# The Search For Ether

# 6.4 The Search For Ether

It was once thought that there exists an invisible substance that permeates all space, known as ether. Since the Earth travels at a considerable speed in a continually changing direction around the Sun, it seemed that Earth must spend most of its time moving relative to this ether frame and hence, the speed of light measured on Earth should be different in different directions. The effect, however, was expected to be very small.

Nevertheless, in 1880, Albert Michelson (1852-1931) and Edward Morley (1838-1923) cleverly devised an interferometer that should have detected the expected discrepancies in the speed of light. To their surprise and disappointment, they found absolutely no variation in their measurement of the speed of light in different directions. Although it yielded null results, this experiment would later be famously known as the Michelson-Morley experiment. For more details involving the experiment, look at: Michelson-Morley experiment.

The Michelson-Morley experiment, together with many different experiments with the same objective, have been repeated and have never found any reproducible evidence of discrepancies in the speed of light relative to Earth. Contrary to expectations, the speed of light is the same in all directions relative to Earth's frame. In other words, it is not true that there is only a unique frame in which light has the same speed in all directions.

This conclusion was so surprising that it was not taken seriously for many years. Instead, several other ingenious theories were put forth to explain the null results of Michelson-Morley experiment, while preserving the idea of a unique ether frame. However, none of those theories were able to explain all of the observed facts, at least not in a reasonable and economical way. Today, almost all physicists have accepted that there is no ether frame and the speed of light is a universal constant in all directions in all inertia frames. The first person who accepted this was Albert Einstein (1879-1955), while first proposing his special theory of relativity.