For and Against the Aether: Einstein's Evolving Position and the Structured Case for a Modern Ether Plenum

Albert Einstein's position on the aether is widely summarized as a single rejection in 1905. The historical record does not support that summary. Einstein's 1905 paper declared the aether superfluous rather than impossible, and he returned to the concept twice, in his 1920 Leiden address and in a less widely discussed 1924 essay, arguing on both occasions that general relativity requires something properly called an ether provided all mechanical connotations, above all any state of motion, are set aside. This paper documents that trajectory against primary sources and addresses directly a misreading that circulates in non-peer-reviewed literature: that Einstein's later writings amount to a repudiation of relativity, and that Dayton Miller's disputed 1925 to 1933 aether-drift measurements vindicated such a reversal. Both claims fail. Einstein's 1920 and 1924 texts explicitly deny that his relativistic ether possesses a rest frame or any mechanical property, and both affirm rather than retract the constancy of the speed of light; Miller's positive result was attributed to thermal gradients by Shankland and colleagues in 1955 and shown consistent with a null result by Roberts in 2006. Having corrected that record, the paper presents a structured case for and against reintroducing a modern medium concept, four arguments on each side, stated at full strength. The arguments for include the non-zero energy density of the quantum vacuum, the Higgs field's vacuum expectation value, Einstein's own position, and the analogue-gravity correspondence. The arguments against include the null results of aether-drift experiments now bounded at approximately one part in ten to the seventeenth, the theoretical economy of dispensing with an undetectable frame, the discrediting of Miller's measurement, and the narrow surviving parameter space of Einstein-aether theory. The paper closes by identifying which of four distinct senses of aether the Ether Plenum of this series invokes, and states the specific empirical burden that invocation carries.