The concept of the "ether" refers to a hypothetical medium that was once believed to pervade all of space. According to the luminiferous ether theory, it was thought that light waves propagated through this medium, similar to how sound waves propagate through air. However, experimental evidence and the development of special relativity in the early 20th century have shown that the existence of such an ether is unnecessary and contradicts our observations of the physical world.
One of the key experiments that challenged the existence of the ether was the Michelson-Morley experiment conducted in 1887. The experiment aimed to detect the motion of the Earth through the ether by measuring the speed of light in different directions. However, the experiment consistently yielded null results, meaning there was no evidence of the expected motion of the Earth relative to the ether. This posed a significant challenge to the prevailing understanding of the time.
Albert Einstein's theory of special relativity, developed in 1905, provided a new framework for understanding the behavior of light and the nature of reference frames. According to special relativity, the laws of physics are invariant under Lorentz transformations, which describe how measurements of space and time differ between observers in relative motion.
In the context of special relativity, the concept of a stationary ether became unnecessary and was replaced by the principle of relativity, which states that the laws of physics are the same in all inertial reference frames. This means that any inertial frame of reference, such as the Earth, can be considered as a valid frame for describing physical phenomena.
The crucial point is that the existence of an ether and the Earth being coincident frames of reference are incompatible with the experimental evidence and the principles of special relativity. The Michelson-Morley experiment and subsequent experiments have consistently shown that there is no privileged reference frame with respect to the propagation of light. The speed of light is found to be constant in all inertial frames of reference, regardless of their motion or orientation relative to one another.
In conclusion, the concept of the ether as a stationary medium through which light propagates has been invalidated by experimental evidence and the development of special relativity. The Earth, as an inertial frame of reference, is perfectly valid for describing physical phenomena, but it is not associated with a stationary ether.