Original sentence: By the early decades of the 20th century physicists finally realized that the reason they couldn't detect the ether was that it didn’t exist.Experiments were conducted to measure the properties but either resulted in confusing or negative results.In the 19th century most physicists believed that all of space was filled with a fluid called the "Luminiferous aether".Keely delivered descriptions of the operating principles of his process on various occasions.Many physicists believed it was plausable that Keely's machines had somehow tapped into this aether.The reasons they had given for thinking it existed had been resolved by the theory of relativity.Today Luminiferous aether is rarely mentioned in textbooks
Edited sentence: Keely delivered descriptions of the supposed principles of his process on various occasions