Cecil, W5DXP wrote:
"But by 1920, Einstein had reintroduced a physical ether as a necessary
condition for the general theory of relativity to be valid."

Recall the high school physics experiment of a clock ticking inside a
bell jar. The air is then pumped out. When the air is nearly all gone,
the clock ticks become nearly inaudible but you can still clearly see
the clock. If ether had been inside the jar it would have been evacuated
too and the EM lightwave would have lacked a medium, making sight of the
clock disappear along with its sound. It seems likely to me that this
experiment indicates that no medium is required for transmission of
light despite the need of a medium to transport sound.

Best regards, Richard Harrison, KB5WZI