The concept of the "ether" as a medium through which light propagates was proposed in the 19th century as an attempt to explain the behavior of light waves. According to this hypothesis, light waves were thought to require a medium, the ether, to travel through, similar to how sound waves require air or water. The ether was envisioned as a stationary frame of reference against which the motion of objects, including the Earth, could be measured.
However, experimental evidence, particularly from the famous Michelson-Morley experiment conducted in the late 19th century, failed to detect the motion of the Earth through the hypothetical ether. This experimental result contradicted the idea of a stationary ether and led to the development of the theory of special relativity by Albert Einstein.
Special relativity replaced the notion of an absolute reference frame, such as the ether, with the principle that the laws of physics are the same for all observers in uniform relative motion. According to special relativity, there is no preferred or absolute frame of reference, and the laws of physics, including the behavior of light, are independent of the observer's velocity. This theory has been extensively tested and confirmed by numerous experiments and observations.
In the framework of special relativity, the concept of the ether as a stationary frame of reference becomes unnecessary. Instead, observers can choose any frame of reference that is convenient for their analysis, as long as they account for the relative motion between different observers.
In summary, the Michelson-Morley experiment and subsequent developments in physics, particularly the theory of special relativity, have shown that the concept of a stationary ether is not supported by empirical evidence. Instead, the laws of physics are the same for all observers in uniform relative motion, and there is no need for a preferred or coincident frame of reference like the ether and Earth.