The belief that the ether composed of matter is not part of the wave theory of light is primarily based on scientific developments and experimental evidence from the late 19th and early 20th centuries.
In the past, scientists proposed the existence of the luminiferous ether as a medium through which light waves were thought to propagate. It was believed that light waves, similar to waves in water or sound, required a material medium to travel through. This idea of the luminiferous ether was supported by observations of wave-like properties of light, such as interference and diffraction.
However, as scientific knowledge progressed, several experiments and theoretical developments challenged the notion of the luminiferous ether. One of the most famous experiments 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. Surprisingly, the experiment yielded null results, indicating that the speed of light was the same in all directions, irrespective of the Earth's motion. This result was incompatible with the concept of an ether that provided an absolute frame of reference.
These findings led to the development of Albert Einstein's theory of special relativity in 1905. Einstein proposed that the speed of light in a vacuum is constant and independent of the observer's motion. Special relativity replaced the idea of an ether with the concept of spacetime, wherein the laws of physics are consistent for all observers moving at constant velocities.
Furthermore, the wave theory of light was further refined and explained through the development of quantum mechanics and the understanding of light as electromagnetic waves. In this framework, light is described as a fluctuation of electric and magnetic fields propagating through empty space without the need for a material medium like the luminiferous ether.
In summary, the abandonment of the concept of the luminiferous ether and its exclusion from the wave theory of light is based on the experimental evidence, theoretical developments, and the formulation of alternative frameworks like special relativity and quantum mechanics that provided a more comprehensive and accurate description of the behavior of light.