of the Milky Way galaxy, and as our galaxy drifts through the cosmos. If there is an “ether wind,” then the speed of light ought to be different in different directions. This, it was reasoned, should occur for the same reason a passenger on a fast-moving truck measures the speed of sound waves coming from the front as faster than the speed of sound waves coming from behind.
In 1887, an experiment was done by two physicists named Albert Michelson and Edward Morley in an attempt to find out how fast the “ether wind” is blowing and from what direction. The Michelson-Morley experiment, as it became known, showed that the speed of light is the same in all directions. This cast doubt on the ether theory. If the ether exists, then according to the results obtained by Michelson and Morley, it must be moving right along with the Earth. This seemed to be too great a coincidence. Attempts were made to explain away this result by suggesting that the Earth drags the ether along with itself. Einstein could not accept that. He took the results of the Michelson- Morley experiment at face value. Einstein believed that the Michelson-Morley experiment would have the same outcome for observers on the Moon, on any other planet, on a space ship, or anywhere in the universe.